Collapse Strength Calculation Model for Coiled Tubing in Ultra-Deep Wells under the Synergistic Effects of Bending and Damage

Abstract The unique operational mode of coiled tubing (CT) inevitably subjects it to bending and damage, making its collapse strength difficult to predict and making it prone to collapse failure. In this study, the tensile mechanical properties of CT material (CT130 steel) under different pre-strains were tested, and a method for calculating the yield strength of CT130 steel after bending was established. The mechanical response of CT during the bending-straightening process was simulated using nonlinear finite element analysis (NLFEA). Calculation models for ovality, eccentricity, and residual stress induced by bending were established and introduced into the collapse strength calculation model as the defect influence factor. Based on the full-wall yield failure principle and the Unified Strength Theory, combined with the methods for calculating the yield strength and the defect influence factors of CT130 steel, a collapse strength calculation model for CT based on four typical yield criteria was constructed. A comparison of the model's calculation results with field data and simulation results shows that the model based on the Geometric Midline (GM) yield criteria is the most accurate model. This model improves the predictive accuracy of collapse strength and helps reduce the probability of collapse failure in CT.