Analysis of Fatigue Life of the Coiled Tubing with Ellipsoidal Defect

To prevent the coiled tubing from breaking down during operation, the fatigue life of the coiled tubing with defects should be predicted. The deformation and fatigue of the coiled tubing with different size of ellipsoid defects without inner pressure during coiling and straightening have been simulated by finite element method. The corresponding fatigue life has been estimated by the equivalent strain method and the critical plane method respectively, and then the corresponding diagrams have been attained. The defect size includes different lengths, depths, and widths given the same other dimensions. The fatigue life changes with the defect size. The effect of defect length change is greater than the defect depth or width change on the coiled tubing fatigue life. The results of the study could help to improve the design level of coiled tubing and the prediction of the service life, and also to reduce the significant loss of personnel and property caused by fatigue.