HOIS09RP2 HOIS RP for Weld Corrosion Inspection

Recommended Practice for the Non - destructive Inspection of Weld Corrosion

HOIS(09)RP2 Issue 1

A Report prepared for HOIS

By

S F Burch and N J Collett, ESR Technology Ltd

December 2009

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HOIS(09)RP2 Issue 1

Authorisation Sheet

Report Title: Recommended Practice for the Inspection of Weld Corrosion

Customer Reference: HOIS

Report Number: HOIS(09)RP2

Issue: Issue 1

Lead Author: S F Burch

7.12.09

Checked: B A Stow

9.12.09

Address for correspondence Dr S F Burch ESR Technology Ltd 16 North Central 127 Milton Park Abingdon Oxfordshire OX14 4SA UK Phone: +44(0)1235 213402 Email: [email protected] © COPYRIGHT ESR Technology Ltd This report is the Copyright of ESR Technology Ltd and has been prepared by ESR Technology Ltd for the HOIS JIP. ESR Technology Ltd accepts no liability whatsoever to any third party for any loss or damage arising from any interpretation or use of the information contained in this report, or reliance on any views expressed therein.

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mailto:[email protected]

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Executive Summary There are currently no international standards or published recommended practices covering inspection for weld corrosion despite its widespread occurrence. The reliable detection and sizing of weld corrosion is not straightforward, because the region of greatest wall loss is usually underneath the weld cap, which prevents direct use of ultrasonic 0º compression wave techniques. Only rarely is it possible to remove the weld cap. In addition, the location and morphology of weld corrosion can be highly variable, and unpredictable. There are often issues of access; these are due to the geometry changes associated with pipe welds, such as weld neck flanges, bends, valves and reducers etc. In addition, misalignment and mismatch can affect interpretation of results. This document presents a unified recommended practice for the in-service inspection for weld corrosion in carbon steel components. It makes recommendations regarding the preferred inspection methods and techniques for a variety of conditions, including: Component geometry (wall thickness and diameter). Access restrictions caused by adjacent weld neck flanges, bends, valves, reducers. Different qualities of surface finish and condition. Effects of raised surface temperature. At present, this document is limited to the inspection for weld corrosion in carbon steel welds only. The preferred technique for weld corrosion inspection is ultrasonic time-of-flight diffraction (TOFD) provided the component falls within the capabilities of the technique. It is also recommended that TOFD should be combined with the use of associated 0º pulse-echo scans to measure the wall thickness on either side of the weld cap, where possible. TOFD involves no radiation safety issues and generally allows accurate sizing around the whole weld circumference. For highest accuracy, issues concerning the sizing of flaws offset from the weld centre line need to be considered and addressed. In cases where application of TOFD is not possible, the alternative recommended technique is tangential radiography provided the arising radiation safety issues can be adequately addressed and sufficient penetration can be achieved given the component diameter and wall thickness. Due to its limited circumferential coverage, the combination of this technique with double-wall double image radiography to locate the most severely attacked section of weld is recommended. If circumstances preclude the usage of these two preferred techniques, consideration may be given to alternative techniques, including double wall radiography which provides only qualitative information on the through-wall extent of weld corrosion. Specialised techniques for inspection through the weld cap using stand-off (water column) 0º pulse-echo probes may also be applicable. It is recommended that these techniques are fully validated with regards to detection reliability and/or sizing accuracy. Based upon HOIS members’ experience, manual angled beam UT can have limitations in terms of detection reliability for this application. Examples of both false calls and false negatives (missed flaws) have been experienced during in-service inspection with this technique. Due to its inherent low sizing accuracy, follow-up using a technique having a higher sizing accuracy is recommended if the presence of weld corrosion is indicated from manual angled beam UT.

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Experience with angled beam PA techniques is limited for weld corrosion inspection, but similar limitations to those for manual angled beam UT are expected, as both techniques are based on very similar physical principles (angle-beam pulse-echo ultrasonics). Detailed guidance is given on the application of the all the above on destructive testing techniques for inspection of weld corrosion. Recommendations are made with regard to Health, Safety and Environmental considerations and inspection personnel and training.

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Contents

1 INTRODUCTION ............................................................................................1

2 SCOPE ...........................................................................................................2

3 GLOSSARY OF DEFINITIONS, TERMS AND ABBREVIATIONS ................3

3.1 Definitions and terms....................................................................................3

3.2 Abbreviations................................................................................................3

4 WELD CORROSION ......................................................................................5

4.1 Mechanisms for weld corrosion ....................................................................5

4.1.1 Introduction ................................................................................5 4.1.2 Location of Degradation.............................................................5 4.1.3 Flow Assisted Degradation ........................................................6 4.1.4 Dissolution (Galvanic) Effects ....................................................6 4.1.5 Longitudinal seam welds............................................................9

4.2 Weld Corrosion Inspection Issues ..............................................................10

4.2.1 Introduction ..............................................................................10 4.2.2 Variable weld corrosion location and morphology ...................10 4.2.3 Measurement of remaining ligament........................................13 4.2.4 Access limitations and geometry changes...............................14 4.2.5 Misalignment/mismatch ...........................................................15

5 WELD CORROSION INSPECTION TECHNIQUES.....................................16

5.1 Introduction.................................................................................................16

5.2 Outline of weld corrosion inspection techniques ........................................16

5.2.1 TOFD .......................................................................................16 5.2.2 Tangential radiography ............................................................16 5.2.3 Double wall radiography ..........................................................17 5.2.4 Manual angle beam UT............................................................17 5.2.5 Angle beam Phased Array (PA)...............................................17

5.3 Technique rankings ....................................................................................18

5.4 Other techniques for weld corrosion inspection..........................................21

5.5 Overall recommendations...........................................................................21

5.6 Surface preparation for ultrasonic inspection. ............................................22

6 TIME OF FLIGHT DIFFRACTION (TOFD) ...................................................23

6.1 Technique Description................................................................................23

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6.2 Application to weld corrosion detection and sizing .....................................26

6.2.1 Relevant standards and documents ........................................26 6.2.2 Advantages ..............................................................................26 6.2.3 Limitations................................................................................27

6.3 General Requirements for TOFD examination of weld corrosion...............27

6.4 Preparation for TOFD testing .....................................................................28

6.4.1 TOFD Setup.............................................................................28 6.4.2 Coverage .................................................................................29 6.4.3 Other aspects...........................................................................30

6.5 Calibration blocks .......................................................................................31

6.6 TOFD scanning of welds ............................................................................31

6.6.1 Modifications to design parameters .........................................31 6.6.2 Sensitivity settings ...................................................................31

6.7 Scanning/Data collection ............................................................................32

6.8 Additional scans .........................................................................................32

6.9 Data processing and Interpretation ............................................................32

6.9.1 Lateral wave straightening .......................................................32 6.9.2 Depth/Ligament Measurement.................................................33 6.9.3 Depth measurement in presence of geometry changes ..........35 6.9.4 Flaws offset from the weld centre line......................................35 6.9.5 Parallel scans...........................................................................39 6.9.6 Analysis of mode converted signals.........................................39 6.9.7 Interpretation............................................................................40

6.10 Reporting of results ....................................................................................41

6.10.1 Standard reporting ...................................................................41 6.10.2 More detailed reporting requirements (optional) ......................41

7 0º PULSE-ECHO FOR THICKNESS MEASUREMENT ADJACENT TO THE WELD CAP...................................................................................................42


7.2 General Requirements for 0º pulse-echo examination of weld corrosion...43

7.3 Equipment for 0º pulse-echo testing...........................................................43

7.3.1 Probes for 0º pulse-echo testing..............................................43 7.3.2 Instruments and data recording ...............................................44

7.4 Calibration ..................................................................................................44

7.4.1 Calibration blocks.....................................................................44 7.4.2 Calibration procedure...............................................................44

7.5 0º scanning for weld corrosion ...................................................................44

7.5.1 Sensitivity settings ...................................................................44 7.5.2 Scanning/Data collection .........................................................44

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7.6 Data processing and Interpretation ............................................................45

7.6.1 Depth/Ligament Measurement for weld corrosion ...................45 7.6.2 Uncorroded wall thickness measurement ................................45


8 TANGENTIAL RADIOGRAPHY...................................................................47

8.1 Technique description ................................................................................47

8.1.1 Introduction ..............................................................................47 8.1.2 Tangential and double wall double image radiography

combined .................................................................................48

8.2 Computed/Digital Radiography...................................................................50



8.4 General Requirements for Tangential radiographic examination of weld corrosion.....................................................................................................52

8.5 Preparation for radiographic examination...................................................56

8.5.1 Source to object distances.......................................................56

8.6 Radiography of small bore branch connections .........................................58

8.7 Examination of welds using tangential radiography ...................................58

8.7.1 Modifications to design parameters .........................................58 8.7.2 Dimensional calibration............................................................59 8.7.3 Radiographic image quality for tangential techniques .............60 8.7.4 Circumferential coverage .........................................................61

8.8 Measurement of wall thickness ..................................................................62

8.8.1 Film Radiographs.....................................................................62 8.8.2 Computed/Digital Radiography ................................................62 8.8.3 Grey-level profile analysis methods .........................................63


9 DOUBLE WALL RADIOGRAPHY................................................................65

9.1 Technique description ................................................................................65

9.1.1 Introduction ..............................................................................65



9.3 General Requirements for double-wall radiographic examination of weld corrosion.....................................................................................................67

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9.4 Preparation for radiographic examination...................................................68

9.4.1 Source to detector distances ...................................................68

9.5 Examination of welds using double-wall radiography.................................69

9.5.1 Modifications to design parameters .........................................69 9.5.2 Radiographic image quality for double wall techniques ...........69 9.5.3 Circumferential coverage .........................................................71

9.6 Measurement of remaining wall thickness..................................................72

9.6.1 Introduction ..............................................................................72 9.6.2 Key points ................................................................................72 9.6.3 Limitations................................................................................73


10 MANUAL ANGLED BEAM UT.....................................................................74


10.2 Issues with application to inspection of weld corrosion ..............................74

10.2.1 Overall approach......................................................................74 10.2.2 Signals expected......................................................................74 10.2.3 Probe angles............................................................................75 10.2.4 Example ...................................................................................75 10.2.5 Issues with sizing .....................................................................76 10.2.6 Standards and training.............................................................77 10.2.7 Trial results and experience of performance............................77


10.3.1 Relevant standards and documents ........................................78 10.3.2 Limitations................................................................................78

10.4 General Requirements for manual angled beam UT examination of weld corrosion.....................................................................................................79

10.5 Preparation for Manual angled beam UT testing........................................79

10.5.1 Probes......................................................................................79 10.5.2 Reference Blocks.....................................................................80

10.6 Manual angled beam UT scanning of welds...............................................80

10.6.1 Modifications to design parameters .........................................80 10.6.2 Sensitivity settings ...................................................................81 10.6.3 Transfer correction...................................................................81 10.6.4 Probe movement......................................................................81 10.6.5 Analysis of results ....................................................................82

10.7 Interpretation ..............................................................................................83

10.8 Additional measurements ...........................................................................83


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11 PHASED ARRAY .........................................................................................84


11.2 Limitations for weld corrosion inspection....................................................85


11.3.1 Relevant standards and documents ........................................86 11.3.2 Advantages over manual angled beam UT..............................86 11.3.3 Limitations................................................................................86

11.4 General Requirements for PA UT examination of weld corrosion ..............87

11.5 Preparation for PA UT testing.....................................................................87

11.5.1 PA probes ................................................................................87 11.5.2 Reference Blocks.....................................................................88 11.5.3 Coverage .................................................................................88 11.5.4 0º pulse-echo measurements of wall thickness .......................88

11.6 PA UT scanning of welds ...........................................................................88

11.6.1 Sensitivity settings and transfer correction ..............................88 11.6.2 Probe movement......................................................................88 11.6.3 Analysis of results ....................................................................89

11.7 Interpretation ..............................................................................................90

11.8 Additional measurements ...........................................................................90


12 INSPECTION THROUGH THE WELD CAP.................................................91

12.1 Introduction.................................................................................................91

12.2 0º scanning over weld cap with stand-off probes .......................................92

12.3 GE flexible probe ........................................................................................92

12.4 SCEXY .......................................................................................................94

13 HEALTH, SAFETY AND ENVIRONMENTAL CONSIDERATIONS. ............95

13.1 General Requirements ...............................................................................95

13.2 Preparation for Site Working ......................................................................95

13.3 Working Precautions ..................................................................................96

13.4 Radiographic techniques ............................................................................96

13.4.1 Size and strength of sources ...................................................96 13.4.2 Source containers and collimation ...........................................96 13.4.3 In-situ inspection of plant .........................................................97

14 INSPECTION PERSONNEL COMPETENCE...............................................98

14.1 General.......................................................................................................98

14.2 Standard techniques...................................................................................98

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14.2.1 Manual angled beam UT..........................................................98 14.2.2 Radiographic techniques .........................................................98

14.3 Specialist techniques..................................................................................99

14.3.1 TOFD and 0º Pulse-echo.........................................................99

14.4 Experience/Competence ............................................................................99

14.5 Recertification.............................................................................................99

15 APPLICATION CONSIDERATIONS ..........................................................100

15.1 Weld datum and numbering system.........................................................100

15.2 Coverage Limitations................................................................................100

15.3 Surface Condition .....................................................................................100

15.4 Inspection History .....................................................................................100

16 CONCLUSIONS .........................................................................................101

17 ACKNOWLEDGMENTS.............................................................................103

REFERENCES........................................................................................................104

Appendices

APPENDIX 1 SUMMARY OF MEMBERS QUESTIONNAIRE AT THE START OF PROJECT...................................................................................................106

APPENDIX 2 FORMULAE FOR ANALYSIS OF TOFD MODE-CONVERTED SIGNALS....................................................................................................111

APPENDIX 3 SUMMARY TABLE........................................................................113

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1 Introduction

Preferential attack in the form of corrosion or erosion associated with welds is a relatively common issue within the oil and gas industry. It affects the in-service integrity of plant, particularly those components manufactured from carbon steel. Indeed there have been a number of un-expected leaks and other incidents caused by this form of degradation. As the age of assets in, for example, the North Sea and elsewhere increases, this form of degradation is set to become an increasingly significant integrity issue. Changes in operational mode from dry gas to wet processing and transportation is also likely to increase this type of damage. Despite the widespread occurrence of weld corrosion, members of the HOIS JIP have identified a lack of well defined inspection methodologies or strategies for its reliable detection and sizing. For example, there are no international standards or published recommended practices covering inspection for weld corrosion. Instead, various different approaches and inspection methods are employed by different operators. The reliable detection and sizing of weld corrosion is not straightforward because the region of greatest wall loss is usually underneath the weld cap, which prevents direct use of ultrasonic 0º compression wave techniques using contact probes. Only rarely is it possible to remove the weld cap. In addition, the location and morphology of weld corrosion can be highly variable, and unpredictable. There are often issues of access; these are generally due to the geometry changes associated with pipe welds, such as weld neck flanges, bends, valves and reducers etc. The recommendations contained in this document were derived from a number of sources. Firstly, feedback from HOIS members with many years experience of inspection of weld corrosion. Secondly, the recommendations take account of the results of a series of evaluation trials, which were based on test specimens containing a variety of examples of naturally occurring weld corrosion supplied by HOIS members (Sarsfield, Collett and Burch, 2009).

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2 Scope

This document is a recommended practice for the non-destructive in-service inspection of carbon steel welded components for weld corrosion on the inside surface. This includes degradation which is variously referred to elsewhere as:

Weld root corrosion (WRC) Weld root erosion (WRE) Preferential weld root corrosion (PWRC)

In this document however the term weld corrosion will be used throughout. For weld corrosion inspection, both detection and through-wall sizing methods are covered. It is assumed throughout this document that the inspection is to be performed with the weld cap in-situ. If instead the weld cap is removed (e.g. by grinding), then contact 0º pulse-echo ultrasonic inspection techniques are applicable1, and the issues addressed within this document are largely irrelevant. However, weld cap removal of already in-service pipework, is often precluded due to the risk of a resulting hydrocarbon or other leak. The inspection techniques recommended in this document are appropriate for weld corrosion that occurs at the weld root, and/or extends beyond the centre line of weld, approximately as far as the edge of the weld cap (external weld toe). This document is not appropriate for forms of corrosion/erosion that occur further from the weld than the edge of the weld cap, which may or may not be associated with the weld, as the inspection issues involved are less significant. The document provides recommendations for the optimum inspection methods and techniques to be used over a variety of conditions, including: Component geometry (wall thickness and diameter), Access restrictions caused by adjacent weld neck flanges, bends, valves, reducers. Different qualities of surface finish and condition. Effects of raised surface temperature The limitations of the individual recommended techniques with respect to the above factors are given. This initial version of the recommended practice is limited to carbon steel welds only, and the focus to date has been on the inspection of circumferential welds and not axial seam welds which very rarely experience this form of degradation. Inspection of austenitic materials, and other corrosion resistant alloys, is possible, but requires specific modifications outside the approach covered in this document at present. Consideration has been given to the best practice for each of the recommended inspection methods and techniques. Reference has been made to relevant international standards and other published documents where appropriate. The document does not cover the use of permanently installed sensors to monitor the progression of weld corrosion over extended time periods.

1 Provided the expected flaw morphology is regular weld corrosion which presents a good reflective surface for a 0° probe. If other flaws are of concern, such as ‘knife edge’ erosion, satellite fusion flaws, or fatigue cracks, an alternative or supplementary technique must be used.

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3 Glossary of definitions, terms and abbreviations

3.1 Definitions and terms

Cavitation is sometimes considered a special case of erosion-corrosion and is caused by the formation and collapse of vapour bubbles in a liquid near a metal surface. Cavitation removes protective surface scales by the implosion of gas bubbles in a fluid. Erosion-corrosion: Erosion-corrosion is associated with a flow-induced mechanical removal of the protective surface film that results in a subsequent corrosion rate increase via either electrochemical or chemical processes. Knife-Line Attack (KLA) is a form of intergranular corrosion of an alloy, usually stabilized stainless steel, along a line adjoining or in contact with a weld after heating into the sensitization temperature range. Weld corrosion Any form of corrosion or erosion associated with a weld. In this document, taken to include all weld associated corrosion/erosion mechanisms referred to by other terms, including weld root corrosion (WRC), weld root erosion (WRE) and preferential weld root corrosion (PWRC). The scope of the present document covers inspection for weld corrosion from the weld root to the edge of the weld cap. For weld corrosion further displaced from the weld other simpler inspection techniques may often be used (e.g. those based on 0º pulse-echo ultrasound).

3.2 Abbreviations

b Component to detector distance BSR Basic spatial resolution. Term used in digital/computed radiography c Ultrasound velocity CR Computed Radiography d Radiation source size for calculation of geometric unsharpness DR Digital Radiography DWDI Double wall double image (radiography). DWSI Double wall single image (radiography). FFS Fitness for service FSH Full screen height HAZ Heat affected zone associated with a weld IS Intrinsically Safe

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MIC Microbiologically influenced corrosion - a mode of corrosion incorporating microbes that react and cause the corrosion or influence other corrosion processes of metallic materials.

OD Outside diameter of a pipe P Ultrasonic compression wave PA Phased Array PWC Preferential weld corrosion. A form of weld corrosion PWRC Preferential weld root corrosion. A form of weld corrosion SDD Source to detector distance, as used in radiography. SNRmeas Measured signal to noise ratio of a CR/DR image. SNR_N Normalised signal to noise ratio, after normalisation using the BSR as used to

measure CR/DR image quality SPD Source to the pipe axis distance. SV Ultrasonic vertically polarised shear wave TOFD Time of flight diffraction – twin-probe ultrasonic technique. UT Ultrasonic (testing). WRC Weld root corrosion. A form of weld corrosion WRE Weld root erosion. A form of weld corrosion w Maximum penetrated thickness through pipe wall at tangential position WT Wall thickness

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4 Weld Corrosion

4.1 Mechanisms for weld corrosion

4.1.1 Introduction

Weld corrosion is a significant concern for the oil and gas industry. There is a particular need to be able to inspect for the onset of damage to allow adequate and timely remediation to be carried out before the equipment integrity is compromised. The assessment of suitable techniques to inspect for weld corrosion is complicated by the fact that weld corrosion does not occur by a single mechanism. There are a number of distinct mechanisms by which the material in the vicinity of the weld may be preferentially attacked. Weld root erosion (WRE) is specifically related to the situation where fluid flow results in disruption of the protective oxide layer, thereby allowing accelerated corrosion in the vicinity of the flow disturbance, typically the weld bead. The term weld root erosion is also often used to cover preferential weld root corrosion (PWRC) which is typically attributable to variations in galvanic potential. This section describes these mechanisms which, collectively, can be termed as “weld corrosion” in carbon steel components. The terminology shown in Figure 4.1 is used in the discussion, where HAZ is the heat affected zone.

Figure 4.1 Terminology.

4.1.2 Location of Degradation

Depending on the principal mechanism which is responsible for the degradation, the material loss will be predominantly from different areas of the weld as shown in the Figure 4.2.

(a) Up-stream corrosion (b) Downstream corrosion

(c) Groove corrosion (d) HAZ/fusion line corrosion

Figure 4.2 Typical locations of different forms of weld corrosion

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The location of the degradation does not of itself define the underlying cause of the corrosion / erosion, as the mechanisms can preferentially affect different regions of the weld zone depending on the precise metallurgy. The various mechanisms which can result in weld root erosion / corrosion are discussed in more detail in the following sections.

