Prediction method on fatigue life of string based on initial imperfection

The initial imperfection of string is one of the key factors that affect its fatigue life. In the traditional string fatigue prediction model, the effect of initial imperfection is often neglected, and consequently the predicted result is higher and the string safety is impacted. In view of the initial imperfection of strings, the extensional stress and bending stress on strings are corrected by using the stress concentration factor based on the force analysis on traditional defect-free strings. Then, the string fatigue life prediction model was built according its crack propagation laws. And finally, the average stress and minimum fatigue life of defect free strings and those with initial imperfection at different depth, semi-length and locations were simulated by means of finite element analysis tool and then compared. It is indicated that the depth and semi-length of initial imperfection have significant effect on the minimum fatigue life of strings. The closer the initial imperfection is to the collar, the more effect is has on the fatigue life of strings. The research results provide the reference for predicting the fatigue life of strings in the process of well drilling accurately so as to guarantee safe and efficient drilling.