Fatigue Life Evaluation of Dented Pipelines During Service and Shutdown Periods Based on the Rerounding Performance Available to Purchase

Current methods for assessing the remaining fatigue life of dented pipelines may lead to unnecessary maintenance interruptions and high-cost procedures. This article studies the impact of dents on the fatigue life of pipelines when subjected to cyclic internal pressure using finite element numerical models. A key contribution of this work is the explicit incorporation of service (pressurization) and shutdown (depressurization) conditions into the fatigue assessment, offering a more realistic and comprehensive understanding of pipeline structural behavior. The study examines the behavior of the dented pipe section during spring-back and rerounding under internal pressure. The results indicate that, for the same initial dent depth, the spring-back coefficient (ratio of the dent depth after spring-back to the initial dent depth) increases with a reduction in the slenderness of the pipe (diameter to thickness ratio) and decreases as the yield strength of the pipe increases. Stress concentration factors (SCFs) in dented pipes were evaluated from numerical simulations for varying pipe dimensions and materials and different indenter shapes (spherical and cylindrical) and dent depths. Based on the obtained SCFs and considering a suitable S–N curve, fatigue analyses were conducted on 240 dented pipe models, and the results indicated a significant decrease in fatigue life with increasing remaining dent depth, characterized after the full process of spring-back followed by rerounding under internal pressure and depressurization, for all slenderness ratios and indenter shapes. The cylindrical indenter (placed perpendicularly to the pipe axis) generated dents with superior fatigue life compared to those produced by the spherical indenter due to lower stress concentration factors resulting from the cylindrical indenter, at the same dent depth. This article provides insight into the structural integrity of dented pipelines and can assist in predicting the remaining fatigue life of these structures after experiencing an accidental collision event.