ABSTRACT Hot tap tees of the full encirclement split tee design are
currently used in the UK gas industry to provide connections onto existing high pressure gas transmission pipelines and above ground installations. The fitness-for-purpose of this type of tee is not covered by the UK design code for above ground gas installations, IGE/TD/9.
A fitness-for-purpose methodology has been developed by Advantica Technologies to determine the integrity of the fitting and attachment welds. The fitness-for-purpose assessment addresses the following:
The compliance of the fitting to plastic collapse,
shakedown and fatigue design criteria. The integrity of the attachment welds onto the carrier
pipe. The use of Engineering Critical Assessments in
conjunction with existing procedures to ensure overall integrity.
INTRODUCTION In-service welding or hot tapping is a technique used to
connect pipes or pipelines to other items of equipment without shutting the pipeline down.
Hot tap tees of the full encirclement split tee design (hereafter referred to as split tee) are currently used by the UK gas operator Transco to provide an economic connection to high pressure pipelines. Transco is responsible for the design, construction, operation and maintenance in accordance with current legislation and appropriate standards to ensure fitness-for-purpose.
Advantica Technologies has been performing the fitness-for-purpose assessments of split tees on behalf of Transco, and have developed a methodology, Fig. 1, for ensuring the integrity of the split tee for its intended design life. These
assessments, integrated with welding and inspection procedures, are used to ensure safe operation of the fittings.
The intention of this paper is to briefly review the design and integrity issues that the methodology in Fig. 1 is intended to address.
NOMENCLATURE AGI Above Ground Installation CTOD Crack Tip Opening Displacement DBA Design-by-Analysis ECA Engineering Critical Assessment FAD Failure Assessment Diagram FEA Finite Element Analysis FE Finite Element HAZ Heat Affected Zone NDE Non-destructive Examination NTS National Transmission System PWHT Post Weld Heat Treatment SCF Stress Concentration Factor SCL Stress Classification Line SMYS Specified Minimum Yield Strength SUTS Specified Minimum Ultimate Tensile
The welding of fittings onto gas pipelines under pressure within the Transco transmission system is covered by the Company specification P9. This procedure was originally developed by the British Gas Engineering Research Station and has been successfully used for over 25 years. The key elements of the procedure include the use of split tee fittings and the precautions to avoid the potential problems of burn through/blow out, hydrogen cracking and lamellar tearing. In recent years, the demand for the installation of split tee fittings onto large diameter, high pressure pipelines has increased. This
has coincided with a requirement for split tees to be designed for higher gas pressures (>70 bar). This has raised two specific issues, namely:
There is a design requirement for heavy-wall split tees,
which according to the welding codes should be subjected to post weld heat treatment (PWHT) in order to relieve the residual stresses. This is potentially a hazardous process and PWHT should be avoided on in-service pipelines, if possible.
The large volume of gas flowing through the pipelines causes an increase in the cooling rate and hence a reduction in time available for welding between preheating cycles.
Ultimately the integrity of the split tees largely depends on
well-established welding procedures, such as P9, and for that to continue the designs of split tees and the integrity of these geometries needs to be reviewed.
Split Tee Design
A typical split tee design is shown in Fig. 2. The Transco split tee Company specification F4 specifies the use of the ‘area replacement’ method in ASME B31.3 process piping code (Ref. 1) to design the fitting. The ‘area replacement’ methodology is used to determine the fitting wall thickness and reinforcement length based on internal pressure alone. The method has been used for many years, and the concept essentially requires that the metal cut out of the opening should be replaced by reinforcement within a prescribed zone around the opening.
The concept is relatively simple and the vast majority of vessels and piping with openings conforming to this design method have given satisfactory performance. The area replacement method is based on internal pressure being the only design load. However, for piping components such as split tees, system loads at Above Ground Installations (AGIs) can be significant. The following section summarizes the current area replacement rules used in the design of split tees, and provides some recommendations to manufacturers to account for system loading.
The rules governing the area replacement method are relatively simple. Where the area provided is greater than the area to be replaced then the design is considered to be satisfactory for internal pressure.
Although these rules are simple to apply, there is no guidance with respect to where the reinforcement should be added. The implications are that some of these split tees will be connected to other pipelines, and possibly be buried, thereby inducing significant system loadings. Using the area replacement philosophy, it is possible to have a design with a nozzle thickness that just meets the pressure requirements but makes no allowance for system loading. Therefore, there is a strong possibility that system loadings on the nozzle may overstress the component and possibly cause the component to fail by plastic collapse, ratchetting (shakedown) or fatigue.
