Research On Pipeline Stress Detection Mechanism Based On Dual Magnetic Field Method

After the long-distance oil and gas pipeline has defects due to corrosion and other reasons,under the action of internal pressure,a stress concentration area will be formed at the defect,and finally promote the crack to expand in depth until the pipeline breaks.This kind of comprehensive defect with coexistence of "macro defect" and "stress" is one of the main factors causing accidents such as sudden pipeline leakage and explosion of in-service pipelines.Therefore,it is very important to evaluate the stress at the defect of pipeline.Magnetic flux leakage detection and magnetic flux weakening detection are two widely used detection methods in pipeline,magnetic flux leakage testing is widely used in in-service pipeline testing because of its strong stability.As a new detection method,weak magnetic field detection can detect cracks,stress concentration areas and other signals.However,in magnetic flux leakage testing,stress has little effect on magnetic signal,so it is difficult to effectively evaluate the stress at the defect;In weak magnetic detection,the magnetic signal is the composite signal of stress and defect,so it is difficult to obtain the influence of stress on the defect magnetic signal.Therefore,it is difficult to effectively evaluate the stress at the defect of the pipeline by magnetizing the pipeline wall with a single external magnetic field.In this paper,the dual magnetic method is used to detect the stress at the defect of the pipeline.Dual magnetic pipeline inner detection method is a new composite detection method in the field of pipeline stress inner detection.It uses the difference between defect and stress magnetic signal under high and low magnetization levels to judge the degree of stress damage at the defect.It has practical significance in the inner stress detection of pipeline defect.However,at present,the mechanism of dual magnetic detection is not perfect.It is of great significance for the development of dual magnetic detection technology to study the mechanism of dual magnetic detection defect stress and establish the model of dual magnetic detection stress.This paper expounds the mechanism of stress detection by dual magnetic field;Based on the J-A theory,the magnetic mechanics relations of dual magnetic detection under elastic stress and plastic strain are established respectively,and the variation laws of the magnetization of the pipe wall under different stress,strain and external magnetic field are analyzed;Combined with the magnetic charge model,the stress magnetic signal model at the defect of the pipeline under different stress and external magnetic field is established,and the influence of stress on the magnetic signal under different external magnetic field is analyzed;The stress extraction model at the defect based on the ratio relationship of dual magnetic signals is established.On the basis of the model,the effects of different defect sizes and different lift off values on the characteristics of dual magnetic signals are calculated.Finally,a systematic experimental verification is carried out.The results show that when the external magnetic field reaches saturation,the discrimination of magnetic signal corresponding to stress is low,and the magnetic signal is mainly reflected in the magnetic flux leakage signal caused by defects;When the external magnetic field intensity is weak,the magnetic signal is significantly affected by stress,and the signal is reflected in the magnetic signal generated by defects and stress.The ratio of dual magnetic signal can reflect the stress at the defect.The stronger the weak magnetic field intensity is,the smaller the variation range of dual magnetic signal with stress is.The defect size and lift off value do not affect the ratio of dual magnetic signals,and the ratio relationship of dual magnetic signals is only affected by stress,which can effectively evaluate the stress at the defect.