Multiaxial Fatigue Life Analysis Of The Base Material And The Bias Weld Of Coiled Tubing

Coiled tubing is widely used in the oil and gas field operation,which can greatly improve the operation efficiency.However,due to various environmental and operational factors,the coiled tubing failed before the designed service life.Coiled tubing is expensive and costly to fix after failure,so preventing failure in coiled tubing operation becomes an important engineering topic.Fatigue failure plays an important role in the failure modes of coiled tubes.In this paper,the fatigue life of coiled tubes is calculated by means of numerical simulation and theoretical model.The process of bending and straightening will occur in each operation of the coiled tubing,and repeated bending and straightening will cause fatigue damage of the coiled tubing.In coiled tubing fatigue test,the specimen after charging pressure under the action of the hydraulic system,first bends against the curved master form,then against the straight master form,and it cycles again and again till the pressure in specimen quickly reduces,then fatigue failure of specimen is considered to occur.According to the fatigue test process of coiled tubing,the finite element analysis model of coiled tube is established,and the calculation results of 3d model and 2d generalized plane strain element model are compared.The results show good agreement between two FEM models.According to the theory of multiaxial low cycle fatigue,the critical plane method is considered the best method to describe the process of the fatigue because most of cracks develop along the plane.Through finite element simulation,the strains of the coiled tubing without bias weld under cyclic load are analyzed,the first possible damage point is found while simulating the fatigue test of the coiled tubing,the critical plane position of the coiled tubing is determined,and the variation rule of the strain parameters on the critical plane is analyzed.Based on the critical plane method,a new multiaxial low cycle fatigue life prediction model for coiled tubing is proposed.The theoretical model is verified by the fatigue test data,and the results show that the life predicted by the multiaxis fatigue life prediction theory model of the coiled tubing proposed in this paper agree very well.The finite element simulation and fatigue life prediction of the coiled tubing under different bending radius and straightening are carried out.Based on the simulation and life calculation of the coiled tube bending and straightening loading conditions,the life of several coiled tubing after straightening is determined to be 50%~70% of that without straightening.The finite element simulation model of the coiled tubing with bias weld was established,and the fatigue life was predicted.The fatigue life of the bias weld zone under high internal pressure is more than 65% of the base material of the coiled tubing.The fatigue life of coiled tubing with different thick of bias weld will be further reduced...