4.1.3 Flow Assisted Degradation

With pipe and flow lines, it is often difficult to control the formation of the weld bead at the inner surface of the weld. Furthermore, it is not cost effective or practical to remove any excess penetration or discontinuity (e.g. due to misalignment). Therefore it is not uncommon for there to be a surface discontinuity which will disturb the flow at the pipe surface. This can lead to erosion-corrosion, as defined below. Erosion-corrosion: Erosion-corrosion is associated with a flow-induced mechanical removal of the protective surface film that results in a subsequent corrosion rate increase via either electrochemical or chemical processes. It is often accepted that a critical fluid velocity must be exceeded for a given material. The mechanical damage by the impacting fluid imposes disruptive shear stresses or pressure variations on the material surface and/or the protective surface film. Erosion-corrosion may be enhanced by particles (solids or gas bubbles) and impacted by multi-phase flows. The morphology of surfaces affected by erosion-corrosion may be in the form of shallow pits or horseshoes or other local phenomena related to the flow direction. Other factors such as turbulence, cavitation, impingement or galvanic effects can add to the severity of attack. At certain flow velocities the resulting turbulence or cavitation can result in erosion of material downstream of the discontinuity. For a description of cavitation, see below. Cavitation is sometimes considered a special case of erosion-corrosion and is caused by the formation and collapse of vapour bubbles in a liquid near a metal surface. Cavitation removes protective surface scales by the implosion of gas bubbles in a fluid. The subsequent corrosion attack is the result of hydro-mechanical effects from liquids in regions of low pressure where flow velocity changes, disruptions, or alterations in flow direction have occurred. Cavitation damage often appears as a collection of closely spaced, sharp-edged pits or craters on the surface. With some metals, e.g. steel, the underlying material is normally protected (passivated) by the formation of an oxide film. However the localised turbulence downstream of the perturbation can hinder formation of this passive layer, by constantly scouring the surface clean, thereby allowing accelerated corrosion to take place. This form of weld corrosion typically results in damage to the downstream surface of the pipe, in the parent material.

4.1.4 Dissolution (Galvanic) Effects

The second mechanism involves the selective corrosion of a part of the weld zone, due ultimately to a difference in the galvanic potential of adjacent zones, correctly identified as Preferential Weld Root Corrosion (PWRC). This may be the result of selective leaching (dissolution), constituent chemical composition or precipitation of elements during the welding process. If the weld material is more susceptible to corrosion than the base material, wash out of the weld causes root corrosion (groove corrosion) as illustrated in Figure 4.3.

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Figure 4.3 Preferential Corrosion of Weld Root (photo courtesy Oceaneering) Figure 4.4 illustrates the mechanism of galvanic corrosion.

Figure 4.4 Galvanic Corrosion of Welds

If the HAZ is more susceptible, then the attack can take the form of grooves either side of the weld bead, typically with widths of 2-3mm. This form of attack is often referred to as HAZ corrosion. [In stainless steels, knife-edge or knife-line attack (KLA) can take place, as defined below. Knife-Line Attack (KLA) is a form of intergranular corrosion of an alloy, usually stabilized stainless steel, along a line adjoining or in contact with a weld after heating into the sensitization temperature range. The corrosive attack is restricted to an extremely narrow line adjoining the fusion line. The

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attack appears razor-sharp (and hence the name of "knife-line" attack), and typically has a width of 0.5 mm. It is possible to visually recognize knife-line attack if the lines are already formed along the weld]. In welded joints, the weld metal and/or HAZ corrode selectively if they are less noble than the parent metal, which is commonly much larger in surface area than the weld metal and HAZ. Additions of alloys such as Cr, Mo, Cu and Ni into the weld material have been used to make the weld metal less anodic. However, this can lead to other problems, such as the HAZ preferentially corroding instead. In carbon dioxide environments, welds with Ni additions can suffer from preferential weld corrosion (PWC). However, the mechanisms are not fully understood and in some cases welds with Ni additions have not experienced PWC. Factors known to be relevant to PWC include:

Electrical properties of the materials and any corrosion cell forming around the weld joint, including:- Electrical resistance, Corrosion current and Potential with respect to the weld metal (WM), heat affected zone (HAZ) and parent material (PM).

Water phase liquid film thickness and conductivity. Temperature and tendency to form corrosion product (protective) scales. Corrosion inhibitor effectiveness, (inhibitor film formation and composition). Pre-corrosion times, (uninhibited period before application of any corrosion inhibitor). Flow induced shear stress. Weld joint metallurgy.

To avoid PWC, the corrosion potential of the weld filler material should be almost the same as that of the parent metals. In practice, however this is rarely the case and frequently they are not exactly the same, due to differences in chemical composition. It has been reported that in some cases, the weld filler material can be as much as 40 mV less noble than the parent material for which they are designed and specified. Depending on the relative potential of the different zones, specific areas will be preferentially corroded. The flow direction can also influence the migration of electrons within the galvanic cell thus giving a preference to corrode on one side rather than the other, as shown in the lower diagram of Figure 4.4. Another mechanism, which can result in differences in galvanic potential in certain materials, is selective leaching. The removal of the least noble metals results in deterioration of the lattice structure in alloys (e.g. dezincification in brass components). The consequential potential difference leads to the establishment of a galvanic cell and preferential corrosion of the anodic side of the cell. Other suggested theories for preferential attack in carbon steel welds include variation in hardness/mechanical properties, and levels of Si in weld metal. Figure 4.5 gives an example of weld root erosion/corrosion in a flowline.

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Figure 4.5 Weld Root Erosion in a Flowline The welding process can also result in chemical segregation effects (impurities have a tendency to be enriched at grain boundaries) or specific phases precipitated at grain boundaries (formation of a continuous network of dendritic carbides of the M6C and Mo2C types in the grain boundaries). Such precipitation can produce zones of reduced corrosion resistance in the immediate vicinity. [A classic example is the sensitization of stainless steels, where chromium-rich grain boundary precipitates lead to a local depletion of chromium immediately adjacent to these precipitates, leaving these areas vulnerable to corrosive attack in certain electrolytes. This problem is exacerbated in the heat-affected zones of welds, where the thermal cycle of welding has produced a sensitized structure, leading to knife-edge corrosion. Note however this mechanism is not relevant to the present document, which covers carbon (ferritic) steel welds only].

4.1.5 Longitudinal seam welds

Longitudinal seam welds are generally only present in pipeline applications and large diameter process pipework (20" diameter and larger). Seams are welded via a mechanised process, whereas circumferential butt welds will normally be welded by a manual/semi-automatic process. The surface profiles of longitudinal seam welds are consequently usually much more uniform in nature than those for a circumferential weld. The root is usually smooth, ripple free, with a shallow and even depth due to the consistent heat input. Because of these factors, longitudinal seam welds are not as prone to this type of corrosion attack as circumferential welds. The presence of weld corrosion in longitudinal seam welds is indeed very rare. The issues involved in the application of NDT techniques to longitudinal seam welds are unlikely to be substantially different from those needed for the circumferential welds in the same component.

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4.2 Weld Corrosion Inspection Issues

4.2.1 Introduction

The reliable detection and sizing of weld corrosion presents several challenges. Unlike the inspection of plain piping or vessels, direct measurements using ultrasonic 0º compression wave probes are generally not possible due to the presence of the weld cap. Furthermore, as described in Section 4.1, there are a number of different mechanisms which cause weld corrosion, which leads to a significant variability in weld corrosion location and morphology. In practice there may be access issues because pipe welds are usually associated with adjacent changes in geometry. These complications are now considered further below.

4.2.2 Variable weld corrosion location and morphology

As described in Section 4.1, there are a number of different mechanisms that lead to wall loss associated with welds. These multiple mechanisms cause complications in the reliable detection and sizing of weld corrosion because both the location and morphology of the weld corrosion can be variable and unpredictable. Some of these variations are illustrated schematically in Figure 4.6.

(a) (b)

(c) (d) Figure 4.6 Examples of different positions and types of weld corrosion: (a) non degraded

weld, (b) moderate weld corrosion near to the weld centre line, (c) moderate weld corrosion offset from the weld centre line, (d) more severe knife edge corrosion in the heat-affected zone, occurs only in stainless and high alloy steels (not covered by the present document).

Figure 4.6 illustrates that the weld corrosion can be located either close to the weld centre line, or significantly offset from it. In addition, the morphology of the more common groove corrosion is inherently variable both in detailed shape and through-wall extent along the welding direction. Groove corrosion is often characterised by reasonably high aspect ratios (i.e. lateral extent/though-wall extent >>1), although examples have been seen of asymmetric groove corrosion (gradual on one side, but with an almost vertical “cliff edge” on the other). An example of variable groove corrosion morphology is shown in Figure 4.7, which gives replicas taken at 90º intervals from a single weld. It can be seen that at all angles, the weld corrosion is asymmetric, with a steeper edge on the right side (downstream) than on the left (upstream). The through-wall extent varied circumferentially from c. 2 mm to c. 5mm, in this example.

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Figure 4.7 Replicas taken at 90º circumferential intervals from a corroded weld in a 3" pipe The less common knife edge attack is characterised by low aspect ratios, with sharp-tipped, almost crack-like morphology at the “top” edge nearer the outer wall. This form of weld corrosion is often offset from the weld centre-line as it is generally associated with the weld heat-affected zone. This form of degradation is associated with stainless and high alloy steels only, beyond the scope of this document. Weld corrosion can be present in different locations in the same weld. For example, there can be present both attack at the weld root and offset from the weld centre-line in two (or more) distinct bands on the same weld. There can also be a combination of corrosion and erosion, with erosion damage some distance from the weld, as illustrated in Figure 4.8. If the erosion is further from the weld than the edge of the weld cap, it’s inspection is beyond the scope of the current document (see Section 2).

Figure 4.8 Examples of both weld corrosion and erosion damage some distance from the

welds. The through-wall extent of the weld corrosion is often variable in the direction parallel to the welding direction (e.g. circumferential for a pipe butt weld) and across the width of the weld, see photos below, as illustrated in Figures 4.9 and 4.10. More extreme examples of this include isolated pits of weld corrosion in welds with very little or no corrosion elsewhere. Thus to locate and measure the minimum remaining ligament, it is often necessary to make measurements at a fine interval, along the full length of the weld.

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Figure 4.9 Illustration of variable depth of weld corrosion parallel to the welding direction

Figure 4.10a Further examples of weld corrosion which are variable in the along weld and

across weld directions. The left hand image shows CO2 corrosion attack of weld metal.

Figure 4.10b Further example of weld corrosion showing the combination of CO2 corrosion

of the parent material and some weld metal attack along with MIC.

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Mismatch and other forms of geometry change from one side of the weld to the other (e.g. tapers from weld neck flanges) can also cause complications for a number of inspection techniques.

4.2.3 Measurement of remaining ligament

For through-wall sizing of the remaining ligament, it is necessary to consider the presence of the weld cap, which is generally raised above the surrounding parent metal, as illustrated in Figure 4.11.

Figure 4.11 Illustration of differing ligament values measured by different NDT techniques Typically ultrasonic inspection techniques (e.g. TOFD – see Section 6) based on probes positioned on the parent material, measure the remaining ligament relative to the inspection surface on the parent metal, ignoring the additional thickness of the weld cap. This distance is shown as “Ligament 1” on Figure 4.11. This dimension is related to the wall loss and the parent material wall thickness by the following relationship: Ligament 1 = Parent material wall thickness – Wall loss Tangential radiographic methods (see Section 8), and 0º ultrasonic techniques which penetrate the weld cap (see Section 12), measure the ligament as shown as the dimension “Ligament 2” on Figure 4.11. This is different from that measured by techniques based on probes positioned on the parent metal, since the weld cap thickness or height is included in Ligament 2 and not Ligament 1: Ligament 2 = Ligament 1 + Weld Cap height Thus for the same weld, the measurements of remaining ligament will not necessarily agree when measured by different techniques, and to obtain consistency it may be necessary to take into account the weld cap thickness. For welds with mismatch, misalignment and other geometry changes, there may also be an issue on how to measure the remaining ligament with a number of techniques. In general, if the wall loss due to weld corrosion is found to be close to breaching performance standards then this issue about definition and measurement of remaining ligament should be given specific attention on a case by case basis.

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4.2.4 Access limitations and geometry changes

Often pipe work welds are associated with a change of geometry. The following are some of the more common forms of geometry changes: Weld neck flanges Bends Valves Reducers These can complicate the application of inspection methods/techniques which require access to both sides of the weld, and even in some cases complicate the application of single-sided techniques. Welds between different wall thickness sections (mismatch) and also welding misalignment can also present complications for methods inspecting from both sides of the welds. An example of a component showing some of the above access limitations (weld neck flanges and bends) is given in Figure 4.12. Note however that the overall spool geometry shown here is extremely unusual and not typical of what is usually found in practice.

Figure 4.12 Component showing welds adjacent to bends and weld neck flanges.

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4.2.5 Misalignment/mismatch

Factors such as mismatch and misalignment can significantly influence the results of an inspection and lead to over or underestimation of flaw sizes, with some inspection techniques. Those most likely to be affected are the ultrasonic techniques including TOFD, manual angled beam UT and angled beam PA – see Sections 6, 10 and 11 respectively. For other techniques such as tangential radiography (Section 8), there are likely to be fewer complications arising from these effects due to the more direct imaging of the component cross-section obtained. In cases where the geometry may affect sizing accuracy and the results are to be used in a Fitness for Service assessment, an analysis of possible sizing errors should be made to ensure that a conservative value of minimum wall thickness is used in the assessment.

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5 Weld corrosion inspection techniques

5.1 Introduction

This section provides an overview of the main NDT techniques for the inspection for weld corrosion/erosion. Detailed guidance on their application to weld corrosion inspection is given in later sections. Comments are also given on known limitations and HOIS members’ experiences of practical application of these techniques to weld corrosion inspection.

5.2 Outline of weld corrosion inspection techniques

Techniques for weld corrosion inspection are described below. For further more detailed information see Sections 6-10.

5.2.1 TOFD

TOFD is an advanced twin-probe ultrasonic technique, requiring access to both sides of the weld cap. TOFD generally involves use of positionally encoded scanners (either motorised or manual) linked to a computer for data capture and display. The technique is based on measurement of signal arrival times, which are then converted to remaining ligament values using a mathematical formula. TOFD requires sophisticated software for data processing and interpretation. It is generally accepted as being capable of high detection reliability and high accuracy in measurement of remaining ligament, provided it is being used within its range of applicability, by suitably trained and competent operators. For further details, see Section 6.

5.2.2 Tangential radiography

Tangential radiography is generally applied in-service to pipes using an isotope source such as Iridium 192, combined with either film or a re-usable imaging plate which is scanned using a laser (computed radiography). The tangential technique gives a direct image of the pipe walls, showing the presence, shape and extent of any weld corrosion present in the weld at the tangent position in the pipe. Dimensional calibration using a comparator or known pipe OD allows direct measurement of the remaining ligament at the tangent position in the pipe. This technique is often combined with double-wall double image radiography (DWDI) on smaller diameter pipes – see Section 5.2.3. There are safety issues associated with use of ionising radiation, but this technique does not require access to the component surface and is unaffected by coatings, poor surface condition or presence of thermal insulation. Detection reliability and sizing accuracy, provided it is being deployed on appropriate components, using suitably trained and competent operators are expected to be good. Note however that the measurement of remaining ligament is made only at the tangent position, and full circumferential measurements are infeasible due to time restrictions. For further details, see Section 8.

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5.2.3 Double wall radiography

Double wall radiography is usually carried out using the same equipment as tangential radiography. For pipe inspection, double wall radiography can be applied as double-wall double image (DWDI), which involves the source remote from the pipe. DWDI is usually used for small diameter pipes (typically 4” OD). For larger diameter pipes the double-wall single image (DWSI) technique is often used, in which the source is positioned close to one pipe wall. There can be limitations due to the presence of liquid product in the pipe which increases the attenuation of the radiation beam. In both forms of double wall radiography, the presence of weld corrosion can be detected by changes in film density or image grey level (for CR). Some qualitative information on the through-wall extent of the wall loss is also available from the amount of the changes in film density or image grey level, but this is not a quantitative technique, unlike tangential radiography. DWDI is often used to provide guidance on the location of the deepest section of the attack, which then allows appropriate circumferential alignment of the source and film/detector to apply the tangential technique to measure the minimum remaining ligament. For further details, see Section 9.

5.2.4 Manual angle beam UT

This is an ultrasonic technique, based on use of a single manually deployed angled shear-wave pulse-echo probe linked to a portable flaw detector (generally now digital) which shows the waveform (A-scan) obtained at the current probe position. Manual angle beam UT is a standard technique used extensively for routine in-service and manufacturing inspection, but can suffer from significant limitations for weld corrosion inspection, in terms of both detection reliability (both false positives and false negatives) and low sizing accuracy. For further details, see Section 10

5.2.5 Angle beam Phased Array (PA)

The phased array (PA) technique is a relatively newly developed ultrasonic method which involves the use of a special multi-element pulse-echo probe linked to a portable computerised instrument. Phased array instruments allow the angle of ultrasound beam to be varied, and various different colour presentations of the resulting ultrasonic data can be generated (Sector scans, C-scans, B-scans etc). This technique has not to date been widely used for weld corrosion inspection, and shares many of the limitations of manual angle beam UT, since both techniques involve use of single-sided pulse-echo probes. PA does however provide some advantages over manual angled beam UT due the presentation of results from multiple probe angles in a single display (sector scan). Also if encoded scanning is undertaken, some instruments allow permanent storage of the collected data, and display of the results obtained at different probe positions in one image. For further details, see Section 11.

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5.3 Technique rankings

Table 5.2 gives subjective rankings of the techniques listed in Table 5.1, for components for which the techniques are applicable. Some notes on the column headings given in Table 5.2 are as follows: Specialist or standard technique

A standard technique is a well established, traditional NDT technique used routinely for many years, with well established international standards. A specialist technique is usually more recently developed and typically involves more advanced, digital data collection, display, storage and analysis involving a computer. Some of the more established specialist techniques have international standards, but more recently developed ones do not.

Detection reliability

Reliability of detecting weld corrosion anywhere in the vicinity of a weld, assuming the technique is being applied in conditions in which it is applicable to the component under inspection.

Sizing accuracy Accuracy of the through-wall sizing capability of the technique, including finding the minimum remaining ligament.

Sensitive to surface condition

Ultrasonic methods are sensitive to poor surface finish, especially loose, or flaking paint/coatings. For information on surface preparation requirements for ultrasonic techniques see Section 5.6. Radiographic techniques are much less affected by poor surface condition.

Safety Issues Lists the main safety issues associated with the deployment of the inspection technique.

Application to complex geometries, e.g. branch connections?

This column is intended to highlight issues associated with complex geometries. Certain geometries such as branch connections cannot be inspected using UT, but radiography is more generally applicable to these types of components, provided double sided access is possible (i.e. source on one side of component, detector/film on the other side).

Ease of deployment

How easily the equipment needed for a technique can be deployed for in-service inspection in a typical oil & gas plant environment.

Ease of interpretation

How easily the results from the inspection technique can be interpreted, i.e. the level of qualifications needed to be competent in analysis.

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Table 5.2 Ranking of main techniques for weld corrosion inspection Technique Specialist

or standard technique

Detection reliability

Sizing accuracy

Sensitive to surface condition

Safety issues

Application to complex geometries, e.g. branch connections?

Ease of deployment

Ease of interpretation

TOFD Specialist High High Yes Equipment not IS

Not possible Medium Medium

Tangential radiography1

Standard (film) Specialist (CR)

High2 Medium/High No Ionizing radiation hazards

Generally OK if double sided access possible

Low High

Double wall radiography1

Specialist (CR) Standard (film)

High Qualitative only

No Ionizing radiation hazards

Generally OK if double sided access possible

Low High

Angle beam manual angled beam UT

Standard Can be Low3

Low4 Yes Equipment not IS

No? High Medium

Angled beam phased Array (PA)

Specialist Can be Low3

Low4 Yes Equipment not IS

No? High Medium

Footnotes:

1 Radiography can be performed using either film (standard technique), or as the more specialised computed radiography (CR).

2 Tangential radiography has high detection reliability if the weld corrosion is present in the tangent position. However, the coverage of this technique is limited to circumferential positions very close to the tangent position. For this reason, this technique is often used in conjunction with double wall double image radiography to locate the circumferential position of the most severe weld corrosion.

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3 If angled beam manual UT or angled beam PA is used for weld corrosion inspection, the operator should be aware that the detection reliability of both these techniques in practice can be low. There were no examples of failure to detect weld corrosion in the HOIS weld corrosion inspection trials. However, in HOIS members’ experiences, historically there have been a number of documented examples in which angled beam manual angled beam UT has either failed to detect the presence of weld corrosion, or has given false calls (indications that are not due to weld corrosion). There is little direct experience of the performance of in-service angled beam PA for this application, but the same limitations could well be present in this technique, as it is based on very similar physical principles.

4 The HOIS weld corrosion inspection trials showed variable sizing accuracy for both manual angled beam UT and angled beam PA. For some

welds, the sizing results were similar to those obtained with the other techniques listed in this table. However, in other cases, the sizing was clearly inaccurate. It is generally acknowledged by HOIS members that the inherent imitations of these single sided ultrasonic inspection techniques are likely to give inaccurate sizing of weld corrosion in some cases, such as for severe weld corrosion in which the minimum ligament position on the flaw is outside the zone of coverage of the ultrasonic probe.

For further details on technique capabilities and limitations, see Sections 5-11, and Appendix 3. .

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5.4 Other techniques for weld corrosion inspection

There are some specialised and less widely used techniques that are not included in Sections 5.2 to 5.3. These include those based on inspection through the weld cap (e.g. 0º compression wave pulse-echo). Information on these techniques can be found in Section 12.

5.5 Overall recommendations

In drawing up the recommendations contained in this section, account was taken of the many years of experience built up by the HOIS membership on the performance of NDT techniques for practical, in-service inspection of weld corrosion. In addition, these recommendations are based on the results from the HOIS technique evaluation trials on components containing a range of examples of naturally occurring weld corrosion (Sarsfield, Collett and Burch, 2009). For all techniques, it is important that they are applied by suitably trained and competent operators. The preferred technique for weld corrosion inspection is ultrasonic time-of-flight diffraction (TOFD) provided the component falls within the capabilities of the technique, as described in Section 6.2. It is also recommended that TOFD should be combined with the use of associated 0º pulse-echo scans to measure the wall thickness on either side of the weld cap, where possible. TOFD involves no radiation safety issues and generally allows accurate sizing around the whole weld circumference. For highest accuracy, issues concerning the sizing of flaws offset from the weld centre line need to be considered and addressed. Surface condition is a key issue affecting the field performance of TOFD and other ultrasonic techniques. See Section 5.6 for further information. In cases where application of TOFD is not possible, the alternative recommended technique is tangential radiography provided the arising radiation safety issues can be adequately addressed and sufficient penetration can be achieved given the component diameter and wall thickness (see Section 8.3 for further details). Due to its limited circumferential coverage, the combination of this technique with double-wall double image radiography to locate the most severely attacked section of weld is recommended. If circumstances preclude the usage of these two preferred techniques, consideration may be given to alternative techniques, including double wall radiography which provides only qualitative information on the through-wall extent of weld corrosion and specialised techniques for inspection through the weld cap using stand-off (water column) 0º pulse-echo probes. It is recommended that these techniques are fully validated with regards to detection reliability and/or sizing accuracy. Based upon HOIS members experience, manual angled beam UT can have limitations in terms of detection reliability for this application. Examples of both false calls and false negatives (missed flaws) have been experienced during in-service inspection with this technique. Due to its inherent low sizing accuracy, follow-up using a technique having a higher sizing accuracy (TOFD or tangential RT) is recommended if the presence of weld corrosion is indicated from manual angled beam UT. Experience with angled beam PA techniques is limited for weld corrosion inspection, but similar limitations to those for manual angled beam UT are expected, as both techniques are based on very similar physical principles (angle-beam pulse-echo ultrasonics).

