Magnetic Flux Leakage Testing For The Defects Of Sucker Rod Body And Its Data Processing

The sucker rod pumping method has become the most significant mechanical oil recovery approach due to its simple structure,long service life and strong adaptability.The sucker rod is one of the "three pumping" equipment in the sucker rod pumping system,and its function is to transmit the power of the ground oil pumping machine to the downhole oil-well pump.In the working process,the sucker rod is subjected to the corrosive and alternating load for a long time,so it is likely to form defects such as corrosion pits and cracks,in some severe cases,it will lead to the occurrence of broken rods and off-rod accidents.Consequently,the detection of the defects on the sucker rod surface and the adoption of targeted measures in advance can reduce the probability of accidents and save maintenance costs.Magnetic flux leakage(MFL)nondestructive testing technology is one of the main approaches for detecting the sucker rod defects,based on this technology,the detection method of the transverse and longitudinal defects on the sucker rod surface has been studied in this thesis.A MFL testing scheme for the defects of sucker rod body has been designed using double-coils DC axial magnetization method,it not only achieved the MFL detection of the two different types of defects,but also solved the problem that the magnetization direction needs to be perpendicular to the defect direction in the defect detection of the sucker rod body.Firstly,the basic principle and magnetization direction of the MFL testing have been analyzed.Based on the magnetic dipole model,the magnetic field gradient has been proposed as a method to evaluate the MFL signals of the defects.The finite element(FE)method has been used to analyze the magnetization effects of single-coil and double-coils DC magnetization modes,the result indicates that the magnetization field in the double-coils DC magnetization method is more uniform and stable,which is more conducive to the formation of the defect leakage magnetic field.The parameters of the multilayer wound magnetizing coil have been derived numerically,and the parameters of the magnetizing coil adopted in the MFL detection have been confirmed.The FE model for the MFL detection of the sucker rod defects has been established,the distribution of the leakage magnetic field and its influencing factors have been studied.The MFL detection scheme of double-coils DC magnetization has been designed,the MFL simulation for the transverse and longitudinal defects of the sucker rod body has been carried out,and the feasibility of the scheme has been verified.The influencing factors of the leakage magnetic field distribution have been analyzed,meanwhile,the variation trends of the leakage magnetic field distribution with the defect depth,defect length and magnetization intensity have been obtained.In addition,the peak value of the leakage magnetic field under different lift-off heights has been gathered,the relationship curve between the peak value of the leakage magnetic field and the lift-off height has been obtained by numerical fitting method,which provides a basis for the selection of lift-off height.The multilayer wound coils for the DC magnetization have been fabricated,a DC-magnetized constant current source(CCS)based on Buck-Boost converter has been developed,and the availability of the CCS in the coil DC magnetization has been verified.A MFL signal conditioning circuit for two-ways Hall output has been designed,moreover,a real-time MFL signal acquisition and processing system has been programed based on Lab VIEW.An experimental platform for the MFL detection of the sucker rod defects has been established,the correctness of the designed scheme has been verified by experiments.The simulated MFL signals after adding different noises have been de-noised using dbN,symN and coifN wavelet families.By comparing and analyzing the de-noising performances,the two wavelet bases with the best de-noising effect,namely sym6 and sym8,have been obtained.On this basis,a multilevel filtering algorithm based on wavelet de-noising and median filtering has been proposed.Finally,the de-noising processing for the experimental MFL signals has been carried out,and the experimental result shows that the improved de-noising algorithm is superior to the conventional wavelet de-noising method in terms of signal-to-noise ratio(SNR)and root mean square error(RMSE).Based on the detected MFL signals,the defect recognition and the two-dimensional(2D)profile reconstruction method have been studied.The leakage magnetic field distribution and magnetic field gradient of the transverse and longitudinal defects with different sizes have been analyzed,and the relationship between the leakage magnetic field distribution and the lift-off height has been gotten in the experiment.The state-space model of the 2D defect reconstruction has been set up,and the inversion calculation of the defects has been studied through particle filter(PF),extended Kalman filter(EKF)and unscented Kalman filter(UKF)approaches.Furthermore,the reconstruction performances and their advantages and disadvantages of the three reconstruction methods have been compared.