Failure pressure prediction by defect assessment and finite element modelling on natural gas pipelines under cyclic loading

In this work, a 3-dimensional finite element (FE) model was developed to investigate the effect of cyclic loading, which is induced by vibration during operation of in-line inspection (ILI) tools, on local stress and strain distributions and failure pressure of an X80 steel natural gas pipeline containing a corrosion defect. Modelling was also conducted on a low-grade X60 steel pipe for comparison. Parametric effects, including internal pressure, R ratio, cyclic frequency and dimension of the corrosion defect (primarily the defect depth), were determined. The cyclic loading greatly increases the von Mises stress and strain at the corrosion defect and reduces the threshold internal pressure to cause plastic deformation at the defect. As the internal pressure increases, both the von Mises stress and the strain increase and the high stress/strain zones expand along the defect length direction. The local stress and strain at the corrosion defect increase with decreased R-ratio and cyclic frequency, resulting in a reduction of failure pressure of the pipeline. An increased defect depth enhances local stress and strain concentrations, reducing failure pressure of the pipeline. A novel method is developed to assess corrosion defect during ILI tool operation and predict the failure pressure of pipelines under cyclic loading for the first time of its kind.