Quantitative Study On The Analytical Model And Propagation Characteristics Of MFL Signals Of Outer Surface Defects In Long-distance Oil And Gas Pipelines

Oil and natural gas energy play an indispensable role in modern energy systems.As one of the primary transportation methods for oil and gas,pipelines are susceptible to macroscopic volume defects caused by aging,corrosion,and other factors,which greatly reduce the service time and safety of pipelines.Magnetic flux leakage（MFL）inspection is an internationally recognized and effective method for pipeline inspection.During the process of using MFL technology to detect outer surface defects,the MFL signal transmission will be affected by and pass through the ferromagnetic pipe wall,which is mainly reflected in the decrease in the amplitude of the MFL signal during internal inspection,resulting in an underestimation of the severity of defects.However,most existing classical magnetic charge analytical models only consider the evaluation of inner wall defects,but do not consider the attenuation effect of ferromagnetic pipe walls on the MFL field of external wall defects.This leads to significant deviations between the quantitative results of classical magnetic charge models and experimental data.Based on magnetostatic shielding theory and magnetic charge model,an analytical model of magnetic leakage internal detection for rectangular external defects in long distance oil and gas pipeline was established.The variation law of propagation compensation factor under different wall thickness and defect depth were analyzed.The effects of wall thickness,external defect depth and defect width on magnetic leakage signal were studied.In order to demonstrate the superiority of the improved analytical model,a three-dimensional model of MFL internal detection of outer defects in pipelines was established by finite element method.The influences of wall thickness and external defect depth on MFL signals were analyzed.The effectiveness of the improved analytical model was verified by systematic experiments.And contrastive analysis among the improved magnetic charge analytical model,classical magnetic charge analytical model and finite element model were conducted according to the experimental results.The research effectively describes the distribution characteristics of the MFL field of the outer surface defects,and has scientific guiding significance for the identification and quantitative evaluation of defects.The results showed that the normal propagation compensation factor increased nonlinearly with the increase of the depth of the outer wall defect,indicating that the deeper the depth of the defects,the smaller the propagation distance,and the stronger the MFL signal that can be effectively conveyed to the internal detection probe.The characteristic values V_p-vand V_maxof the normal and tangential components of the MFL signal of external wall defects show a first-order exponential decline with the increase of the wall thickness.The accuracy difference between the classical model and the improved model was analyzed by error analysis method.The accuracy difference parameter,（35）E showed that the normal signal accuracy of the improved analytical model was improved by 75.06%and the tangential signal accuracy by 55.40%.With the increase of the depth of external defects,the characteristic values of magnetic leakage signal V_p-vand V_maxof external defects showed a first-order exponential growth relationship with the increase of the depth of external defects.The accuracy difference parameter,（35）E shows that the normal signal accuracy of the improved analytical model was improved by 9.78%,and the tangential signal accuracy was improved by 12.19%.It showed that the improved analytical model was more consistent with the characteristics of magnetic signal.Compared with the finite element numerical results,the V_maxand V_p-vshowed a first-order exponential decline with the increase of wall thickness.With the increase of external defect depth,V_maxshowed an upward trend,while V_p-vshowed a logarithmic or nearly linear increase.The comparative analysis showed that the improved analytical model was more consistent with the variation characteristics of the outer surface defect signal in the magnetic leakage detection of long distance oil and gas pipelines.