Study On The Mechanical Behavior Of Coiled Tubing And Low Cycle Fatigue Life

The technology of coiled tubing(CT)has been widely used in well drilling,well completion,oil recovery and well washing.CT plays an increasingly important in the petroleum industry and enjoys the laudatory of "universal operating equipment".However,due to the effect of bad working condition,complex coupling load and defect,the service time of CT can’t be guaranteed.Therefore,it’s urgent to study the ultimate bearing capacity and fatigue life of CT by using theoretical methods and finite element(FE)techniques to obtain the corresponding master parameters and optimization parameters.This paper based on the working conditions on site,the 2.375" CT is selected for studying.In order to study the ultimate bearing capacity of CT under the multi-group load,based on the classical mechanics model,the mathematical model and FE model of CT is established.CT will be subjected to tensile load,bending deformation load,internal pressure and collapse load in the course of work.To obtain the main control loads which affect the ultimate bearing capacity of CT.The fluctuating load distribution method is selected to analyze the change of stress value of CT under different loads.When obtain the main control loads,the multiaxial principle of CT is expounded.Based on the classical Tresca rule and Von Mises criteria,a new plastic modulus is established to explain the multiaxial ratcheting and the stress-strain relationship of CT.At the same time,based on the new plastic modulus,the FE model of 2.375"CT is established to study the change of the diameter of CT under the main control load.According to the actual conditions of CT under bending and straightening cycles,the reason of low-cycle fatigue life of CT is illustrated.And based on plastic strain,the low-cycle fatigue life model and FE model of CT is established.The correctness of the low-cycle fatigue life model and FE model of CT is verified by the relevant experimental dates.Besides,the influence of internal pressure,bending radius and wall thickness on fatigue life of 1.25",1.75"and 2.375" CT are analyzed.Based on the local strain approach and the fatigue life model of the non-defective CT,the stress concentration coefficient and strain concentration coefficient are introduced and the low-cycle fatigue life model of the defective CT is established.The defect depth,defect circumferential spread,defect axial length and defect shape are selected.And the effect of these defects on fatigue life of 1.25",1.75" and 2.375"CT are studied respectively.This paper not only obtains the main control loads of ultimate bearing capacity of CT under the multi-group load,but also establishes the low-fatigue life model of the non-defective CT and the defective CT by the CT actual working condition.