Ensuring Overmatching Strength in Pipeline Girth Welds
Guidance is given to prevent girth weld failures in newly constructed pipelines
By William A. Bruce and Russell Scoles
William A. Bruce is senior principal consultant, welding technology, at DNV, Dublin, Ohio and Russell Scoles is senior specialist welding engineer, pipeline integrity engineering, at Enbridge, Houston, Tex.
Reprinted with permission: The AWS Welding Journal
In a previous Welding Journal article (Ref. 1), several girth weld failures in newly constructed cross-country pipelines were described. These failures occurred in pipelines constructed using conventional stove-pipe welding methods (i.e., cellulosic-coated AWS EXX10-type electrodes) at relatively low global strain levels either during preservice hydrostatic pressure testing or in service soon after the pipelines were commissioned. Some of these failures were attributed to undermatching weld strength and/or heat-affected zone (HAZ) softening in what would otherwise be considered acceptable girth welds per applicable codes and standards.
Background Basics
A fundamental rule of welding engineering is that, in general, the strength of a weld should be greater than the strength of the base materials being joined. This is sometimes taken to mean that the strength of the weld should be greater than the specified minimum strength of the base materials being joined. There are often no requirements for the strength of the weld to be greater than the actual strength of the base materials being joined. Because there are no requirements for overmatching the actual strength of base materials in the majority of industry codes and standards used around the world for pipeline construction, good engineering judgment must be used to choose appropriate acceptance criteria for tensile testing during procedure qualification. For many applications, the acceptance criteria may be above the minimum requirements in the applicable code or standard.
Recent industry trends that have contributed to pipeline girth weld failures include strength levels for as-received line pipe toward the upper end of the acceptable range in API Specification 5L:2018, Line Pipe (Ref. 2), the use of an alloying strategy for the line pipe that results in a very lean chemical composition that increases the probability for softening in the HAZ, the use of undermatching strength electrodes for root pass welding, and the use of cellulosic-
coated electrodes in general for intermediate and cover pass welding. Measures that can be taken in the short term to prevent these failures were described in the previous article. Since that time, much progress has been made at implementing these and other measures, and the progress is described in this follow-up article.
Areas of Improvement
The pipeline industry in North America, and in particular, pipeline operator Enbridge, Houston, Tex., has implemented measures in a number of areas to mitigate these low global strain girth weld failures (Ref. 3). These areas include line pipe procurement and girth welding practices. These measures are intended to control or limit pipe material strength, increase weld metal strength, and minimize HAZ softening.
Line Pipe Procurement
In terms of line pipe procurement practices, Enbridge now requires pipe material tensile testing in the longitudinal direction in its pipe material purchase specifications, whereas API 5L currently requires tensile testing only in the circumferential direction for larger-diameter line pipe. The pipeline operator also specifies maximum allowable yield and ultimate tensile strength levels in the longitudinal direction that are no more than 17 and 22 ksi (117 and 152 MPa), respectively, over the specified minimum values for both Grades X70 and X65 line pipe material. For example, the pipeline operator requires a maximum allowable yield and ultimate tensile strength for Grade X70 line pipe material of 87,300 and 99,700 lb/in.2 (602 and 722 MPa), respectively. This is less than the API 5L-allowed maximum yield and ultimate tensile strength for Grade X70 line pipe material of 92,100 and 110,200 lb/in.2 (625 and 760 MPa), respectively.
In addition, the pipeline operator specifies pipe material chemical composition limits in its pipe material purchase specifications to control and/or limit softening in the HAZ. These limits include a carbon content of no less than 0.040% and a Pcm carbon equivalent of no less than 0.140%. The intent of specifying minimum chemical composition limits is to prevent the loss of strength that occurs because the HAZ of a girth weld is not subjected to accelerated cooling or control rolling, both of which contribute to strength in modern line pipe material.
Girth Welding Practices
Measures implemented by Enbridge intended to increase weld metal strength for higher-strength line pipe materials (Grade X65 and higher) include the use of higher-strength electrodes for root pass welding and restricting and/or eliminating the use of cellulosic-coated electrodes for intermediate and cover pass welding. Lower-strength electrodes (e.g., E6010) have been traditionally used for root pass welding to control the risk of hydrogen cracking in the HAZ of higher-carbon-equivalent line pipe materials. Now that modern high-strength line pipe has a much leaner chemical composition, and a subsequently high resistance to hydrogen cracking in the HAZ, matching-strength electrodes are now appropriate for root pass welding. Recent trials have demonstrated that E8010 electrodes are acceptable for root pass welding in Grade X70 line pipe in terms of operability (e.g., root pass quality), resistance to hydrogen cracking, and an improvement to overall girth weld strength level (Ref. 4). Some welder training may be required because E8010 electrodes tend to produce a slightly softer arc than E6010 electrodes. From a weld strength perspective, the use of higher-strength electrodes for root pass welding is particularly useful for thinner-wall materials (e.g., 0.500 in. and less) where the root pass represents a greater portion of the weld thickness.
For intermediate and cover pass welding, E8010 electrodes have difficulty matching the longitudinal strength of modern Grades X65 and X70 line pipe material, and the use of cellulosic-coated electrodes with a strength level greater than that of E8010 has been known to produce a significant risk of hydrogen cracking in the weld metal for all but relatively thin-wall (0.250 in. and less) pipelines constructed in relatively flat terrain in warm climates. For these reasons, the pipeline operator has moved toward the use of low-hydrogen welding consumables and/or processes with higher weld metal strength for intermediate and cover pass welding. The preferred option in this area for manual welding is the use of low-hydrogen downhill electrodes (e.g., E9045). For applications where mechanization is appropriate, the use of gas-shielded flux-cored arc welding (FCAW-G) with a minimum of an E91T1 consumable strength level is the preferred option. Both welding procedure options typically specify the use of E8010 electrodes for root pass welding.
