Contribution of finite element analyses to crack-like flaw assessments in a gas pipeline

When defects reach a critical size, failure occurs in engineering components. The criticality assessment of defects is hence a key aspect of the structural integrity of gas pipelines in service. Fitness-For-Service (FFS) assessments are generally employed for evaluating the criticality of a crack-like flaw in structures using simplified assumptions in relation to geometry and material properties. Whilst the errors resulting from these modelling simplifications prove acceptable in many cases, there are situations where it will be necessary to take into account the nonlinearities in geometry and/or material behaviour. This can be either to avoid excess conservatism or, on the contrary, to ensure the results are safe. In such cases it becomes essential to develop a finite element model of the structure to account for such real-engineering complexity. Welding, the most prevalent technique to join pipe, often brings about a misalignment between two pipes and hence complex crack shape is formed. The aim of this study is to develop an elastoplastic finite element model of a gas pipeline possessing a crack in a misaligned weld. The remaining life of the pipeline is determined using a Failure Assessment Diagram (FAD) and the Paris law. The results obtained from the finite element method to determine the stress intensity factors are compared to results derived using the API-579 for stress intensity factors calculations.