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5.6 Surface preparation for ultrasonic inspection.

Successful application of ultrasonic inspection for weld corrosion relies on suitable surface condition where the probes are applied. Excessive surface roughness and/or loose paint can severely affect the amplitude and consistency of signals obtained. Weld spatter left in situ can also affect amplitudes and lead to irregular movement of the probes. Field experience with weld corrosion inspection indicates that surface preparation is one of the most important factors determining the capability of the inspection and the timeframe for completion. To ensure that the surface condition does not significantly affect inspection capability this document makes reference to a number of standards, e.g. CEN/TS 14751:2004, that define the minimum required condition and the relevant requirements should be followed. It is important to note, however, that cleaning/surface preparation should be considered well in advance of the inspection in order to enhance inspection productivity. In many cases, the time required for surface preparation of each weld exceeds the time for inspection. Hence surface preparation can become a bottleneck if started at the same time as the inspection or only a short time beforehand. It is recommended that the cleaning/surface preparation requirements be assessed for each weld by carrying out a survey well in advance of the inspection itself so that the sequencing of preparation and inspection can be optimally planned. For paint in poor condition, the best results are usually achieved by removal of the paint adjacent to the weld. Repainting following inspection is usually costly and time consuming and is therefore not generally an option, particularly where re-inspection is envisaged within a reasonably short timeframe. In these situations it is recommended that a temporary coating be applied to the regions over which paint has been removed. There are two approaches in use in this respect, (i) use of a viscous non-water soluble coating covered by a polymer film and (ii) use of protective adhesive tapes.

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6 Time of flight diffraction (TOFD)

6.1 Technique Description

Time-of-Flight Diffraction (TOFD) is a specialised NDT technique for detection, sizing and monitoring of weld flaws. It was originally developed for the through-wall sizing of crack-like defects by analysis of diffracted signal arrival times (see e.g. Silk, 1984). For weld corrosion inspection, the technique name is not fully appropriate as diffracted signals are not generally involved (although diffraction does provide an important mechanism for detection and sizing of the tip of knife-edge corrosion found only in stainless and other corrosion resistant materials). TOFD is based on two probes (separate transmitter and receiver), placed on either side of the weld, as illustrated in Figure 6.1. Probes giving short (high bandwidth) pulses are used, to allow good resolution and accuracy of time measurement. The technique generally uses 60º to 70º compression wave transducers and frequencies between 5 and 15 MHz. A number of signals are typically obtained, three of which are illustrated in Figure 6.1.

Figure 6.1 Schematic of weld corrosion inspection using TOFD The first signal to arrive is the lateral wave, which is a compression wave taking the shortest path between the transmitter and receiver probes. For the case of weld inspection, as illustrated in Figure 6.1, the shortest path between the two probes is along the component surfaces, and directly through the weld cap. The second signal is a compression wave signal scattered, reflected or diffracted from the point on the weld corrosion closest to the inspection surface. The third signal shown in Figure 6.1 is a reflected compression wave signal from the backwall. Note that for moderate or severe examples of weld corrosion, the backwall signal may be absent or very weak since it is masked by the presence of the weld corrosion. The arrival time of the weld corrosion signal can be used to derive, using Pythagoras, the minimum ligament (depth) between the uppermost point on the weld corrosion and the inspection surfaces on either side of the weld cap, given the probe separation, probe delay and material velocity. The beamwidths of the probes used in TOFD are generally large, so that coverage of almost the full weld volume can often be achieved with a single pair of probes, as illustrated schematically in Figure 6.2.

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Figure 6.2 Schematic of compression wave coverage achieved for weld corrosion

inspection using TOFD. The coloured region represents the area of optimum flaw response.

The standard formula used to derive depth from signal arrival time assumes that the scattering point on the weld corrosion is midway between the two probes, i.e. on the weld centre line (assuming the probes are symmetrically placed either side of the weld). If the weld corrosion is substantially offset from the weld centre line, there can be significant errors in the derived flaw depths, unless additional measurements are taken (see Section 6.9.4). In TOFD, signals arriving later than the compression wave backwall signal are often observed. These are generally mode-converted signals, in which one path from probe to flaw is a shear-wave and the other path is a compression wave. These signals generally involve asymmetric paths, as illustrated in Figure 6.3, with a mode conversion occurring at the flaw. These signals arise because, due to their very wide beamwidths, TOFD probes both generate and receive shear waves in the component, via a mode conversion at the inspection surface. Mode-converted back-wall signals are also generally observed, unless they are obstructed by the weld corrosion.

Figure 6.3 TOFD inspection of offset weld corrosion, showing a mode converted signal, in

which the path Tx to weld corrosion is a compression wave (P) and the path weld corrosion to Rx is a vertically polarised shear wave (SV).

Figure 6.4 shows an example of the typical coverage achieved using the mode converted signals (P-SV and SV-P), with good sensitivity to flaws in the coloured regions, either side of the weld centre line. As can be seen, the use of the mode converted signals extends the coverage achieved for weld corrosion which may be substantially offset from the weld centre line.

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Figure 6.4 Schematic of the coverage achieved using mode converted signals (P-SV and

SV-P) for weld corrosion inspection using TOFD. The red lines show the SV beams and the blue lines the P-wave beams. The coloured regions represent the coverage areas for mode converted signals (P-SV and SV-P).

The use of mode converted signals in the interpretation and analysis of TOFD data from weld corrosion inspections is discussed further in Section 6.9.6. The results from scanned TOFD inspections of welds are generally recorded as grey-scale B-scans or D-scans, which show probe position horizontally and increased signal arrival time vertically downwards. Grey-scale coding is used to show the different amplitudes of the unrectified RF waveforms. A TOFD D-scan showing the presence of an isolated but substantial example of weld corrosion is shown in Figure 6.5. This example also shows the effects of mismatch (two backwalls evident, especially in the mode-converted signals) and also evidence for offset of the weld corrosion to one side of the weld (since only one of the two mode-converted backwall signals is affected by the presence of the weld corrosion).

Figure 6.5 TOFD D-Scan of an isolated area of severe weld corrosion/erosion. Illustration

courtesy of Sonomatic Ltd.

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6.2 Application to weld corrosion detection and sizing

TOFD is the preferred method for weld corrosion inspection, provided the component falls within the capabilities of the technique, as given in Section 6.3 below. It is recommended that the technique is used in conjunction with the 0º pulse-echo technique (see Section 7), to measure the wall thickness of the component immediately adjacent to the weld and also a Mimic profile at four cardinal locations to plot the cap profile and misalignment of external surfaces.

6.2.1 Relevant standards and documents

TOFD should be applied in accordance with following documents: CEN/TS 14751:2004 Welding – Use of time-of-flight diffraction technique (TOFD) for

examination of welds ENV 583-6 Non-destructive testing – Ultrasonic examination – Part 6: Time-of-flight diffraction

technique as a method for detection and sizing of discontinuities. The specific recommendations for weld corrosion inspection are given in the Sections below.

6.2.2 Advantages

TOFD is recommended for weld corrosion inspection, wherever possible, because of the following main advantages: The twin-probe inspection geometry and physical basis of TOFD is well suited to measurement

of the remaining ligament between the inspection surfaces, and the point on the weld corrosion closest to that surface.

The TOFD signals from weld corrosion are usually strong, and readily detectable since they are

primarily due to a reflection or strong forward scattering from the ”top” of the weld corrosion (i.e. the point nearest the inspection surface).

In many cases, a single TOFD scan gives a direct profile on one image of the variations in depth

in the along weld direction of the point on the weld corrosion closest to the inspection surface. This allows the location and value of the minimum remaining ligament to be measured reliably, even for this variable form of degradation.

The trial results given in Sarsfield, Collett and Burch (2009) show excellent agreement with the

mechanical measurements of the weld corrosion, to within typically ±0.5 mm, provided the recommended additional techniques for sizing of offset flaws are used. Other trials are also understood to show similar levels of accuracy.

A permanent record of the results is obtained.

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The technique is repeatable and can be used for monitoring the progression of the degradation over a period of time, through repeat inspections.

The technique can be applied to tapers and mismatched welds, although special measures are

then required. The use of TOFD in conjunction with the 0º wall thickness measurement technique allows

measurement of both remaining ligament and the through-wall extent of the wall loss.

6.2.3 Limitations

There are however some limitations which need to be considered when applying this technique: TOFD can significantly underestimate the remaining ligament for weld corrosion which is

substantially offset from the weld centre line. In the case of substantial offsets, additional techniques should then be applied, such as 0º pulse-echo using probes adjacent to the weld cap, performing additional offset circumferential (non parallel) scans or axial (parallel) scans with increased PCS and/or analysis of the mode converted TOFD signals.

The accuracy of sizing extensive weld corrosion that approaches the inspection surface is

limited. For small ligaments, the weld corrosion signal will merge with the lateral wave. Under these circumstances, limited or no sizing will be possible, but the presence of a near surface area of wall loss can be recognised by the absence of a backwall signal and potentially the mode converted backwall signal. A through-wall flaw will give a loss of the lateral wave signal as well. In practice however, lateral wave durations can generally be restricted to a depth of 3mm or less, and in many situations a remaining ligament of <3mm would be cause for taking the weld out of service.

In practice, surface condition can be problematic for ultrasonic techniques, without prior surface

preparation. For other limitations, see Section 6.3 below.

6.3 General Requirements for TOFD examination of weld corrosion

Access TOFD requires sufficient space on both sides of the weld cap to allow placement and scanning of probes. Miniature probes and specially shaped shoes can assist in obtaining sufficient access to permit application of the technique, e.g. on the small neck of material between the weld cap and a taper. If necessary, TOFD probes can be scanned along a taper or other change of geometry, although there will be a consequent effect on the accuracy of the measurements (see Section 6.9.3 for further details). Insulation/coatings TOFD, in common with other ultrasonic techniques, cannot be performed on insulated surfaces. The presence of paint and other coatings, provided they are in good condition may not generally preclude application of TOFD. Performance does however need to be verified on any coated components. Coatings which have been in-situ for extended periods can cause issues associated

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with loss of coupling due to lack of adhesion to the surface, and should generally be removed prior to TOFD inspection. See Section 5.6 for further details. Wall thickness The wall thickness for TOFD examination of weld corrosion should normally be a minimum of about 6 - 8 mm. In practice, the minimum wall thickness is likely to be influenced by coating type and condition which can cause extension of the lateral wave signal and hence reduced capability for inspection of thin sections. Performance should be verified using representative test samples. Surface Condition Surface condition should be adequate to permit scanning and achieve sufficiently good coupling, as stated in Sections 8.5 and 11 of CEN/TS 14751:2004. See also Section 5.6 for further details and recommendations concerning surface preparation for UT inspection. Component surface curvature TOFD in common with other ultrasonic techniques is not generally applied to components having outside diameters of less than about 3", due to issues with coupling the probe to the surface, and distortion of the ultrasonic beam caused by the surface curvature. Temperature For temperatures below 50-60º C, conventional probes and couplant can be used. Between 50º-60ºC and 120ºC, high temperature probes and couplant are required. Between 120º C and 400-500º C, special measures are required, including safety issues. For higher temperatures, no ultrasonic techniques are generally applicable for weld corrosion inspection.

6.4 Preparation for TOFD testing

Preparation for TOFD testing shall be in accordance with Section 8 of CEN/TS 14751:2004. In drawing up a work instruction for the testing, the following must be considered.

6.4.1 TOFD Setup

Probes should be selected in accordance with Table 2 of CEN/TS 14751:2004, a section of which is reproduced in Table 6.1. Probes made from piezo-composite materials are recommended as these have a good combination of high sensitivity and broad bandwidth. Table 6.1 Recommended TOFD set-ups for simple butt-welds, taken from CEN/TS

14751:2004. Thickness

t /mm Number of TOFD set-

ups

Depth-range t/mm

Centre frequency F / MHz

Beam angle /º

Element size /mm

Beam intersection

6 – 10 1 0 - t 15 70 2- 3 2/3 of t 10 – 15 1 0 - t 15 - 10 70 2- 3 2/3 of t 15 – 35 1 0 - t 10 - 5 70 - 60 3- 6 2/3 of t 35 – 50 1 0 - t 5 - 3 70 - 60 3- 6 2/3 of t

The probe separation should initially be specified in accordance with the probe beam angle and beam intersection depth as given in Table 6.1.

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If possible, information should be obtained in advance on the weld preparation details, and the extent of the weld cap, which may require an increase in the probe separation beyond that from Table 6.1. All values given in Table 6.1 should be regarded as nominal values and deviations must be verified using representative test samples.

6.4.2 Coverage

It is recommended that proprietary software tools are used, where available, to demonstrate satisfactory design of the inspection; particularly to ensure that the selected probes and probe separation give adequate coverage of the weld volume. This is especially important for any inspections involving a change in geometry adjacent to the weld (e.g. a flange taper). An example of the application of software tools to demonstrate coverage is shown in Figure 6.6.

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(b)

Figure 6.6 Example of TOFD coverage verification using Sonovation’s ScanPlan

software (illustration courtesy of Sonovation Holding BV). The full red line shows the centre line of the compression wave beam, and the weaker red line shows the compression wave beam edges. The blue lines show the extent and centre of the shear wave beams. The lower picture includes the locus curves for the compression (red) and mode-converted (green and purple) waves.

6.4.3 Other aspects

Refer to the relevant sections of CEN/TS 14751:2004 for other aspects of preparation: Scanning surfaces Temperature Couplant Datum points

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6.5 Calibration blocks

It is recommended that a calibration block of similar material and wall thickness to the test component is used containing side-drilled holes at different depths (for example see Annex A of CEN/TS 14751:2004). Scans of the calibration block should be made to verify the depths derived from TOFD data are in good agreement with the known depths of the side-drilled holes, to within at least ±0.2 mm. This verifies the linearity of the time base, and that the probe separation/probe index points are measured to acceptable accuracy. Alternatively, blocks with notches may be used.

6.6 TOFD scanning of welds

6.6.1 Modifications to design parameters

In practice, it may be found that the work instruction prepared in advance of the inspection may need modification when the actual component is inspected. The modifications to the original inspection plan may include: Changes to the planned PCS, due to the extent of the weld cap being different from that

assumed. Reduced frequency probe needed due to poor surface condition or high material noise levels. Any changes to the design PCS or probes must be recorded in the inspection report. In the event of any of the above changes, it is necessary to ensure that the actual achieved coverage is adequate, using for example proprietary coverage modelling software (Section 6.4.2). In case of any limitations of the modelling software (e.g. reduction of probe frequency due to coating effects), the set-up should be verified using representative test samples.

6.6.2 Sensitivity settings

As recommended in CEN/TS 14751:2004, the amplitude of the lateral wave signal from the weld shall be adjusted to be between 40% and 80% of full screen height (FSH), with the preferred value for weld corrosion inspection being at the lower end of this range (i.e. c. 40%). If use of the lateral wave is not appropriate, then the amplitude of the backwall signal shall be 18 db – 30 dB above FSH. If neither the lateral wave nor backwall signals are appropriate, the material grain noise should be 5- 10% FSH. Random noise shall be reduced by means of signal averaging.

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6.7 Scanning/Data collection

The minimum scans required are as follows: Positionally encoded TOFD scan around the weld (i.e. parallel with the welding direction), with

the probes on either side of and as close as possible to the weld cap. This is usually referred to as a TOFD non-parallel scan (or sometimes as a D-scan), with the scan direction orthogonal to the probe beam directions.

Associated 0º pulse-echo measurements using probes scanned along the weld, on both sides

of the weld cap. Ideally, these measurements should take the form of recorded RF waveform data, recorded and displayed in a similar format to TOFD data.

As recommended in CEN/TS 14751:2004, a TOFD scan increment of no more than 0.5 mm should be used for wall thicknesses up to 10 mm, and no more than 1 mm for thickness between 10 mm and 150 mm. The time gate of the recorded TOFD data should be set to include the lateral wave signal, the compression wave (P-P) backwall signal and the mode converted (P-SV) backwall signal from the component.

6.8 Additional scans

Where the presence of weld corrosion significantly offset from the weld centre line is indicated from the initial TOFD and 0º scans (see above and Section 6.9.4), consideration should be given to making additional TOFD scans, as follows. These recommended additional scans should be made, access permitting, using a larger PCS to allow “parallel scans” to be made across the weld (i.e. scan direction parallel with the probe beams). The aim of these scans would be to obtain improved accuracy of the measurement of the remaining ligament for weld corrosion offset from the weld centre line (see also Section 6.9.5). Alternatively, additional non-parallel TOFD scans, offset from the weld centre line, may be used to establish to which side the WRC is offset. The arrival times of the indications obtained in these scans can also be correlated to provide improved sizing accuracy for offset flaws (see also Section 6.9.4).

6.9 Data processing and Interpretation

6.9.1 Lateral wave straightening

Due to a variety of effects, including changes in coating thickness and/or coating condition (causing varying couplant thickness), the TOFD D-scans in the “as collected” form generally show substantial variations in the arrival time of the lateral wave, as for example shown in Figure 6.7. If these variations in lateral wave arrival time are present, the TOFD scan should be processed using a routine which “straightens” the lateral wave, so that the lateral wave arrival time is then constant for all probe positions, as illustrated in Figure 6.7. Lateral wave straightening lines up the arrival times of all the signals in the scan (e.g. backwall, weld corrosion), to provide a more accurate assessment of variations in these signals with scan position.

HOIS(09)RP2 Issue 1

Figure 6.7 Example of straightening of lateral wave signal in a TOFD scan of weld

corrosion in a weld. Left – raw data, Right – straightened data, such that the lateral wave has a constant arrival time with probe position.

6.9.2 Depth/Ligament Measurement

Calibration of times Calibration of the measured arrival times in a TOFD weld corrosion scan is best achieved by measurement of the arrival time of the lateral wave, assuming that the material velocity and probe separation are known. This calibration essentially gives the probe delay (i.e. the time taken for the ultrasound to propagate through both probe shoes and any coating if present) for the TOFD setup in use. Subtraction of the probe delay from the measured times then gives the propagation time in the test component material. Use of the lateral wave arrival time for calibration of times minimises any errors due to the presence of coatings which may be present on the test component and not on a calibration block, since the measured signal times are then corrected for the propagation times in the coating where present. Measurement of times/Allowance for phase changes The measurement of signal arrival times on a TOFD scan needs to be made on a consistent feature in the ultrasonic pulse, taking account of any phase changes present in the signals. The phases of both the weld corrosion and backwall signals should be the same, and 180º different from that of the lateral wave, due to the reflection experienced at the flaw and backwall.

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If the signals present do not show these phase differences, further investigation is required, because this may indicate the presence of a different type of flaw (e.g. the bottom edge of a near surface breaking crack). For an example of consistent time measurements, see Figure 6.8. In Figure 6.8, the lateral wave calibration was made using the peak of the first negative going half cycle of the pulse. It is then necessary to allow for the 180º change in phase of the signal from the “top” of the weld corrosion, relative to the lateral wave. Hence, the time of the peak of the first positive going half cycle on the signal from the weld corrosion should then be measured. Similarly, if the first positive going zero crossing is measured on the lateral wave, the first negative going zero crossing must be measured on the weld corrosion and backwall signals.

Lateral Wave

WRC signals

Figure 6.8 Measurement of TOFD arrival times, allowing for 180º phase difference between

lateral wave and weld corrosion signals. Derivation of depths The standard formula for derivation of depth/ligament assumes the flaw is symmetrically positioned midway between the probes, as shown in Figure 6.9.

Tx Rx S/2 S/2

d

P P

Figure 6.9 TOFD geometry for a flaw midway between the probes.

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The flaw depth below the inspection surface is then given by the formula:

220 S t-t c0.5 d (6.1)

Where d is the flaw depth below the inspection surface c is the ultrasound velocity t is the flaw signal arrival time t0 is the combined probe delay S is the probe separation

6.9.3 Depth measurement in presence of geometry changes

Accurate sizing and reliable detection in the presence of geometry changes adjacent to, or associated with the weld, such as mismatch/misalignment and nozzle tapers is dependent on the use of experienced operators for collection and interpretation of data. If misalignment/mismatch are expected to influence the detection or sizing performance of the inspection, the capability of techniques and operators should be qualified on a case by case basis using suitable samples. Advanced modelling and loci plotting software can be used to improve measurement accuracy.

6.9.4 Flaws offset from the weld centre line

For TOFD, the standard formula for depth sizing (i.e. distance of the indication below the inspection surface), assumes that the indication is located symmetrically between the probes. If the flaw offset from this position is small, then the errors in sizing are negligible (e.g. Charlesworth & Temple, 2001). However for offsets that are a significant fraction of half the probe separation, then the errors can become more significant. Moreover, for weld corrosion inspection, these errors are non conservative, i.e. remaining ligament is overestimated. An illustration of the effect of flaw offset on TOFD depths derived using the standard formula is shown in Figure 6.10. This gives errors in depth sizing using a probe separation of 50mm, for a flaw at a true depth of 10mm below the inspection surface. It can be seen that the sizing errors exceed 0.5 mm for offsets greater than 8.5mm, and exceed 1 mm for offsets greater than 11.5mm. Offsets of this magnitude, or larger are not uncommon in examples of weld corrosion provided by HOIS members.

