Study On Deformation Behavior And Fatigue Life Prediction Of Coiled Tubing

With the increasing difficulty of oil and gas exploration and production,and continuously falling of international oil prices,for the needs of oilfield reduce operation costs.Because of low operation costs,coiled tubing has become indispensable equipment in the field of oil and gas exploitation,particularly in the process of unconventional oil and gas operations.Coiled tubing deformation is a complex plastic large deformation problem under the action of internal pressure,axial load and cyclic bending,short work life is the main bottleneck restricting the development of coiled tubing technology.CT80 is taken as the research object.First of all,the simplified mechanics model was established with aiming at the coiled tubing deformation under the action of internal pressure,axial load and bending cycle.The true stress-strain curve of CT80 coiled tubing was obtained through simple material tensile test.Stress distribution of coiled tubing with winding bend was calculated through the theory of differential geometry and elastic-plastic mechanics;the theoretical analysis was carried out.Then the change law of coiled tubing's diameter and ovality varying with the internal pressure and axial load was obtained through the deformation finite element simulation.It shows that the winding bend is the origin of ovality,but the influence of internal pressure on ovality,which is the main cause of diameter increases,also cannot be ignored.After that the bending cycle deformation of finite element simulation on coiled tubing was proceeded under the action of a certain internal pressure and axial load.As a result,the distribution law of coiled tubing plastic strain and cross section parameters along the circumferential direction and the change law of that cross section parameters(diameter,wall thickness and ovality)varying with the bending cycles times were obtained.It indicates that with the increase of cycling times,the diameter of tensile side in cross section which is vertical to curved neutral layer is the most dangerous place and the plastic deformation failure first popular in its inner wall.Finally,the influence of coiled tubing's cross section diameter change rate,wall thickness reduction ratio and ovality on fatigue life were operated curve fitting.According to the result,the residual life prediction model is put forward which has used the three parameter power function energy method and considered the influence of diameter growth,wall thickness thinning and ovality.And the reasonable suggestions to improve the working life of coiled tubing was put forward.