Welding the First erW X80 grade PiPelineBarbaro F J, Bowie G F and Holmes W
ABSTRACT
Pipeline materials and construction costs are the most significant components of major transmission pipelines. In Australia these costs have been contained over the past two decades by the utilisation of high strength thin walled pipe. API 5L X70 grade pipe is common place and 13km of X80 grade pipe has been installed in a looping section of the Roma - Brisbane pipeline. The aim of the Roma - Brisbane looping project was to fully evaluate the economic benefits associated with the use of 8.8mm thick, 406mm diameter X80 grade pipe. The evaluation involved development of weld procedures using both the conventional cellulosic manual metal arc (MMA) process and a mechanised gas metal arc welding (GMAW) system to determine the influence of weld metal strength on allowable girth weld defect tolerance.
Although currently available cellulosic consumables have been shown to undermatch the strength of X80 pipe, the full section pipe tension test demonstrated tolerance to both Tier 1 and Tier 2 girth weld defect allowances. These results support recent research which has shown that the tolerable level of weld metal strength undermatching is related to the pipe wall thickness and the defect depth assumption
Weld metal strength matching with an appropriate level of toughness was shown using engineering critical assessment procedures to provide increased defect tolerance. Defect tolerance under axial yield stress loading is more accurately determined using destructive test methods.
Frank J Barbaro, Chief Development Officer and Graham F Bowie, Senior Development Officer, BHP Steel Flat Products, Port Kembla Steelworks, New South Wales and William Holmes, Technology Manager Pipeline & Coatings, Agility Team Build, Fyshwick Canberra, ACT
First published in the proceedings of the Welding Technology Institute of Australia International Conference on ‘Pipeline Construction Technolog’, 4-5 March 2002, Novotel North Beach, Wollongong, Australia.
The significant cost of pipeline materials and construction in conjunction with competition with other energy sources has driven the development of high strength linepipe for transmission of natural gas. The cost savings associated with high strength pipe arise from a reduction in pipe wall thickness which reduces both required steel tonnage and also welding cost. In comparison with X70 grade pipe, X80 grade pipe represents approximately 12% reduction in total steel weight and up to 25% less deposited weld metal. These benefits however are balanced by any increase in the pipe / weld consumable costs and require that field welding productivity is not compromised.
In Australia, where thin walled small diameter pipe is commonplace, maximum economic benefits have been obtained by the use of high strength linepipe up to and including X70 grade pipe [1]. The continued use of conventional manual metal arc (MMA)welding using cellulosic consumables for such pipe designs has enabled field construction rates which have been as high as 8kms per day.
The strength of X80 however, challenges the continued use of cellulosic welding consumables because of their limit in strength and also high inherent hydrogen content. The main issues in the welding of high strength linepipe are resistance to hydrogen assisted cold cracking (HACC) and sufficient weld metal strength to match the pipe [2, 3]. Extensive investigations have shown that under normal field construction practices HACC can be avoided [4, 5]. The limited strength of cellulosic consumables is a more serious concern and has been shown to undermatch the yield strength of X80 grade pipe [6] and even X70 grade pipe at the upper end of the normal strength range [7, 8]. It is pertinent to point out however, that it is not simply the weld metal yield strength that is the governing factor but rather the level of defect
tolerance relative to the pipe design. From an economic viewpoint adequate weld metal strength matching is required to ensure sufficient tolerance to the typical weld defects which occur during pipeline construction in order to avoid unnecessary repairs.
There is an important difference between weld metal yield strength matching and weld metal strength matching. The latter is directly related to weld defect tolerance, which not only depends on the actual yield strength of the weld metal and the pipe, but also the specified defect limits (particularly depth) and pipe wall thickness. Yield strength matching will provide maximum defect tolerance but is difficult to determine [7], particularly where different yielding phenomena can occur in different suppliers of high strength pipe grades.
To address these issues of weld metal requirements and field welding productivity, AGL Pipelines undertook to construct a section of the Roma to Brisbane Looping line using 8.8mm thick, 406mm diameter API 5L X80 grade pipe. This particular looping project was selected because the pipe dimensions closely represented a number of proposed pipelines which could also benefit economically by the use of X80 grade pipe. The justification for the use of X80 grade pipe involved evaluation of different welding processes with particular emphasis on the assessment of defect tolerance. Cellulosic MMA and two commercial automatic GMAW processes were evaluated by full section pipe tension (FSPT) [9] tests to determine limits in girth weld defect tolerance. Further evaluation was undertaken using approved fracture mechanics methods to support the FSPT tests.
