Differential eddy current method for full circumferential defect detection of small diameter steel pipe: Numerical analysis and experimental study

•A coil probe for eddy current testing of duplex stainless steel (DSS) coiled tubing (CT) was designed. The array probe is placed alternately on the surface of CT, enabling 360° fully covered detection of coiled tubing with a diameter of less than 50.8 mm.•By establishing the finite element model, the detection parameters are optimized.•The detection capability of the device is verified by processing sample tubes for experiments.•It realizes the detection of longitudinal and axial defects, breaking through the limitation that MFL detection can only detect unidirectional defects and carbon steel. Duplex stainless steel (DSS) coiled tubing (CT) is widely used in offshore and onshore operations because of its high plasticity, high toughness, and high corrosion resistance. However, the online detection technology of DSS CT has not yet been achieved. In this paper, we propose an eddy current testing method using a differential curved surface probe designed for CT inspection. The probe parameters are optimized using numerical simulation. The array probe is placed alternately on the surface of CT, enabling 360° fully covered detection of coiled tubing with a diameter of less than 50.8 mm. The method detects longitudinal and axial defects and overcomes the limitation of magnetic flux leakage (MFL) detection, which can only detect unidirectional defects in carbon steel. Finally, we processed and tested a sample tube, demonstrating that the device meets the testing standard of coiled tubing.