Effects of Loading Sequences on Remaining Life of Plain Dents in Buried Liquid Pipelines

The integrity assessment of dents in liquid pipelines, as regulated by codes and standards, is based mostly on depth and threat integration, with strain analysis incorporated as a nonmandatory recommendation for gas pipelines. There have been incidents in which the current regulatory criteria have not successfully predicted imminent failures of dents, leading to operators using noncodified, more conservative approaches to maintain safety, typically resulting in poor dig efficiency. The dent assessment methods currently available in the industry do not account for the sequences of loading by which a dent may have formed. In this paper, the effects of different load sequences on the remaining life of plain dents in liquid pipelines are demonstrated using a validated finite-element analysis. A parametric study is carried out to confirm that the findings hold true for a range of pipe geometries, materials, and dent depths. The findings indicate that the severity and/or remaining life of a dent cannot be fully assessed based on its depth alone. The restraint condition, indenter shape, loading sequence, and pressure-cycling history should be considered for a reasonable assessment of the remaining life of plain dents.