Stress Intensification and Crack Growth in the Presence of Dents on Pipelines

Historically, ruptures in near-neutral pH SCC environments on gas transmission pipelines have been associated with some kind of stress raiser, generally the long-seam weld, corrosion, or dents/gouges. The current work provides an assessment of the effect of dents on environmental cracking. A dent assessment model was combined with a recently developed crack growth rate model to simulate crack growth in high-stressed areas caused by the presence of a dent. The work suggests that unrestrained dents (indenter removed) provide higher stresses and result in faster crack growth than restrained dents (indenter held in place). A dent in a lower ductility, higher strength X-65 steel further promoted the formation of high stresses and crack growth. Small crack growth and crack growth on gas lines, when governed by a strain rate based crack growth mechanism, may be less sensitive to dent parameters than crack growth governed by corrosion fatigue. It should be noted that neither the dent assessment model nor the crack growth model have been fully validated. Further work using elastic-plastic Finite Element Analysis is needed to determine more accurately the extent of stress intensification of a crack growing in or near a dent.