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TOFD: Effect of flaw offset on sizing error

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

-20 -15 -10 -5 0 5 10 15 20

Offset from midpoint between probes (mm)

Siz

ing

err

or

(mm

)

Figure 6.10 Example of errors in TOFD sizing, if the offset of the flaw from the probe

midpoint is ignored. The errors are for an example with a probe separation of 50mm, and an actual flaw depth of 10 mm below the inspection surface.

The sizing error as shown in Figure 6.10 is an absolute error. Often, repeat measurements are used to monitor progress of weld corrosion. With proper procedures in place, during each repeat inspection the same absolute measurement is made. Repeatability of such measurements has been shown to be possible to accuracies of 0.2 mm, which means that defect growth measurement accuracies can also be in this range. Practical reasons, such as coating being present, poor access conditions etc may necessitate that for calculation purposes sometimes larger inaccuracies are taken into account. The example below depicts an inaccuracy of ±0.5 mm used.

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Figure 6.11 Example of WRC repeat measurements transferred into an RBI database.

Illustration courtesy of Sonovation Holding BV. From a single TOFD D-scan, there is no information on the flaw offset from the probe mid-point if only the standard compression wave signals are analysed. However, evidence for large offsets can often be obtained by comparison of the relative amplitudes of the compression and mode-converted weld corrosion signals. If the mode-converted signals are weaker than the compression signals, it is likely that flaw offsets are small, and hence the standard sizing technique is unlikely to be significantly in error. If the mode-converted signals are stronger than the compression signals, this could well be due to an offset flaw, as illustrated in Figure 6.12.

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Compression WRC signal

Mode converted WRC signal

Figure 6.12 Example of weld corrosion offset from the weld centre-line giving mode

converted signals stronger than the compression signal. It should be noted that any weld corrosion offset can vary around the circumference of a weld. Therefore strong mode converted signals, relative to the compression signals, may not be obtained for all scan positions, as shown in Figure 6.12. If the TOFD D-scan shows evidence for significantly offset weld corrosion, the following are recommended: If possible, increase the PCS and carry out short parallel scans at the appropriate positions. The

aim is to better measure the arrival time of the “top” of the arc from the weld corrosion. Alternatively, two or three non-parallel TOFD scans may be performed having different offsets

from the weld centre line (note that an increased PCS will be needed for scans offset from the weld centre line).

If increased PCS parallel or non-parallel scans are not possible, for example due to geometric

constraints, additional information can be obtained by analysis of the times of the compression and mode converted signals, as described in Section 6.9.6.

Examination of the 0º pulse-echo data obtained with probes adjacent to the weld cap to obtain

values for the depth of any weld corrosion indications, for comparison with the TOFD values.

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6.9.5 Parallel scans

The parallel scans should be made with increased probe separation, across the weld, at circumferential locations where offset flaws are suspected. In practice, these scans may not be possible due to geometric constraints. With an increased probe separation it is necessary to verify that coverage is still achieved, and lateral wave, flaw and backwall signals can be clearly detected. An example of a “short” parallel scan is shown in Figure 6.13, obtained from a component containing two bands of weld corrosion, one of which was substantially offset from the weld centre line. The arc from the more central indication (Weld corrosion signal 1) clearly shows the minimum arrival time, corresponding to zero flaw offset. Even for the more offset indication (Weld corrosion signal 2), the minimum arrival time can be accurately estimated, although only one side of the arc is obtained.

Lateral Wave

Weld corrosion signal 1 Weld corrosion signal 2

Backwall signals

Figure 6.13 Example of a short parallel TOFD B-scan, obtained using an increased probe

separation to allow scanning across the circumferential weld in the axial direction. To allow scanning in the presence of the weld cap, the probe separation was increased by 20mm from 44mm to 64mm.

6.9.6 Analysis of mode converted signals

For a parallel sided test component, Figure 6.14 shows the TOFD inspection geometry for an indication offset from the probe mid-point.

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Tx Rx S/2 S/2

x

d P1 P2

Figure 6.14 TOFD geometry for offset weld corrosion, for straight sectioned butt welds. Appendix 2 gives formulae for the arrival times of the compression and mode converted signals, assuming that all signals originate from the same point on the flaw. There are three signals which can be detected:

1. P-P 2. P-SV 3. SV-P

If two of the above three signal arrival times are measured, then this gives two simultaneous equations, with two unknowns, x and d. These equations can then be solved for the unknowns x, d to derive both the flaw offset and its’ actual depth, taking account its offset. Similar results can be obtained with loci-plotting software, and the above approach can be extended to more complex geometries such as those involving one probe on a taper. Hence, measurements of the P-P and one mode converted signal (either P-SV or SV-P) allow both the flaw offset and depth to be determined, provided both signals arise from the same point on the flaw.

6.9.7 Interpretation

In certain cases, it may not be possible to ascertain whether a TOFD indication is connected to the internal surface or is buried with no connection. This is particularly the case where weld corrosion propagates through a cloud of porosity or scattered minor flaws such as inclusions. The use of parallel or offset scans may aid the interpretation. Also, manual shear wave or phased array UT may be used to provide further information. However, there will inevitably be situations where positive discrimination cannot be provided. In such cases, the indications are to be classified as ‘suspect weld root corrosion’ and assessed as weld root corrosion until proven otherwise.

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6.10 Reporting of results

For each weld examined, a test report shall be produced, in accordance with Section 13 of CEN/TS 14651:2004.

6.10.1 Standard reporting

For any weld corrosion indications reported, it is important to record the following information: Minimum depth/remaining ligament measured anywhere in the weld. The typical, or average depth/ligament of the weld corrosion below the inspection surface Estimate of the wall thickness in areas which have not been corroded (from 0º pulse-echo

and/or TOFD results). Description of the circumferential location and circumferential extent of the corrosion,

particularly for any regions with a ligament less than the corrosion allowance (CA).

6.10.2 More detailed reporting requirements (optional)

There can be a need for more extensive reporting requirements in some cases, for example where a detailed fitness for service (FFS) assessment needs to be made of the weld. Optional additional reporting requirements for this situation include:

The through-wall extent of the wall losses corresponding to the minimum ligament values, derived using the plate thickness values measured using the separate 0º inspection technique.

External Mimic profile from at least four cardinal points?

Circumferential profile of the weld corrosion minimum ligament at regular intervals as

appropriate for the FFS assessment (typically 10mm), with recording of local minima within these intervals.

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7 0º pulse-echo for thickness measurement adjacent to the weld cap


The pulse-echo technique uses a single ultrasonic probe both to excite a pulsed beam of ultrasound into the component, and to receive any reflected echoes. The arrival time of the return echo provides information on the range to the reflector, given the ultrasound velocity. If 0º compression wave probes are used, measurements can be made of the local remaining wall thickness immediately below the probe, using the time of the return signal. For weld corrosion inspection using TOFD, as an additional requirement, it is recommended that a 0º pulse-echo probe is scanned in a direction parallel with the weld, with the probe placed adjacent to the weld cap. Two scans are recommended to cover both sides of the weld, provided access is possible, as illustrated in Figure 7.1.

Figure 7.1 Schematic of weld corrosion inspection using 0º pulse-echo probes applied on

both sides of the weld cap. The aims of these supplementary 0º pulse-echo scans are as follows: To measure the uncorroded wall thickness (as illustrated on the left side of the weld in Figure

7.1), on both sides of the weld. To detect and measure the remaining ligament for any weld corrosion that is sufficiently offset

from the weld centre line to emerge from underneath the weld cap (as illustrated on the right side of the weld in Figure 7.1).

Example 0º pulse-echo B-scans obtained from a weld containing weld corrosion that is offset from the weld centre-line are given in Figure 7.2. This shows two circumferential scans, obtained from each side of the weld cap. The indications from the weld corrosion are shown on the right-hand plot only.

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Weld corrosion indicationsFirst backwall

Second backwall

Figure 7.2 Example of 0º pulse-echo B-scans from both sides of the weld cap, obtained

from a weld containing offset weld corrosion, which extends beyond the weld cap on one side of the weld only (right-hand B-scan).

7.2 General Requirements for 0º pulse-echo examination of weld corrosion

The 0º pulse-echo examination method for thickness measurement is recommended as a supplement to TOFD examination of a weld for weld corrosion. The 0º pulse echo method should be applied in general accordance with EN14127:2004. In general, if a weld is suitable for TOFD examination in terms of access, coatings, surface condition and wall thickness, then the 0º pulse-echo technique should also be applicable.

7.3 Equipment for 0º pulse-echo testing

7.3.1 Probes for 0º pulse-echo testing

For offset weld corrosion inspection, the recommended 0º probes are twin-crystal, with a frequency of 4 - 5 MHz, and a crystal size of 10-12 mm (both elements combined). Standard twin-crystal contact probes are adequate for testing of remaining ligaments measuring from 10 to 50 mm. For smaller ligaments (down to c. 2mm), high quality probes are recommended (e.g. a Krautkramer/GE Inspection Technologies model DA301 probe).

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7.3.2 Instruments and data recording

It is recommended that the digital ultrasonic instrument used for TOFD is also used to record the 0º pulse-echo scan data. The 0º pulse-echo data should then be recorded as full waveforms, and stored as B-scan or D-scan files. Alternatively, conventional flaw detectors, capable of displaying the full ultrasonic waveform can be used.

7.4 Calibration

7.4.1 Calibration blocks

It is recommended that a calibration block of similar material and wall thickness to the test component is used. Calibration blocks should comply with international standards, such as the V1 block (EN 12223) or V2 block (EN 27963). Calibration blocks containing step wedges may also be used, provided they comply with a recognised standard.

7.4.2 Calibration procedure

The ultrasonic instrument should be calibrated in accordance with the manufacturer’s recommendations for ultrasonic pitting inspection (mode 1, as per EN 14127:2004). As a minimum, the calibration procedure will establish the probe delay for the probe and shoe in use, given the thickness of the test block (or step) and the material velocity. Some calibration procedures also allow determination of the ultrasound velocity for the calibration material, by means of analysis of multiple signals (e.g. repeat backwall signals).

7.5 0º scanning for weld corrosion


To achieve sufficient sensitivity for detection of irregular weld corrosion pitting indications, the amplitude of the first backwall on the test component should be set to about full screen height + 6dB. Alternatively, the second backwall should be set to about 80% full screen height.

7.5.2 Scanning/Data collection

0º pulse-echo measurements should be made using probes scanned along the weld, on both sides of the weld cap. It is recommended that the time gate used for recording is sufficient to include both the first and second backwall signals. Waveform averaging is recommended to improve signal to noise ratio, if available on the equipment in use.

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7.6 Data processing and Interpretation

7.6.1 Depth/Ligament Measurement for weld corrosion

The arrival time of any weld corrosion type indications should be measured. This can be used to derive the depth of the remaining ligament, given the probe delay and assumed material velocity, based on the following formula:

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0 t-t c 0.5 d (7.1)

Where d is the flaw depth below the inspection surface c is the ultrasound velocity t is the flaw signal arrival time t0 is the probe delay found by calibration This formula is usually implemented within the flaw detector or UT instrument, and hence is not needed by the operator. Note that if there is a coating present on the test component, the value for d will be an over-estimate of depth, due to the additional propagation time in the coating. The velocity of ultrasound is lower in the coating than the steel and so the effective steel equivalent coating thickness is substantially larger than the actual coating thickness. In practice, this can lead to overestimates in the measured depths of about 0.5 - 1 mm, depending on coating thickness and type.

7.6.2 Uncorroded wall thickness measurement

In the absence of significant corrosion, the measurements of the thickness of the parent metal on both sides of the weld can indicate the presence of a change of section across the weld. The uncorroded wall thickness can be measured from the first backwall signal, using equation 7.1 above. If coatings are present on the test component, then this value will be an overestimate of the actual steel thickness, as discussed in Section 7.6.1. However if there is a requirement to measure accurately the true steel thickness, with allowance for the coating thickness, the calculation can be made using the time difference between the first and second backwall signals, in the following formula:

12 t- t c 0.5 WT (7.2) Where WT is the wall thickness c is the ultrasound velocity t1 is the arrival time of the first backwall signal t2 is the arrival time of the second backwall signal

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Information obtained from the 0º pulse-echo examination of the weld, using probes positioned either side of the weld cap, should be reported as follows: Estimate of the wall thickness in areas which have not been corroded, on both sides of the

weld. For any offset weld corrosion indications detected, the following should be reported: Minimum depth/remaining ligament measured anywhere in the weld. The typical, or average depth/ligament of the weld corrosion below the inspection surface The through-wall extent of the wall losses corresponding to the above values, derived using the

plate thickness values derived using the separate 0º inspection technique Description of the circumferential location and circumferential extent of the corrosion,

particularly for any regions with a ligament less than the corrosion allowance (CA).

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8 Tangential Radiography

8.1 Technique description

8.1.1 Introduction

In service radiography of pipes and some small diameter vessels is usually performed using an isotope source, typically Iridium 192. Other sources used less commonly are Selenium 75 (for thinner walled components) and Cobalt 60 for thick sections. The use of ionizing radiation has associated safety implications because exposure of any part of the human body to X-rays or gamma-rays can be highly injurious to health. Appropriate legal requirements must be applied wherever X-ray equipment or radioactive sources are in use. Local or national or international safety precautions when using ionizing radiation shall be strictly applied. Traditional radiographic film is generally used for manufacturing weld radiography. For pipes, single or double walled radiographic techniques are employed, in which the manufacturing flaws can be detected by the changes in radiographic density they produce. In-service inspection often employs a different technique called tangential radiography (sometimes referred to as profile radiography) for weld corrosion inspection. A direct radiographic image of the pipe wall is obtained at the position(s) at which the radiation beam forms tangent(s) to the pipe surfaces, as illustrated in Figure 8.1. The technique can be applied with the source symmetrically positioned on the pipe centre-line, or offset to examine a single wall.

X or gamma-ray source

Detector

Extended area of corrosion

Image of reduced thickness pipe wall

Image of normal thickness pipe wall

Insulation

Extended area of corrosion


Detector

Image of reduced thickness pipe wall

Figure 8.1 Principle of tangential radiography with the source on the pipe centre-line (left)

and offset from the centre-line (right). The geometries shown in Figure 8.1 give a radiograph which directly images the pipe walls, and allows any large-scale wall loss flaws in the pipe wall to be detected. Both internal and external wall loss flaws can be detected. Small corrosion pitting type defects may not be well detected with this technique, mainly for larger penetrated thicknesses.

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The image of the pipe wall(s) allows the remaining ligament(s) to be measured directly from the radiographic image, provided appropriate calibration techniques are used to allow for the enlargement (or “blow-up”) of the image. Note that, compared with double-wall inspection techniques, the maximum penetrated path for the tangential technique is substantially higher, and occurs for the position of the tangent to the inner pipe wall (sometimes referred to as the chord length). This increased penetrated thickness can limit the applicability of the tangential technique to thicker wall and/or larger diameter pipes.

8.1.2 Tangential and double wall double image radiography combined

For relatively thin-walled, small diameter pipes, a single radiographic image has sufficient dynamic range and size to show the presence of wall loss by both of the tangential and double wall double image (DWDI) techniques, illustrated in Figure 8.2. The tangential method inspects only a small extent of the circumference of the pipe for a single source/detector position, and so full coverage can only be achieved by use of a large number of radiographic images taken circumferentially around the pipe. Because of this, the DWDI technique is often used to detect and locate circumferentially areas of wall loss. These can then be sized using the tangential method by rotating the source and detector so that the wall loss is then at the tangential position. This method does not work well for small pitting flaws, but is usually effective for more extended areas of wall loss. If the weld corrosion is significantly variable circumferentially, it is important to note that there can be issues in ensuring that the area of greatest wall loss (minimum remaining ligament) is positioned accurately at the tangent position, especially given practical, on-site constraints. For liquid product filled pipes, there is increased attenuation for the double walled radiographic techniques. With the tangential technique, the presence of liquid product does not increase the maximum penetrated path, but the radiographic contrast at the key ID location is reduced. This reduced contrast can give noticeably poorer definition of the ID position and hence the remaining ligament, particularly for pipes having maximum penetrated thickness values close to the limit for the selected isotope source.

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Detector

Figure 8.2 Radiography combining both the tangential and double wall double image (DWDI)

techniques in a single radiograph An example of combined tangential and DWDI computed radiography (CR) is given in Figure 8.3, which shows a weld in a 3” OD, 10mm wall thickness pipe containing substantial wall loss weld corrosion, with variable through-wall extent. The CR image shows clearly, on the right-hand side, the asymmetric morphology of the weld corrosion, which is sharp edged on the lower side of the weld and much more gradual on the upper side. The through-wall extent of the weld corrosion is lower on the left-hand side, which results in greater penetrated thickness. Improved definition of this area can be obtained using different contrast and brightness settings.

Figure 8.3 A computed radiography image of a pipe butt weld, showing extensive weld

corrosion both by the tangential and double-wall double image techniques. The presence of the weld corrosion in the tangential image of the right-hand pipe wall is highlighted.

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8.2 Computed/Digital Radiography

The computed radiography (CR) technique is often used for tangential radiography. CR is based on the use of imaging plates, which replace the X-ray film. These imaging plates contain photostimulable storage phosphors, which build up a latent image when exposed to ionising radiation. This image is read out using a scanning laser beam, linked to a computer. Unlike film, the same imaging plate can be re-used many times. The resulting CR images are displayed and analysed using computer-based software packages. With CR, the dimensional measurements can be made conveniently on-screen using either interactive cursors, or by methods which allow analysis of grey-level profiles extracted from the CR image perpendicular to the pipe axis. Digital Radiography (DR) is a newly developed X-ray technology with some applications to in-service tangential radiography of pipes. Unlike CR, the detectors used in DR provide real-time imaging and are based on two-dimensional arrays of X-ray sensitive detectors (‘flat-panels’). These are generally classified into either direct detectors in which X-ray energy is converted directly into electric charge (using amorphous Selenium/Silicon) and indirect conversion detectors in which the X-ray energy is first converted into light using an X-ray scintillator (e.g. Caesium Iodide or Gadolinium Oxysulphide). Some indirect devices use structured scintillators based on elongated Caesium Iodide crystals. DR detectors are not intrinsically safe, and not robust. However, they are often more efficient detectors of X-rays than CR systems, allowing reduced exposure times. Devices with a pixel size of 200m have a basic spatial resolution (BSR) comparable with some CR systems, and are recommended for tangential radiographic inspection of weld corrosion. Interactive software tools for dimensional sizing can be applied to DR images in the same way as for CR images.


If circumstances preclude the use of the TOFD/0º pulse-echo technique described in Sections 6 and 7, then tangential radiography is the preferred method for weld corrosion inspection, provided the component falls within the capabilities of the technique, in Section 8.4.


Tangential radiography should be applied in accordance with following documents: The HOIS recommended practice on CR inspection of pipes for wall loss flaws (Burch, 2009). The recommendations made in the subsequent sections of the present recommended practice. Other relevant standards include for general film and computed radiography:

EN 444, Non-destructive testing - General principles for radiographic examination of metallic materials using X-rays and gamma-rays.

EN 584-1, Non-destructive testing - Industrial radiographic film - Part 1: Classification of film systems for industrial radiography

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EN 462-1 to EN 462-5, Non-destructive testing - Parts 1 to 5: Image quality of radiographs. EN 1435, Non-destructive examination of welds - Radiographic examination of welded joints EN 14784-1, Non-destructive testing - Industrial computed radiography with phosphor

imaging plates - Part 1: Classification of systems EN 14784-2, Non-destructive testing - Industrial computed radiography with storage

phosphor imaging plates - Part 2: General principles for testing of metallic materials using X-rays and gamma rays

E 1647-98a, ASTM Standard Practice for Determining Contrast Sensitivity in Radioscopy. None of the above cover in-service tangential radiography directly.

8.3.2 Advantages

The main advantages of tangential radiography, and combined tangential radiography/DWDI for weld corrosion inspection are as follows: The tangential technique gives a direct image of the weld corrosion profile, which allows

quantitative measurement of remaining ligament and wall loss. The image shows the detailed morphology of the weld corrosion and its location relative to the weld centre line.

The sizing accuracy is not affected by any displacement of the weld corrosion from the weld

centre line (unlike TOFD). The technique is applicable to welds containing geometry changes (e.g. mismatch, section

changes, and tapers) and to welds on bends, and other complex geometries. Applicable to high temperature components, as direct contact with the surfaces is not needed. Direct access to the metal surfaces on one or both sides of the weld cap is not needed. The technique can be applied through lagging/insulation and is largely unaffected by surface

condition and surface coatings. Surface preparation is not required. The technique can be used to check for changes since a previous inspection of the same weld

(monitoring). A permanent record of the inspection results is obtained.

8.3.3 Limitations

The use of ionizing radiation is a significant safety hazard, and in accordance with the Ionising Radiation Regulations (IRR), its use should be minimised wherever possible.

There can be operational issues with radiation sources due to the need to maintain controlled

areas. This may require a plant shutdown and interfere with plant sensors such as UV detectors, Gammatrols etc.

Limited circumferential coverage. The tangential technique only measures the remaining wall

thickness at the exact tangent position. For weld corrosion which varies circumferentially, finding the minimum wall thickness would require a prohibitive number of exposures. For this reason, the technique is often used in conjunction with DWDI to locate the deepest area of wall loss.

HOIS(09)RP2 Issue 1

The accuracy of the wall thickness measurements can be affected by the large variations in penetrated thickness across the pipe wall from the pipe ID to the OD.

The presence of liquid product increases the overall radiographic attenuation and gives reduced

contrast for the tangential technique. This can consequently give poorer definition of the ID position.

For further details on limitations of applicability, see Section 8.4 below.

