Flexible differential eddy current array sensor for thread defects of drill pipes based on biased magnetization

Threaded connections at oil drill pipe ends are critical joints between pipe segments bearing continuous operational loads. Prolonged service induces root cracks, corrosion and other failures, which may cause catastrophic joint failure during drilling, highlighting the imperative of thread inspection. This paper introduces a magnetised flexible printed circuit board (FPCB) differential coil high-frequency eddy current detection method, where three pairs of differential coils enable comprehensive defect detection across thread crests, flanks and root surfaces. Permanent magnets are employed to magnetise the thread surface, enhancing the probe’s sensitivity to defects in critical transition regions. Numerical simulations were conducted to optimize coil design by analysing parameter impacts on detection performance. The effects of defect angle, depth, length, and width on the detection signal were systematically investigated. A more effective strategy for enhancing detection capability within a limited space is to increase the number of coil turns, rather than to expand the wire width or spacing. This conclusion is supported by both simulation and experimental findings. Finally, an eddy current inspection system incorporating magnetised FPCB differential coils was developed, which achieves full-coverage inspection of drill pipe threads with custom scanning assemblies and shows great potential for petroleum drilling field use.