A Crack Quantification Method to Suppress the Lift-off Effect in Alternating Current Field Measurement

Offshore pipelines are key facilities for transporting oil and gas. However, its long-term work in the harsh seawater environment is very prone to cracks. Alternating current field measurement (ACFM) is an emerging non-destructive technique for detecting cracks in conductive material. During the detection of offshore pipelines using ACFM, the unevenness of the pipeline surface can produce a lift-off effect that affects the quantification accuracy of the cracks. To address the above problem, a new crack quantification method that can suppress the lift-off effect of ACFM is proposed. First, the Bx detection signals of ACFM at different lift-off distances are normalized twice. Then, the amplitudes corresponding to the troughs of the Bx curves are extracted as the new feature for crack depth quantification. The simulation and experimental results of the crack detection by ACFM at different lift-off distances show that the Bx curves after twice normalization vary in a very small range, and the troughs characterizing the same crack in the curves approximately coincide to one point, respectively. The proposed method can improve the interference of the lift-off effect on the Bx signals and enhance the detection accuracy of the cracks.