Girth Welds for Strain-Based Design Pipelines

From the standpoint of structural integrity, girth welds are the weakest link in strain-based design pipelines. Girth welds (weld metal and heat affected zone) contain inherent defects, are a common location for geometric eccentricities like high-low misalignment, and potentially contain less resilient microstructures compared to linepipe steel. For these reasons, work was undertaken to study the effects of welding process on X80 girth welds. In the first part of this study, welds were produced using a number of shielded metal arc welding (SMAW) electrodes. Both low hydrogen, vertical down (LHVD) and traditional low hydrogen uphill electrodes were tested. It was found that most of these electrodes did not meet the target properties. Additionally, diffusible hydrogen measurements indicate that the LHVD electrodes tested can generate relatively high levels of hydrogen. It is concluded that typical SMAW electrodes should be enhanced for high-strain X80 applications, although some electrodes might be suitable for less demanding strain-based scenarios. In a second part of the study, gas tungsten arc welds (GTAW) were produced as a laboratory exercise to investigate the influence of this clean process on weld metal toughness (tearing resistance) and strain performance. Single edge notch bend specimens and curved wide plate tests were conducted using unloading compliance to quantify tearing resistance. These welds demonstrated dramatic improvements in structural performance. Despite the practical limitations of the GTAW process for field pipeline welding, these results provide significant insight for future welding developments.