Low cycle fatigue assessment tool for wrinkled energy pipelines

Abstract

The majority of energy pipeline systems are buried. They may be subject to differential ground movement which as a consequence impose axial forces and bending moments onto pipeline sections that may result in local deformation of the pipeline section such as ovalization, wrinkling and buckling. For these pipelines with local plastic damage, there exists limited knowledge and guidance on the pipe mechanical integrity and remaining low-cycle fatigue life. This uncertainty influences management considerations with respect to pipe operations, repair and intervention that have potential implications for cost and safety. Using continuum finite element methods, a numerical simulation tool was developed to simulate the local damage, using an analogue “strip test” coupon, and assess the remaining low-cycle fatigue life response. The numerical modelling procedures were calibrated using third-party data and experimental investigations performed in this study, which is a unique contribution on this subject. The low-cycle fatigue response was influenced by the residual strain and incremental damage associated with strain energy during a loading cycle, which was characteristic of a Coffin-Manson type power law response. The low-cycle fatigue response was also correlated with other key parameters including damage radius of curvature to pipe wall thickness ratio, imposed stroke amplitude and opening or closing mode of deformation. Recommendations to refine the numerical modelling procedures and further establish confidence in the use of the analogue “strip test” for the assessment of low-cycle fatigue response of damaged pipelines is provided.