Prediction of the low-cycle fatigue life of the coiled tubing based on ovality and wall thickness parameter

The coiled tubing experiences 6 times of bend-straighten plastic deformation in each pipe lifting process, and the low-cycle fatigue failure accident occurs occasionally. Based on the work state of coiled tubing, a set of tailor-made experimental device and specimen on fatigue life was designed. The fatigue life test was carried out on the coiled tubing of ?60.325 mm. The values of ovality and wall thickness of different cycles were obtained in the experiment. The formulas of ovality and wall thickness were generated based on the mathematical regression method, and the formula of low cycle fatigue life was given. The experimental results show that the fatigue life calculation coincidence rate was over 90%. Therefore, the calculation method could be a theoretical basis for the coiled tubing failure prediction and control.