Failure prediction of gas and oil drilling rig pipelines with axial defects

Abstract

Working conditions of casing pipes in drilling rigs can significantly influence the initiation and development of damage in the material, and therefore also the safe service of the entire system. In this work, an integrity assessment of a steel pipe with initial defect (machined surface crack) is presented. The position of this defect is on the external surface; unlike transport pipes, where internal surface is often endangered due to the contact with the fluid, casing pipes are often exposed to damages at the external surface. Analyzed crack is in axial direction, bearing in mind that this type of defect most significantly decreases the load carrying capacity of the cylindrical pressurized components. A pipe segment exposed to internal pressure is analyzed experimentally and numerically, using the finite element method. The experimental setup included the tracking of crack mouth opening displacement (CMOD) values, as well as J integral, which is determined by application of direct measurement. Criteria for pipe failure are determined on finite element models of the pipe; fracture initiation and plastic collapse are considered as failure mechanisms. The size of the crack is varied in the finite element models; several 3D models with different crack sizes (length and depth) are evaluated. Dependence of maximum internal pressure on the defect size is obtained. 2D plane strain models are also examined, with an aim to determine the applicability limits of this simplified approach. Based on the obtained results, integrity assessment criteria for the analyzed geometries are discussed.