| With the rapid development of the national economy and the continuous improvement of people’s quality of life,the demand for oil and natural gas has been gradually increasing.Long-distance oil and gas pipeline transportation is cost-effective and convenient,making it widely used in the oil and gas industry.However,long-distance oil and gas pipelines are inevitably affected by adverse environmental conditions and third-party human factors during service,leading to defects such as cracks,corrosion,and perforation in the pipelines.With the increase in service life,these defects can easily cause pipeline failures and pose a serious threat to the national economy and people’s safety.Therefore,pipeline transportation safety issues have become urgent.Pipeline internal inspection technology is a crucial non-destructive testing method that plays a significant role in ensuring the safe operation of oil and gas pipelines.Among various pipeline non-destructive testing techniques,pipeline magnetic flux leakage detection technology is recognized internationally as a mainstream approach due to its advantages of not requiring coupling agents,strong anti-interference capabilities,and support for non-contact dynamic online inspection.With continuous exploration and research by researchers and technical professionals in the field of pipeline inspection,the quantitative relationship between leakage magnetic signals and defect size has become an important indicator for pipeline safety inspection.However,existing analytical models of magnetic flux leakage theory assume uniform distribution of magnetic charge density at defect locations for convenient computation and analysis of magnetic signals.This approach neglects factors such as the distribution of magnetic charges on the sidewalls of defects in service pipelines,leading to inaccurate calculations of the intensity of leakage magnetic fields and the inability to accurately quantify the relationship between defect size and signal characteristic values.Therefore,determining the distribution of magnetic charges on the defect surface is of significant importance for evaluating the risk of pipeline defects.This thesis investigates the influence of defect sidewall magnetic charge density distribution on magnetic signals.Based on the characteristics of finite spatial integration of zero electric field intensity and the interaction force between magnetic charges,an analytical model for the non-uniform distribution of defect sidewall magnetic charges is established.This model is combined with the classical magnetic charge model to form a non-uniform magnetic charge model.The variation patterns of magnetic charge density under different sizes of defects are investigated.The effects of defect size and lift-off value on leakage magnetic signals are studied.The changing trends of characteristic values of leakage magnetic signals with different defect sizes and lift-off values are analyzed.A gradient parameter K for the characteristic values of leakage magnetic signals is proposed,and the changing patterns of K under different defect sizes and lift-off values are experimentally validated.The non-uniform magnetic charge model and analysis methods proposed in this paper have important value and significance for the research of pipeline leakage internal detection technology and the quantification of leakage magnetic signals.The research results indicate that compared to the uniform magnetic charge model,as the length of the defect increases,the magnitude of the characteristic parameter gradient K gradually decreases,and the maximum value of Kmaxshows a nonlinear decreasing trend and gradually converges.As the depth of the defect increases,the magnitude of the characteristic parameter gradient K gradually increases,and the maximum value of Kmaxshows a nonlinear increasing trend.With the increase in the lift-off value,the magnitude of the characteristic parameter gradient K gradually decreases,and the maximum value of Kmaxshows a nonlinear increasing trend.Through experimental verification,it is observed that as the length of the defect increases,the accuracy of the fitting curves for the axial and radial components of the non-uniform magnetic charge model improves by 12%and 11%,respectively,compared to the fitting curves of the uniform magnetic charge model.As the depth of the defect increases,the accuracy of the fitting curves for the axial and radial components of the non-uniform magnetic charge model improves by 10%and 12%,respectively,compared to the fitting curves of the uniform magnetic charge model.With the increase in the lift-off value,the accuracy of the fitting curves for the axial and radial components of the non-uniform magnetic charge model improves by 9%and 10%,respectively,compared to the fitting curves of the uniform magnetic charge model. |
