Development of Surface Flaw Interaction Rules for Strain-Based Pipelines

Strain capacity prediction is important for strain-based design (SBD) of welded pipelines. Current strain capacity prediction models consider only a single flaw. Practical engineering tools are needed to assess cases in the presence of multiple defects. In the current paper, it is shown that the existing flaw interaction rules developed for allowable-stress design are not applicable for SBD, which can lead to overly conservative or nonconservative assessment. A combined numerical modeling and experimental testing investigation was performed to develop interaction rules for coplanar surface flaws specific to SBD pipelines. Advanced finite element (FE) modeling was used to simulate flaw interaction, and full-scale tests (FSTs) were conducted to study the effects of flaw spacing on pipeline strain capacity. The FE modeling was first validated against the experiments and then used to develop surface flaw interaction rules through a parametric FE study. Using the rules, multiple flaws were converted into an equivalent single flaw resulting in the same pipeline strain capacity. The developed flaw interaction rules provide a useful tool for conducting engineering critical assessment (ECA) for strain-based pipelines in the presence of multiple flaws.