Pre-bending Coiled Tubing And Its Prediction For The Fatigue Life

Currently, a large number of coiled tubing units and technology over the world areput into in oil-gas field applications. With its many advantages, such as small footprint,low operating costs, easy relocation and installation, protection reservoir, wide application,etc., coiled tubing unit is compared to“universal operating unit”. However, undercombined actions of bending, tension, internal pressure and other stress, coiled tubing inoperations will be fatigue fracture easily, which reduces working life, increases operatingcosts, and makes the application to be greatly restricted. Therefore, the study on the newtechnology improving and predicting the service life of coiled tubing is of great economicvalue and guiding significance for petroleum industry.In this paper, the development history, unit, and technology of coiled tubing areintroduced. Pre-bending coiled tubing technology is proposed, which may improve thefatigue life of coiled tubing by making coiled tubing in the curved shape to reduce thebending stress. The new mathematical models of the mechanical analysis and strengthcheck from the reel to the injection head of pre-bending coiled tubing are established. Thestress cycling characteristics of pre-bending coiled tubing in field operations is analyzed;on the basis of fatigue experimental data under the symmetric cycle and the pulsatingcycle, with fitting and interpolation method, the fatigue life prediction model of pre-bendcoiled tubing under arbitrary cycle is established. The fatigue life of pre-bending coiledtubing with that of the straight coiled tubing is simplly predicted.Through simple comparison of fatigue life of the two kinds of coiled tubing , it isconcluded that the fatigue life of pre-bending coiled tubing is more than two times of thatof straight coiled tubing under the pure bending state, and pre-bending coiled tubingtechnology may multiply improve the fatigue life of coiled tubing.