Due to the lack of the efficient and accurate detection method, the axial crack of pipeline seriously threatens the safe operation of oil pipeline, and therefore the application of micromagnetic nondestructive testing technology in the field of pipeline axial crack detection is of practical significance. To study the characterization of axial crack by micromagnetic signal, a numerical model of micromagnetic signal to detect the axial crack of pipeline is established based on the micromagnetic theory in this article. The characteristics of micromagnetic detection signals of cracks with different sizes and directions are calculated. The propagation law of micromagnetic detection signal is analyzed, and the experimental study of X70 pipeline is carried out to verify the correctness of theoretical model and simulation. The results show that at each tip of the axial crack, the axial component of the micromagnetic detection signal has a peak and a valley, and the radial component has an extreme value. The amplitude of the micromagnetic internal detection signal at the axial crack linearly increases as the depth or length of the crack increases. The propagation law of micromagnetic signal conforms to the falling exponential function, specifically, the smaller the crack depth, the faster the signal decays and eventually stabilizes. The micromagnetic detection signal of the probes to detect the crack having a certain angle with the axial direction is arranged with equal displacement spacing.
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.
Quantitative online detection of microcracks in long-distance oil and gas pipelines is an international problem, and the effective detection method is still lacking. In this paper, a mathematical model of non-uniform distribution of crack magnetic charges is established based on the stress distribution laws of pipeline cracks under internal pressure. The weak magnetic signal characteristics of pipeline cracks with different sizes are analyzed. The internal pressure increasing factor of weak magnetic signals are extracted to analyze the corresponding relationship between crack size and weak magnetic signals. The experimental study of the X70 pipeline is carried out. The results show that the axial component of the weak magnetic signal at the crack has a maximum value near the tip, and a minimum value appears in the middle of the crack. The internal pressure increasing factor is introduced to quantify the weak magnetic signal, the crack is in a safe state (not expanding) when the internal pressure increasing factor is positive, the weak magnetic signal has a linear relationship with the crack size. However, the crack is in a dangerous state when the internal pressure increasing factor is negative, and the pipeline crack will expand as the internal pressure increases.
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