Application of eddy currents induced by permanent magnets for pipeline inspection

This paper presents an alternative to the common concentric coil method to induce low-frequency eddy currents in ferromagnetic pipe and tubes. Pairs of permanent magnets rotating around the central axis of these cylinders in proximity of the surface can be used to induce high current densities in the material that is the object of the inspection. Anomalies and wall thickness variations are detected with an array of sensors that measure local changes in the magnetic field produced by the current flowing in the material. This electromagnetic technology is being developed for pipeline inspection platforms that either crawl slowly inside a pipe to maneuver past physical barriers or are pushed by flexible rods. These devices move down the pipeline independent of the product flow, and potentially stop for detailed defect assessment. Fundamental finite element modeling analysis and experimental investigations performed during this development have led to the derivation of a first-order analytical equation for designing rotating exciters and positioning sensors. The rotating permanent magnet system has the potential for broader application because the sensor configurations can be small in physical size, allowing them to pass obstructions that currently prevent inspection using available NDE implementations.