Study on Natural Crack Initiation and Propagation of Coiled Tubing with Spherical Defect

Abstract

The crack initiation and propagation at the surface defect of coiled tubing (CT) are easily caused by stress concentration, which leads to failure of CT. To explore the critical load and its influencing factors for crack initiation of CT with spherical defect, based on the maximum principal stress criterion and damage evolution criterion, the extended finite element method is applied. In this paper, the ABAQUS software is used to establish a CT model with spherical defects, and to study the crack initiation and propagation of the CT with spherical defects under the combined loads of tensile load, internal pressure, and bending displacement. The results show that the critical tensile load for crack initiation is 274.5 kN, the critical bending displacement is 86.6mm, and the critical internal pressure load is 88.8 MPa at the stage of crack initiation at a spherical defect with a radius of 2 mm. Meanwhile, in the crack propagation stage, the crack length under 430 kN tensile load is 97.8 mm, the crack length under 145 MPa internal pressure is 161.8 mm, and the crack length under 120 mm bending displacement is 130.4 mm. The research results provide a reference for judging whether the CT with defect can continue to be used, and lay a foundation for the formulation of the standard for the use of CT with defect.