ESTABLISHMENT AND APPLICATION OF FATIGUE LIFE PREDICTION MODELS FOR COILED TUBING

Coiled tubing (CT) technology has been developing in recent years. Analyzing the factors that affect CT fatigue life and getting a more accurate life prediction are of great significance. The work process of CT is one that goes through large deformation and multi-axial low-cycle fatigue, which suggests that the strain-life method should be used. Considering tri-axial strain state, this paper uses finite element software to fit the expression of strains in three directions so as to get equivalent strain. This paper also carries out tensile and fatigue experiments to get necessary fatigue parameters for calculating CT life. Comparing to the experiment results, the error is within 10%, which is considered to be acceptable. Based on the life prediction model above and field information, the software for predicting CT life is developed, through which the operators can store and recall data as well as export and print figures and tables. Besides, one can predict CT residual life before each operation and it shows the distribution of damage along the tube, providing basis for CT removal and repair.