Residual stress measurement in pipeline steel for studying the effect of residual stress on cracking induced by SCC and denting

During operation of gas/oil pipeline, cracking can occur through the mechanism of stress corrosion cracking (SCC) or denting which have led to in-service or hydrotest leaks. The formation and propagation of such cracks is highly affected by the local stress state both on the surface and through thickness. The sources of stress can be a combination of externally applied environmental stress from rock/soil movement while underground, internal gas/oil pressure, and local residual stress due to manufacturing/welding and in-field bending (i.e., during installation). While the global stress state of the pipeline (e.g., due to soil movement) can be modeled by finite element analysis (FEA), the effect of residual stress that varies locally is believed to have a significant impact on where cracking occurs through SCC, or in dent areas. Therefore, neutron diffraction will be utilized to measure the three principal residual stresses on ex-service pipelines, which either remained SCC free (despite FEA predictions of potential risk) or have developed various levels of SCCs, and also sections where dent occurred with evidences of cracking. The measured residual stresses will be correlated with macroscopic FEA modeling and post-mortem local microstructure crack-tip examination to understand the driving mechanism for cracking through SCC or denting, and providing useful information to revise the current service for fitness code of pipeline operation on controlling crack growth.