Fracture Control Offshore Pipelines: Advantages of Using Direct Calculations in Fracture Assessments of Pipelines

Abstract

Surface cracks pose major challenges for the structural integrity of pipelines. In fracture assessment programs the use of constraint parameters, such as the T-stress, along with K, J or CTOD are important to account for the limitations of single-parameter fracture mechanics. However, the three-dimensional nature of surface cracks precludes detailed 3-D finite element modeling for routine calculations. Here line-spring/shell-element models are demonstrated to be an efficient and reasonably accurate tool for constraint estimation even under large deformation levels when general yielding prevails in the pipe. Envisaging the potential use of this procedure in fracture analysis of pipelines, a new software, LINKpipe, has been developed. The program has been developed as a part of the Joint Industry project Fracture Control Offshore Pipelines. The objective of this project is to study the behaviour of defected girth welds in pipelines subject to construction and operational loads ever experienced before. The calculations have been performed in close cooperation with the project participants; see presentations of project-colleagues at OMAE 2005: Bruschi et al (2005), O̸stby (2005), Nyhus et al (2005) and Sandvik et al (2005). In this paper the line-spring calculations are compared with 3-D FE calculations and computations according to BS 7910. A pipe geometry, with OD = 400mm, was selected for the comparisons. The line-spring calculations were close to the 3-D calculations, while BS7910 was very conservative for long cracks and unconservative for short cracks. In highly ductile materials, such as pipeline steels, considerably amount of stable crack growth can be tolerated prior to the final failure of the structure. A simple method for simulating ductile tearing in surface cracked pipes with the line-spring model has been developed. A detailed parametric study has been performed to examine the effect of ductile tearing for pipes loaded in tensile, bending and with internal pressure. A significant reduction in deformation capacity from the stationary case is noticed. As the crack depth increases, the effect of ductile tearing becomes more important. And under biaxial loading a significant reduction of the deformation capacity is found as the internal pressure is increased. The development of the line-spring methodology paves the way for a transition from to-days rule-based design to direct calculations.