Analysis Of The Effect Of Surface Defects On Low-cucle Fatigue Life Of Coiled Tubing

Compared with the conventional thread connecting tubing,the coiled tubing has the advantages of high efficiency,safety and reliability.It can solve many difficult problems that conventional operation technology is difficult to deal with.In the engineering application,it involves drilling,completion,oil and gas development and other fields.But during coiled tubing operation,the main reason for low cycle fatigue failure is the combined action of winding bending,tensile load and internal pressure.Meanwhile,the poor service environment caused the volume defect on the coiled tubing surface and intensified the premature fatigue failure of coiled tubing.In this paper,the low cycle fatigue life prediction of coiled tubing with surface defects is systematically studied by combining theory with experiment.Firstly,according to the actual conditions of the coiled tubing,the paper analyzes the mechanical behavior of the coiled tubing in the cylinder.By calculating the tension of the coiled tubing at different angles of relaxation,the optimum relaxation angle of the coiled tubing at the roller is obtained.Secondly,the fatigue life test data of QT800 coiled tubing is obtained by the full size coiled tubing fatigue test machine.On the basis of the test results,the effect of different internal pressure and surface defect damage on the low cycle fatigue life of coiled tubing is analyzed.The mechanism of crack propagation on coiled tubing surface is explored,and a simplified grinding repair method is proposed by analyzing the parameters of surface volume defects.Finally,according to the basis of the fatigue life data of the coiled tubing without surface defects,the stress concentration factor and the strain concentration factor are introduced to characterize the effect of internal pressure and surface defects on the fatigue life of coiled tubing by local strain method.Assuming that the average bending strain amplitude is the only factor affecting the fatigue life of the coiled tubing,a low cycle fatigue life prediction model with a coiled tubing with a surface defect damage is established,and the error analysis is carried out with the test results,and the feasibility of the prediction model is verified.According to the mechanical analysis,the paper explores the optimum relaxation angle of the cylinder of the coiled tubing,which has a certain reference value for prolonging the service life of the coiled tubing in the actual working condition.Through the combination of theoretical analysis and experimental research,the factors affecting the low cycle fatigue life of coiled tubing are integrated,and a low cycle fatigue life prediction model for coiled tubing with surface defects is established.The prediction error is within the allowable range of the project and has certain prediction accuracy.The research results can predict the service life of the coiled tubing in the actual working condition,avoid the hidden danger caused by the premature fatigue failure of the coiled tubing,and provide theoretical support for the field construction.