8.4 General Requirements for Tangential radiographic examination of weld corrosion

Access Sufficient clearance and access is required on both sides of the pipe weld, to allow positioning of the film or detector in close proximity to the pipe wall, and the radiation source on the opposite side of the pipe, at a distance of typically 0.3 – 1.0m from the pipe. Insulation/coatings Tangential and tangential/DWDI radiography can be applied to insulated components. The presence of paint and other coatings, does not affect application of the technique. Surface Condition Surface preparation is not usually required for radiography. Generalised wall loss/pitting corrosion With tangential radiography, inspection and sizing of generalised wall loss due, for example, to erosion is the most common application of the technique, for which a standard quality of inspection is usually sufficient. However, detection and reliable sizing of localised pitting flaws is a more demanding application for this technique, and usually requires a higher quality of inspection. Recommendations are given below for these two classes of inspection – standard and higher quality. Wall thickness/pipe diameter For a pipe with wall thickness WT and outside diameter OD, the maximum penetrated thickness, wmax, through the pipe wall occurs for a line forming a tangent with the inner diameter. This maximum path is shown in Figure 8.4 and is given by

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WT- ODWT 2 w max (8.1)

HOIS(09)RP2 Issue 1

Detector

wmax

Source

Figure 8.4 Maximum penetrated thickness, wmax, for the tangential technique Values for the maximum penetrated thickness, wmax, through pipes of various diameters and schedules are given in Figure 8.5 and Table 8.1, for ease of reference. Note that these penetrated thickness values are generally much larger than twice the wall thickness of the pipe. Also shown on Figure 8.5 are the approximate maximum penetrated thicknesses for Se75, Ir 192 and Co 60, recommended for CR inspection using the standard quality tangential technique. These values are given in Table 8.1, which also gives recommended reduced values for higher quality tangential inspection (for sizing of pitting flaws, which are more difficult to detect and size than generalised wall loss). For weld corrosion that is approximately uniform circumferentially, standard quality inspection would be adequate. However, if the weld corrosion is very variable in through-wall extent, then a higher quality inspection should be performed.

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Tangential path lengths

0

20

40

60

80

100

120

140

160

180

200

0 2 4 6 8 10

Pipe nominal bore (inch)

Tan

gen

tial

pat

h (

mm

)

12

Schedule 40Schedule 80Schedule 160Limit for Se 75Limit for Ir 192Limit for Co 60

Se 75

Ir 192

Co 60

Figure 8.5 Maximum (tangential) path lengths through the walls of pipe of different

diameter. The maximum recommended penetrated thicknesses for different isotope sources are also shown.

Table 8.1 Maximum tangential paths in steel for different isotope sources

Isotope Source Maximum tangential path (mm)

Standard quality

(for generalised wall loss) Higher quality

(for pitting flaws) Se 75 c. 55 c. 40 Ir 192 c. 85 c. 60 Co 60 c. 140 c. 100

Table 8.2 shows which pipes can be inspected using the standard and higher quality tangential techniques with the different isotope sources.

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Table 8.2 Maximum paths through different schedule pipes of various diameters, together with

applicable isotope sources for tangential CR.

Isotope applicability2

Nominal Bore

(inches)

Outside diameter,

OD

(mm)

Schedule Nominal wall

thickness, WT1 (mm)

Nominal maximum Tangential

path1 (mm)

Se 75 Ir 192 Co 60 1 33.4 40 3.4 20.2 80 4.5 22.8 160 6.4 26.3 XXS 9.1 29.7

1.5 48.3 40 3.7 25.7 80 5.1 29.7 160 7.1 34.2 XXS 10.2 39.4

2 60.3 40 3.9 29.7 80 5.5 34.7 160 8.7 42.5 XXS 11.1 46.7

3 88.9 40 5.5 42.8 80 7.6 49.7 160 11.1 58.8 X XXS 15.2 66.9 X

4 114.3 40 6.0 51.0 80 8.6 60.3 X 120 11.1 67.7 X 160 13.5 73.8 X XXS 17.1 81.5 X

5 141.3 40 6.6 59.6 X 80 9.5 70.8 X 160 15.9 89.3 X X XXS 19.0 96.4 X X

6 168.3 40 7.1 67.7 X 80 11.0 83.2 X 120 14.3 93.9 X X 160 18.3 104.8 X X XXS 21.9 113.2 X X

8 219.1 40 8.2 83.2 X 80 12.7 102.5 X X 120 18.3 121.2 X X XXS 22.6 133.3 X X 160 23.0 134.3 X X

10 273.0 40 9.3 99.0 X X 80 15.1 124.8 X X 120 21.4 146.8 X X XXS 25.4 158.6 X X X 160 28.6 167.2 X X X

12 323.8 40 10.3 113.6 X X 80 17.5 146.4 X X X 120, XXS 25.4 174.1 X X X 160 33.3 196.7 X X X

1 Note that if corrosion/erosion is present, the wall thickness and maximum penetrated thickness can be substantially reduced, allowing application beyond the limits implied by the nominal wall thickness shown in this table.

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2 key to isotope applicability:

Both standard and high quality

Only standard quality

Neither

A pipe that contains liquid product does not have a larger maximum penetrated path, but the radiographic contrast at the key ID location is reduced. This can give noticeably poorer definition of the ID position and hence the accuracy of the measurements of remaining ligament will be reduced for pipes having maximum penetrated thickness values close to the limit for the selected isotope source. Temperature As direct contact with the pipe is not required, the temperature of the component does not normally affect the applicability of the technique. If necessary, a layer of insulating material can be placed between the pipe surface and the film/IP holder (either cassette or light-tight envelope), to minimise the possibility of thermal damage. Standard safety precautions for working in close proximity to a hot surface should be observed.

8.5 Preparation for radiographic examination

In drawing up a work instruction for the testing, the following must be considered.

8.5.1 Source to object distances

8.5.1.1 Source on pipe centre line

For tangential radiography with the source on the pipe centre line, the distances and dimensions are given in Figure 8.6.

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SDD

b'

Source, size d

Film/Detector

SPD

Figure 8.6 Dimensions and distances for tangential radiography with the source on the

pipe centre line. There are two main factors which affect the accuracy of wall thickness measurements and hence the recommended source to detector distances (SDDs):

Geometric unsharpness.

Dimensions measured in the CR image are progressively distorted away from the source axis due to the finite source to detector distance. This has differing effects on the accuracy of the various methods used for calibration of dimensions in the CR images.

It is recommended that for tangential radiography with the source on the pipe centre line, the minimum source to pipe centre distance, SPD, should be at least 3.5 times the pipe OD. Hence SDD ≥ 3.5 OD + b’ (8.2) In addition, the source to detector distance should be increased if either of the following values is larger the value given in equation 8.2: For standard quality: SDDmin = (d . b’)/0.6 (8.3) For higher quality (pitting inspection): SDDmin = (d . b’)/0.3 (8.4) Where d is the source size b’ is the distance from the pipe centre to the film/detector.

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8.5.1.2 Source offset from pipe centre line

For tangential radiography, with the source offset from the pipe centre line, the effects of the different dimensional calibration techniques on measured wall thickness accuracy are much less significant than with the source on the pipe centre line. The recommended SDD is then given by equations (8.4) or (8.5) above, only, and not equation (8.2).

8.5.1.3 Combined tangential/DWDI image radiography

For smaller diameter pipes (< 4" - 6" diameter) , the double-wall double image (DWDI) technique is often combined with the tangential technique with the source on the pipe centre line. The SDD for this combined technique should be those as for DWDI or that given in equation (8.2), whichever is the greater. The SDD for DWDI recommended by Burch (2009) is SDDmin = (d . b)/0.6 (8.5) Where d is the source size b is the distance from the source side of the pipe to the film/detector.

8.6 Radiography of small bore branch connections

Radiography of small bore branch connections can involve particular considerations regarding film/detector placement and access. For further information and recommendations, see Burch and Collett (2005).

8.7 Examination of welds using tangential radiography


In practice, it may be found that the work instruction prepared in advance of the inspection may need modification when the actual component is inspected. The modifications to the original inspection plan may include: Changes to the planned source to detector distances and film/detector position due to access

limitations. Any changes to the design parameters must be recorded in the inspection report.

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8.7.2 Dimensional calibration

For tangential radiography, when making dimensional on-screen measurements of wall thickness, it is important to calibrate the distances involved in the radiography, to allow for the image enlargement or “blow-up”. The recommended dimensional calibration method involves the use of a ball bearing or similar dimensional comparator. This is an effectively radiation opaque object (usually spherical) with a known diameter, which is placed close to the pipe, and in approximately the same plane as the tangent position on the pipe wall, as illustrated in Figure 8.7.

Detector

Comparator, diameter c

c

Figure 8.7 Tangential radiography showing use of comparator for dimensional calibration.

The comparator should be placed as close to the pipe wall as possible, without overlapping it.

Measurements of the imaged size of the comparator then enable the pipe wall thickness measurement to be calibrated in mm, hence allowing for the radiographic magnification or “blow-up”. Note that if the comparator cannot be placed adjacent to the pipe tangent position, due for example to the presence of external insulation, it is recommended that the source is offset from the pipe centre-line to be aligned with the pipe wall as shown in Figure 8.8.

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Detector

Comparator

Figure 8.8 Tangential radiography showing use of offset source position with comparator for

dimensional calibration, for insulated pipes, where the comparator should be placed as close to the outside of the insulation as possible.

As an alternative to a separate dimensional comparator, the imaged width of the pipe OD can also be used, provided this dimension is known accurately, and is clearly recorded on the radiography or associated documentation.

8.7.3 Radiographic image quality for tangential techniques

For tangential radiography, conventional wire or step/hole IQIs are not directly applicable, because they cannot be positioned near to the tangential pipe position, and the rapid changes in penetrated thickness in this part of a radiographic image makes it impossible to assess IQI visibilities in any meaningful way. As far as possible given in-service constraints, the source to detector distance should comply with the values recommended in Section 8.4.1.

8.7.3.1 Film radiography

The radiographic film type should be in accordance with the requirements of CEN class 5 or higher (for example Agfa D7 or similar). The following radiographic quality control for tangential film radiography is recommended. Ensure that the optical film densities are within the following ranges:

o Optical density on the pipe centre line ≥ 1.5. o Optical density in the un-impeded beam 3.5 – 4 (max). o Optical density on the pipe ID ≥ 0.5.

These values are those recommended by Zscherpel and Ewert (2006).

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8.7.3.2 Computed/Digital radiography

For tangential radiography, conventional wire or step/hole IQIs are not directly applicable, because they cannot be positioned near to the tangential pipe position, and the rapid changes in penetrated thickness in this part of a radiographic image makes it impossible to assess IQI visibilities in any meaningful way. However, very noisy CR images will give lower wall thickness measurement accuracies than less noisy CR images. In addition, the unsharpness of the CR image will influence the WT measurement accuracy. Hence, some form of quality control for tangential CR radiography is considered necessary, as follows.

Ensure that the SDD’s recommended in Section 8.3 are used.

The exposure time should be adjusted so that the un-impeded radiation beam outside the pipe wall does not exceed 80-90% of the CR imaging system’s saturation value (see Sections 12 and 14.4 for further information on exposure times and burn-off effects).

The resulting exposure time depends on the gain setting of the CR system. It is important

that a sufficiently low system gain is used to allow the SNR_N values given below to be achieved.

If the available software provides this function, the normalised signal to noise ratio (SNR_N)

should be measured with appropriate image analysis software. The average SNR_N values obtained in the free beam outside the pipe should be at least 70 for the standard quality and 110 for the higher quality classes of wall loss inspection. Note that an SNR_N of 110 for the higher quality class may be difficult to achieve due to fixed pattern noise levels on some imaging plates. If this is the case, use of an exposure twice that needed to achieve a value of 70 would be permissible.

If the pipe centre line is available for measurement of SNR_N, then the DWDI values of 50

and 80 for standard and higher qualities respectively can be used as an alternative to the above free beam SNR_N values.

Note that in all cases when measuring SNR_N, it is important that the image is in a form having the image grey levels directly proportional to radiation intensity, otherwise the values can be misleadingly high.

8.7.4 Circumferential coverage

The tangential radiographic technique only measures the remaining wall thickness at the exact tangent position. Hence, full coverage can only be achieved by use of a prohibitively large number of radiographic images taken circumferentially around the pipe. Because of this, the DWDI technique is often used to detect and locate circumferentially areas of wall loss. These can then be sized using the tangential method, by rotating the source and detector until the wall loss is then at the tangential position. For DWDI, recent measurements (Burch, 2009b) suggest a single image can give adequate circumferential coverage provided the maximum tangential path through the pipe wall is 50mm or less, when using Iridium 192 (see Table 8.2). For pipes have larger tangential penetrated thickness, at least two exposures 90º apart are recommended.

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8.8 Measurement of wall thickness

For the tangential radiography technique, it is important to note that the wall thickness measured generally includes the thickness of the weld cap, whereas the measurements made using TOFD or other ultrasonic probes positioned on the parent metal do not (Section 4.2.3). Thus, measurements obtained with tangential radiography will be higher than those from TOFD on the same component, unless a correction is applied for the weld cap thickness.

8.8.1 Film Radiographs

Dimensional measurements from film radiographs can be made with callipers, of both the pipe wall thickness and an object of known dimension for calibration purposes (e.g. the ball-bearing comparator or known pipe OD –Section 8.7.2).

8.8.2 Computed/Digital Radiography

8.8.2.1 Interactive on-screen measurements

Many CR/DR systems contain software options which allow on-screen interactive dimensional measurements using a cursor overlaid on the CR/DR images. The user then judges by eye the locations in the image of the inner and outer edges of the pipe wall. The same method is used to measure an object of known diameter (e.g. ball-bearing comparator or pipe OD –Section 8.7.2). This allows the distances in the CR/DR image in pixels to be converted into a true dimension at the position of the pipe wall tangent. This method can however be prone to error, especially if the penetrated thickness of the pipe wall at the tangent position is approaching the maximum possible (Section 8.3), given the radiation source in use. This makes the exact positions of the outer and especially inner walls difficult to determine by eye. Furthermore, the apparent locations can be affected significantly by the contrast and brightness settings in use on the CR/DR image at the time. Thus, the WT measured using this technique can change appreciably as the contrast and brightness settings are varied on the same image. If the on-screen measurement method is used, it should, if at all possible, first be checked for accuracy using the current contrast and brightness settings of the displayed image, by application to a section of the pipe with known wall thickness (e.g. known to be uncorroded or not eroded). For pipes having maximum tangential paths which approach the maximum permissible for the source in use (Table 6.2), the interactive on-screen measurement method is best applied to CR/DR images having logarithmic response functions to radiation exposure/dose, which reduces the overall dynamic range of the image. This improves the visibility of the position of internal pipe wall (ID). Logarithmic CR/DR images can be obtained from logarithmic-response CR/DR scanners (e.g. the GE Inspection Technologies CR100) or by application of an appropriate logarithmic look-up table (LUT) to CR/DR images resulting from scanners with linear response functions. In addition, use of high-frequency spatial filtering (sharpening) may improve the accuracy of this measurement method, by emphasizing the positions of the edges of the pipe wall in the CR/DR images.

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8.8.3 Grey-level profile analysis methods

Many CR systems have software which allows the user to mark lines on the CR image orthogonal to the pipe wall axis. The software extracts a grey-level profile along this line, which is then generally presented on-screen, superimposed on the image. Measurements of wall thickness can be obtained by either interactive or automated analysis of these grey-level profiles. Automated routines Automated analysis routines can increase the reliability of the measured wall thickness values, unless the maximum tangential penetrated thickness (wmax) is approaching the maximum possible, given the radiation source in use (see Tables 8.1 and 8.2). In addition, other factors such as the presence of external scale, corrosion products or irregular internal/external corrosion may affect the accuracy of these automated routines. In these cases, the automated routines are subject to uncertainties, and the operator should check the consistency of the derived values with the density profile. Interactive methods In the absence of automated routines for wall thickness analysis, the operator should use the available interactive facilities for analysis of the image profiles. Accuracy is likely to be improved, especially for pipes having larger wmax values, if the CR images have a logarithmic response and are high-pass filtered. Figure 8.9 shows an example of interactive measurement of wall thickness, using cursors on a grey level profile across the pipe wall, after applying a logarithmic look-up table to the CR image, and high-pass filtering to enhance details. The position of the outer diameter corresponds to a clear peak in the profile, and the location of the inner diameter is given by the minimum and pronounced change in gradient of the profile. This method, combined with a visual assessment of the image, is recommended. It should be noted that the accuracy of all measurement methods decrease as the tangential penetrated thickness, wmax, approaches the maximum value recommended for the isotope in use (see Tables 8.1 and 8.2), since the location of the inner wall becomes increasingly difficult to determine with any reliability due to lack of contrast and increased noise.

HOIS(09)RP2 Issue 1

Figure 8.9 Example of interactive wall thickness measurement using cursors

superimposed on a grey level profile taken across the pipe wall.


For any weld corrosion indications reported, it is important to record the following information: Minimum depth/remaining ligament measured from each radiograph/CR image. Estimate of the wall thickness in areas which have not been corroded. Description of the circumferential location and circumferential extent of the corrosion, especially

for any regions with a ligament less than the corrosion allowance (CA).

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9 Double Wall Radiography

9.1 Technique description

9.1.1 Introduction

A description and recommendations for tangential radiographic inspection of weld corrosion is given in Section 8, which includes a combination of tangential radiography for sizing with the double-wall double image (DWDI) technique for flaw detection. Section 8 also contains general information on radiography, including safety hazards, different types of detector etc, which will not be repeated here. As indicated in Section 5, in cases where tangential radiography is inapplicable, information on the presence of weld corrosion can be obtained using double wall radiography alone without tangential radiography. For in-service inspection, there are two main forms of double-wall radiography, as indicated in Figure 9.1. In the double wall single image (DWSI) inspection technique, illustrated in Figure 9.1(a), the source is generally in contact with or close to the pipe wall. The detector or film is placed adjacent to the opposite pipe wall and wrapped around the pipe OD. In the double wall double image (DWDI) inspection technique, illustrated in Figure 9.1(b), the radiography is usually carried out with the source directly in-line with the centre of the pipe. A sufficiently large source to detector distance (SDD) is used to ensure that wall loss can be detected in the portion of the pipe wall nearer the source, as well as that closer to the detector.


Pipe

Detector

Corrosion

Image of wall loss on detector


Pipe

Detector

Corrosion

Images of wall loss on detector

(a) (b)

Figure 9.1 Principle of double-wall single image (DWSI) radiography (left) and double-wall

double image (DWDI) radiography (right).

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DWDI is usually applied to pipes with diameters of typically 4” or less, with DWSI being applied to larger diameter pipes. For both techniques, wall loss flaws such as weld corrosion are detected by the increases obtained in the transmitted radiation intensity which give corresponding changes in film density or image grey level for digital radiography. Approximate estimates of the through-wall extent of the weld corrosion can be obtained by analysis of the film densities or image grey levels.


If circumstances preclude the use of the TOFD/0º pulse-echo technique described in Section 7, and tangential radiography (see Section 8) then double-wall radiography is recommended, where applicable, for detection of the presence of weld corrosion, with some approximate information available on the through wall extent.


There are no international standards which cover the application of double-wall radiography to in-service pipe inspection. However, many standards cover the more demanding requirements of manufacturing weld inspection, as detailed below.

EN 444, Non-destructive testing - General principles for radiographic examination of metallic materials using X-rays and gamma-rays.

EN 584-1, Non-destructive testing - Industrial radiographic film - Part 1: Classification of film systems for industrial radiography

EN 462-1 to EN 462-5, Non-destructive testing - Parts 1 to 5: Image quality of radiographs. EN 1435, Non-destructive examination of welds - Radiographic examination of welded joints EN 14784-1, Non-destructive testing - Industrial computed radiography with phosphor

imaging plates - Part 1: Classification of systems EN 14784-2, Non-destructive testing - Industrial computed radiography with storage

phosphor imaging plates - Part 2: General principles for testing of metallic materials using X-rays and gamma rays

E 1647-98a, ASTM Standard Practice for Determining Contrast Sensitivity in Radioscopy. None of the above cover in-service double wall radiography directly. The HOIS CR Recommended Practice covers in-service CR inspection of pipes for wall loss flaws, including weld corrosion (Burch, 2009).

9.2.2 Advantages

The main advantages of double wall radiography are as follows: Can be applied to pipes in which the combination of wall thickness and OD does not allow

tangential radiography. The technique is applicable to welds containing geometry changes (e.g. mismatch, section

changes, and tapers) and to welds on bends, and other complex geometries. Applicable to high temperature components, as direct contact with the surfaces can be avoided.

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Direct access to the metal surfaces on one or both sides of the weld cap is not needed. The technique can be applied through lagging/insulation and is largely unaffected by surface

condition and surface coatings. Surface preparation is not required. The technique can be used to check for changes since a previous inspection of the same weld

(monitoring). A permanent record of the inspection results is obtained.

9.2.3 Limitations

The use of ionizing radiation is a significant safety hazard, and in accordance with the Ionising Radiation Regulations (IRR), its use should be minimised wherever possible.

There can be operational issues with radiation sources due to the need to maintain controlled

areas. This may require a plant shutdown and interfere with plant sensors such as UV detectors, Gammatrols etc.

Double-wall radiography only gives limited information on through-wall extent. The presence of liquid product increases the overall radiographic attenuation and gives reduced

contrast. For further details on limitations of applicability, see Section 9.3 below.

9.3 General Requirements for double-wall radiographic examination of weld corrosion

Access Sufficient clearance and access is required on both sides of the pipe weld, to allow positioning of the film or detector in close proximity to the pipe wall, and the radiation source on the opposite side of the pipe, either at or near to the pipe wall (DWSI) or at a distance of typically 0.3 – 1.0m from the pipe (DWDI). Insulation/coatings Double-wall radiography can be applied to insulated components. The presence of paint and other coatings, does not affect application of the technique. Surface Condition Surface preparation is not usually required for radiography. Wall thickness Pipe wall thickness affects the recommended radiation sources, as given in Table 9.1.

Table 9.1 Source selection for the double wall inspection techniques(DWDI & DWSI)

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Standard quality wall loss inspection

class Radiation Source

Penetrated thickness, w

(mm)

Pipe WT

(mm) Yb 169 1 w 15 0.5 WT 7.5 Se 75 5 w 55 2.5 WT 27 Ir 192 7 w 85 3.5 WT 42 Co 60 40 w 200 20 WT 100

For product filled pipes, the additional radiation attenuation caused by the product should be allowed for in selection of sources. For a fully product filled pipe, the penetrated thickness, w, in Table 9.1 should be increased by approximately ID/9 for water, as measured by Burch (2009a). For oil, the factor is estimated as ID/11 on basis of relative densities of water and oil with 0.8 gm/cm3 but no measured values are available. Temperature As direct contact with the pipe can be avoided, the temperature of the component does not normally affect the applicability of the technique. If necessary, a layer of insulating material can be placed between the pipe surface and the film/IP holder (either cassette or light-tight envelope), to minimise the possibility of thermal damage. Standard safety precautions for working in close proximity to a hot surface should be observed.

