Deformation And Reliability Analysis Of The Coiled Tubing Based On The Incremental Theory Of Plasticity

With the increasing difficulty of oil and gas exploration and production,and especially the development of unconventional oil and gas well,coiled tubing technology has been widely used.However,in the process of working,coiled tubings need to withstand a combination of steady high internal and cyclic bending-straightening and progressive plastic deformation and fatigue failure can occur under loads.Therefore,the research of coiled tubing deformation and low cycle fatigue failure has great significance for guiding the production and operation of coiled tubing.This paper takes CT80 coiled tubing as the research object.The deformation and fatigue life of coiled tubing under cyclic bending and high internal pressure are calculated based on fatigue test,theoretical study and finite element simulation.The specific studies are as follows:Firstly,this paper takes CT80 coiled tubing as the research object.The variation of coiled tubing's diameter and thin thicks with bending cycles is obtained through coiled tubing fatigue test under different bending radius and different internal pressure.Secondly,this paper presents a new reasonable numerical algorithms to calculate diametral growth and wall thinning under loading of cyclic plastic bending and internal pressure based on incremental plasticity theory and virtual work principle.The algorithms include three main parts of nonlinear ring theory,the incremental elastoplastic constitutive relation and virtual work principle and A-F hardening rule is chosen in incremental elastoplastic constitutive relation.Moreover,the numerical algorithms are evaluated by coiled tubing cyclic bending tests under internal pressure.Meanwhile,based on the Chaboche hardening rule,the three-dimensional finite element model of the coiled tubing under internal pressure and cyclic bending and straightening is established by ANSYS finite element.The variation rule of the coiled tubing deformation with the bending cycles is obtained.Finally,this paper use artificial neural network theory to predict the low cycle fatigue life of coiled tubing with surface defects.Based on the Self-Organizing Feature Map(SOFM)neural network and Radial Basis Function(RBF)neural network,and considering the effect of coiled tubing surface imperfection,a hybrid network model is built for predicting the life of coiled tubing.The result shows that the hybrid network model is superior to BP neural network in accuracy and stability.