Quantization of magnetic flux leakage internal detection signals for composite defects of gas and oil pipelines

Abstract

This study aimed to solve the quantitative problem of the magnetic flux leakage (MFL) internal detection signals of long-distance oil and gas pipelines. Based on the magnetic charge theory, the numerical–analytical model for MFL internal detection of pipeline complex defects was constructed by introducing stress factor in this work. This model was used to investigate the changes of magnetic signals caused by different division sizes of the defective end-face. The distribution pattern of MFL signal under different geometric sizes and stresses of defective end-face was analyzed, and its influence on the characteristics of magnetic signals was assessed. The effectiveness of the model was verified through experiments. The results indicated that the overall accuracy of magnetic signal analysis by determining the optimal division size on the end face defects was improved by 35% compared with the conventional magnetic charge model, in which the calculation accuracy for defects with depth less than 25% of wall thickness was 41%. The axial signal component had extreme values. The radial component showed sinusoidal fluctuations, accompanied by peaks and troughs. The magnetic signal component increased non-linearly with the increase of the depth and width of the defect, and the incremental change increased first and then decreased. The magnetic signal components exponentially decreased with the increase of defect stress under magnetic saturation, and the decay rate gradually decreased with the increase of stress.