This is one issue that split tee manufacturer’s could address at the design stage if the magnitude of the system loading for which the split tee is to be designed was known. However, this information is not known until a flexibility analysis of the layout has been performed. A fitness-for-purpose assessment method of split tees is therefore required to determine the significance of the system loads, once the design of the split tee to internal pressure has been confirmed and once the flexibility analysis results are available.
A supplement to Transco specification F4 was issued in April 2000, in an attempt to consider system loadings at the design stage. The supplement specifies that the ratio of reinforcement area provided to reinforcement area required should not be less than 1.5. In addition, the supplement stipulates that the total reinforcement area provided should be added to the header and nozzle in the same ratio as the nominal diameters of the carrier pipe to attached branch pipe. It should be noted that this guidance does not exempt the fitting from plastic collapse, ratchetting (shakedown) or fatigue assessments, but the modified design guidance will result in lower stresses from the same system loadings.
Avoidance of Post Weld Heat Treatment
For large diameter (>36”), high pressure (>70 bar) gas transmission pipelines, use of the area replacement rules may result in large wall thicknesses for the split tee run and nozzle. These could, typically, be in excess of 50mm. The ASME code would require welds of these thicknesses to undergo PWHT.
As shown in Fig. 3, the F4 specification requires each end of the split tee run to be chamfered to a thickness of twice the carrier pipe wall thickness. These are the locations where the circumferential fillet welds are made. It is considered that this technique is used to control the welding operation, by reducing the number of weld runs, and also as a method of ensuring that the fillet welds avoid the requirement for PWHT.
For large diameter high pressure pipelines, fillet weld sizes equal to twice the carrier pipe wall thickness should be subject
to PWHT. This is impractical and highly dangerous since heating during the process would result in a substantial loss in strength of the pressurized carrier pipe. Therefore, the integrity of these welds requires further assessment and is addressed in this paper.
Figure 3 - Fitting Taper at Fillet Weld
STRESS ANALYSIS Process pipework, like pipelines, is designed according to
the level of internal pressure. Transco requires its AGIs to conform to the design recommendations in IGE/TD/9 (Ref. 2), which subsequently refers to a pipe stress analysis to be performed in accordance with IGE/TD/12 (Ref. 3). The pipe stress analysis is used to ensure that any fitting does not fail by plastic collapse (sustained), ratchetting (shakedown) or fatigue.
An IGE/TD/12 assessment of split tees cannot be undertaken using pipe stress analysis programs, as the stress concentration factors (SCFs) for split tees have not been evaluated and incorporated into any code or pipe stress analysis programs. This information would be difficult to collate, as there are different areas of peak stress for different geometries of split tee. It would also be difficult to model a split tee in a pipe stress model, as the full internal pressure is present in the annulus between the fitting and the carrier pipe, causing the circumferential fillet welds to be highly stressed. Figure 4 shows pressure acting to open the annulus between the carrier pipe and split tee run, causing a high stress at the root of the circumferential fillet weld. This is the response of the split tee that cannot be adequately modelled using the simple beam elements available in a pipe stress model.
The pipe stress model does serve to provide system forces and moments to which the split tee would be subjected. This is achieved by modelling the stiffness of the split tee as a full encirclement tee. Using the forces and moments from the pipe stress analysis, a more detailed assessment can be performed using a three-dimensional finite element (FE) model.
Modelling Figure 2 shows a half model of a typical split tee. The split
tee mesh is constructed of approximately 40,000 20-noded reduced integration brick elements of ABAQUS (Ref. 4) type ‘C3D20R’ with 4 elements through the thickness. To date all the analyses performed by Advantica Technologies have been linear elastic, with solution times of approximately 1 hour. In determining an adequate mesh, particular attention is given to the circumferential fillet welds and the run to nozzle intersection region.
No gap is modelled between the carrier pipe and the fitting. However, one of the main verification aspects of the analysis is to determine whether any contact occurs between the carrier pipe and split tee run. Where contact is predicted then a non-linear contact analysis should be performed.
The pre and post-processing is carried out using PATRAN 2001 r2a (Ref. 5) and the analysis code is ABAQUS Version 6.21.
