Detection, distinction, and quantification of pipeline surface and subsurface defects by DC magnetization scanning induction thermography

Abstract

Pipelines are the backbone of the oil and natural gas industry. In order to guarantee its quality, DC magnetization scanning induction thermography (DCMSIT) is proposed to detect, distinguish and quantify the pipeline surface and subsurface defects. The exciting method, consisting of Helmholtz DC magnetization coils and a toroidal AC induction heating coil, is designed to adapt the pipeline’s annular shape and realize full coverage automatic detection. Under DC magnetization, surface and subsurface defects cause distorted permeability distribution in the skin layer relating to defect depth. Under AC magnetization, the distorted permeability produces abnormal thermal distribution, indicating the characteristics of defects. Based on the different thermal distributions in shapes and intensities of surface and subsurface defects, continuous wavelet transform (CWT) is used for thermal signal spatial-frequency analysis to distinguish surface and subsurface defects. The principle of the proposed method is analyzed based on Faraday’s law, Joule’s law, and thermal propagation. Then, finite element simulation is conducted to investigate magnetic permeability distribution for pipeline surface and subsurface defects with different depths. Finally, the DCMSIT system for pipelines is designed and developed to validate the proposed method. Experimental results show that the proposed method can evaluate pipeline surface and subsurface defects from 1.0 mm to 5.0 mm in depth.