The Prediction of the Low Cycle Fatigue Life About the Coiled Tubing with Ovality and Wall Thickness

Abstract

With respect to conventional tubing with threaded connections, coiled tubing has the advantage of not screwing on and off in the lifting pipe string. It is widely used in drilling and repairing wells. Coiled tubing experiences 6 bend-straighten plastic deformations in each lifting pipe string. Low working life and high cost are the key technical issues for coiled tubing in downhole operations. Based on the work state of the coiled tubing, a dedicated experimental device for fatigue life and specimen is designed in this article. Fatigue life tests were carried out on $2{\raise0.5ex\hbox{$\scriptstyle 3$} \kern-0.1em/\kern-0.15em \lower0.25ex\hbox{$\scriptstyle 8$}}^{{\prime \prime }}$ diameter tubes which are commonly used in industry. The experiments were carried out on the bending and straightening devices for the coiled tubing with the damage of ovality and reduction of wall thickness monitored. A large number of experimental data were obtained. The formulas for ovality and wall thickness with the cycle of bending–straightening were fitted based on mathematical regression method, and then the formulas for the low cycle fatigue life with damage of ovality and reduction of wall thickness are given for the coiled tubing. Comparing with the results of theoretical formula and laboratory experiments, the maximum error was 3.03%. Therefore, the calculation method will provide a theoretical basis for the coiled tubing failure prediction and control.