[228] A numerical investigation into the behaviour of fatigue circumferential cracks in ultra-deep water pipelines

Pipelines are used extensively in several applications and, in particular, they are commonly used for offshore oil and gas transportation. The goal of this paper is to analyse the behaviour of semi-elliptical circumferential surface cracks in ultra-deep water (>2100 m) pipelines by modelling their response to structural loads. The assessment of cracks in pipelines under varying loads is highly dependent on an accurate estimation of crack driving force, such as the stress intensity factor and the 3D J-integral or its equivalent crack-tip opening displacement. An accurate prediction of the structural integrity fof the pipeline is therefore important in order to prevent unnecessary and expensive downtime, failures leading to leakage or spillage of pipeline contents to the surrounding environment, and ultimately improve the reliability of the pipeline. This paper presents a numerical study to predict the fatigue lifetime of cracks in pipes, determining the J-integral and stress intensity factors for pipes containing a range of circumferential surface cracks by using the finite element method (FEM) and its implementation in a commercial FEM software package. A pipe segment is structurally loaded and numerically evaluated. Based on the results, a number-of-cycles-to-failure curve is obtained through a parametric study of different crack geometries, loading conditions and pipe geometries. Furthermore, a regression analysis is performed to propose a parametric equation for the fatigue crack growth life as a function of the crack depth and width, the pipe geometry and the applied loads.
Keywords: finite element method, fracture mechanics, J-integral, circumferential surface crack, fatigue crack growth life, ultra-deep water pipeline.