Fatigue study on an offshore pipeline with a 3-D surface crack

Abstract

Over the years, demand for long distance offshore pipelines to transport oil and gases has been increasing steadily due to the rising cost of energy consumption. To build an economical offshore pipeline, carbon steels are normally utilised with considerable plastic deformation capacity. The use of carbon steels to build economical offshore pipelines comes with considerable plastic deformation capacity. For pipelines connected by the girth welding method, the processes of installation and operation subject them to excessive internal pressure. This is also accompanied by large plastic strains which give rise to mechanical defects such as surface or embedded elliptical cracks as seen at the weld area. Such defects can potentially jeopardise the structural reliability of the offshore pipelines. With the advancement of technology in material assessment, methods like non-destructive testing (NDT) and crack assessment protocols offers a safe and reliable way of inspecting the serviceability of the pipelines. However, NDT methods can be costly and inaccurate while some crack assessment protocols are proven to be too restrained which may impose difficulties of achieving the recommended fracture toughness of the pipes. This report aims to analyse and indent parameters which may impact the response of pipeline fracture by using 3D Finite Element Analysis (FEA) software, ANSYS Workbench 19.2. One of the half-pipes contains one semi elliptical embedded crack at one end of the pipe. The other one consists of a semi elliptical surface crack placed on the surface of the pipe. This report will present the results at the crack regions of the pipe subjected to an internal pressure. The understanding of fracture mechanics will also be studied and the effects of displacement condition on the crack growth will be presented in this report.