Strain Assessment of Pipeline Dents Interacting With Girth Welds Using Finite Element Method

Abstract The recent publication of the “Gas Transmission Final Rule” by PHMSA allows operators to develop engineering critical assessments (ECA) to evaluate dents and other mechanical damage in pipelines [1]. One of the requirements of ECA is to evaluate the strain level associated with the dent and any nearby welds using Finite Element Analysis. When the dent apex sits on top of a weld, strain spikes can be generated at the weld location, creating an integrity risk to the pipeline. As the dent apex moves away from the weld, the effect of the weld on the dent strain will gradually reduce to negligible amounts. The existing guidelines such as API RP 1183 do not explicitly provide a method for calculating the dent strain associated with welds and there is no guidance on identification or quantification of the degree of dent-weld interaction with respect to strain behavior. To address these gaps, this paper conducted a comprehensive parametric study to examine the influence of welds on dent strain. The study considered various pipe attributes, pipe materials, dent depths, indenter sizes, indenter shapes, indenter orientations and restraint conditions and most importantly, the distance from dent apex to nearby girth welds. Three-dimensional finite element (FE) models were developed to simulate pipeline dents and girth welds. Mesh sensitivity analysis was then performed to ensure the accuracy of the FE models. A Python script was created to automate 580 representative FE models. The primary objective of the FE simulation was to investigate two scenarios involving the dent apex: directly on top or within proximity of a weld. In instances where the dent apex coincides with a weld, the dent strain amplification factors due to various pipe diameter and wall thickness ratios, dent depths and indenter sizes were calculated and compared with the factors provided by a recent PRCI report [2]. Additional aspects such as the indenter shape, indenter orientation, pipe material property and restraint conditions not considered in the PRCI report have also been examined in this study. For cases where the dent apex was not directly on top of a weld but rather adjacent to it, an interaction identification criterion was proposed. The dent-girth weld interacting distance was then determined based on the identification criterion and compared with the interacting distance calculated using the dent-girth weld interaction for fatigue equation from API RP 1183 [3]. The study found that dent strain increased with increasing dent-girth weld interaction. The interaction was highly dependent on the distance from dent apex to girth weld, and the dent geometry (depth, length), rather than the pipe steel grade. The restraint condition had little influence on the dent-girth weld interaction effect for small diameter pipelines (i.e., 12 in), but can be a primary factor influencing the dent-girth weld interaction effect for large diameter pipelines (i.e., 36 in). Based on the findings from this study, a dent-girth weld interaction rule for strain assessment was developed using a logistic regression algorithm applied on the 580 FE cases, which is a function of pipe geometry (D, t), dent geometry (d, L50%d), dent apex to girth weld distance and restraint condition. Additionally, a framework for evaluating dents associated with girth welds was created that aligns with the requirements outlined in CFR 192.712(c) [1].