Fracture assessment of post-buckled offshore pipeline using eXtended finite element method

Shallowly buried offshore pipelines operating under high pressure and high-temperature conditions are susceptible to upheaval buckling. Such pipelines may contain pre-existing defects, including fabrication- or operation-induced cracks. If an offshore pipeline with an initial defect experiences vertical movement due to upheaval buckling, the crack can propagate in the tensile stress region, leading to fracture. This study presents a numerical modelling technique using an eXtended Finite Element Method (XFEM) to analyze the initiation and propagation of tensile fractures in a post-buckled pipeline. Conventional fracture mechanics commonly employ damage initiation criteria based on maximum principal stress (MAXPS) or maximum principal strain (MAXPE) with fixed values. However, these criteria have limitations when considering crack-tip constraints (stress triaxiality and Lode angle) during the numerical analysis. A modified Mohr-Coulomb (MMC) fracture criterion is implemented in the finite element program, Abaqus, using a user-defined subroutine to address this limitation. The MMC criterion considers shear slip and ductility, providing a more realistic representation of ductile materials than MAXPS and MAXPE models. This study also examines the influence of various fracture parameters under different damage degradation models. The findings provide practical insights for assessing crack initiation and propagation in post-buckled offshore pipelines.