Elastic–plastic fracture analyses for pipeline girth welds with 3D semi-elliptical surface cracks subjected to large plastic bending

Abstract

Offshore pipelines are subjected to large plastic strain conditions, such as bending accompanied by high internal pressure, during the installation and operation processes. These pipelines are usually constructed by the girth welding method and weldments may contain naturally occurring surface and embedded planar and/or volumetric imperfections and/or defects. It is widely recognized that the existing fracture assessment procedures which are based on the load-controlled method, are not explicitly designed for situations with large plastic deformation. The main objective of the current paper is to find a routine to make fracture assessment for a pipeline subjected to plastic bending. In this paper, based on 3D elastic–plastic finite element analyses, the influences of various parameters on the fracture responses (CTOD) of the cracked pipelines are investigated and a CTOD estimation formula is proposed. Furthermore, comparison with fracture assessments made by BS7910:2005 code and finite element analyses indicates that the former gives over-conservative predictions. Based on the proposed formula, a failure assessment diagram (FAD) for strain-based fracture, is presented for the specific geometry and material properties of the particular pipe with semi-elliptical surface cracks under large plastic bending, where the global strain is up to 3%.