Crack Propagation Of Coiled Tubing Based On Extended Finite Element Method

After more than 30 years of development,coiled tubing technology has been used for more than 20 purposes,including paraffin removal,acidification,well killing,sand washing,drilling,well logging,and tubing production.Compared with conventional connected tubing technology,coiled tubing technology has the advantages of high efficiency,labor saving and high safety.However,the coiled tubing operation also has the problems of low working life and less recycling times.In particular,the fatigue life of coiled tubing becomes the key problem to limit the operation of coiled tubing.This is because in the process of production and application of coiled tubing,stress concentration will occur in the welding seam and defect parts,and cracks will inevitably appear.Once crack growth leads to structural fracture,low-cycle fatigue fracture will occur,which will bring great economic losses to coiled tubing operation.Therefore,it is of great significance to study the crack propagation of coiled tubing.In this thesis,the crack propagation problem of coiled tubing alloy materials is simulated by using the extended finite element method?XFEM?.Firstly,the cyclic fatigue finite element model of CT110 steel in coiled tubing is established to simulate the crack propagation process of alloy material in coiled tubing.The crack growth rate of CT110 alloy was obtained based on Paris formula.Obtained the CT110 material specimens under different stress ratio of lg??da?/?dN??-lg?K curve of crack extension phase,also got stress crack propagation threshold value ?K_th of CT110 steel under different stress ratio.In order to further study the influence of crack defects on different types of coiled tubing under load during coiled tubing operations,three groups of crack defects factors?crack depth?,crack length L,and crack Angle??were set up to analyze the influence of crack defects on coiled tubing crack propagation,the effects of crack defects on the tensile,bending and internal pressure properties of coiled tubing were studied.The variation rules of tensile strength,bending moment and internal pressure resistance of coiled tubing at different crack depths,crack angles and crack lengths were obtained.Finally,the orthogonal test is established in combination with the actual working conditions to study the sensitivity analysis of the influence of the type of load on the crack growth of the coiled under composite load.the result of research shows that:the calculation of the stress intensity factor by the extended finite element method is close to that by the manual,and the maximum relative error is less than 10%.When the maximum stress is constant,stress ratio by R=0.02 increases to=0.5,the threshold value fell by 37%.By comparing,and,the specific stage of coiled tubing crack growth can be determined.In the study the influence of crack defects on the mechanical properties of coiled tubing,When the crack depth is 0.25¹.5 mm,the crack length is 0^C/4?C is the circumference of the outer wall of the coiled tubing?,crack angle is 30°⁷5°,The tensile and bending properties of coiled tubing vary greatly.When the crack depth is3.25⁴.44 mm,the crack length is 0^C/12,the crack angle is 0°⁴5°,Its resistance to internal pressure varies greatly.When the coiled tubing is subjected to compound load,the tensile load has obvious influence on the crack propagation behavior of the coiled tubing.