Study On Magnetic Flux Leakage Testing For Longitudinal Crack Of Sucker Rod

As an important component in oil production process,sucker rod is affected by the erosion of corrosive medium and the effect of asymmetrical cyclic loading for a long time in the course of service,it is easy to generate defects including transverse and longitudinal cracks,and may even lead to broken rod accident.At present,the device of longitudinal crack detection has complicated structure and the production cost is higher.This thesis has studied the magnetic flux leakage(MFL)method for longitudinal crack of sucker rod,achieving results in modeling,lift-off value and experimental device.The main work of this thesis is as follows:1.The principle of MFL testing has been studied,and the models of radial magnetization,single-coil and double-coils axial magnetization have been established using ANSYS.What's more,the scheme of double-coils axial magnetization has been determined by comparing the effects of magnetization in sucker rod.According to the theory of electromagnetic field and finite element analysis,a finite element model of MFL testing for longitudinal crack of sucker rod has been established,and the distribution characteristics of leakage magnetic field in longitudinal crack has been solved and analyzed,the variation trends of leakage magnetic field with external magnetic field,crack size and the lift-off value have been gotten.2.The experimental device of MFL testing for longitudinal crack of sucker rod has been designed,including the winding of double excitation coils,the selecting of detection element,the designing of signal adjusting circuit and acquisition system.Wherein,the double excitation coils are used to magnetize the sucker rod,the detection element is used to collect the MFL signal,and the signal conditioning circuit and the acquisition system are used to achieve the saving and taking of detection signal.3.The longitudinal crack of sucker rod has been detected in the experiment,and the defect signal has been denoised by using wavelet technology,the sudden changing characteristics have been effectively retained.The influence of lift-off value on MFL signal has been analyzed,the limited value of defect signal upon the crack has been gotten.The function relation between the lift-off value of sensor and the limited value of acquisition signal has been obtained by data fitting.In this thesis,a MFL testing scheme with good magnetization effect,simple structure and low cost has been designed.The scheme has been studied from the aspects of modeling,simulation and experiment,and the MFL testing for the longitudinal crack of sucker rod has been achieved.