Due to the extremely small size and arbitrary orientation of the cracks, a highly sensitive sensor based on the balanced-field electromagnetic technique was designed for in-line inspection of oil and gas pipeline cracks. A balanced-field electromagnetic technique sensor mutual inductance model was established and used to theoretically analyze the parameters affecting sensitivity. Finite element simulation was used to analyze the specific effects of the magnetically conductive medium, the number of coil turns, and the sensor lift-off height on the sensor output, respectively, and the sensor parameters of high sensitivity were determined. The detection effect of the sensor on the pipeline crack was tested by the single-sensor experiment and the pulling test. The results show that the designed balanced-field electromagnetic technique sensor is effective in detecting both circumferential and axial cracks of 0.5 to 6 mm in depth. As the crack depth increases, the sensitivity decreases and the detection voltage amplitude increases linearly. The sensitivity of the sensor is highest when detecting circumferential and axial cracks of 1 mm in depth at 1.76 and 0.87 mV/mm, respectively. In addition, the amplitude of the circumferential crack signal at the same depth is approximately twice that of the axial crack signal.
The balanced field electromagnetic technique as an effective in-line inspection method for cracks in long-distance oil and gas pipelines uses the pipeline inspection gauge (PIG) as the detection tool. PIG is characterized by the employment of a large number of sensors, but as each channel uses its crystal oscillator as a signal source, it inevitably generates frequency difference noise, which affects crack detection. A method of eliminating the frequency difference noise by using same-frequency excitation is proposed to solve the problem. Combining the principle of electromagnetic field propagation with the detection signal processing process, the formation process and characteristics of the frequency difference noise are theoretically analyzed, and the specific impact of frequency difference noise on crack detection is analyzed. The method of unified clock excitation for all channels is adopted, and a same-frequency excitation system is developed. The correctness of the theoretical analysis and the validity of the proposed method are verified by platform experiments and pulling tests. The results show that the effect of the frequency difference on noise follows the whole detection process, and the smaller the frequency difference, the longer the noise period. The frequency difference noise distorts the crack signal and is of comparable magnitude to the crack signal, which tends to drown out the crack signal. The same-frequency excitation method can eliminate frequency difference noise at the source and has a high signal-to-noise ratio. The method can provide a reference for multi-channel frequency difference noise cancellation in other AC detection technologies.
The balanced-field electromagnetic technique is an effective in-line inspection method for pipeline cracks. To address the problem that the interference signal generated by the tilt jitter of the sensor during the detection process affects the judgment of cracks, this paper proposes a method to differentiate the crack detection signal from the sensor jitter signal by using an amplitude–phase composite figure. The generation principle of the detection signal was analyzed by using the mutual inductance model, and the amplitude–phase composite figure was constructed by using the components of the detection signal after quadrature demodulation. The feasibility of using the phase as a signal discrimination feature was illustrated by finite element simulations, and the characteristics of the amplitude–phase composite figure were determined. The validity of the proposed method was verified experimentally. The results show that the crack detection signal and the signal generated by the sensor jitter are of the same frequency with similar waveforms and significantly different phases. The phase base value of the crack detection signal ranges from 35° to 55°, and the phase base value of the jitter signal is −4°. In terms of the characteristics of the amplitude–phase composite figure, the crack detection signal distribution is symmetrical about the origin in the first and third quadrants, and the axial crack is closer to the Y-axis than the circumferential crack; the jitter signal is distributed in the second and fourth quadrants and has a very small angle to the X-axis. In addition, the proposed method effectively weakens the observation of the phase noise region in the detection signal of the balanced-field electromagnetic technique.
The balanced field electromagnetic technique is an effective way of in-line inspection to detect cracks in pipelines. A signal demodulation method based on phase characteristics is proposed for the problem of interference signals generated by the sensor tilt shaking during the detection, which affects the judgment of the cracks. The method uses a reference signal whose phase is orthogonal to the signal generated by the sensor shaking to demodulate the detection signal to eliminate the shake interference. The generation principles of crack detection signals and interference signals generated by sensor shaking are analyzed, and the influence of sensor lift-off on detection is compared. A demodulation model is established based on the characteristic of that same frequency and different phases of crack and shake signals. The feasible conditions of the method are analyzed by simulation, and the phase value of the reference signal in the demodulation method is determined. The platform detection experiment and pulling tests at different speeds are carried out, respectively, to verify the effectiveness of the proposed method. The results show that there is a significant phase difference between the signals generated by the sensor shaking and the crack. For carbon steel pipelines, the signal phase of different shake angles is −4°. When the sensor structure and excitation frequency in this study are used, the reference signal phase is chosen to be 86°. The method preserves the detection signal characteristics before processing and enables the linear output responses to be obtained for different depths of cracks.
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