Strain response of API 5L X80 pipeline subjected to indentation

Abstract

Dent is a common type of defect in offshore pipelines. In this paper, the API 5L X80 unpressurized pipeline subjected to indentation was studied by experimental and numerical methods. Firstly, a full-scale experiment of API 5L X80 pipeline subjected to indentation was completed, and the axial and circumferential strains in the dent area were measured accurately. Then, a finite element model of dented pipeline was established, and the accuracy and reliability of which were verified by comparing with experimental results. Based on the above model, the stress and strain response of dented pipeline were studied in detail. Results obtained from the experiment and FE analyses show that the strain variations at and near the dented region are largely dependent on their location. The influence range of the indenter on the pipe's strain is limited. In most cases, the closer to the center of the dent, the larger the strain amplitude. The strains of longitudinal and hoop direction change dramatically with the increasing indenter displacement, while the strain variation in the 45° direction is small. Tensile-compressive stress transformation occurs at the flank of the dent. The axial stress and strain changes once along the longitudinal direction in "decreasing–increasing–decreasing" which is expressed as the wavy curve. A large amount of plastic deformation is mainly concentrated at the dent center. The dent creates a strong stress concentration, and the location of high-stress zone is dependent on the dent depth, pipe diameter and indenter radius. The plastic collapse strain of the dented pipes was evaluated by twice elastic slope criterion. The plastic collapse strain increased with pipe thickness increasing, pipe diameter and indenter radius decreasing.