This paper details the results of the investigation and some field welding experience using automatic GMAW.
01 INTRODUCTION
02 pROCeDURe
2.1. Pipe materialThe pipe used in this program was 406mm diameter, 8.8mm thick seam welded using the electrical resistance welding (ERW) process. The chemical composition of the pipe (Table 1) is characterised by a low carbon content and controlled additions of microalloys required for advanced thermomechanical rolling to optimise the level of strength and toughness as well as control of weldability.
2.2. Mechanical propertiesTensile tests were performed in both the longitudinal and transverse direction of the pipe. Pipe body Charpy impact tests were carried out over a range of temperatures. Girth weld Charpy impact tests were carried out at the minimum design operating temperature which was defined as 0°C
2.2.1. Welding procedurePipe girth welding trials involved conventional MMA welding using cellulosic consumables and two different commercially available automatic GMAW processes.
The cellulosic welding procedure, C1, employed the standard pipe mill prepared end bevel, which consisted of a 60° included angle with a 1.5mm root face. Root opening varied between 1 and 1.5mm.
The first GMAW process, which will be referred to as G1, utilised the standard pipe mill bevel but was ground just prior to welding to remove the root face. The pipe ends were aligned without a gap and relied on the welding procedure to ensure full penetration. Internal segmented copper shoes were employed to prevent excessive internal weld reinforcement and/or burnthrough.
The second GMAW process, which will be referred to as G2, utilised a narrow gap J type preparation machined on site just prior to welding. This welding process employed a newly developed mode of metal transfer, the surface tension transfer (STT) technique, to deposit the root pass which avoided the need to use internal copper shoes. Detailed welding conditions are presented in (Table 2.).
2.3. Defect tolerance determination.Defect tolerance was determined using a FSPT test, which was developed by the Cooperative Research Centre for Welded Structures. The test basically involves loading a complete section of pipe, containing a girth weld and the defined defect, in uniaxial tension up to the point of fracture. Defects were produced in the root pass on the inside surface of the pipe using electro discharged machining to a depth of 3mm which is the assumed maximum depth of a girth weld defect. Assessment of the complete pipe diameter eliminates the conservatism associated with other smaller scale tests such as the well-known wide plate test. The test rig used in this investigation is described elsewhere [6, 7, 9].
The aim of the test is to demonstrate that gross section yielding (GSY), and not net section yielding (NSY), occurs before fracture. The GSY criteria, which is defined below, is designed to ensure that the weld metal containing the defect has sufficient strength to transfer strain to the adjacent pipe and so ensure a reasonable level of overall elongation of the
pipe before failure. NSY occurs when the strain is concentrated in the weld metal and fails at low levels of elongation. It is important to state that the GSY criteria is not designed to prevent catastrophic failure but to ensure a defined level of defect tolerance.
The GSY criteria was originally defined by the European Pipeline Research Group (EPRG) [10] and requires that the girth weld, containing the maximum allowable defect, under load in uniaxial tension achieve a:
maximum test stress >= the parent pipe yield stress,•
total elongation >= 0.8%, and,•
remote or parent pipe strain >= 0.5%•
In the FSPT test the maximum load is determined using a calibrated load cell. Total elongation was measured with a linear displacement transducer attached along the length of the welded test pipes. Remote or parent pipe strain was measured using strain gauges. Recorded strain levels are verified by subtracting the weld strain, measured by a clip gauge opening across the weld defect, ie crack mouth opening displacement, on the inside of the pipe, from the total elongation.
The crack mouth opening displacement (CMOD) not only provides a check on strain levels but also uniquely defines the onset of strain transfer to the pipe body. At the point of strain distribution to the parent plate (or yielding of the pipe) the CMOD is interrupted as the uniaxial load increases.
03 ReSULTS aND DISCUSSION
The Australian Pipeline Standard AS2885.2 has a 3 tier approach to assessment of girth weld defects, which is designed to improve economics in pipeline construction. An increased level of weld imperfections is permitted provided the girth weld possesses a minimum level of strength matching and toughness. Tier 1 is a workmanship level which, in general, permits 25mm long surface breaking defects and 50mm long embedded defects. Tier 2 defect limits however, are a function of pipe diameter and wall thickness and for the 406mm diameter 8.8mm wall thickness pipe in the present investigation, the maximum defect length is 84mm, irrespective of its position through the wall thickness.