Design Conditions
Split tees on Transco’s high pressure National Transmission System (NTS) are designed for at least 70 bar pressure. IGE/TD/9 specifies that the maximum design temperature is +60°C, and the minimum design temperature is -20°C. Piping expansion and contraction is based around a thermal stress-free datum temperature referred to as the tie-in temperature.
For the plastic collapse analysis, pressure and dead weight loadings are applied on the split tee model. If the split tee is to be buried then the soil effects should be included by extracting the forces and moments from the flexibility analysis.
For the incremental plastic collapse or ratchetting (shakedown) assessment, both primary (pressure) and secondary (thermal) loadings are required. For this assessment
the loadings giving rise to the maximum stress range are required.
For the fatigue assessment, Transco require a fatigue life of 40 years that usually comprise the following cycles:
Commissioning/recommissioning full design pressure
and temperature cycles. Compressor operation, pressure and temperature
fluctuations. Winter diurnal, pressure and temperature fluctuations. Summer diurnal, pressure and temperature
fluctuations. The above analyses are conducted using the PD5500:2000
(Ref. 6) & ASME VIII (Ref. 7) stress categorization design-by-analysis (DBA) criteria. These are used to determine the conformance of the fitting to the required plastic collapse, shakedown and fatigue assessment. This is discussed in the following section.
ASSESSMENT CRITERIA IGE/TD/12 code allowable criteria are specific to pipe
stress analysis results. When a more detailed FE model of a fitting has been created, DBA rules are more appropriate, and give a better understanding of any onset of failure. A method is given in PD5500 and ASME VIII, known as the ‘stress categorization route’.
The method requires the stresses from a finite element analysis (FEA) to be categorized as primary, secondary and peak. These stresses are then combined, as appropriate, and assessed to allowable design stress limits. For the split tee fitting, appropriate locations are chosen through the thickness where the stress categorization is to be undertaken. These locations are termed stress classification lines (SCLs). At the SCLs, the through wall stresses need to be separated into membrane and bending components; this process is termed ‘linearization’. Figure 5 shows typical SCLs and they are selectively chosen in regions of high stress. It must be noted that SCLs should be taken at various angles around the split tee fitting for a comprehensive assessment.
It is important to note that these rules were designed to guard against the failure mechanisms using linear elastic FEA. It is essential to appreciate that plastic collapse and incremental plastic collapse failure mechanisms cannot be readily dealt with using an elastic analysis, as the failure mechanism is inelastic. The elastic analysis approach does not make use of the ductility of the material and results in a conservative margin of safety.
With the continuing development of computer hardware and software, performing inelastic FEA is becoming the norm. A new design route incorporating inelastic analysis has been developed and is included in the new DBA manual (Ref. 8). The analysis being proposed is to be incorporated in the draft CEN unfired pressure vessel standard prEN 13445 (Ref. 9).
Figure 5 - Location of Stress Classification Lines
Plastic Collapse Plastic collapse falls into three categories, namely general
primary membrane (a global plastic collapse failure mechanism), local primary membrane (a local plastic collapse failure mechanism) and local primary membrane plus primary bending (a local plastic collapse failure mechanism). The difference between the two failure mechanisms is that global plastic collapse is associated with piping free from any stress concentration features and local plastic collapse addresses fittings that have areas of high stress concentration features.
For the detailed assessment of the failure mechanisms, SCLs, such as those shown in Fig. 5, can be selected for the appropriate assessment.
SCLs 2, 6 and 7 are used to assess against the general primary membrane stress limit. For this assessment, the ‘stress intensity’ (also known as Tresca stress) should not exceed, 0.67 of the specified minimum yield strength (SMYS), or 0.44 of the specified minimum ultimate tensile strength (SUTS), whichever is the lowest.
SCLs 1, 3, 4 and 5 are used to assess against local plastic collapse. For the local primary membrane and local primary membrane plus primary bending assessments, the Tresca stress should not exceed the lower of the SMYS or 0.67 of the SUTS.
Where the above criteria are exceeded then further analyses can be performed to determine the extent of yielding by performing an elastic-plastic analysis. Generally, the area replacement method provides sufficient reinforcement to discount failure due to plastic collapse.
Shakedown
Shakedown is a design criterion that is required to ensure that a component remains in an elastic condition during any cyclic loading. The objective of the assessment is to ensure that if any plasticity does occur, a residual stress is created that will eventually ensure that the component shakes down to a purely elastic response.
