Study On Low Cycle Fatigue Property And Fatigue Life Prediction Of CT90

In recent years,CT90 coiled tubing was used extensively and it was the highest strength level in the engineering.In the bending and internal pressure cycle,the tubing was prone to low cycle fatigue.Research on the low cycle fatigue property and the most important fatigue life of the CT90 was beneficial to improve the safety of CT90 coiled tubing technology.In this paper,the bending fatigue process of coiled tubing in field operation was simulated by low cycle fatigue testing machine,and the bending fatigue property of the CT90 sample was analyzed by applying different internal pressure.The influence of tube microstructure and weld on the fatigue life of coiled tubing was analyzed,and the diameter growth law of the coiled tubing was studied.A prediction model for calculating fatigue life of the coiled tubing was established by theoretical analysis and the finite element method in this paper.The error was less than 15% between the predicted results and the number of cycles,and the accuracy of CT90 coiled tubing life prediction model was verified.At the same time,the fatigue damage of coiled tubing was calculated by non-linear cumulative rule.Based on fatigue life prediction model,the influence of key factors such as internal pressure,wall thickness,outside diameter and bending radius on the fatigue life of coiled tubing was analyzed quantitatively.The results showed that the life of the coiled tubing increases with the increase of wall thickness,the life of the coiled tubing increases with the increase of the radius of the spool and the guide arch,as the diameter of the coiled tubing increases,the life of the coiled tubing decreases.And the yield strength of the tube has a great influence on the fatigue life of the high pressure zone,and the elastic modulus of the material has a great influence on the fatigue life in the low pressure zone.Based on the fatigue experiment,it was found that the non-linear damage theory was more suitable for engineering practice.The model of the coiled tubing diameter growth limit was established,which provides a theoretical basis for judging the scrap of coiled tubing.