Research On Pipeline Stress Internal Detection Technology Based On Weak Magnetic Method

With the rapid social and economic development,the demand for natural gas and petroleum products is also growing rapidly,which promotes the construction and development of long-distance oil and gas pipelines,but at the same time it also brings more safety hazards.Oil and gas pipelines have the characteristics of high risk of transportation media and difficulty in monitoring their operating conditions.Leakage or explosion accidents occur from time to time,posing a greater threat to the pipeline and surrounding areas.The occurrence of pipeline accidents is inseparable from the stress damage on the surface of the pipeline.When the long-distance oil and gas pipeline structure is unreasonably arranged or the pipeline material has defects,it will often cause stress damage areas on the surface under the action of external load,high temperature and high pressure,resulting in stress concentration.Some of these stress-damaged areas have reached the critical yield point or have undergone plastic deformation,and may even cause oil and gas pipeline bursts and other disasters due to the interaction of stress and crack propagation.Therefore,it is of great significance for the safe operation of pipelines to carry out on-line internal stress detection on a regular basis.However,due to the complexity and diversity of the pipeline stress damage area in terms of surface structure and stress distribution,it is difficult to accurately detect and evaluate the surface stress of the traditional pipeline stress detection technology.The strong and weak magnetic stress detection technology relies on the two magnetization systems of strong magnetic and weak magnetic to magnetize the pipeline in two sets of strong and weak,and then obtain more information about the magnetization of the pipeline’s stress-damaged part,so as to realize the effective internal stress detection of the pipeline-damaged part.However,as an emerging non-destructive testing technology,strong and weak magnetic stress testing technology is still in its infancy in terms of testing mechanism and testing model establishment.This also affects the accurate quantitative assessment of pipeline stress damage by this technology.For this reason,this paper,from the perspective of the micro-magnetic properties of ferromagnetic materials,calculates and analyzes the magnetic properties of the pipeline under an external field,so as to clarify the mechanism and characteristics of strong and weak magnetic stress detection.Based on the magneto-electric coupling theory of ferromagnetic materials,the calculation of magnetism is introduced into Maxwell’s equation,the quantization equation of the detection signal is established,and the signal detection characteristics are analyzed and calculated.A finite element simulation model of pipeline stress damage is established,and the internal detection characteristics of the pipeline’s strong and weak magnetic stress are simulated and calculated,and the magnetic potential matrix is used to extract the stress signal to realize the effective detection of the surface stress of the pipeline crack.The thesis first studied the excitation and magnetomechanical properties of ferromagnetic materials under external magnetic fields and stress lines.It is clear that the micro-magnetic properties of ferromagnetic materials are the main reason for the formation of strong and weak magnetic stress detection technology.Furthermore,the excitation and magnetomechanical properties of ferromagnetic materials are further calculated and analyzed from the two microscopic perspectives of magnetic domain and quantum.In terms of magnetic domains,based on the free energy in ferromagnetic crystals,the changes of magnetic domain characteristics such as magnetic domain wall shifting,magnetic domain moment rotation and pinning effect are calculated with the changes of external magnetic field and stress.In terms of quantum,based on the solid energy band theory,the inevitable connection between electronic motion and solid stress,external magnetic field and material magnetic properties is calculated and analyzed.Based on the theory of the microscopic magnetic properties of ferromagnetic materials based on the JA model,the paper calculates the changing laws of reversible magnetization,irreversible magnetization and ideal magnetization of ferromagnetic materials under different stresses and external fields.Then the magnetization equation of ferromagnetic material under external magnetic field is established,and the magnetization curve and hysteresis loop of the pipeline under different stress are calculated by using this equation,and then the detection characteristics of strong and weak magnetic stress are studied.Studies have shown that: the detection signal in the strong excitation stage contains good information about the defect in the damaged area;the detection signal in the weak excitation stage contains good information about the stress in the damaged area,and the stress signal first increases with the increase of the external magnetic field and then becomes smaller.There is the best Excitation magnetic field.The thesis further introduces the obtained magneto-mechanical calculations into Maxwell’s equations,and establishes the quantization equation of the detection signal,and uses this equation to calculate the magnetomechanical characteristics and signal characteristics of the detection signal on the three axis of the pipeline.Studies have shown that as the stress increases,the three-axis detection signal changes linearly,and the Villari reversal point of the magnetodynamic curve occurs at the same time on the three axes.After the inversion,the pipe magnetism sensitivity decreases but the linear characteristics become more obvious.The detection signal has a peak value in the axial direction,and has a peak-to-peak value and a zero-crossing point in the circumferential and radial directions.The peak point and the zero point can be used to locate the location of the stress damage zone,and the circumferential detection signal strength is weak.The paper uses the current distribution characteristics of the electrostatic field to introduce the magnetic scalar potential,and then introduces the demagnetization field as a secondary field source into the detection signal quantification calculation,and realizes the calculation and analysis of the demagnetization effect in the detection,so as to reduce the signal interference and accurately detect the signal equation.The research results show that the demagnetizing field transfer direction in the transmission medium is the same as the detection signal direction,adding demagnetization effect calculation can reduce the calculation error rate of the mathematical model by 27%;the demagnetizing field transfer direction in the pipe wall material is opposite to the detection signal direction,resulting in detection The signal attenuation speeds up.The paper uses finite element simulation software to establish a simulation model of the pipeline stress damage zone.Using this model,the detection signal transfer characteristics and excitation characteristics are calculated.In the transmission medium,through the comprehensive analysis of the attenuation amplitude,linear phase relationship,and signal strength of the detection signal in the transmission process,the transmission law of the detection signal in the transmission medium is clarified,and the best interval for the detection signal collection is found.In the pipeline wall material,the detection signal transmission equation is established to analyze the signal transmission characteristics of the outer wall damage,and the stability and effectiveness of the normal detection signal to detect the stress damage of the outer wall of the pipeline are clarified.Fit the excitation characteristic curve to clarify the excitation characteristic of the pipeline by the external magnetic field.The thesis calculated the magnetic characteristics and excitation characteristics of the pipeline crack based on the magnetic charge theory.Under strong and weak magnetic detection,the magnetic potential matrix in the finite element matrix equation is used to extract the stress signal to complete the effective detection of the stress at the fatigue crack of the pipeline.A finite element simulation model of pipeline magnetism under an external magnetic field was established,and the excitation characteristics,magnetism characteristics and magnetic potential matrix stress extraction characteristics of pipeline cracks were further calculated,which confirmed the correctness of the mathematical model calculation.Through the pipeline interception test piece tensile and excitation characteristics experimental platform,relevant experiments are carried out to verify the correctness of the theoretical analysis.