The following results detail the pertinent characteristics of girth welds produced in API 5L X80 grade pipe using different welding procedures and its ability to meet the above mentioned defect limits.
3.1. Radiography of weldsAll welds fabricated as part of this investigation were examined by conventional radiographic techniques and complied with the requirements of AS2885.2. It was however noted that radiographs of welds produced by the GMAW, G1 process contained marks which corresponded with artefacts produced on the surface of the root pass by the segmented copper shoes. In general this did not interfere with the inspection process.
3.2. Chemical composition and microstructure of weldsThe chemical composition of final cap weld deposits is given in (Table 3). It will be noted that the carbon equivalent of the GMAW consumables were markedly different. The carbon equivalent of G1 was the highest of all assessed and is related to the addition of Ni along with the Cr and Mo levels. The carbon equivalent of G2 was extremely low and is reflected in mechanical properties. The MMA weld C1 also had a high carbon equivalent which predominantly relied upon a relatively high carbon content and additions of Mo and V for strengthening.
The relative level of deposited weld metal strength however depends upon welding conditions and it is evident from the macrophotographs presented in (Figure 1), that both GMAW welds were welded at low heat inputs as evidenced by the narrow width of visible weld HAZ. It is apparent from (Figure 1.) that the MMA weld was carried out at weld heat inputs greater than both GMA welds, refer (Table 2.).
The microstructure of weld C1 primarily consisted of ferrite and pearlite throughout the entire weld thickness. This as mentioned above is related to the weld heat input and also the alloy design. A high weld heat input produces a low cooling rate which promotes the formation of coarse equilibrium microstructures and also increases the extent of recrystallisation
of previously deposited underlying weld runs. The level of heat input employed in weld C1 was evident by the complete recrystallisation of the root pass (Figure 2a.) which was up to 4mm of the girth weld thickness.
Both GMA welds were characterised by distinct columnar structures which persisted throughout the weld thickness. The low weld heat input employed limited the extent of recrystallisation of underlying weld runs. As a result both welds contained relatively fine grained acicular ferrite and martensite microstructures outlined by columnar grain boundary ferrite (Figure 2b and c). The difference between welds G1 and G2 can be related to the consumable alloy design. Weld G1 with a carbon equivalent some 16 points higher than G2 contained significantly higher levels of martensite. This was most prominent in the root pass of weld G1 where rapid cooling over the copper shoes employed during root pass welding further enhanced martensite formation. This observation was supported by the hardness results presented later.
3.3 Mechanical propertiesThe results of tensile tests carried out on the parent pipe in both the transverse and longitudinal directions are presented in Table 4. Recorded yield strengths in the transverse direction were within a tight range with the maximum less than 70 MPa above the minimum specification.
Conventional cross weld tensile tests demonstrated that all weld procedures satisfied traditional workmanship requirements with a tensile strength greater than that of the specified minimum of the pipe (Table 5.). It should be noted however, that both welds C1 and G2, with the weld reinforcement removed, failed in the weld metal at a strength less than that of the pipe.
Assessment of weld metal strength matching as determined by the notched tensile test, although acknowledged as difficult to interpret, revealed significant differences in weld metal yield strength (Table 6.). Clearly the MMA weld C1 undermatched the yield strength of the pipe by approximately 17% while the high carbon equivalent of weld G1 provided a considerable level of overmatching, approximately 8%. It is interesting to note that the low carbon equivalent GMAW weld G2, that indicated slight tensile strength undermatching in the standard cross weld tensile test, in fact demonstrated yield strength matching in the notched tensile test.
Clearly the recorded weld metal strength is directly related to the alloy design and the welding conditions as evidenced by the weld metal microstructures detailed above. The results also highlight that the level of weld metal yield strength undermatching may go unnoticed in the standard cross weld tensile test. It is however important to emphasise that, as mentioned in the introduction, strength matching is not the only characteristic of a girth weld that influences weld defect tolerance.
3.4. HardnessThrough thickness hardness (HV5) profiles were conducted on all weld deposits, refer (Table 7). As expected the higher strength GMA weld, G1, recorded the highest hardness with values in the root pass up to 50 points higher than the pipe. Both welds C1 and G2 recorded hardness values which were below that of the pipe which supports the cross weld tensile tests discussed previously. The level of hardness of weld C1 could again be attributed to weld microstructure.