9.4 Preparation for radiographic examination


9.4.1 Source to detector distances

DWSI In the DWSI technique the recommended source to detector distance (SDD) given in EN1435 for basic class A inspection is

3/2

mm

b 7.5dbSDD

(9.1)

Where b Component to detector distance (from nearer ID to detector/film). d Source size for calculation of geometric unsharpness For larger diameter pipes, this formula allows the source to be placed adjacent to the pipe wall opposite the detector (as is conventional practice for DWSI). This may not be true for pipes with diameters of 4" or less, or those covered by insulation, so that the detector could not be placed in close contact with the pipe wall. The SDD should then be calculated according to the formula above, and the source positioned accordingly.

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For pipes with diameters of less than about 3" to 4", the DWDI technique (see below) will be preferable in some cases as greater axial coverage can be achieved in a single exposure. DWDI For the DWDI technique, the SDD is increased compared with DWSI inspection, allowing inspection of both pipe walls. The film/detector is then usually flat, and not wrapped around the pipe wall. In this case, the relevant object plane is the external surface of the pipe closest to the source. The recommendations below on SDD follow those given in Burch (2009). For the standard quality wall loss inspection: SDDmin = (d . b)/0.6 (9.2)

9.5 Examination of welds using double-wall radiography


In practice, it may be found that the work instruction prepared in advance of the inspection may need modification when the actual component is inspected. The modifications to the original inspection plan may include: Changes to the planned source to detector distances and film/detector position due to access

limitations. Any changes to the design parameters must be recorded in the inspection report.

9.5.2 Radiographic image quality for double wall techniques

As far as possible given in-service constraints, the source to detector distance (SDD) should comply with the values recommended in Section 9.4.1.

9.5.2.1 Film radiography

The radiographic film type should be in accordance with the requirements of CEN class 5 or higher (for example Agfa D7 or similar). The following radiographic quality control for double-wall radiography is recommended. The optical density of the pipe centre line should be at least 2. The quality of images should also be assessed in terms of single-wire DIN type IQI wires, placed in the region of interest. Target IQI values are given in Burch (2009b) for CR images, and in the absence of additional information, these can be used for film radiography as well.

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9.5.2.2 Computed/Digital radiography

Normalised signal to noise ratio The image quality control criteria given in Burch (2009) for double-wall computed radiography (CR) and digital radiography (DR) are recommended. It is recommended that CR/DR image quality is assessed by measuring the normalised signal to noise ratio (SNR_N) in the area of interest, using appropriate software if available. Note that it is important when using this software with images from non-linear CR scanners to select the correct look-up table (LUT), to achieve a linear relation between radiation intensity and CR image grey level. Different CR scanners have different characteristics (e.g. logarithmic or square-root, as well as linear), and the correct LUT must be selected for the scanner used. Otherwise the values can be misleadingly high. For typical CR images, SNR_N values will typically be in the range 50 – 200. Values in excess of this may be due to use of an incorrect LUT, and should be checked carefully. When making this measurement, it is important to ensure the analysis area does not include any significant variations in grey level due to changes in penetrated thickness. Thus the SNR area should not include for example component edges, areas of wall loss, or any other image areas where there are significant changes in penetrated thickness. The size of the SNR area should be 20 pixels horizontally x at least 55 pixels vertically to provide a dataset of at least 1100 values, as required by EN 14784-1. In all cases, SNR_N measurements should be made at several locations (minimum of four) within the area of interest, and a mean value derived. The SNR measured on a CR/DR image needs to be normalised using a factor which depends on the basic spatial resolution (BSR) of the CR/DR system (see EN 14784-1, equations 2 and 3). The formula to be applied to calculate the normalised signal to noise ratio is: SNR_N = SNRmeas (88.6/BSR) (9.3) Where SNR_N is the normalised signal to noise ratio SNRmeas is the signal to noise ratio measured on the CR image using the BAM IC software or

similar BSR is the basic spatial resolution of the CR system in microns (depends on the scanner

pixel size and the CR plate type). 88.6 is the length of the side of a square having the same area as a circle with diameter

100 microns (see EN14784-1, p11 for explanation) The normalised signal-to-noise ratio (SNR_N) as calculated from equation (9.3) in the pipe centre should be at least 50. IQI wires Alternatively to the above SNR_N quality criteria, the quality of images should be assessed in terms of single-wire DIN type IQI wires, placed in the region of interest. Target IQI values are given in Burch 2009b.

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9.5.3 Circumferential coverage

DWDI The number of DWDI exposures which should be taken to ensure adequate circumferential coverage is given in Section 8.7.4. DWSI For DWSI, (Burch 2009), recommends values based on a maximum permissible increase in penetrated thickness of 20%, as given in EN 1435 for class A weld radiography. The minimum number of exposures is then given by Figure A.4 of EN 1435 : 1997 (page 17). However, if the detector is offset from the pipe wall due for example to the presence of insulation, then this figure is not applicable, and the values given in Figure 9.2 should be used instead. In Figure 9.2, note that the vertical axis is the pipe OD divided by the distance from the source to the pipe axis (SPD). To obtain the circumferential angular difference (in degrees) between exposures, the following formula should be used for DWSI: Angular difference = 360 / (Number of exposures)

Minimum number of exposures DWSI

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2

2.1

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

WT/OD

OD

/SP

D

> 3

> 4

> 5

> 6

> 7

> 84

5

7

6

89

3

Figure 9.2 Minimum number of DWSI exposures circumferentially around a pipe, as a

function of the ratios WT/OD and OD/SPD, where SPD is the distance from the source to the pipe axis (centre).

However, recent trials have indicated that for wall loss inspection, the maximum permissible increase in penetrated thickness can be greater than 20%, without significant loss of sensitivity for pitting type flaws in pipes.

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Thus for DWSI, it may also be possible to reduce the number of exposures below the values shown in Figure 9.2. However further experimental trials would be needed to investigate this. For DWSI large variations in source to detector distance occur across typical images, which result in significant changes in background image brightness. This effect should also be considered if new criteria are to be established for DWSI circumferential coverage.

9.6 Measurement of remaining wall thickness

9.6.1 Introduction

For digital and computed double wall radiography, computer analysis of the image grey levels can, in certain conditions, be used to estimate wall thickness changes. It is important to note however that this is a relatively new method which has not yet been fully validated. This method is described in Burch (2009). This method should therefore be used with caution, and validated in advance for the component under inspection using test components with closely similar wall thickness and pipe OD. These test components should contain areas of wall loss of known through-wall extent, to allow validation of the penetrated thickness analysis method, described below. It should be emphasised that this is a relative method for measurement of wall loss, and unlike the tangential method (Section 8), the penetrated thickness method does not provide direct measurements of remaining wall thickness. For weld corrosion, this method will be further complicated by the presence of the additional material in the weld cap, which will have a corresponding effect on the film density/CR image grey levels.

9.6.2 Key points

The key points for this technique are:

CR image grey levels must be linearised using the correct look-up table for the CR scanner, if a non linear amplifier is built into the CR scanner.

A step wedge must be used to measure the effective attenuation coefficient, and located

such that the local penetrated thickness is close to that for the wall loss being measured. The reference and measurement areas should be as large as possible given the size of

features in the image to measure. (Small areas are more affected by noise). The reference and measurement areas should be as close to together as possible, given the

limitations of the image content. With this technique, initial results suggest that accurate (±1mm) results can only obtained for relatively small wall losses of c. 30 – 50%. This result was however not obtained on weld corrosion components, where the presence of the additional material in the weld cap will reduce the accuracy of this method.

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9.6.3 Limitations

If this method is used to estimate wall loss, it is important to appreciate its limitations and to note that it is a relatively new method which has not been fully validated. Practical issues can include:

1. The additional material weld cap will complicate the analysis due to the relatively large resulting effects on film density/image grey level.

2. For the DWDI method, if wall loss occurs on both pipe walls (those near to and far from

the source) the corresponding radiation intensity changes can be superimposed on the CR/DR image, leading to an overestimation of the wall loss, if it is assumed to be present in a single pipe wall only.

3. For DWSI it can be difficult to place a step wedge between the pipe wall and detector,

without distorting the detector and increasing the object to detector distance significantly (which can lead to a need to increase the SDD – see Section 8.1).

4. For DWSI, the CR images can contain large variations in background grey level, due the

rapidly changing SDD and penetrated pipe wall thickness values across the image. These background variations introduce uncertainties into the measurement of an appropriate reference grey level for the area of wall loss.

To estimate the background image grey level “underneath” the area of wall loss, analysis methods based on grey-scale profiles extracted from the CR image are recommended. Alternatively, the reference value should be derived from the average grey level of two areas, placed on either side of the area of wall loss. Given the above, for weld corrosion inspection, it is probable that in most cases only a qualitative indication of the severity of the wall loss will be obtainable with this technique.


For any weld corrosion indications reported, it is important to record the following information: Description of the circumferential location and circumferential extent of the corrosion. Qualitative description of the effect on film density/image grey level caused by the weld

corrosion. Optional Estimated wall loss of flaw derived by penetrated thickness analysis of image grey levels (digital

images only). If weld corrosion is detected using this technique, a follow-up inspection using TOFD or tangential radiography, if either is applicable, is recommended to obtain more quantitative information on remaining ligament.

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10 Manual angled beam UT


Manual angled beam UT is a standard ultrasonic NDT technique, and has been in routine use for many years for both manufacturing and in-service weld inspection. Manual angled beam UT is based on a single hand-held pulse-echo shear-wave ultrasonic probe which both excites a pulsed beam into the component and receives any reflected echoes. A hand portable flaw detector is used for excitation of the probe, and display of the arising waveforms. Recent developments have included the widespread use of digital flaw detectors which have additional capabilities compared with their more traditional analogue counterparts. Only a single operator is needed to apply this technique, unlike the more specialised techniques such as TOFD that generally require at least a two man team.

10.2 Issues with application to inspection of weld corrosion

10.2.1 Overall approach

For weld corrosion inspection, manual angled beam UT needs to be applied with different procedures and probes than those used for manufacturing weld inspection. The pulse-echo probe needs to be placed on the parent metal close to the weld cap, and the use of angled-beams generally allows coverage of the weld root region, as illustrated in Figure 10.1. Coverage for weld corrosion offset from the weld centre line away from the probe will be limited.

Figure 10.1 Schematic of pulse-echo weld inspection. The coloured area represents the

coverage achieved using the probe and the green lines show illustrative ray paths for signals from the base of the weld corrosion, where it meets the backwall, and also the position of greatest wall loss (smallest remaining ligament).

10.2.2 Signals expected

For typical examples of weld corrosion, positioned either close to the weld centre-line, or on the probe side of the weld, scanning of the probe in a direction towards/away from the weld cap (i.e. varying the offset between the probe index point and the weld centre line) should generally show a

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number of signals, provided the inspection is carried out at high sensitivity, with due allowance for any transfer losses between calibration specimen and test component. In practice, there is generally a signal seen at or close to the range of the backwall, which corresponds to a corner echo from the base of the wall loss, where it meets the component ID (the left-hand green arrow in Figure 10.1). The optimum probe angle for detection of corner echoes is 45º as weaker signals are generally obtained with 60º and 70º probes due to mode conversion losses. However, there can be potential for confusion with other signals that can arise from the weld root region, due to manufacturing issues such as lack of penetration, protruding weld beads (excess penetration), mismatch etc. Hence signals from the range of the backwall alone are not reliable indicators of the presence of weld corrosion.

10.2.3 Probe angles

Different facets on the weld corrosion itself may also give rise to a number of other indications having ranges less than that of the backwall. Given the likely morphology of the flaw, use of a higher angle (70º) may be beneficial in detecting signals from the flanks of the corrosion. However, with a single-sided ultrasonic technique, any signals from the location on the flaw corresponding to the minimum ligament position are likely to be weak and irregular as the majority of the ultrasound energy will be scattered in the forward direction, away from the probe (see right hand green arrow on Figure 10.1). Hence, for weld corrosion inspection the choice of probe angles (see Section 10.5.1 for more details) does not follow the traditional ultrasonic procedure for manufacturing weld inspection. For example, there is no need to use a 60º probe for a 60º included weld angle, as the detection of lack of sidewall fusion via a reflection off the backwall is not a relevant issue.

10.2.4 Example

An example of weld corrosion exhibiting unusually clear manual angled beam UT indications is given in Figure 10.2, which shows (a) a strong corner signal at the backwall range (weld root) and (b) a weaker signal at a small range, from a point on the weld corrosion nearer the inspection surface. It should be emphasised that it is unusual for examples of weld corrosion to give signals as clear cut as these.

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(a) (b)

Figure 10.2 Examples of manual pulse-echo examination of a weld containing root corrosion.

(a) shows a strong signal at the backwall range – a corner signal from the base of the weld corrosion. (b) shows a weaker signal at a smaller range, from a point on the weld corrosion nearer to the inspection surface. Not all welds give as clear cut signals as these.

10.2.5 Issues with sizing

The back-scattered ultrasonic energy from the point on the flaw closest to the inspection surface (minimum ligament position) is likely to be weak and variable, depending on the detailed morphology of the flaw. In some cases, the signal observed with the shortest range will correspond to the position on the weld corrosion having the minimum ligament. Given the probe beam angle, and the minimum range of the weld corrosion indication(s), an estimate can then be obtained of the minimum remaining ligament. In other cases, the minimum ligament point on the weld corrosion will not give a detectable signal, and consequently the measured remaining ligament will be overestimated (value too high). In some cases, this overestimate can be substantial. For near-surface breaking weld corrosion, it is likely that the point on the weld corrosion closest to the surface will not be covered, as illustrated in Figure 10.2. This problem will be compounded if the weld corrosion were offset from the weld centre line, in a direction away from the probe. For near-surface breaking weld corrosion there is therefore unlikely to be any indication from manual angled beam UT of this condition, as signals from the flanks of the flaw can be mistaken for signals from the minimum ligament position.

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Figure 10.2 Schematic of pulse-echo weld inspection for near surface breaking weld

corrosion, showing lack of coverage of the minimum ligament position even using a high angle probe.

10.2.6 Standards and training

Although manual angled beam UT for weld inspection is a standard NDT technique, all the relevant international standards, and standard PCN, ASNT training courses, relate primarily to manufacturing inspections and do not cover in-service flaws such as weld corrosion. Recently some courses aimed at in-service inspection are however being established.

10.2.7 Trial results and experience of performance

Blind trials (e.g. PISC, NORDTEST, NIL) have shown that angled-beam manual angled beam UT has lower reliability and reduced flaw sizing accuracy than other more specialised NDT techniques such as TOFD. However none of these trials examined performance for weld corrosion inspection. The HOIS weld corrosion trials carried out in support of the development of this document (Sarsfield, Collett and Burch, 2009) showed that sizing accuracy for manual angled beam UT inspection of weld corrosion was also significantly lower than that of the other techniques trialled. Detection reliability was not examined in these trials. HOIS members’ past experiences of manual angled beam UT for weld corrosion inspection have shown unreliability in detection, with examples of both false-calls (indications misdiagnosed as weld corrosion), and false-negatives (i.e. the genuine presence of weld corrosion not being reported). There may be benefit in improving the procedures and training used for weld corrosion inspection. It is possible this would lead to somewhat greater detection reliability with fewer false calls and false negatives (misses). However, fundamentally there are inherent weaknesses in this technique for weld corrosion inspection, as noted above, and accurate sizing is unlikely to be achievable for the reasons outlined above.


If manual angled beam UT is used for weld corrosion inspection, then the guidance given in this Section should be followed. It is important to emphasise however that there is currently no verification that these recommendations will lead to significantly improved detection reliability and

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sizing accuracy in practice. There is therefore a significant possibility of false-calls and missed flaws in some cases. If manual angled beam UT indicates the presence of weld corrosion, it is important that a follow up inspection is made using a technique with greater sizing accuracy, as defined in Section 5, to confirm/deny the presence of the weld corrosion, and also to obtain more reliable sizing information. Little or no weight should be given to the sizing information provided by manual angled beam UT.


Manual angled beam UT should be applied in accordance with following documents: The recommendations given in the subsequent sections of this document. Relevant standards include: BS EN 1714 1998 Non destructive testing of Welds - Ultrasonic testing of welded joints BS EN 583-1:1999 Non-destructive testing. Ultrasonic examination. General principles BS EN 583-2:2001 Non-destructive testing. Ultrasonic examination. Sensitivity and range setting BS EN 1330-4:2000 Non-destructive testing. Terminology. Terms used in ultrasonic testing BS EN 1712:1997 Non-destructive examination of welds—Ultrasonic examination of welded

joints — Acceptance levels. BS EN 1713:1998 Non-destructive examination of welds—Ultrasonic examination —

Characterization of indications in welds. BS EN 12668-1:2000 Non-destructive testing. Characterization and verification of ultrasonic

examination equipment. Instruments BS EN 12668-2:2001 Non-destructive testing. Characterization and verification of ultrasonic

examination equipment. Probes BS EN 12668-3:2000 Non-destructive testing. Characterization and verification of ultrasonic

examination equipment. Combined equipment None of the above standards however directly cover in-service weld inspection for weld corrosion.

10.3.2 Limitations

Experience to date indicates unreliability in detection and sizing of weld corrosion. Is likely to provide no inspection capability for near surface breaking weld corrosion. In practice, surface condition can be problematic for ultrasonic techniques, without prior surface

preparation. Interpretation of results is operator dependent. Weld corrosion inspection is not covered by standard operator training courses in manual

angled beam UT for weld inspection. No permanent record of the inspection data. For further details on limitations of applicability, see Section 10.4 below.

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10.4 General Requirements for manual angled beam UT examination of weld corrosion

Access Manual angled beam UT requires sufficient space on one side of the weld cap to allow placement and scanning of probes both circumferentially, and towards/away from the weld cap. However, when possible, scanning should be carried out from both sides of the weld. Insulation/coatings Manual angled beam UT, in common with other ultrasonic techniques, cannot be performed on insulated surfaces. The presence of paint and other coatings, provided they are in good condition may not generally preclude application of manual angled beam UT. Performance does however need to be verified on any coated components. Coatings which have been in-situ for extended periods can cause issues associated with loss of coupling due to lack of adhesion to the surface, and should generally be removed prior to ultrasonic inspection. See Section 5.6 for further details. Wall thickness The minimum remaining thickness for manual angled beam UT examination of weld corrosion should normally be a minimum of about 6 - 8 mm. Surface Condition Surface condition should be adequate to permit scanning and achieve sufficiently good coupling, as stated in Sections 8.5 and 11 of CEN/TS 14751:2004. See also Section 5.6 for further details and recommendations concerning surface preparation for UT inspection. Component surface curvature Manual angled beam UT in common with other ultrasonic techniques is not generally applied to components having outside diameters of less than about 3", due to issues with coupling the probe to the surface, and distortion of the ultrasonic beam caused by the surface curvature. Temperature For temperatures below 50-60º C, conventional probes and couplant can be used. Between 50º-60ºC and c.120ºC, high temperature probes and couplant are required. Between c. 120º C and c. 400-500º C, special measures are required, including safety precautions. For higher temperatures, no ultrasonic techniques are generally applicable for weld corrosion inspection.

10.5 Preparation for Manual angled beam UT testing


10.5.1 Probes

For general weld inspection, single crystal shear wave probes typically 10mm diameter, frequency 4-5MHz, are usually used. Lower frequency probes (1-2MHz) are permissible for thicker sections or attenuating materials.

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For weld corrosion inspection, the probe angle should be selected taking into account the weld geometry, the width of the weld cap and the likely remaining ligament. The probe angle should be sufficiently high to achieve coverage of the weld root area. Critical weld root scans are often carried out using 45º probes. Use of 45º probes at one and a half skips is recommended as this can improve “reach” into the weld, in cases where the extent of the weld cap, combined with the physical size of probe, prevents coverage of the weld root with a 45º probe via the usual half a skip approach. However, use of one and a half skip inspection increases the required standoff of the probe from the weld cap, which may be limited by geometry. Also, if the effectiveness of this method will be reduced if there is internal corrosion, which will weaken the reflected signal from the backwall. For this reason, it is recommended that if inspections are performed at one and a half skips, the absence of internal corrosion should be first confirmed using a 0º pulse-echo applied using probes adjacent to the weld caps. Higher probe angles (typically 70º or more) are recommended for detection and depth sizing of signals from the sides (flanks) of the weld corrosion. These higher angles are likely to be needed to achieve coverage of the position on the weld corrosion having greatest wall loss, depending on the wall thickness, extent of the wall loss and the width of the weld cap. Even then it needs to be appreciated that for near-surface breaking weld corrosion, coverage of the point on the weld corrosion nearest the inspection surface may not be achieved (see Figure 10.2).

10.5.2 Reference Blocks

Use of the IIW V1 and V2 reference blocks and the IOW beam profile blocks are recommended for manufacturing and in-service weld inspection Other types of reference block as noted in standards ASTM E164-03 or BS1714 are also permissible. The reference block should be of a similar material to that of the item under examination. Use of a calibration block with typically 1.5mm diameter side drilled holes is recommended for setting the sensitivity required.

10.6 Manual angled beam UT scanning of welds


In practice, it may be found that the work instruction prepared in advance of the inspection may need modification when the actual component is inspected. The modifications to the original inspection plan may include: Changes to the probe beam angle, due to the extent of the weld cap being different from that

assumed. Reduced frequency probe needed due to poor surface condition or high material noise levels. Any changes to the design parameters must be recorded in the inspection report.

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For general weld inspection, a calibration block is generally used to make a distance amplitude correction (DAC curve) in accordance with the methods in ASTM E164-03 or BS EN 1714. For weld corrosion inspection, establishing a DAC curve may not be necessary. The minimum sensitivity for angled beam inspection of weld corrosion should be set as follows. The signal from a 3 mm hole at the component thickness in depth should be set to Full Screen Height plus 6 dB. If using skip and a half, then the appropriate range should be used. Anything less than this may prove to be inadequate for detection purposes.

