Numerical investigation of encircling eddy current signals for improved sizing of localised corrosion under insulation in steel pipelines

Corrosion under insulation (CUI) inspection is crucial for asset owners using insulated steel pipes susceptible to CUI, such as steam transport pipes. Current systems are effective for areas of corrosion above 0.01 m 2 but are liable to miss small, deep localised corrosion below this size, which is the focus of the work presented. Defect sizing equations for a new encircling coil eddy current system have been developed from 2D simulation results. Tests of these equations on randomly sized defects in 2D simulations have shown an average error of 10%. In addition, 3D simulations confirmed that 2D results effectively translate to 3D, assuming a minimum circumferential extent. Initial lab tests of regular defects showed results comparable to simulations, but minor sensor placement errors can cause significant variations. This system’s potential to detect corrosion as short as 5 mm in the axial direction has been shown with a wall thickness error of 10%. However, this effectiveness is limited to specific corrosion and pipe morphologies; further study is required to expand these limits and to produce a system capable of detecting pitting corrosion in a wide range of applications.