Research On Magnetic Field Distribution Characteristics Of Weak Magnetic Source

As the main energy of today’s society,the demand of oil and natural gas is increasing day by day,and the transportation capacity of energy is put forward higher requirements.Oil and gas pipelines are laid underground,under the sea or exposed to the air.In addition to the natural wear and tear of materials,pipelines are subject to corrosion from the environment in which they are erected and the materials they carry,resulting in loss of or damage to the metal of the pipe body,which affects energy transmission and reduces its service life.Regular safety monitoring of oil and gas pipelines is therefore essential to ensure the proper delivery of energy,to meet domestic and production supplies and to protect the natural environment from serious pollution caused by oil and gas leaks.A variety of pipeline safety testing methods have been developed to detect different safety issues during the operation of pipelines.One of is magnetic tomography method,an emerging non-destructive inspection technique outside pipelines based on the principle of metal magnetic memory.At present,the research and application of this technology is still in its infancy,and the discrimination of defects from inspection data is based on qualitative analysis,and the characteristics and localisation of pipeline defects cannot be quantitatively identified from the pipeline magnetic memory signal.Therefore,it is necessary to carry out theoretical research related to this detection method in order to improve the reliability,applicability and accuracy of this technology in pipeline safety inspection applications.With this aim in mind,this paper presents a study of the problem.Magnetic tomography method is a method to determine the type and location of defects in a pipe based on the magnetic anomaly characteristics of the spatial distribution of the detected magnetic field signal.A model of the magnetic field characteristics in the stress concentration zone of the inner wall of a pipe is established based on the theory of magnetic anisotropy of the material,starting from the microscopic magnetization theory of ferromagnetic materials and the macroscopic approach to magnetic field distribution.In-depth study of the boundary conditions of the space where the pipe is located,the boundary area of the magnetic field signal transmission process is delineated,a theoretical model of the distribution of the magnetic memory signal in the stress concentration zone of the inner wall of the pipe outside the pipe segment is established,and the model is solved numerically to analyse the spatial magnetic field characteristics.The distribution of magnetic anomalies at different locations on the pipe wall and outside the pipe were studied by modeling the stress concentration zones on the inner wall of the pipe through finite element analysis.An experimental platform was built to carry out field measurements on commonly used fittings to verify the validity of the theoretical findings.The method of using spatial magnetic field magnetic flux intensity correlation and energy correlation is proposed as a method of screening the valid signals from the measured weak magnetic signals.Statistical analysis of the distribution of the spatial magnetic field yields the spatial distribution law of the magnetic memory signal outside the pipe in the stress concentration zone of the inner wall of the pipe.The mechanism of magnetic tomography method is analysed in detail based on an in-depth study of the magnetic principle of ferromagnetic materials.The mechanism of magnetic anisotropy in metallic media is studied based on the magnetic anisotropy theory,the magnetic dipole equivalence model and the Biot-Savart law,a model of the equivalent field in the stress concentration zone of the pipeline is established.The model is solved by integration to determine the magnetic field distribution characteristics and distribution law.The stress concentration zone equivalent field model is used as the source of the weak magnetic excitation field in the spatial distribution model of the magnetic memory signal of the pipeline to form the stress concentration zone magnetic field in the inner wall of the pipeline.The spatial distribution field of the magnetic memory signal in the stress concentration zone of the pipe is analysed in detail based on the uniqueness of the solution of the magnetic field problem and the theory of continuity of the magnetic field distribution,and the field is divided according to the different media environments inside and outside the pipe through which the magnetic lines of force penetrate the pipe wall.The boundary conditions at the media junction of the magnetic field signal in the stress concentration zone of the inner wall of the pipe are determined in the propagation process.The magnetic field distribution model of the stress concentration zone of the inner wall of the pipe in the outer space of the pipe with boundary conditions is established based on the magnetic anisotropic magnetic vector potential and the Biot-Savard law.The distribution characteristics of the spatial magnetic memory signal in the stress concentration zone are obtained by numerical solution and analysis of the model.The spatial distribution of the magnetic field in the stress concentration zone at different locations on the inner wall of the pipeline is studied,and the spatial magnetic field distribution characteristics of the magnetic memory signal in the stress concentration zone at different locations are analysed by solving the model.It is found that when the stress concentration zone is located at the top of the inner wall of the pipeline,the magnetic field of the magnetic memory signal outside the pipeline is the strongest and can well reflect the anomalous characteristics of the magnetic field signal in the stress concentration zone,and the location of the peak normal magnetic induction intensity and the tangential over-zero point corresponds best to the magnetic field centre of the stress concentration zone.This explains that the magnetic tomography method is effective in detecting the stress concentration zone.The results show that the method is able to obtain the main characteristics of the magnetic memory signal in the stress concentration zone of the pipeline,with the normal component of the spatial magnetic flux showing a peak value in the stress concentration zone and the tangential component peaking past the zero point.The spatial magnetic memory signal detected at different lift-off values outside the pipeline is consistent with the magnetic field law and distribution trend in the stress concentration zone,which can well reproduce the magnetic anomaly characteristics of the stress concentration zone.With the increase of the lift-off value,the magnetic flux intensity of the spatial magnetic field signal decays in a negative exponential law.By analysing the peak point and cross-zero point of the magnetic distortion signal for the different lift-off values,it is concluded that the difference in position between the centre of the stress concentration zone measured in practice and the origin of the current carrying loop set up in the experiment can meet the requirements of the field application.The spatial variation law of the magnetic field in the stress concentration zone of the pipeline is analysed from the perspective of magnetic flux intensity and magnetic energy.The correlation between the spatial magnetic signal and the magnetic field in the stress concentration zone of the pipeline can be analysed to extract the target signal from the measurement data in order to improve the detection efficiency and accuracy.The time domain characteristics such as peak-topeak values and gradients of the spatial magnetic field signals are analysed to provide evaluation indicators for identifying stress concentration zone and defect in pipelines from the inspection data.