Fatigue of coiled tubing and its influencing factors: A comparative study

Coiled tubing (CT) has been widely used in oil well drilling and servicing. The operation of coiled tubing requires it to travel through the reel and gooseneck repeatedly under high internal pressure loading, which leads to bending fatigue damage of coiled tubing. Hence, it is very important to understand the mechanisms of the fatigue damage and to predict the working life of coiled tubing. The fatigue damage of coiled tubing has been extensively studied by using experiments, finite element simulations, and analytical models. It has been found that most of the fatigue damage of coiled tubing happens under low cycle fatigue conditions. Both stressbased and strain-based models have been employed to describe the fatigue damage of coiled tubing. Due to the severe operating conditions of coiled tubing inside a well, corrosion, ovality, pre-bending, rotation, and mechanical damage should all be considered in the fatigue analysis of coiled tubing. In this paper, mechanisms and influencing factors for the coiled tubing fatigue damage are reviewed, and existing models for the fatigue of coiled tubing are comparatively studied.