Quantitative study on the propagation characteristics of MFL signals of outer surface defects in long-distance oil and gas pipelines

The magnetic flux leakage (MFL) internal detection is one of the most effective methods for assessment of long-distance oil and gas pipelines. To quantify the outer surface defect through MFL signals, which are affected by pipe wall and its shielding effect, propagation compensation factor (PCF) was proposed to apply to a classical magnetic charge model. Using the established model for outer surface defects in long-distance oil and gas pipelines, characteristics of MFL signals with different wall thicknesses and defect sizes along with their propagation patterns were analyzed. To verify the analytical model, X70 pipes with different wall thicknesses were used for systematic experiments. The results showed that the pipe wall thickness had a significant attenuation effect on the MFL signals of external defects. As the wall thickness increased from 7 to 17 mm, the MFL signals exhibited a first-order exponential decay, with a maximum error of 25.8% and a minimum error of 2.5%. When the defect depth increased from 0.6 to 7.2 mm, the MFL signals showed a first-order exponential upward trend, with a maximum error of 18.8% and a minimum error of 2.0%. This study effectively described the propagation characteristics of MFL signals of outer surface defects, and facilitated identification and quantitative evaluation of outer surface defects.