It is important to note that the level of hardness recorded in the root pass of weld G1 could not be solely attributed to the carbon equivalent or weld conditions. The root pass of this weld was carried out with the use of internal copper shoes which has increased the cooling rate to produce the high levels of martensite in the microstructure.
3.5. ToughnessGirth weld toughness was evaluated using the Charpy test at 0°C and results are presented in Table 8. All welds satisfied the minimum requirement of 22J minimum individual and 30J minimum average specified in AS2885.2 which is required to ensure that in the event of girth weld failure, fracture would occur by plastic collapse and not in a brittle manner.
It is however evident that weld G2 clearly possessed a superior level of toughness compared to both welds C1 and G1. The low carbon equivalent of weld G2 along with the controlled low heat input welding appear to have combined to provide a fine grained microstructure. The outcome is an optimum balance of toughness and, as shown later, adequate strength for the welding of X80 grade pipe. Although a similar fine grained ferritic microstructure was produced in weld G1, the higher carbon equivalent and weld cooling rate has increased the level of martensite to the detriment of toughness. The toughness of the conventional MMA weld C1 can be explained by the relatively coarse ferritic microstructure.
CTOD fracture toughness values for both weld C1 and G1 are consistent with the measured Charpy impact test results Table 8. Unfortunately weld G2 was not tested, but based on Charpy toughness, a CTOD significantly exceeding that achieved with C1 and G2 would be expected.
3.6. Full section pipe tension (FSPT) testsLimited girth weld samples prevented a complete assessment of all weld procedures. Four tests covering defect lengths of 75-150mm were carried out on weld C1. Unfortunately only two tests were carried out on weld G2 while insufficient material prevented any tests on weld G1. The artificial defects produced were accurately controlled around the maximum assumed depth of 3mm and provided a thorough assessment of tolerance as defined by Australian Standard AS2885.2.
It is apparent from Table 9. that GSY was satisfactorily demonstrated in weld C1 with a defect length up to 100mm. The 125mm defect, which did not meet the gross stress requirement by just 2 MPa could also be considered
satisfactory if the slight increase in defect depth of this test is taken into consideration. Despite this however, the results clearly demonstrate that the maximum defined limit of the less conservative Tier 2, i.e. 84mm, was quite easily achieved.
The defect lengths selected for GMAW procedure G2 was based on the level of weld metal yield strength and previous experience and unfortunately for the two tests carried out, neither completely satisfied the GSY criteria. Interpolation of the data however, in the form of a plot of the maximum stress versus defect area (Figure 3.) strongly suggests that the defect limit to be a length approximately 170mm which is significantly greater than AS2885.2 Tier 2 limit of 84mm.
3.7. Field weldingThe production sequence consisted of the normal pipe stringing and alignment with an internal compressed air clamp. The welding technique employed was identical to that of weld procedure G1 above including pipe end preparation and the use of internal segmented copper shoes. Only one welding station was employed however.
The welding system consisted of two welding bugs each with twin heads, which travelled around a metal band attached to the pipe. Welding commenced by deposition of the root and hot pass using one welding bug down one side of the pipe from the 12 o’clock position to the 6 o’clock position. Before completion of this first run the weld start position was ground in preparation for a similar run down the other side of the pipe, which commenced as soon as possible but generally on completion of the first side. As the root / hot pass on the second side of the pipe was being deposited, preparation for the fill and cap was underway in a similar sequence to the root / hot passes. Unfortunately however, severe arc blow was experienced during welding of the root / hot pass run on the second side of the pipe. Efforts to eliminate the effect indicated that the root cause may originate from induced magnetic effects from the twin welding heads. As a result of these issues the majority of the X80 section was successfully welded with MMA cellulosic consumables in accordance with procedure C1.
3.8. Engineering critical assessment of girth weld defect limitsThe determination of critical defect dimensions in a girth weld using fracture mechanics is not only dependent on the mechanical properties of the weld metal and pipe but also the assumed operating stresses. For a gas transmission pipeline the stress imposed, which could be considered normal, includes the field hydrostatic test and the maximum allowable operating pressure. These are defined levels of stress for which a defect limit can be estimated. More recent attention however, has focussed on the capacity of girth welds, containing defects, to withstand displacement controlled loading, i.e. axial yield stress loads, which could occur in areas of unstable ground.