10.6.3 Transfer correction

It is important to apply for a transfer correction between the calibration block and the test component. One method for establishing the transfer correction between calibration block and test component is given in EN 583-2. This is based on use of two identical pulse-echo probes, directed towards one another to form a full V path with the backwall. This combination of two probes is used in pitch-catch mode, with one acting as transmitter and the other as receiver. The difference between the backwall amplitudes from test component and calibration reflector then gives the transfer correction. However, for in-service weld corrosion inspection, this twin-probe method for establishing the transfer correction may be impractical in which case the following simpler method is recommended. On the test component, the gain should be increased until the general “grass level” or “rumble” from the backwall (half skip) and front-wall (full skip) reaches 5% FSH. Any indications exceeding the general grass level shall be reported and analysed. Note that this method for transfer correction needs to be in accordance with the minimum sensitivity setting given in Section 10.6.2 above.

10.6.4 Probe movement

A fixed offset weld root scan is unlikely to be adequate for weld corrosion inspection. Instead, the probe should be scanned towards/away from the weld cap with the probe front edge in contact with the weld cap at the point of closest approach. The full circumference of the weld should be inspected. For a typical scan pattern see Figure 1in EN 1417, an adaptation of part of which is reproduced in Figure 10.3.

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Figure 10.3 Probe movement for inspection for weld corrosion using a manual pulse-echo

shear-wave probe. The probe offset from the weld centre line at the furthest point shall be sufficient to cover at least the backwall below the weld toe (i.e. probe index point at least ½ skip from weld toe). A magnetic strip placed at a suitable distance from the weld could be used as a guide to define the maximum offset from the weld to be used for scanning. If access permits, measurements should be made from both sides of the weld.

10.6.5 Analysis of results

For half skip inspection, for any indication above the general grass level, occurring at an apparent depth equal to the backwall or less, the probe should be moved towards/away from the weld to peak the signal. For inspection using one and half skips, any indication above the general grass level, occurring at an apparent depth between two and three times the first backwall, the probe should be moved towards/away from the weld to peak the signal. At this probe position, the index point of the probe in relation to the weld centre line should be noted, and the arrival time (or range in steel), with correction from the probe delay, should be recorded. This should then be converted to minimum ligament (depth below the inspection surface) in the usual way. For some examples of weld corrosion, a pair of signals can be seen at different ranges, which peak at different probe positions. These may correspond to a corner echo from the base of the weld corrosion, and a back-scattered signal from the point on the weld corrosion nearest the inspection surface. However, other locations on the “flank” of the weld corrosion can also give rise to reportable signals, but the point closest to the inspection surface may not always give a reportable signal. The possibility that the weld corrosion is extensive and near surface, resulting in lack of coverage of the minimum ligament location shall be considered (see Figure 10.2).

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10.7 Interpretation

When interpreting the manual angled beam UT indications obtained from weld corrosion inspection, given previous experience, the likelihood of unreliability in both detection and sizing needs to be considered. To minimise the risk of false-calls, indications from the weld root region alone (range close to that of the parent metal backwall) should not be interpreted as arising from weld corrosion, due to potential for confusion with other weld root features (mismatch, protruding weld bead, slight lack of penetration etc). Reduction in false-calls may be achievable by also requiring detection of signals having ranges indicating scattering from parts of the weld corrosion extending above the parent metal backwall. It is likely that any sizing results from manual angled beam UT inspection of weld corrosion will be inaccurate, and in some cases the remaining ligament may be substantially less than that obtained from manual angled beam UT examination. If the presence of weld corrosion is indicated from manual angled beam UT examination, it is strongly recommended that a follow-up inspection is made using one of the recommended sizing techniques described in this document (see Section 5).

10.8 Additional measurements

Use of a 0º compression wave probe is also recommended in conjunction with the angled-shear wave measurements, to make measurements of the parent metal wall thickness, and to check for the presence of any generalised internal corrosion near to the weld. In addition, measurements can be made of remaining ligament for any weld corrosion which is sufficiently offset from the weld centre line to be covered by a 0º probe positioned adjacent to the weld cap (see Figure 7.1). The 0º pulse echo method should be applied in general accordance with EN14127:2004, and see also the relevant parts of Section 7.


For any weld corrosion indications reported, it is important to record the following information: Description of the circumferential location and circumferential extent of the corrosion, especially

for any regions with a ligament less than the corrosion allowance (CA). Estimated minimum depth/remaining ligament measured anywhere in the weld. Estimate of the wall thickness in areas which have not been corroded (from 0º pulse-echo). If the presence of weld corrosion is indicated from manual angled beam UT examination, it is strongly recommended that a follow-up inspection is made using one of the recommended techniques described in this document (see Section 5).

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11 Phased array


Ultrasonic phased array (PA) instrumentation is an example of a recently developed technology, which exploits the latest advances in computers and micro-electronics to provide novel inspection capabilities. Until recently, phased array systems required relatively large electronic/computer units which limited their portability. However, in the last few years, a number of much smaller units have been developed, which are highly portable. When used in conjunction with a hand-held phased array probe, the overall instrumentation package is not dissimilar to that needed for manual angled beam UT using a digital flaw detector, as described in Section 10. Phased array ultrasonic probes consist of multiple small piezo-electric elements, usually arranged in a line to form a linear array. Phased array technology generates a pulsed ultrasonic beam and software enables the setting of beam parameters such as angle, focal distance, and focal point size. As with conventional probes, an angled shoe is used to generate angle-beam shear waves in the component by refraction at the component surface. By introducing progressive electronic delays into the firing of the elements, and the reception of the received pulses, the effective angle of the ultrasonic beam can be dynamically steered or focussed. Linear delays cause beam steering (change of angle), while quadratic delays allow focussing. Another important operation mode of a phased array involves moving the effective probe position electronically within the overall length of the phased array, i.e. scanning within the array. The application of phased arrays to in-service inspection is at an early stage, and is not covered in detail by any recognised international standards or procedures. Weld corrosion inspection using phased arrays is effectively similar to inspection using conventional pulse-echo probes, in that both are single-sided pulse-echo techniques. For weld corrosion inspection, the capabilities of the PA instrumentation to provide variable beam angles is often used, as illustrated in Figure 11.1. This increases the coverage achieved compared with a conventional pulse-echo probe at a fixed angle. Data from the PA probe at a single probe position is then displayed as a sectorial or S-scan, in which indications are displayed as a function of both range from the probe and angle in a geometrically correct image. Signals from indications such as facets on weld corrosion typically appear as short curved arcs on a sectorial scan.

Figure 11.1 Schematic of pulse-echo weld inspection using a variable-angle phased array

probe. The coloured areas show two different beam angles, but the phased array generates a continuous range of angles, between user specified limits.

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A ‘good’ example of a PA sectorial scan obtained from an example of moderate weld corrosion is given in Figure 11.2. In this case, two main signals from the weld corrosion were seen. The first was at a depth corresponding to the backwall, and corresponds to a corner echo. The second was from a point on the weld corrosion close to the “top” of the flaw – nearest to the inspection surface.

Figure 11.2 Example of a PA sectorial scan from inspection of weld corrosion. The two

highlighted signals are firstly from the corner formed by the backwall and the weld corrosion, and secondly from a point on the weld corrosion near to the “top” of the flaw, i.e. nearest to the inspection surface. Example courtesy of GE Inspection Technologies.

11.2 Limitations for weld corrosion inspection

Despite the advances in technology given in the section above, phased arrays suffer from many of the same limitations as conventional pulse-echo for weld corrosion inspection because, fundamentally, the inspection geometry and physics of the inspection are closely similar in the two techniques. In particular, with any single-sided inspection method, the majority of the ultrasound energy from the weld corrosion will be scattered in the forward direction, away from the probe, and hence the back-scattered ultrasonic energy from the shallowest point on the flaw is likely to be weak and variable, depending on the detailed morphology of the flaw. Coverage may also be incomplete for substantial weld corrosion, such that the minimum ligament position is not within the probe beam. The results obtained using PA for weld corrosion in the inspection trials (Sarsfield, Collett and Burch, 2009) indicate a considerable variability in performance. Reliable detection and accurate sizing results were obtained on some examples of weld corrosion (typically those with low to moderate through-wall extents), while for others the sizing results were significantly less accurate. As application of PA to in-service inspection is in its infancy, HOIS members have little or no experience of its reliability for weld corrosion detection and sizing.

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Due to the limitations described in the above section, the application of PA techniques is not recommended for weld corrosion inspection. The inherent physical shortcomings of the technique are likely to result in unreliable detection, a high false call rate and limited sizing accuracy. If however PA UT is the only technique available for weld corrosion inspection, then the information given in this Section may lead to improved performance. It is important to emphasise however that there is currently no verification that these recommendations will lead to significantly improved detection reliability and sizing accuracy in practice.


ASME Section V, Article 4 covers PA in general terms. ASME Code Cases 2541, 2557, 2558, 2599 and 2600 cover phased array inspection of construction welds. There is however no standards or code cases which cover in detail the application of PA techniques for weld corrosion inspection.

11.3.2 Advantages over manual angled beam UT

The advantages of PA UT over manual angled beam UT are as follows: Compared with manual angled beam UT, PA can give improved coverage due to use of variable

beam angles and less need for scanning towards/away from the weld PA technology includes increased data display and visualisation capabilities compared with flaw

detectors used for manual angled beam UT, including the ability to show the results from a complete scan of the weld in a single presentation.

Permanent recording of data is available for encoded scans, but not for manual (un-encoded)

scanning.

11.3.3 Limitations

Experience to date indicates variable sizing accuracy of weld corrosion. Detection reliability has not been assessed adequately.

Is likely to provide no inspection capability for near surface breaking weld corrosion. In practice, surface condition can be problematic for ultrasonic techniques, without prior surface

preparation. PA probes and shoes can be larger than conventional manual shear wave probes, so access

limitations can be more significant and coupling on surface with poor surface condition can be more problematic. However, PA probes with similar sizes to conventional probes are now available and should be used for this application.

Interpretation of results is operator dependent. No recognised PCN/ASNT training course for PA inspection of weld corrosion.

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For further details on limitations of applicability, see Section 11.4 below.

11.4 General Requirements for PA UT examination of weld corrosion

Access PA UT requires sufficient space on one side of the weld cap to allow placement and scanning of probes both circumferentially, and towards/away from the weld cap. However, when possible, scanning should be carried out from both sides of the weld. Insulation/coatings PA UT, in common with other ultrasonic techniques, cannot be performed on insulated surfaces. The presence of paint and other coatings, provided they are in good condition does not generally preclude application of PA UT. Performance does however need to be verified on any coated components. Coatings which have been in-situ for extended periods can cause issues associated with loss of coupling due to lack of adhesion to the surface, and should generally be removed prior to ultrasonic inspection. See Section 5.6 for further details. Wall thickness The minimum remaining thickness for PA UT examination of weld corrosion should normally be at least about 6 - 8 mm. Surface Condition Surface condition should be adequate to permit scanning and achieve adequate coupling. Component surface curvature PA in common with other ultrasonic techniques is not generally applied to components having outside diameters of less than about 3", due to issues with coupling the probe to the surface, and distortion of the ultrasonic beam caused by the surface curvature.

11.5 Preparation for PA UT testing


11.5.1 PA probes

For weld corrosion inspection, PA probes are typically c. 15 mm in length with a frequency of c. 4-5MHz. Beam angles are generally between about 30º and 70º , the PA beam angle steering capabilities provide a continuous range of angles between these limits for one design of probe shoe. PA probes are also available up to ~15 MHz and less than 10 mm length. Also, wedges are available for higher angles if appropriate. Compact designs for probe shoes are recommended to minimise access limitations. Care should be taken in shoe design to minimise standing echoes which can confuse data interpretation.

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11.5.2 Reference Blocks

Use of a calibration block with typically 3mm diameter side drilled holes is recommended for setting the sensitivity required, as described in Section 11.6.1 below.

11.5.3 Coverage

It is recommended that proprietary PA software modelling tools are used, where available, to demonstrate satisfactory design of the inspection, particularly that the selected probes and probe offset from the weld centre line give adequate coverage of the weld volume.

11.5.4 0º pulse-echo measurements of wall thickness

Prior to the PA inspection of a weld, use of a 0º compression wave probe is recommended to make measurements of the parent metal wall thickness. This allows the distances to the backwall echoes to be marked correctly on the display of the PA data, for example using horizontal graphics lines superimposed on a sectorial scan. This assists the reliable interpretation of the PA sectorial scan data, since corner echoes from the weld corrosion/backwall should then coincide in depth with the first backwall.

11.6 PA UT scanning of welds

11.6.1 Sensitivity settings and transfer correction

For manual angled beam UT, it is recommended that a minimum sensitivity is set using the signal from a 3mm diameter side drilled hole at the same depth as the component thickness, which should be set to Full Screen Height + 6 dB. An allowance for transfer corrections between calibration block and test component should also be made (see Section 10.6.2) For PA UT testing, similar procedures need to be used to establish the minimum inspection sensitivity and make allowance for the transfer correction between a calibration block and test component.

11.6.2 Probe movement

With a variable angle PA probe, it can be possible to inspect a weld containing slight or moderate weld corrosion with the probe at a fixed standoff from the weld, illustrated in Figure 11.3. As mentioned in Section 11.5.3, PA software modelling tools should be used to set the stand-off and to ensure adequate coverage of the weld.

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Figure 11.3 PA inspection with the probe at a constant stand-off from the weld. Depending on the weld geometry and beam coverage, for moderate to severe weld corrosion, it is likely that the PA probe will need to be scanned towards and away from the weld, as well as along the weld, as shown for conventional manual probes in Figure 10.3. If access permits, measurements should be made from both sides of the weld.

11.6.3 Analysis of results

For any indication occurring at ranges corresponding to the backwall (1/2 skip) or shorter, the PA instrumentation shall be used in accordance with the manufacturers instructions, to measure the depth below the inspection surface, typically using interactive cursors. When measuring the depth of an indication, it is recommended that cursors are aligned with the centre of the arc, not some arbitrary point above centre. The depth of weak signals from the backwall should be checked for consistency with the marked first backwall line (i.e. the line should go through centre of the weak “rumble” signals from slight roughness on the backwall, and any corner echoes from weld corrosion). For some examples of weld corrosion, a pair of signals can be seen at different ranges and offsets from the probe (see Figure 11.2). These may correspond to a corner echo from the base of the weld corrosion, and a back-scattered signal from the point on the weld corrosion nearest the inspection surface. It should be noted that other locations on the “flank” of the weld corrosion can also give rise to reportable signals, and the point closest to the inspection surface may not always give a reportable signal. The possibility that the weld corrosion is extensive and near surface, resulting in lack of coverage of the minimum ligament location shall be considered (see Figure 10.2).

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11.7 Interpretation

When interpreting the PA indications obtained from weld corrosion inspection, the likelihood of unreliability in both detection and sizing needs to be considered. To minimise the risk of false-calls, indications from the weld root region alone (range close to that of the parent metal backwall) should not be interpreted as arising from weld corrosion, due to potential for confusion with other weld root features (mismatch, protruding weld bead, slight lack of penetration etc). Reduction in false-calls may be achievable by also requiring detection of signals having ranges indicating scattering from parts of the weld corrosion extending above the parent metal backwall. It is likely that any sizing results from PA inspection of weld corrosion will be inaccurate, and in some cases the remaining ligament may be substantially less than that obtained from PA examination. If the presence of weld corrosion is indicated from PA examination, it is strongly recommended that a follow-up inspection is made using one of the recommended techniques described in this document (see Section 5).

11.8 Additional measurements

Measurements using a 0º pulse-echo probe (PA or conventional) should be made of remaining ligament for any weld corrosion which is sufficiently offset from the weld centre line to be covered by a 0º probe positioned adjacent to the weld cap (see Figure 6.1). The 0º pulse echo method should be applied in general accordance with EN14127:2004, and see also the relevant parts of Section 7.


For any weld corrosion indications reported, it is important to record the following information: Description of the circumferential location and circumferential extent of the corrosion, especially

for any regions with a ligament less than the corrosion allowance (CA). Estimated minimum depth/remaining ligament measured anywhere in the weld. Estimate of the wall thickness in areas which have not been corroded (from 0º pulse-echo). If the presence of weld corrosion is indicated from PA examination, it is strongly recommended that a follow-up inspection is made using one of the recommended techniques described in this document (see Section 5).

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12 Inspection through the weld cap

12.1 Introduction

Depending on the characteristics of the weld cap, detection and sizing of weld corrosion by inspection using a 0º pulse-echo probe through the weld cap may be possible, as illustrated in Figure 12.1.

Figure 12.1 Schematic illustration of 0º pulse-echo examination of weld corrosion through

the weld cap. In general, the unevenness of the weld cap surface precludes the use of conventional 0º twin-crystal contact probes. However, with certain specialised probes or scanning systems it may be possible to obtain useful results, as detailed in the following sections. In all cases, it is important to be able to move or scan the probe over the weld cap surface, both in the across weld and along weld directions, to find the position of the minimum remaining ligament. With inspection through the weld cap, it is also important to note that the measured ligament includes the height of the weld cap, under the probe, whereas for ultrasonic techniques applied from the parent metal, the weld cap height is not included in the measured ligament values – see Figure 4.9. Note that a measurement through the cap may still be the minimum ligament remaining dependent on the location of the weld corrosion. The probes and/or techniques described below are not currently sufficiently well developed, or there is insufficient experience of their use, to be recommended for general weld corrosion inspection. However, in some specialised applications, principally those involving lower profile or less rough weld caps, these methods could be considered further, especially if the more generally recommended TOFD or CR techniques are not applicable for some reason (e.g. access constraints). The main limitation of these techniques is that the irregular weld cap surface will inevitably distort or deflect the ultrasonic beam generated by the probe, which may lead to unpredictable results, and weak reflected signals from any weld corrosion in the weld.

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12.2 0º scanning over weld cap with stand-off probes

Stand-off 0º probes which use a water column for coupling (so called ‘squirter’ probes) have been reported as allowing inspection of weld corrosion through weld caps (Cole 2006) when used with a fast corrosion mapping scanning system (LSI from Physical Acoustics. Given sufficient stand-off from the surface, the probes can be scanned over the parent metal and weld cap in the same scan. With this equipment, single crystal immersion probes are used which give a strong reflection from the near surface of the component. If the measured arrival times are triggered by this strong surface reflection, the variable water path does not affect the accuracy of the measured weld corrosion depths. In initial trials of this method, Cole (2006) reported good agreement between the results of the ultrasonic measurements of remaining ligament, compared with those measured physically, as illustrated in Figure 12.2. The agreement is good, with a maximum difference of < 1mm.

0

2

4

6

8

10

12

14

16

18

0 2 4 6 8 10

Measurement position

Wal

l lo

ss (

mm

)

12

Mechanical measurement

Measured by 0 deg UT

Figure 12.2 Wall loss values derived by mechanical measurements compared with those

from 0º pulse-echo squirter probes, from Cole (2006). It is apparent, from these initial studies that useful results can be obtained on some welds. Although performance may be dependent on the height and unevenness of the weld caps under inspection, this method is now used routinely by one HOIS member and good results are reported.

12.3 GE flexible probe

GE Inspection Technologies have recently developed a 0º ultrasonic probe which is made of highly flexible material which can be made to conform to some weld caps, by manually pressing the probe onto the surface. The probe is shown in Figure 12.3.

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Figure 12.3 The new flexible 0º probe from GE Inspection Technologies This probe can be used in conjunction with conventional flaw detectors. However, initial experience suggests that in these cases the signals from weld corrosion below weld caps can be weak and difficult to interpret. In an initial trial, improved results were obtained by using the probe in conjunction with digital ultrasonic data recording equipment, and display using grey-scale B-can imaging techniques (similar to those used conventionally for TOFD). It was found that the probe could be scanned manually over the weld cap, at selected circumferential locations, whilst maintaining adequate coupling. As predicted in Section 12.1, variable results were obtained with the probe on the weld cap due to the uneven surface which distorts and deflects the ultrasound beam. Nevertheless, on the weld inspected, the B-scan imaging techniques allowed detection and measurement of signals from the weld corrosion. An example results is shown in Figure 12.4. The ligaments measured from the 0º B-scan data agreed well with those obtained from mechanical measurements with a typical difference of c. 1mm.

Figure 12.4 B-scan results of inspection of a weld containing both central and offset weld

corrosion using the flexible 0º probe from GE Inspection Technologies

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Prolonged usage of this probe involving scanning across a surface may cause significant wear to the probe face. This would need to be investigated further before this method could be recommended for general usage. Nevertheless, this technique could be considered for applications in which the recommended techniques of TOFD or CR are precluded, perhaps due to access constraints.

12.4 SCEXY

A technique known as SCEXY has been used in certain cases for weld corrosion inspection through the weld cap. It is understood this technique involves use of epoxy resin applied to the weld cap surface, to provide a more even surface for 0º ultrasonic measurements using conventional twin-crystal probes. Experience of this technique is limited, but the need to apply an epoxy and wait for it to cure will lead to slow inspection speeds, and there will also be limitations due to distortion/deflection of the ultrasound beam at the epoxy/weld cap surface. As with the other 0º techniques described in this section, this is a technique to consider if the preferred techniques are not applicable.

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13 Health, Safety and Environmental Considerations.

13.1 General Requirements

Compliance with all health and safety legislation is a legal requirement of all employers. Below is a list of some of the key legislation relating to inspection.

Title Publication Number

Approved Code of Practice (ACOP)

Issuing Body

Management of Health and Safety at Work Regulations 1999

SI 1999/3242 L21 HSE

The Personnel Protective Equipment at Work Regulations 1992

SI 1992/ 2966 L25 HSE

Provision and use of Work Equipment Regulations 1998

SI 1998/ 2306 L22 HSE

Manual Handling Operations Regulations 1992

SI 1992/ 2973 L23 HSE

Control of Substances Hazardous to Health 2002 (COSHH)

SI 2002/ 2677 L5 HSE

The Confined Spaces Regulations, 1997 SI 1997/ 1713 L101 HSE Health and Safety Regulations… A Short Guide

HSC13 HSE

The Ionising Radiations Regulations 1999 L121 HSE IEE Guidance Document, EMC and Functional Safety. http://www.iee.org.uk/PAB/EMC/core.htm

IEE

The asset managing organisation and the inspection service company need to be aware of all the requirements and make appropriate provisions. The inspection team and any local site support staff need to be aware of all requirements and comply with them. Any appropriate work permits will need to have been obtained prior to the commencement of any inspection. Prior to the commencement of any inspection all operators must comply with local safety requirements and complete local induction and safety briefings.

