Analyses and application of the magnetic field at girth welds in pipelines

This paper proposes a novel method of utilizing weld identification to improve the positioning accuracy of an in-pipe detector in pipelines. The distributions of the magnetic field inside a single pipe are analysed using the equivalent magnetic charge method. Then the causes and characteristics of abnormal magnetic fields near the welds in the pipelines when pipes are welded together are discussed. A spherical carrier equipped with a magnetic sensor is designed and is used to measure the magnetic field inside an annular experimental pipeline when the carrier is pushed forward by the fluid in the pipeline and is rolling along the pipeline. Theory and experimental research show that there are very obvious abnormalities of the magnetic field at the girth welds in the pipelines. The abnormal magnetic field near the welds can be remarkably enhanced and accurately located using the signal enhancement method of continuous wavelet transformation and the peak detection method based on quadratic polynomial fitting respectively, thus enabling the identification and location of the welds. Different approaches are adopted to process magnetic field signals of different intensities in the pipelines in different directions. Finally, by considering construction information on the pipelines recorded previously, the positioning error is kept to less than 5 cm.

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