Research On Several Issues Of Low-frequency Electromagnetic Extern Detection Technique In Oil And Gas Pipelines

With the continuous advancement of modern industrialization and technology,the energy such as oil and natural gas plays an extremely important role,and the pipelines are the main medium for the transportation and storage of chemical raw materials such as oil and natural gas.They are always affected by the acid-base environment consists of the soil,air,and water during the working process.Therefore,the phenomena with corrosion,deformation,stress concentration often occurs for the pipelines.The health of oil and gas pipelines is not optimistic,and the potential dangers are also increasing sharply.Ensuring the safe and stable operation of oil and gas pipelines is of great significance to the development of the national defense industry and the national economy.For the internal pipelines of oil and gas field stations,there is also a risk of corrosion.Because the diameters of the pipelines in this area are different and the outer surface contains coatings or anti-corrosion paints,the traditional nondestructive methods such as the eddy current,ultrasound,penetration,radiation,and magnetic flux leakage are no longer applicable.The low-frequency electromagnetic detection technique(LFET)is regarded as a new type of non-destructive testing method that has developed rapidly in recent years,whcih mainly uses unsaturated AC magnetization with variable magnetization intensity to complete the detection of the internal and external corrosion defects of the tested material and has the advantages of small residual magnetism,light equipment,and convenient use.LFET can appropriately adjust the parameters of excitation amplitude and driving frequency according to the properties(material quality,thickness,etc)of the inspected material,effectively increasing the diversity of excitation methods.The following three problems will be mainly faced when carrying out the comprehensive inspection of internal pipelines in oil and gas field stations based on low-frequency electromagnetic inspection technology.Firstly,due to the diversity of excitation methods and the characteristics of unsaturated magnetization,it is impossible to accurately predict and evaluate the corrosion degree of defect on oil and gas pipelines.Secondly,it is impossible to distinguish whether the currently detected defects are located on the upper surface or the lower surface of the oil and gas pipeline,which brings inconvenience to the later maintenance.Thirdly,due to the increase of the lift-off value,the low-frequency magnetic flux leakage field formed by defects(wall damage,welds,large corrosion points)on the outer surface of the oil and gas pipeline with coating is extremely weak and difficult to detect.In summary,this thesis has carried out a lot of theoretical calculations,simulation analysis,and experimental verification mainly based on the related problems of low-frequency electromagnetic technology for the detection of internal pipelines in oil and gas field stations.From the perspective of Maxwell's equations in this thesis,the vector diffusion equations of electric potential and magnetic potential in the low-frequency space electromagnetic field have been derived and established,and the magnetic refraction effect of the external excitation magnetic field at the interface between the defect of the tested material and the air has also been elaborated,which providing a theoretical basis for the analysis of the low-frequency magnetic flux leakage field distribution at defect.At the same time,the detection mechanism of low-frequency electromagnetic technique is demonstrated,and the mutual inductance mathematical model of the double air-core coil based on the combination of the excitation and the inductive magnetic sensor detection coil is introduced to theoretically derive and calculate the influence factors of the output voltage and phase of the detection coil,which will provide a strong theoretical support for the subsequent experimental platform construction,the experimental parameter setting and the analysis and processing of testing data.In this thesis,the mathematical model of the detection coil is constructed and the specific size relationship is given based on the principle that the output signal of the detection coil of the inductive magnetic sensor should have a linear relationship with the magnetic field intensity at its geometric center point.The low-frequency electromagnetic detection experimental system was designed and developed combined with the low-frequency electromagnetic detection mechanism.And the composition structure and working mechanism of the detection system were explained in detail.At the same time,the multi-channel excitation probe array,the excitation circuits,the signal conditioning and sampling circuits,the storage and reading integration circuits were completed and realized the systematic circuit modules design based on the STM32 controller.The consistency between the system detection results and the theoretical research is verified through experiments when embedded with a four-parameter sine fitting algorithm,which reflects the high stability and high reliability of the developed low-frequency electromagnetic detection experimental system and provides an excellent experimental basis for the further development of low-frequency non-destructive testing technique.The low-frequency magnetic flux leakage field distribution models for different types of defects(V-shaped,trapezoidal,semicircular and rectangular groove crack defects)have been derived and established based on the magnetic dipole theory in this thesis.The composite array probe composed of a high-sensitivity magnetoresistive sensor and an inductive magnetic sensor detection coils are designed as a medium to effectively pick up the low-frequency leakage magnetic field,which can collect more defect features.The experimental results show that the established magnetic dipole numerical model can be used to analyze the change trend of the upper and lower envelope difference curves of the tangential and normal components outputed by the magnetoresistive sensor.In addition,the single-peak change value?H(x)of the tangential component envelope difference curve and the double-peak change value?H(y)of the normal component envelope difference curve obtained under different excitation conditions(excitation amplitude,driving frequency,lift-off)are all linearly positively correlated with the cross-sectional area along the detection direction,which provides a theoretical basis for the later evaluation of the corrosion degree about the surface defects of the tested materials.From the perspective of distinguishing the location(upper surface or lower surface)of the detected defects in oil and gas pipelines,this thesis proposes to embed the spontaneous and self-receiving high-frequency eddy current coil array in the inductive magnetic sensor detection coil array probe,and complete the defect location judgement on the basis of the skin effect.The relevant influencing factors of the eddy current coil(the thickness h,the inner-outer diameter difference(r₁-r₂)and the number of turns N,etc)are obtained through theoretical calculations,and the mutual influence mechanism between the eddy current detection coil and the inductive magnetic sensor detection coil is discussed.And the really existed interference for the eddy current coil from the low-frequency excitation field has been verified through raleted experiment.On this basis,the concept of voltage-frequency product is put forward.In the experimental environment of this thesis,the limit excitation condition for single-channel eddy current coils to identify 10%wall thickness defects on the surface of the tested material is given(the voltage-frequency product is approperimately 760VHz).The experimental results show that the eddy current detection coil basically does not affect the detection result of the inductive magnetic sensor detection coil,and allows the low-frequency excitation coil to be adjusted in a larger range according to the properties of the tested material.The multi-layer conductors(metal protective layer,corrosion defects,etc.)mathematical model of low-frequency pulse eddy current detection technology for oil and gas pipelines containing coatings has been builded,and the vector magnetic potential distribution of the excitation coil in the probe area has also been deduced theoretically in this thesis,which pointing out the relevant influencing factors.In addition,a pulsed eddy current detection coil based on the litz wire is proposed and the mathematical analysis model of the winding coil upon the litz wire is implemented and the interdependence between its eddy current loss and the parameters including the excitation frequency,the coil turns,the number of strands and the wire diameter are given accordingly.Meanwhile,the coil winded by copper wire owning the characteristics of the same cross-sectional area,the same number of turns and the same material was utilized as a comparative object.The experimental result demonstrates that compared with the copper wire,the quality factor Q value of the litz wire inspection coil is significantly improved,the signal-to-noise ratio is optimized,and the eddy current loss is reduced in a larger frequency range.