Research On Calculation Method Of Leakage Magnetic Field Of Pipeline Defect

The increasing demand for oil,natural gas and other energy use in recent years has forced an increasing need to transport energy by pipeline.The pipeline is mostly ferromagnetic material,which is affected by external environment and transport medium to form crack defects in the long-term use process.Once a leak,explosion and other accidents can cause severe economic losses.The theoretical study of pipeline leakage detection within the magnetic detection is mainly for the geometry of the regular defects,through the sensor to capture the leakage signal at the defect,the distribution of the defect signal for the main purpose of interpretation.The pipeline in the project is affected by corrosion to form a crater,which is similar to an ellipsoid,so the ellipsoidal defect model is chosen for the study in this thesis.In this thesis,the mathematical relationship between ellipsoidal defect geometry and defect leakage magnetic field in the leakage magnetic internal inspection in the case of non-uniform magnetization of external magnetic field is investigated based on the pipeline leakage magnetic internal inspection technology,magnetic dipole theory,etc.The method of solving the leakage field of ellipsoidal defects is analyzed,and after weighing the advantages and disadvantages,the leakage field generated by ellipsoidal defects is equated to the cumulative sum of leakage fields of multiple ellipsoidal defects using differential and integral ideas,and the leakage field is solved by applying curve integral and magnetic dipole method to each ellipse.In this thesis,the derivation process of the three-axis component equation of the leakage magnetic field is given,and the demagnetization factor is introduced to compensate for it in order to make it more consistent with the actual defect model.The resulting analytical equation for the leakage magnetic field of ellipsoidal defects is programmed and modeled using MATLAB software,and the effects of the length,width and depth of ellipsoidal defects on the spatial distribution of the leakage magnetic field are simulated and analyzed.The effect of different geometric defect sizes on the leakage field is analyzed by experimentally creating different ellipsoidal defects in the steel plate and verifying the obtained ellipsoidal analytical formulations.The distribution pattern of the obtained leakage magnetic signal in these two ways is in good agreement,which is in accordance with the characteristic variation trend of the influence of the geometric size of the defects on the leakage magnetic field.The results show that the spatial distribution of the leakage magnetic field obtained from the ellipsoidal defect model of the pipe based on the magnetic dipole method is in good agreement with the leakage magnetic field distribution pattern obtained from the experimental analysis,which verifies the accuracy of the obtained analytical formula.It provides a method for calculating the leakage field generated by surface defects.The analytical solution of the ellipsoidal defect model shows that the distribution of the leakage magnetic field of ellipsoidal defects is linearly related to the length,width and depth of ellipsoidal defects,as follows: the leakage magnetic field intensity of axial and radial components is negatively correlated with the length and width,and positively correlated with the depth;the peak-to-peak spacing of radial components is basically independent of the length and depth,and positively correlated with the width.