Abstract
Pipeline magnetic flux leakage (MFL) internal detection technology is the most widely used and effective method in the field of long-distance oil and gas pipeline online detection. With the improvement of data quantization precision, the influence of stress on MFL signal has been paid more and more attention. In this paper, the relationship between stress and saturation magnetization is introduced based on J-A theory. The analytical model of MFL detection signal for pipeline composite defects is established. The MFL signal characteristics of composite defects are quantitatively calculated. The effect of stress on MFL signal is studied. The theoretical analysis is verified by experimental data and excavation results. The researches show that the saturation magnetization of ferromagnets decreases exponentially with the increase of stress in strong magnetic field. The MFL signal of composite defect is weaker than that of volumetric defects of the same dimension. The axial amplitude and radial peak-to-peak value of MFL signal decrease with the increase of stress around the defect. The axial amplitude and radial peak-to-peak value of MFL signal increase non-linearly with the increase of width and depth of defects. When using MFL signal to judge the defect depth, it is necessary to make clear whether there is stress concentration phenomenon around the defect because the stress will lead to underestimation of the defect depth.
Highlights
Pipeline transportation plays an important role in the transportation of oil and natural gas
Many scholars have studied the relationship between pipe stress and magnetic signal, mostly using magnetic memory measurement (MMM) technology [11,12]
Compared with traditional magnetic flux leakage (MFL) detection signals, the MMM signals are unstable and easy to disturb, so the MMM technology has not been widely used in the online internal detection of long-distance oil and gas pipelines
Summary
Pipeline transportation plays an important role in the transportation of oil and natural gas. Many scholars have studied the relationship between pipe stress and magnetic signal, mostly using magnetic memory measurement (MMM) technology [11,12]. Compared with traditional MFL detection signals, the MMM signals are unstable and easy to disturb, so the MMM technology has not been widely used in the online internal detection of long-distance oil and gas pipelines. In order to adapt to the widely used technology of MFL internal detection, some scholars have studied the magnetic signals of defects with stress under excitation state [16]. The traditional mathematical model of MFL internal detection ignores the influence of stress on magnetic signal and has errors in the analysis of the dimension and damage degree of composite pipe defects. A numerical model of composite defect in pipelines for internal detection is established based on the magnetic charge model introducing the stress operator. The research lays a foundation for quantitative analysis of composite defects in the future
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