Key Considerations for Elastic Finite-Element Modeling of Pipeline Dents for Fatigue Assessments

Abstract

The assessment of the remaining life of dents in pipelines generally relies on characterizing the structural response to pressure loading and combining a known pressure history with S-N curves to determine a fatigue life. A robust method for determining the structural response of a dent to pressure loading is through the determination of a Stress-Concentration Factor (SCF) derived from the modelling of the dent using Finite-Element Analysis (FEA). For simplicity, most SCF assessments rely on the use of unrestrained models derived directly from deflection data recorded by ILI or laser scan; however, this application can lead to overly high predictions of SCF values when evaluating restrained dents. Explicit modelling of restraint using bespoke indenter profiles and elastic-plastic material models can be used to derive more appropriate SCF values for restrained dents; however, this requires significantly more analytical effort and can sacrifice the fidelity of the shape for complex geometry. An approach that utilizes the efficient modeling and high fidelity of unrestrained elastic models would provide the industry with a reliable and repeatable process for evaluating the fatigue response of restrained dents. The methodology presented within this paper will seek to validate reasonable bounds for unrestrained elastic models that can be applied to cases where restrained dents are indicated. This paper will investigate the feasibility of a plasticity-restraint correction factor that could be applied to elastic SCFs and discuss the implications for dent fatigue assessments. The response of restrained elastic-plastic models will be compared to the response to elastic models for a range of indenter shapes to show the feasibility of this correction factor.