13.2 Preparation for Site Working

Suitable risk assessments must be carried out prior to the arrival of any inspection team at the inspection site. Such risk assessments should be performed in consultation with the asset owners; personnel involved in the day to day running of the site; inspection operators and their management. Before entering any site this risk assessment should be reviewed by the inspection team leader. If there are any new risks identified or inadequate provision to mitigate identified risks then these must be addressed to the satisfaction of the inspection team leader, prior to the commencement of any inspection.

http://www.iee.org.uk/PAB/EMC/core.htm

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At all times during an inspection the safety of the inspection personnel will be the responsibility of the inspection team leader. The correct PPE must be provided and worn as required and highlighted within the risk assessment(s).

13.3 Working Precautions

The equipment being used can be heavy and difficult to move. Further to these considerations it is necessary to ensure that adequate breaks are taken during the inspection to ensure operator fatigue does not become an issue. This can be a particular problem in hot or confined environments. Often rope access is required to deploy the inspection techniques described in this document. All relevant safety precautions for rope access must be complied with.

13.4 Radiographic techniques

The radiographic techniques described in this document involve the use of ionising radiation. The use of ionizing radiation leads to safety issues as exposure of any part of the human body to X-rays or gamma-rays can be highly injurious to health. Wherever X-ray equipment or radioactive sources are in use, appropriate legal requirements must be applied. Local or national or international safety precautions when using ionizing radiation shall be strictly applied. In the UK, the relevant legislation is the Ionising Radiation Regulations (IRR 1999).

13.4.1 Size and strength of sources

The strengths (activities) of isotope sources used for radiography need to comply with local regulations. Radiography contractors should state the maximum strength isotopes within their local rules as required by IRR 1999.

13.4.2 Source containers and collimation

The source containers should conform to the requirements for source containers given by ISO3999-1:2000 or BS5650:1978 ISO 3999-1977 and any applicable national standards. Conventional projection equipment can be used, provided the requirements of the current radiation safety regulations are complied with. For these systems, a large radiation controlled area is normally needed, which often requires out of hours working, or even shutdown of plant. Systems which keep the source within a single container or single container/collimator assembly are now available that allow a much smaller size of controlled area (of order 1 m - 5 m). These systems reduce radiation doses to operators, and the small size of the controlled area generally means that plant operation does not need to be interrupted while site radiography is underway.

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The ratings of a source container must be checked for compliance with the type and strength of the isotope to be used. The selection of an appropriate source container and deployment system depends on the balance of the above factors for each individual site and inspection application, together with economic considerations. Careful collimation of the sources is recommended to minimise unwanted radiation, and to reduce the effects of scatter on the radiograph.

13.4.3 In-situ inspection of plant

Use of gamma ray radiography equipment for in-situ inspection of plant involves significant safety issues associated with the use of ionising radiation. The appropriate mandatory safety regulations appropriate to the plant must be adhered to (IRR 1999 in the UK). These include the construction and maintenance of radiation controlled areas, by means of appropriate barriers. Pre-planning of the inspection work to be carried out on a plant is required, to include both a risk assessment and a practical assessment of how the source container and shielding will be placed (IRR 1999).

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14 Inspection Personnel Competence

14.1 General

For the inspection of weld corrosion, the number of personnel and their qualifications and training depend upon the inspection technique being applied. The requirements are lower for standard techniques such as manual angled beam UT than for specialist techniques such as TOFD. In general, the selection of personnel and their training should be in accordance with either of the following standards:

EN 473:2008 - “Non-destructive testing. Qualification and certification of NDT personnel. General principles”

ANSI/ASNT CP-189 - Standard for Qualification and Certification of NDT Personnel (2006)

BS EN ISO/IEC 17020:2004 General criteria for the operation of various types of bodies

performing inspection Especially for specialist techniques, employers should have a written practice based upon a document such as ASNT Recommended Practice SNT-TC-1A to ensure the competence of staff in the specific technique. The written practice should detail the competence requirements including experience of personnel carrying out weld corrosion inspections, and ensure the proper preparation of procedures, work instructions, training, to assure that the work is carried out in accordance with the company’s quality system and client specification.

14.2 Standard techniques

14.2.1 Manual angled beam UT

For manual pulse-echo ultrasonics, a single operator is adequate, having appropriate relevant qualifications (level II PCN or ASNT for manual pulse-echo weld inspection). Experience of inspection for in-service flaws including weld corrosion is required, in addition to the PCN or ASNT qualification. It is recommended that operators complete a training course specifically aimed at manual angled beam UT examination for in-service applications, including weld corrosion.

14.2.2 Radiographic techniques

For the radiographic techniques described in this document, it is important for safety reasons to have a minimum working team of two personnel. At least one of these personnel shall have a standard relevant EN 473 or ASNT level II qualification (e.g. in film radiography of welds). Additional training in the tangential technique, if being used, is also required in accordance with the employer’s written practice.

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14.3 Specialist techniques

14.3.1 TOFD and 0º Pulse-echo

Specialist NDT techniques involving the use of computer equipment and scanners require an inspection team of at least two individuals. The roles required of the inspection team can be described as:

Level II TOFD operator (Team Leader) Scanner operator/Technical Assistant (TA)

Each member of the team can accept the responsibilities of more than one role. The Scanner operator should be trained for the specific application in accordance with the employer’s written practice to ensure proper coupling conditions during scanning, properly aligning the scanner assembly relative to the weld centre line (centred or offset), and informing the Team Leader of conditions related to any geometrical or other condition having a possible adverse influence on the quality of the data. In addition, scanner operators should be trained to select and check equipment in accordance with a written work instruction, assemble scanners, et cetera. The level II TOFD operator shall have an EN 473 level II qualification in TOFD, with additional training in and experience of WRC inspection in accordance with the employers’ written practice. He shall be able to carry out the inspection in accordance with a procedure, analyse and report the data.

14.4 Experience/Competence

In addition to an individual gaining the qualifications described above there is a requirement to build up experience. This can be achieved partially through the training courses, but given the difficulty of arranging classroom training of a practical nature for weld corrosion inspections, personnel will need to gain an understanding of weld corrosion inspection requirements by on-the-job experience. The timing and extent of the experience shall be described in the employers’ written practice.

14.5 Recertification

An individual shall be re-certified periodically by a company in accordance with their own written practice and the requirements of SNT-TC-1A. All NDT Personnel may be re-examined at any time at the discretion of a Company and have their certification extended or revoked accordingly.

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15 Application Considerations

15.1 Weld datum and numbering system

The proposed weld referencing system should be clear before starting inspection and agreed, in advance, with the client. A simple and unambiguous labelling system should be defined for the welds within a plant to be inspected and referenced to some defined point or datum. All reported positions, both circumferential and axial must then be reported relative to this datum point. The coordinates system relative to the datum must also be defined and recorded in both the circumferential and axial directions.

15.2 Coverage Limitations

In some cases, it may not be possible to achieve 100% coverage of welds due to physical access constraints. If this is the case, any coverage limitations shall be clearly recorded in the inspection reports.

15.3 Surface Condition

This is covered in Section 5.6 for UT techniques and for the individual technique sections, but special attention should be given to surface conditions for UT inspection techniques. The quality of inspection is directly dependent upon the quality of the surface condition and the true value of the inspection will only be achieved if the surface condition is optimised which usually means any coating should be removed.

15.4 Inspection History

It is recommended that site owners should ensure access, where available, to weld design and inspection history. This should include wall thickness and details of weld preparations and any coatings including thickness.

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16 Conclusions

Despite the widespread occurrence of weld corrosion, there are currently no international standards or published recommended practices covering inspection for weld corrosion. The reliable detection and sizing of weld corrosion is not straightforward because the region of greatest wall loss is usually underneath the weld cap, which prevents direct use of ultrasonic 0º compression wave techniques. Only rarely is it possible to remove the weld cap. In addition, the location and morphology of weld corrosion can be highly variable, and unpredictable. There are often issues of access due to the geometry changes associated with pipe welds, such as weld neck flanges, bends, valves and reducers etc. This document presents a unified recommended practice for the in-service inspection for weld corrosion, and gives recommendations regarding the preferred inspection methods and techniques, for a variety of conditions, including: Component geometry (wall thickness and diameter). Access restrictions caused by adjacent weld neck flanges, bends, valves, reducers. Different qualities of surface finish and condition. Effects of raised surface temperature. At present, this document is limited to the inspection for weld corrosion in carbon steel welds only. The preferred technique for weld corrosion inspection is ultrasonic time-of-flight diffraction (TOFD) provided the component falls within the capabilities of the technique. It is also recommended that TOFD should be combined with the use of associated 0º pulse-echo scans to measure the wall thickness on either side of the weld cap, where possible. TOFD involves no radiation safety issues and generally allows accurate sizing around the whole weld circumference. For highest accuracy, issues concerning the sizing of flaws offset from the weld centre line need to be considered and addressed. In cases where application of TOFD is not possible, the alternative recommended technique is tangential radiography provided the arising radiation safety issues can be adequately addressed, and sufficient penetration can be achieved given the component diameter and wall thickness. Due to its limited circumferential coverage, the combination of this technique with double-wall double image radiography to locate the most severely attacked section of weld is recommended. If circumstances preclude the usage of these two preferred techniques, consideration may be given to alternative techniques, including double wall radiography which provides only qualitative information on the through-wall extent of weld corrosion. Specialised techniques for inspection through the weld cap using stand-off (water column) 0º pulse-echo probes may also be applicable. It is recommended that these techniques are fully validated with regards to detection reliability and/or sizing accuracy. Based upon HOIS members experience, manual angled beam UT can have limitations in terms of detection reliability for this application. Examples of both false calls and false negatives (missed flaws) have been experienced during in-service inspection with this technique. Due to its inherent low sizing accuracy, follow-up using a technique having a higher sizing accuracy is recommended if the presence of weld corrosion is indicated from manual angled beam UT. Experience with angled beam PA techniques is limited for weld corrosion inspection, but similar limitations to those for manual angled beam UT are expected, as both techniques are based on very similar physical principles (angle-beam pulse-echo ultrasonics).

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Detailed guidance is given on the application of the all the above on destructive testing techniques for inspection of weld corrosion. Recommendations are made with regard to Health, Safety and Environmental considerations and inspection personnel and training.

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17 Acknowledgments

Malcolm Miller from Shell is thanked for many valuable discussions during the preparation of earlier drafts of this document, and his invaluable overall guidance during the course of this project. In addition, Ian Bradley (BP) provided significant contributions to Section 4 on weld corrosion mechanisms. Sonovation bv and Sonomatic Ltd supplied useful information and examples concerning TOFD in Section 6. Several other members of the HOIS JIP provided useful comments on earlier drafts of this document.

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References Burch S F (2009). Recommended Practice for the in-service inspection of wall loss in pipes by

computed radiography, Published HOIS report HOIS(09)RP1, December 2009. Burch S F (2009a). Results from HOIS CR Trials in Bergen October 2008. HOIS report HOIS(08)R8

Issue 1. January 2009 Burch S F (2009b). Results from HOIS CR Trials in Bergen May 2009. HOIS Report HOIS(09)R5

Issue 1. August 2009. Burch S F and Collett N J (2005). “Recommended practice for the rapid inspection

of small bore connectors using radiography”. HSE Research Report 294. Available at http://www.hse.gov.uk/research/rrhtm/rr294.htm

Charlesworth J P and J A G Temple, ‘Engineering applications of ultrasonic time-of-flight diffraction’,

Research Studies Press Ltd, Second Edition, 2001. Cole P T (2006), ‘Ultrasonic Measurement of Weld Root Erosion’, ECNDT 2006 - Poster 194. DD CEN/TS 14751:2004 “Welding – Use of time-of-flight diffraction technique (TOFD) for

examination of welds”. Sarsfield H R, Collett N J and Burch S F (2009) “Trials of techniques for inspection of weld

corrosion”. HOIS Report HOIS(09)R3 Issue 1, August 2009. Silk M. G. (1984) "The use of diffraction-based time-of-flight measurement to locate and size

defects" Brit. J. NDT vol 26 1984 pp 208-213. Zscherpel U. and Ewert U. (2006). Presentation at HOIS CR workshop, March 2006.

http://www.hse.gov.uk/research/rrhtm/rr294.htm

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Appendices

APPENDIX 1 SUMMARY OF MEMBERS QUESTIONNAIRE AT THE START OF PROJECT...................................................................................................106

APPENDIX 2 FORMULAE FOR ANALYSIS OF TOFD MODE-CONVERTED SIGNALS 111

APPENDIX 3 SUMMARY TABLE........................................................................113

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Appendix 1 Summary of members questionnaire at the start of project

A1.1 Introduction

A HOIS members questionnaire seeking information about their experience with weld corrosion, and the inspection techniques used to detect and size this form of degradation was prepared and issued in January 2007. A compilation of the responses obtained is given below. Responses were received from 10 members. Appendix 1 of HOIS2000(07)R6 compiles the questionnaire responses. The main findings can be summarised as follows:

A1.2 Level of Interest in project

The great majority of questionnaire respondents rated this as high (7), with only two stating it was of medium interest.

A1.3 Plant Locations

Weld corrosion was noted as occurring in all the listed plant locations, both onshore and offshore.

A1.4 Location

Examples of weld corrosion were found at all locations specified, ranging from upstream of the HAZ/parent metal interface, through the HAZs and weld, to downstream of the HAZ/parent metal interface. The highest number of occurrences were on the weld centre line.

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A1.5 Type

All specified types of weld corrosion were reported, with the most common being groove corrosion, closely followed by erosion including erosion/corrosion.

A1.6 Materials

Members reported weld corrosion occurring most frequently in carbon steel (9), with relatively few examples in duplex (2) and super duplex (1) steels.

A1.7 Process fluids

The following process fluids were listed by members as being associated with weld corrosion Produced fluids before separation – water/gas/oil (and sand) Produced water - oxygenated Wet hydrocarbons, HP Water Injection Stabilized crude oil containing produced water. Glycol regeneration in gas drying plant. Scale inhibitor injected in lift gas. De-oxygenated low sulphate water (Water Injection) Produced water and flowlines Wet gas Dryish gas Various media including hydrocarbon/wet gas systems, produced water, sea-water injection.

A1.8 Usage of inspection techniques

The following techniques were listed as being used frequently by at least one member, without removal of the weld cap: TOFD (with digital data recording giving position encoded B or D-scan display). Manual angled beam pulse-echo. Double wall radiography (DWDI or DWSI). Tangential Radiography. With weld cap removal, only one member reported frequent usage of a technique (manual pulse-echo).

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A1.9 Experiences with Inspection techniques

A1.9.1 TOFD

TOFD is a widely used technique for weld corrosion inspection, and has been applied to many different components. Opinions on performance generally ranged from adequate to performed very well, with one respondent rating it disappointing. The main benefits of the technique were listed as: Permanent record. Accuracy, repeatability and reliability of inspection. Fast. The main limitations of the technique were given as: Equipment reliability. Good surface finish required . Good access required for probes. Sometimes high temperature an issue. Cost. Geometry limits. Availability of competent personnel. Occasional interference (electrical) ?

A1.9.2 Manual Angled Pulse-echo

Manual angled pulse-echo is a widely used technique for weld corrosion inspection, and has been applied to many different components. Opinions on performance generally ranged from disappointing to performed well, with the highest “vote” for adequate. Only one respondent rated it as performing very well. The main benefits of the technique were listed as: Fast. Effective. Standard capability of core crew. Found the defects in components with simple geometry. Cheap and easy to deploy. The main limitations of the technique were given as: Lack of credibility. Difficult to compare with results from previous inspections due to lack of permanent record. More reliant on individual NDT technician capability – some better than others. Limitations on sizing. Limitations on geometry. Limitations on reliability.

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A1.9.3 Double wall Radiography

Double wall radiography is a reasonably widely used technique for weld corrosion inspection, and has been applied to many different components. Opinions on performance were generally that it performed well, although one respondent rated it as only adequate and another as disappointing. Presumably because of its relatively slow inspection speed, one respondent mentioned it is not used as a primary screening technique, but it may be used in support (presumably follow-up). The main benefits of the technique were listed as: Good detection capability with through lagging / insulation capability. Record of results for repeat inspection. Use of digital RT systems gives more flexibility and can significantly reduce exposure durations. The main limitations of the technique were given as: Safety risk (although can be reduced by utilising SafeRad / SCAR type systems). No quantified measure of wall loss. Requires UT follow up to derive figure. Some access problems for placing film due to geometry. Limitation on maximum thickness. Liquid product in pipe affects results.

A1.9.4 Tangential Radiography

Tangential radiography was given as being used frequently by one respondent. Two respondents rated it as performing well. The other described it as disappointing because the radiography was not effective due to an excessive chord length issue (maximum tangential path through the pipe wall being too great for the specific wall thickness and pipe diameter of interest). The main benefits of the technique were listed as: Remaining wall thickness, at tangential locations, can be measured reasonably accurately. Hard copy image generated. The main limitations of the technique were given as: Restricted to small diameter; relatively thin wall Safety and nucleonic aspects of performing RT on working plant. Slow. Inappropriate technique for thicker sections.

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A1.9.5 Other techniques

Other less widely used weld corrosion inspection techniques included: 0º UT through weld cap. Manual TOFD. 0º pulse-echo applied with a water column between the stand-off probe and the component SCEXY.

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Appendix 2 Formulae for analysis of TOFD mode-converted signals

For a parallel sided test component, Figure A2.1 shows the TOFD inspection geometry for an indication offset from the probe mid-point.

Tx Rx S/2 S/2

x

d P1 P2

Figure A2.1 TOFD geometry for offset weld corrosion, for straight sectioned butt welds. From Figure A2.1, it can be seen that:

22

1 x2

SdP

(A2.1)

2

22 x

2

SdP

(A2.2)

Where x is the flaw offset from the probes’ midpoint S is the probe separation d is the depth P1 is the path from probe 1 to the flaw P2 is the path from probe 2 to the flaw The observed signal arrival times of the compression and mode converted signals are then, assuming all signals originate from the same point on the flaw, as follows:

pp cc21

pp

PPt (A2.3)

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sp cc21

ps

PPt (A2.4)

ps cc21

sp

PPt (A2.5)

Where: tpp is the compression wave signal arrival time tps is the P-SV mode converted signal arrival time tsp is the SV-P mode converted signal arrival time cp is the compression wave velocity cs is the shear wave (SV) velocity If two of the above three signal arrival times are measured, then this gives in two simultaneous equations, with two unknowns, x and d. These equations can then be solved for the unknowns to derive both the flaw offset and its actual depth, taking account of its offset. Similar results can be obtained with loci-plotting software, and the above approach can be extended to more complex geometries such as those involving one probe on a taper.

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Appendix 3 Summary table

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The table below gives a summary of an initial consideration of the techniques for inspection of weld root erosion showing advantages and limitations, based on the responses obtained from the members’ questionnaire, supplemented where appropriate with additional information.

Table A3.1 Technique Summary table Weld root erosion inspection technique

Advantages Limitations

TOFD (automated) Good detection and sizing capabilities Permanent data record Accuracy and reliability Easy to compare with results from previous

inspection

Expensive Access needed to both sides of weld Good surface finish needed Difficulties with complex geometries (bends,

nozzles) Can undersize wall loss if this occurs to one side

of the weld (e.g. HAZ) Can be problems with equipment reliability Availability of competent personnel

Tangential Radiography Gives quantitative through-wall sizing information

Permanent data record Easy to compare with results from previous

inspection Works through lagging/insulation and

surface coatings Applicable to complex geometries Does not require surface preparation

Limitations for pipes with chord lengths > 85 mm when using Ir192 (lower limit with Se 75)

Radiation Safety & permits Operational issues with radiation sources (UV

detectors, Gammatrols etc) Can be comparatively slow for certain

components Two man team needed Some access issues in deploying source and

film/detector on opposite sides of pipe Not applicable to vessels

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Double Wall Radiography (DWDI or DWSI)


inspection Works through lagging/insulation and

surface coatings Applicable to complex geometries Does not require surface preparation

Does not give through-wall extent (although estimates may be available using the new penetrated thickness analysis technique with CR data)

Radiation Safety & permits Operational issues with radiation sources (UV

detectors, Gammatrols etc) Can be comparatively slow for certain

components Two man team needed Some access issues in deploying source and

film/detector on opposite sides of pipe Not applicable to vessels

Angled pulse-echo (manual) Fast Low cost, standard capability of core crew Readily available Access only needed to one side of weld

Problems with false calls and low POD Through wall sizing accuracy poor or non

existent Interpretation operator dependent No permanent record Difficult to compare with results from previous

inspection Good surface finish needed Difficulties with complex geometries (nozzles)

Phased arrays/automated angled pulse-echo

Permanent data record in some cases (encoded scans with appropriate instrumentation)

Access only needed to one side of weld

Expensive Similar limitations to manual angled beam pulse-

echo likely in terms of detection reliability Through wall sizing accuracy poor or non

existent Potential for false-calls (normal weld root

signals) Good surface finish needed Difficulties with complex geometries (nozzles)

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0º pulse-echo through weld cap (manual)

Quick and easy Works well at points where weld cap ground

flat

Requires grinding of flats on weld cap Limited circumferential coverage Could miss erosion/corrosion at HAZ and

upstream/downstream of weld Corrosion Mapping using 0º water column “squirter” probes


inspection Little additional cost if corrosion mapping

being applied in same area – welds are a bonus

Weld always gives some loss of signal Results very dependent on weld cap profile

SCEXY – build up Araldite over weld to provide flat smooth surface – then scan with conventional 0º probe

Simple Quantitative wall loss information

Time taken to apply epoxy and harden COSSHH limitations Bubbles in epoxy and bonding with weld cap Deflection of UT beam by irregular weld caps,

especially away from centre line – low POD for HAZ wall loss?

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HOIS09RP2 HOIS RP for Weld Corrosion Inspection

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