Research On Forward Theory And Applied Technology Of Non-Contact Magnetic Memory Testing For Pipelines

Non-contact Magnetic Memory testing of Pipeline is a novel nondestructive testing(NDT)method which is based on the principle of metal magnetic memory.The method can identify buried pipelines' defects without excavation by measuring the signal of self-magnetic leakage field(SMLF)generated by ferromagnetic pipelines.It is an outside testing method which does not require magnetization and is not limited by pipe size and trend.Nowadays,the identification of defects based on this method is conducted through qualitative analysis and lacks forward theory between defects and SMLF.Besides,many problems need to be solved since research on this technology is still in early stage.Therefore,it is necessary to perform research on forward theory to improve its accuracy,reliability and adaptation,and then to prevent pipeline accidents.Based on the J-A model,the stress magnetization model of the pipeline in the geomagnetic field was established.Then,SMLF models of the pipeline without defects and the pipeline with defects were established based on the magnetic dipole theory.And the change law of the SMLF was obtained through the solution and analysis of the model.Finally,combined with the field application,the testing equipment and the defect identification method were improved on the basis of the forward theoretical analysis.The research contents and main achievements are as follows:(1)Based on the theory of pipeline mechanics,magneto-mechanical effect and electromagnetism theory,the mechanism of Non-contact Magnetic Memory testing technology was studied,and the causes,influencing factors of SMLF were determined.Then the stress magnetization model of the pipeline in the geomagnetic field was established by means of the J-A model based on the theory of magnetic mechanical effect.(2)The forward models of non-defect pipelines was established based on stress magnetization model and magnetic dipole theory using infinitesimal and integration method.The change rules of SMLF generated by non-defect pipelines were obtained by solving the model and analyzing the model parameters.(3)The existence of defects leads to stress,metal volume and shape change of pipelines.Based on discrete grid method,the forward theory of pipeline with defects can be obtained.Firstly,the defect and affected area of pipelines is separated into tiny elements by grid,and then the SMLF of each grid element is solved respectively.Finally,the SMLF of the defective pipeline is obtained by vector superposition.The change rules and characteristics of SMLF generated by defect pipelines were obtained by solving the model and analyzing the model parameters.(4)The actual model and method proposed in this paper were verified by the actual test and analysis.The results show that the method can obtain the main characteristics of the SMLF of the pipeline.The maximum error of the gradient modulus of the SMLF is 9.6%,which can meet the requirements of field applications.Based on the forward modeling method proposed in this paper,the existing detection methods were analyzed,and the shortcomings of the current engineering application were pointed out.The detection equipment and the detection method were improved,and the defect identification method was improved.This research lays the foundation of quantitative and trenchless identification in the future.