Critical Ultimate Load and Ultimate Bearing Failure of CT110 with Initial Crack Defect

Abstract

CT110 is ultra-high-strength coiled tubing (CT). It has been applied in the drilling, completion, fracturing and stimulation process of deep wells, ultra-deep wells and large displacement horizontal wells. Under complex load, the CT will crack and eventually failure. Fracture toughness of CT materials can be measured using Crack Tip Opening Displacement (CTOD). In this paper, a three-point bending finite element model (FEM) of CT110 material was established, and its critical CTOD value is obtained by calculation as 0.732 mm. The FEM of three-dimensional CT with initial crack was established and its feasibility was verified. The ultimate bearing failure analysis of CT with initial crack defects shows that under the same working condition, internal pressure accelerates the crack initiation and propagation speed, so that the crack of tubing body reaches the critical value in advance, and the maximum value at 130 MPa is close to the critical value. Under the combined loads, the maximum CTOD value of the CT with a crack increases with the increase in the crack depth and length, and the deeper the crack depth, the greater the influence of the crack length on its maximum value. This study on crack initiation and propagation of CT with defects provides a basis for predicting the fatigue life and revealing the failure mechanism of CT.