Welding Procedure Qualification
Welding procedure qualification can be used to demonstrate that the selected welding processes, consumables, and welding parameters are capable of producing acceptable girth welds. Acceptable in today’s context includes overmatching strength where the deposited weld metal has a strength (yield and tensile) that matches or overmatches the actual strength of the pipe material and there is no significant HAZ softening. Even though there are no requirements for the actual strength of the weld to exceed the actual strength of the pipe material in the majority of industry codes and standards used around the world for pipeline construction, supplemental requirements should be incorporated into construction contract documents, when practical, such as for large construction projects, that require welding procedure qualification on actual project pipe (not just pipe of the same grade) and cross-weld tensile testing failures to occur in the base material away from the weld. While not always practical, the length of pipe selected for welding procedure qualification should ideally be at the upper bound of the strength distribution for the pipe delivered for the project.
When project pipe is used for welder qualification at the beginning of a project, cross-weld tensile testing provides another opportunity to demonstrate overmatching strength when failures occur in the base material away from the weld.
Welder Training
Welder training in the use of low-hydrogen downhill electrodes tends to be necessary for welders who are accustomed to welding downhill using cellulosic-coated electrodes. Differences in technique between the two include the need for higher current levels and faster travel speeds. Other differences include arc initiation techniques, required electrode angles, arc length limitations, techniques for breaking the arc, grinding of starts/stops, and low-hydrogen electrode storage practices. Increased travel speeds and tighter control over welding parameters tend to result in reduced heat input levels compared to the use of cellulosic-coated electrodes, an effect that aids in avoiding HAZ softening.
Field Implementation/Industry Acceptance
Recent experience has demonstrated that the use of E8010 electrodes for root pass welding followed by intermediate and cover pass welding using either low-hydrogen downhill electrodes or mechanized FCAW-G are all viable options in terms of resulting girth weld strength level and industry acceptance.
Acceptance among North American welding contractors starts with a clear explanation of what is driving the need to utilize higher-strength consumables and lower-heat-input welding practices. Additional technical support and engagement from pipeline operators is key to fostering this change. Lack of experience with alternatives to legacy stove-pipe welding practices creates the risk of increased repair rates, production disruptions, and commercial impacts, all of which can create discomfort from a contractor perspective. Training of welders is critical in mitigating these risks. Many of the various contractor organizations and electrode manufacturers are beginning to modernize their training programs to address these changes.
It should be noted none of the recent pipeline failures have involved girth welds made using mechanized gas metal arc welding (GMAW) equipment, which is the obvious choice for girth weld strength matching when the size of the project justifies the expense. Many projects of varying sizes do not justify the use of mechanized welding and are better suited to the use of hybrid processes (manual or semiautomatic root and second pass welding followed by mechanized FCAW-G intermediate and cover passes). This is an excellent step toward full mechanization without major impacts to conventional construction practices when using a front-end pipe gang, V-prep welding bevels, and conventional radiographic testing or digital/real-time radiographic inspection. Root pass welding, using GMAW with advanced waveform power supplies, is also being evaluated, and applied in some cases, for cross-country pipeline applications.
The use of E8010 electrodes for root pass welding followed by low-hydrogen uphill (e.g., EXX18-type) electrodes for intermediate and cover pass welding is not a primary preference, but Enbridge makes this available for contractors/welders who prefer low-hydrogen uphill welding. It should be noted that additional oversight and control measures are often necessary during welding procedure qualification and field welding to avoid HAZ softening due to the excessively high heat input levels when welding uphill due to the nominally slow travel speeds.
Areas for Additional Improvement
In addition to the primary measures intended to control or limit pipe material strength, increase weld metal strength, and minimize HAZ softening, close attention must be paid to quality control during pipeline construction activities to make sure that excessive axial tension and/or bending strains are avoided. This includes making sure the profile of the pipe string matches the profile of the ditch during field bending operations and providing adequate pipe support so that high axial stresses do not occur after lowering-in from soil loading in nonflat terrain and at points of inflection.
Conclusion
The use of matching-strength girth welds prevents longitudinal strains from accumulating in the weld region, which is a natural stress concentration and is more likely to contain imperfections than the pipe material. Matching strength in this context means deposited weld metal strength that matches or overmatches the actual strength (yield and tensile) of the line pipe material and no significant HAZ softening. This is most important for large-diameter pipelines constructed using modern high-strength line pipe materials, particularly those in hilly terrain or subject to subsidence or other forms of ground deformation. WJ
Acknowledgments
The authors would like to acknowledge all the welders, contractors and contractor organizations, field inspection personnel, mechanical testing labs, consumable manufacturers, and others involved over the last four years of steady progress into the future of what is next with onshore pipeline construction. The folks on the ground are critical to overall success. The authors would also like to acknowledge Steve Rapp at Enbridge and Ken Lee, Melissa Gould, and Matt Boring at DNV for their contributions to this stream of work and to this article.
References
Bruce, W. A. 2019. Pipeline girth weld strength matching requirements. Welding Journal 98(10): 56–60.
API Specification 5L:2018, Line Pipe.
Interstate Natural Gas Association of America, Electronic References. Retrieved January 8, 2024, from ingaa.org/x70-jip-enhanced-girth-weld-performance-for-newly-constructed-grade-x70-pipelines.
Lee, K., Bruce, B., and Gould, M. 2022. Evaluation of higher strength E8010 pipe root pass welding. Proceedings of the ASME 2022 13th International Pipeline Conference (IPC 2022), paper no. IPC2022-87295. Calgary, Alberta, Canada.
Photo courtesy of Lincoln Electric/Enbridge.
