Pre-bending coiled tubing and its fatigue life prediction

Coiled tubing is widely used in oil drilling and production operations,however,the extremely strong stress variation of coiled tubing during processes of pulling out,rolling in the reel and passing through the gooseneck make coiled tubing fatigue easily.Thus,it is of great importance to increase the coiled-tubing fatigue life.We introduced a new technique capable of improving the fatigue life of coiled tubing,i.e.the pre-bending coiled tubing technique,made a mechanical analysis and strength check of the pre-bending coiled tubing,and characterized its stress cycling in field operations.Based on data of fatigue tests under the symmetric and pulsating cycles,we established a prediction model of the pre-bending coiled-tubing fatigue life under an arbitrary cycle with fitting and interpolation methods,and simply compared the fatigue life of the pre-bending coiled tubing with the conventional straight coiled tubing.Preliminary calculations showed that the fatigue life of coiled tubing would be multiplied several times by applying the pre-bending coiled tubing technique.