Research on multi-frequency electromagnetic detection methods for outer wall metal loss defects in buried steel pipelines

There is a lack of effective trenchless inspection methods for detecting metal loss defects on the outer walls of buried steel pipelines where internal inspection is not feasible. In this study, by investigating the skin depth of alternating currents of different frequencies on steel pipelines, the excitation current frequencies for multi-frequency electromagnetic detection were determined. The current distribution at the locations of metal loss defects on the outer wall was studied, clarifying the characteristics of changes in current density and direction at defect locations, and identifying the patterns of magnetic field anomalies caused by metal loss defects on the pipeline outer wall at different frequencies. Using electromagnetic field signals obtained at a horizontal line position directly above the pipeline and perpendicular to its axis, a model for identifying metal loss defects on the pipeline’s outer wall was established. Furthermore, by calculating the ratio of the vertical component amplitude of the multi-frequency electromagnetic field to its horizontal component amplitude (perpendicular to the pipeline axis), the differences in energy loss caused by frequency variations and variations in soil electromagnetic properties at different pipeline locations during the propagation of multi-frequency electromagnetic cylindrical waves in the soil were eliminated. Finally, experiments verified the feasibility of the multi-frequency electromagnetic detection method for metal loss defects on the outer walls of buried steel pipelines and the reliability of the defect identification model.