It must be noted that predicting the onset of failure due to shakedown using the results from a linear elastic analysis is not a strictly accurate assessment. However, the current design rules state that any possible incremental plasticity should be avoided and the maximum Tresca stress range at any location within the fitting should be less than the lower of twice the SMYS or 1.33 of the UTS. All SCLs can be used to determine the stress intensities at the critical stress locations within the fitting. Once again, SCLs should be taken at various angular positions for a detailed assessment to be made.
If the above criterion is exceeded, then remedial action is necessary. Such action may be to reduce the level of system loads on the split tee by increasing the flexibility of the piping layout.
Fatigue
The maximum principal stress (at any location) for each of the fatigue loadcases should be used to undertake the fatigue assessment. For the circumferential fillet weld, the ‘Class W’ S-N fatigue curve from PD5500 should be used. Typically, the circumferential welds have the highest principal stresses and in reality are the welds most susceptible to cracking. For the seam weld, the ‘Class D’ S-N fatigue curve in PD5500 should be used. For the fillet weld at the split tee intersection, the ‘Class F’ S-N fatigue curve in PD5500 should be used.
It is important to note that for the fatigue analysis of any fillet weld, the stresses are very much mesh dependent, and likely to be artificially high at that location due to numerical singularities. This effect can be taken into account by linear extrapolation of the maximum principal stress to the root/toe of the fillet weld. This stress is known as the ‘hot spot’ stress. The hot spot stress is considerably lower than the ‘peak’ stress but provides a consistent definition of stress range for application with the fatigue curves in PD5500.
For all the fatigue duty cycles, the maximum principal stresses should be used to determine the number of allowable cycles. A Miner’s Law summation can then be performed to determine the total fatigue usage.
Where the Miner’s Law summation is > 1.0, then the largest damaging fatigue cycle is identified and loadings contributing to the most damage are identified for possible reduction. Where loadings cannot be reduced, then aspects of the design should be considered for modification.
Circumferential Fillet Weld Integrity
Stresses in fillet welds are complex, and are not covered specifically in the previous design criteria. BS7910 (Ref. 10) provides some guidance on the maximum allowable shear stress on the net minimum throat area. It stipulates that this maximum shear stress should not exceed 0.48 of the SMYS of the weld metal.
For the welding of linepipe of material grades X52 to X70, P9 states that basic coated low-hydrogen electrodes are suitable. Data from the FILARC electrode handbook (Ref. 11) for 27P electrodes (which Transco use for P9 welding) quotes
SMYS and SUTS values of 460 MPa and 550 MPa respectively for the weld metal.
This sustained assessment for the fillet welds should consider loads from the design pressure plus dead weight forces and moments. Where stresses exceed the allowable stress then the fillet weld is required to be re-designed.
Under internal pressure, the pipe stress analysis does
not address the behaviour of this type of split tee. IGE/TD/12 or other piping codes do not contain any
relevant geometrical SCFs for split tees. Current philosophy is to ensure code compliance to
PD5500 DBA criteria, which covers plastic collapse, shakedown and fatigue. It must be noted that the IGE/TD/12 allowable stress criteria are inappropriate to use for detailed FEA.
The integrity of the fillet welds is determined by limiting the maximum shear stress to 0.48 of the SMYS.
The split tee is considered to be fit-for-purpose when all of the above assessments have been satisfied.
Where the split tee fails to comply with the above criteria, recommendations are made to the operator to either reduce the system loadings (by increasing the flexibility of the piping arrangement) or to alter the design of the split tee.
ENGINEERING CRITICAL ASSESSMENTS & NDE One of the areas of concern is possible cracking at the fillet
welds attaching the split tee to the carrier pipe. Since the site welds are unlikely to be subjected to any PWHT or site hydrotest that can act as a source of stress relief, Engineering Critical Assessments (ECAs) can be used to determine what sizes of defects are likely to cause failure and whether the non-destructive examination (NDE) would detect these reliably.
As shown in Fig 6, the ECA evaluates the proximity to failure of a defective weld due to both brittle fracture and plastic collapse. The assessment result provides the analyst with some guidance on the theoretical size of a defect that could cause failure. The evaluated defect size is termed the critical defect size.
Key inputs to the fracture mechanics calculations are detailed stresses from the FEA and fracture toughness data (from material testing or estimates).
