Ratcheting response of dented pipes under monotonic and cyclic axial loadings

Abstract

Offshore pipelines are usually subject to hostile environments. Temperature changes and repetitive start-up/shutdown in offshore pipelines generate cycles of axial compression and relaxation. Consequently, the pipes may become subject to loading magnitudes that could cause them to locally buckle and wrinkle, and if subject to repeated loading, they could undergo ratcheting. Pipelines could also be subject to damage in the form of dents. This form of damage would have a profound effect on a pipe’s ratcheting response when subjected to cyclic loading. In this article, the ratcheting response of locally dented pipes that were subject to axially applied cyclic loading has been experimentally investigated. Scaled pipes (tubes) were first laterally indented with a cylindrical indenter and then were subjected to a monotonic axial compressive load that initiated small amplitude wrinkles on them. The pipes were subsequently subjected to repeated axial strain. The experiments were conducted on low carbon steel pipes (API 5L-grade B), with D_o/t ≃ 35, which were indented to three different indentation depths. Significant variation in pipes’ responses was observed, and the number of cycles prior to collapse dramatically decreased by as much as 75% when the indentation depth was marginally increased.