Research On Several Issues Of Pipelne Stress Detection Technology Based On Magnetoelastic Effect

Oil,gas and other pipelines are buried under the ground or in the seabed.The pipeline service process suffered from high temperature,high pressure,external loads and other factors,inevitably formed a local stress concentration,metal fatigue and pipe bending and other stress damage phenomenon.Therefore,the stress detection of in-service oil and gas pipelines is of great significance for pipeline integrity assessment,national economic improvement and industrial technology development.Early stress damage only affects the microstructure of oil and gas pipelines(displacement,rotation and dislocation slippage of crystals in the internal magnetic domain walls of the pipeline material)and does not result in metal loss defects in the macroscopic sense of the pipe wall.The traditional ultrasonic testing,eddy current testing and magnetic flux leakage testing methods have a relatively complete theoretical system in the detection of metal loss defects.However,all of the above methods have limitations in the practical application of online detection of early metal stress damage in pipelines.The pipeline stress detection technology based on the magnetoelastic effect mainly uses unsaturated alternating magnetization to detect stress changes in ferromagnetic materials,which has the characteristics of high sensitivity,small remanent magnetization,no coupling agent,fast and efficient,etc.There are good prospects for the application of pipeline stress detection engineering and it has been widely used in the engineering practice of stress detection.However,the mechanism of magnetoelastic effect is an urgent problem to be studied.Most of the existing stress detection theory of ferromagnetic materials is the explanation and summary of experimental phenomena,and there is a lack of complete mechanism research on the magnetization process and magnetic mechanics relationship of ferromagnetic materials.A mature analytical model of the spatial magnetic field distribution on the surface of ferromagnetic materials and the variation of magnetic flux inside the excitation circuit under coupled stress and external magnetic field has not yet been developed.In summary,the thesis focuses on the problems related to pipeline stress detection technology based on the magnetoelastic effect and carries out a lot of mechanistic research,simulation analysis and experimental verification work.In view of the spatial magnetic field in the local stress concentration area of oil and gas pipelines and the variation law of magnetic flux in the excitation circuit under the combined action of stress and external magnetic field,this paper discusses the motion law and magnetic energy distribution of magnetic domains in the stress and external magnetic field environment.Aiming at the shortcomings of complex structure and poor passing ability of pipeline stress detection equipment,a pipeline stress detection technology based on magnetoelastic effect is proposed.The detection mechanism was studied,and the structures of two induction coils of the space magnetic field induction sensor and the magnetic flux induction sensor were analyzed,which provided a strong theoretical basis for the mechanism research,numerical simulation,experimental platform construction and detection data processing of the subsequent pipeline stress detection technology based on magnetoelastic effect.Since the relationship among magneto-strictive strain,stress intensity and material magnetization in the Jiles-Atherton(J-A)model cannot effectively reflect the variation of magnetostriction coefficient of ferromagnetic material with stress intensity and material magnetization in the nonlinear relationship.In this paper,an improved J-A magnetization model is established,which can reflect the influence of elastic stress on the magnetization process of ferromagnetic materials under external magnetic field.The influence of elastic stress on the magnetization curve,hysteresis loop and a series of magnetic parameters is discussed.After verification by numerical simulations and real measurements,it is proved that in lower magnetic fields,the stress intensity will inhibit the magnetization of the ferromagnetic material,so that the magnetization intensity of the material under stress conditions decreases without stress action conditions,and the magnetization curve and hysteresis lines of the ferromagnetic material show a decreasing trend with increasing stress intensity.To address the problem of the internal magnetic charge distribution of ferromagnetic materials under continuous stress and the practical engineering application of the method of detecting the spatial magnetic field to achieve the measurement of the local stress concentration area of ferromagnetic materials.The paper combines the magnetoelastic effect of ferromagnetic materials under hysteresis conditions and the model of body magnetic charge distribution due to continuous stress changes in the local stress concentration zone of the pipe,establishes an improved magneto-mechanical model for ferromagnetic materials under complex stress conditions,and derives an improved magnetic charge model for the local stress concentration zone of the pipe.The effect of stress intensity on the magnetization strength and surface magnetic field of ferromagnetic materials was investigated by numerical simulation and experiment,and the effect of the length and depth of the local stress concentration zone on the distribution pattern of the spatial magnetic field was analyzed in conjunction with theory.Multiple regression analysis is used to quantify the size of the stress concentration zone and the stress intensity.This provides strong theoretical support for subsequent work on the design of sensor structures and magnetic circuit analysis based on the magnetoelastic effect.The thesis explores the choice of magnetization method for pipeline stress detection devices,analyses the effect of stress on the output electric potential at both ends of the space induction coil under AC current source excitation conditions,and realises the force-magnetism-electricity measurement conversion process.Combined with the stress detection principle of magnetoelastic effect and magnetic circuit theory,the stress detection experimental system based on magnetoelastic effect is designed and developed.The systematic design of excitation probe,excitation circuit,signal conditioning sampling circuit and upper computer display and storage software is completed.The tensile test platform of magnetic steel bars and the pipeline pressure test were designed and built.The corresponding relationship between the stress and the variation of the spatial induction signal on the surface of the measured material was analyzed by real-time detecting the influence of stress on the surface magnetic field of the measured material during the loading process.The experimental results are consistent with the variation law of magneto-mechanics calculated by the magneto-mechanics model.Based on improved magnetization and magneto-mechanical models of ferromagnetic materials,the thesis investigates the effect of stress on the magnetic flux inside the excitation loop.On the basis of the improved magneto-mechanical model and magnetic circuit theory,the relationship between stress and the output electric potential at the two ends of the excitation coil under the excitation condition of AC current source is established,and the influence law of stress intensity and lifting off value on the output electric potential at the two ends of the excitation coil is analyzed.The effects of stress intensity,lift-off value and excitation frequency on the magnetic flux and magnetic circuit impedance inside the excitation loop are discussed in detail through numerical simulations and pipeline bending stress simulation experiments.These works provide an idea for the engineering application of stress detection techniques based on the magnetoelastic effect for pipeline stress detection and quantitative analysis.