The calculations are conducted according to the defect assessment procedures in BS7910. A Level 2 failure assessment diagram (FAD) is used, with no partial safety factors.
The circumferential fillet welds are the main concern. There are five credible planes where defects can occur. Planes 1 and 2 are from the fillet weld toe and root respectively, through the carrier pipe wall thickness. Plane 3 is the net minimum throat area of the fillet weld. Plane 4 is the buttering run fusion plane, and plane 5 is the external surface of the completed fillet weld, to account for any transverse cracking. This plane nomenclature is depicted in Fig 7.
Two loadcases are considered for the ECA. These are:
The design pressure, dead weight and hot temperature. The design pressure, dead weight and cold
temperature. Stresses from pressure and dead weight are classified as
primary, and stresses from temperature states are classified as secondary.
Stresses normal to planes 1, 2 and 4 are extracted from the FE model, the maximum shear stress is extracted along plane 3 and the hoop stress is extracted along plane 5. These stresses are linearized into membrane and bending components for input into the fracture mechanics calculations.
For defects postulated along planes 2, 3 and 4, the crack face pressure is added to the primary membrane stress.
Welding residual stress distributions are estimated using the recommendations in BS7910 Annex Q. For defects postulated along planes 1 and 2, the welding residual stress is usually assumed to be linear bending yield magnitude through the carrier pipe wall thickness. For defects postulated on planes 3, 4 and 5, the welding residual stress is usually assumed to be membrane yield magnitude. All welding residual stresses are assumed to be secondary.
The assessment of the fillet welds will usually bound the assessment of the axial seam welds. This is because the seam welds are usually thicker, and the stress will be dominated by hoop stress, which is usually low in the seam welds.
Once the ECA is complete, the calculated critical defect sizes can be used to envelope the extent of real defects and are used to set an appropriate inspection policy as part of the welding procedure.
Figure 7 - Identification of Defect Assessment Planes Fracture Toughness
The ECA assumes that fracture toughness data is known, or can be estimated, for the carrier pipe heat affected zone (HAZ) and weld metal. If toughness is not known, then a target toughness can be set. This is based on limiting the critical defect size to that detectable by NDE, or a defect size that is
considered incredible. The calculated target toughness may be sufficiently low that it can be assumed it will be met, otherwise it will have to be demonstrated that the actual toughness exceeds this target value. This could involve either performing a literature review to obtain better toughness data or actual material testing to generate crack tip opening displacement (CTOD) toughness data at the minimum design temperature.
Development of NDE Procedures
The scope of any NDE procedures will be based on the findings of the ECA. If the critical defect size cannot be reliably detected, then an attempt to improve the reliability of the inspection will be reviewed. Where no further improvements can be made, then further fracture toughness tests, possible PWHT or modification to the design may be recommended.
SUMMARY This paper details a methodology developed by Advantica
Technologies that is used to determine the fitness-for-purpose of encirclement split tees. The paper has highlighted numerous assessments that gas pipeline operators should perform when implementing such split tee designs.
The paper has highlighted the need to supplement the area replacement and fatigue analysis of split tees with FEA. FEA can further analyze the suitability of the circumferential fillet welds when subjected to system loadings.
ACKNOWLEDGMENTS The authors would like to thank the Stress Analysis Team
at Advantica Technologies in the development of this methodology.
REFERENCES 1. The American Society of Mechanical Engineers: ‘Process
Piping.’ ASME B31.3, 1999. 2. The Institution of Gas Engineers: ‘Offtakes and Pressure-
Regulating Installations for Inlet Pressures Between 7 and 100 bar, Recommendations on Transmission and Distribution Practice.’ IGE/TD/9 : 1986, 1986.
3. The Institution of Gas Engineers: ‘Pipework Stress Analysis for Gas Industry Plant, Recommendations on Transmission and Distribution Practice.’ IGE/TD/12 : 1985, 1985.
4. ABAQUS/Standard Version 6.21, Hibbitt, Karlsson & Sorenson, Inc., 2001.
6. British Standards Institution: ‘Specification for Unfired Fusion Welded Pressure Vessels.’ PD5500 : 2000, 2000.
7. The American Society of Mechanical Engineers: ‘Rules for Construction of Pressure Vessels.’ ASME Boiler and Pressure Vessel Code, Section VIII, July 1998.
