Non-Magnetization Detection of Arbitrary Direction Defects in Coiled Tubing Based on Fluxgate Sensor

The presence of axial and circumferential defects on a coiled tubing (CT) surface leads to high risks of failure when the CT is used under acidic conditions in high-pressure and high-temperature wells. As the CT cannot rotate, it is difficult to identify the axial defects using traditional magnetic flux leakage methods. In this paper, a new method based on fluxgate sensor weak magnetic detection technology is proposed for detecting arbitrary direction defects under non-magnetization conditions. The method uses tri-axial fluxgate sensors and avoids complicated designs such as rotating magnetization. A data acquisition system is designed, and the amplitudes and coordinates of magnetic anomalies are obtained using the magnetic field gradient method. Sample tubes with arbitrary direction defects are tested. The test results show that the method can quickly locate and identify the arbitrary direction defects in the CT. The magnetic induction intensity signals obtained from the pre-fabricated arbitrary direction and circumferential defects show noticeable variations. The measured signal strength also varies significantly with the same lift-off height and different types of defects. The tri-axial fluxgate detection method can solve the problem of arbitrary direction defect detection.