Integrity of Longitudinal Welds in Pipelines: Bridging Scales

Abstract

Toughness testing of the heat affected zone (HAZ) of longitudinal welds is increasingly often required in pipeline standards and specifications. This includes simple tests such as the Charpy impact test that was designed to serve as quality test as well as enhanced methods including crack tip opening displacement (CTOD) tests that are necessary to conduct an engineering critical assessment (ECA). If occasional low toughness values are observed, the question turns towards assessing the impact of such numbers and how representative they are of the behavior of a pipe in service. The significance of low toughness values measured in laboratory testing can be judged on basis of ring expansion and hydraulic burst tests. The current study summarises an extensive test series to quantify the toughness of submerged arc welds (SAW) obtained by different test methods. The tested pipes cover a wide range of material including medium strength X70 up to high strength X100. Their welds are characterized in terms of fracture toughness properties with single edge notch tension (SENT) and single edge notch bending (SENB) tests. Different constraint levels are obtained within each series by introducing notches of standard depth as well as shallow notches. Structural behavior is characterized with burst tests as well as ring expansion tests containing notches in the longitudinal weld. The experimental results are assessed within dedicated finite element studies. The assessment is conducted for pipes serving as pressure containment, thus having circumferential stress resulting from internal pressure. Based on the results achieved the conclusion can be drawn that the standard route including high constraint CTOD leads to overly conservative results concerning the integrity of longitudinal welds. A better representation of structural behavior is observed in ring expansion tests.