Some crack problems in offshore pipelines
Date of Issue2016-12-07
School of Mechanical and Aerospace Engineering
With the ever-rising cost of energy consumption, the demand for crude oil and natural gas has drastically increased in recent years. Offshore pipeline, one of the most economically form of transportation tool are commonly utilized for the energy resource. To construct an economically competent offshore pipeline, carbon steels with considerable plastic deformation capacity are commonly utilized. They consist of short pipeline segments connected by girth welding method and often experienced large plastic strains. This is normally due to mechanical flaws such as surface or embedded elliptical cracks observed at welding zone which posed a potential threat to the reliability of the offshore pipelines. In addition to the pipelines installed by reeling method, the large plastic bending load accompanied by high internal pressure could also pose tremendous challenges to the structural integrity of pipelines especially to the girth welds. Some advance and already used methods such as Non Destructive Testing (NDT) and crack assessments protocols are used to evaluate the properties of the pipelines without causing damage. However, not all NDT methods are suitable to inspect. Also, the NDT methods are usually not accurate to small fracture behaviors and are expensive. Crack assessments protocols are proven to be too restrained and conservative to be applied. In this project, commonly encountered crack cases in offshore pipes due to welding such as semi elliptical surface cracks are investigated simultaneously by using the 3D Finite Element (FE) analysis software package, Abaqus 6.13. The evolution of a crack will follow the fracture parameters given (Input), which is the Crack Tip Opening Displacement (CTOD) under two different loading conditions. Furthermore, two Failure Assessment Diagrams (FADs) for strain based fracture are established to perform fracture assessment on the specific pipelines. Relationships between CTOD, half length of the minor axis which is the surface crack depth denoted by a, half length of the major axis denoted by b, aspect ratio denoted by b/a, internal pressure and Stress Intensity Factor (SIF) concept are established and analyzed in this report.
Final Year Project (FYP)
Nanyang Technological University