An engineering critical assessment (ECA) was carried out on the current pipe design using The Welding Institute software program ‘CRACKWISE 3’ which is based on British Standard BS7910 - 1999 Guide on methods for assessing the acceptability of flaws in metallic structures, Level 2 analysis. (Table 10.) presents the calculated critical length of 3mm deep defects for the above three stress conditions as a function of fracture toughness and enables a direct comparison with the current experimental FSPT test results.
Evident from Table 10. is a significant difference in the ECA calculated critical defect length and the measured FSPT test value under yield stress loading conditions. It is also apparent that with the ECA under this loading condition (600MPa), fracture toughness does not effectively influence defect tolerance since predicted tolerance is very low (length <6mm). Clearly the ECA approach employed for this particular pipe design has grossly underestimated defect tolerance and would appear to be related to an assumption that failure occurs by a brittle or tearing mechanism, whereas EPRG has found that if fracture toughness exceeds a CTOD value of 0.1mm minimum, 0.15mm average, fracture would occur by plastic collapse.
For the MMA weld C1 with a fracture toughness of 0.156mm, the calculated critical defect length increases to 239mm under hydrotest conditions (1.4 x 0.72 x 0.3 x SMYS (552) = 167MPa) and 465mm for maximum allowable operating conditions (0.3 x 0.72 x SMYS = 119MPa). Clearly under these stress conditions defect tolerance is influenced by fracture toughness and would be considered reasonable predictions based on the current destructive tests.
The results of this investigation have demonstrated that the 8.8mm thick API 5L X80 grade pipe can be welded with both the conventional MMA cellulosic process and mechanised GMAW systems. The mechanical strength of the girth welds varied significantly but, for the pipe design assessed, did not compromise the structural integrity of the pipeline and meets the accepted requirements of GSY for currently specified defect limits in both Tiers 1 and 2 of Australian Standard AS2885.2.
MMA cellulosic weld, C1, provided adequate toughness and defect tolerance despite a degree of weld metal yield strength undermatching. In fact, the measured defect limit is some 15mm greater in length than currently specified in Tier 2 of AS2885.2. GMAW processes offer a greater range of weld metal strengths and increased defect tolerance.
Engineering critical assessments provide a reasonable assessment of defect tolerance under normal operating conditions. Under axial yield stress loads destructive tests would provide a more accurate estimate however.
05 aCKNOWLeDGeMeNTSThe authors would like to thank colleagues at Agility and BHP Steel for their many contributions.
06 RefeReNCeS
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Barbaro F J, Bilston K, Fletcher L, Kimber M and Venton 2. P July 1999. Research shows that X80 pipe can be economically and safely welded by conventional methods, Australian Pipeliner 22-23.
Barbaro F J March 1999. Types of hydrogen cracking in 3. pipeline girth welds. WTIA/APIA/CRC-WS International Conference on Weld metal cracking in pipeline girth welds, Wollongong, Australia.
Barbaro F J, Meta A, Williams J G and Fletcher L September 4. 1995. Weldability of high strength ERW X80 grade pipe. Pipeline Technology Conference II, Ostend, Belgium.
Alam N, Dunne D P and Barbaro F J March 1999. Weld metal 5. crack testing for high strength cellulosic electrodes. WTIA/APIA/CRC-WS International Conference on Weld metal cracking in pipeline girth welds. Wollongong, Australia.
Barbaro F J and Bowie G F October 2000. Assessment 6. of workmanship defect acceptance levels in high strength 5mm wall thickness pipeline girth welds. IIW Asian Pacific International Congress, Melbourne, Australia.
Bowie G F and Barbaro F J July 2000. Defect acceptance 7. levels and fracture risk in pipeline girth welds. CRC-WS final report 98-62.
Barbaro F J, Bowie G F, Stathers P A and Williams J G 8. November 1997. Factors controlling defect acceptance levels in 5mm thick high strength pipeline girth welds. Int’l Welding and Joining Research Conference and WTIA 45th Annual Conference, Melbourne, Australia.
Bowie G F and Barbaro F J July 1998. Defect acceptance 9. levels in 5mm thick high strength pipeline girth welds. CRC-WS final report 96-31.
Hopkins P and Denys R May 1993. The background to the 10. proposed European pipeline research group’s girth weld defect limits for transmission pipelines. Joint EPRG/PRC Conference.
Full section pipe tension test results of Weld G2. Maximum stress plotted against defect area in conjunction with measure parent strain enables estimation of the maximum defect area to meet the GSY criteria
