Research On Key Issues Of Pipeline Crack Detection Based On Balanced-Field Electromagnetic Technique

Pipelines have become the most common form of oil and gas storage and transportation with their advantages of long transport distances,large transport scale,and low maintenance costs.However,the special service environment of pipelines will make it inevitable that defects will appear with increasing service time due to external loads,media corrosion,aging of the main body,and other factors,which will lead to pipeline failure making it a risk of leakage,explosion,and other accidents.Among the many failure factors,cracks play a decisive role in the fracture of a pipeline.Therefore,the effective detection of cracks in pipelines is of great importance to ensure the safe and smooth operation of pipelines and to extend their service life.However,on the one hand,cracks have the characteristics of extremely small openings,different directions,and random locations in the pipe wall,and on the other hand,due to the limitations of the engineering environment of in-line pipeline inspection,the applied in-pipe inspection technology has certain limitations in terms of detection direction and scope of application.At present,there is no effective in-line inspection method for pipeline cracks.The research team’s previous research into the balanced-field electromagnetic technique has demonstrated its applicability and great potential for the detection of cracks in pipelines.Research has been carried out on the theoretical basis,sensor design,and full-angle detection,and has resulted in a surface crack detection method based on the balanced-field electromagnetic technique.However,in the actual pipeline in-line detection,there are still some problems,such as the difficulty in-line detection of buried cracks in any direction,the tendency of electromagnetic noise in engineering environment space to drown the crack signal,and the influence of sensor jitter on crack judgment,and there is a lack of inline detection system which can effectively detect the surface and buried cracks of pipelines in any direction and meet the engineering service conditions.Therefore,based on previous research,this topic focuses on solving the above key problems,proposing an in-line detection method for buried cracks in pipelines,developing a high signal-to-noise ratio balanced-field electromagnetic detection system,and proposing a jitter interference noise differentiation method and elimination method.Comprehensively improve the detection capability and application scope of the balanced-field electromagnetic technique,and carry out a lot of research work according to the technical route of theoretical analysis,numerical simulation,experimental verification,and engineering application.The study starts from the fundamental equations of time-varying electromagnetic field properties and analyses the intrinsic relations of the medium as well as the boundary conditions at the partition interface.The process of magnetization of the medium and the formation of induced voltages are investigated based on the basic laws of electromagnetic fields.The electromagnetic field penetration equation is used to analyze the influence of the skinning effect on electromagnetic field propagation.At the same time,the sensor structure of balanced-field electromagnetic technique is used to discuss the detection principle of balanced electromagnetic technology in terms of the formation process of the balanced electromagnetic field and the generation of the detection signal.The arbitrary direction of detection is also illustrated in terms of the spatio-temporal relationship between the magnetic flux and the induced current generated by the balanced-field electromagnetic technique,and a model of the detection voltage in the form of a simple harmonic signal is established.In addition,an equivalent model of the detection voltage of the balanced-field electromagnetic technique is established based on the mutual inductance effect,and the self-zero characteristics of the sensor structure are elucidated based on the Norman formula.On the other hand,a numerical calculation model of the crack detection of the balanced-field electromagnetic technique is established to support the solution of the finite element analysis solution problem in this study.The study investigates the buried crack detection method with balanced-field electromagnetic technique,analyses the dynamic change of permeability of the pipeline under the hysteresis effect in the Rayleigh region,solves the relationship between magnetic field strength and magnetic induction strength using the Jiles-Atherton model,theoretically calculates the hysteresis return of the pipeline,and further finds the change of permeability with the applied magnetic field strength in the Rayleigh region.On the other hand,the solution of the time-harmonic field penetration equation is derived,and the relationship between the field volume and the surface field volume of the pipeline when the electromagnetic field propagation depth is greater than the skin depth is clarified,which provides a theoretical basis for the establishment of the buried crack detection method.In addition,the relationship between excitation frequency and buried crack detection is analyzed and the excitation detection is analyzed and the excitation frequency is determined.A finite element model of buried cracks in equilibrium electromagnetic technology is constructed by combining the solved hysteresis lines in the Rayleigh zone of carbon steel pipelines,and the normalized results of different excitation currents are analyzed to determine the excitation currents that can be used for buried crack detection.To meet the application situation of a large number of sensors and the complex application environment of in-pipeline inspection systems,the thesis develops a detection system with a high signal-to-noise ratio against inter-channel interference.Considering the problem that the detection signal is weaker due to the use of weak field excitation,the frequency selection characteristics of parallel resonance are analyzed,and a parallel resonant circuit with the same resonant frequency as the excitation frequency is designed with optional capacitors.In the meantime the eight-channel acquisition and transmission system is designed according to the requirements of the project,and the frequency difference interference between multiple channels is analyzed.Unified excitation source circuit is developed to achieve the same frequency excitation to eliminate the interference caused by the difference of crystal oscillation.The thesis proposes a distinction method based on the amplitude-phase two-dimensional figure and a signal processing method based on rotational demodulation,respectively,for the distinction of jitter signals from crack detection signals and for the elimination of jitter signals.Based on the electromagnetic field distribution of the balanced-field electromagnetic technique and the established mutual inductance model,the principle of sensor jitter signal generation is analyzed,and the signal-free characteristics of vertical jitter and the co-frequency of tilt jitter and crack are proved by combining the self-zero property;the amplitude and phase of the crack detection signal and the jitter signal are calculated numerically using finite elements,and the similarity of the waveform and the phase difference between the two are illustrated.The amplitude-phase two-dimensional figure is constructed,and the difference in distribution between the two is used as a distinguishing mark;at the same time,the signal demodulated to balance the electromagnetic technique detection signal orthogonal to the phase of the jitter signal is selected according to the difference in phase,and the interference signal generated by the sensor tilt jitter is suppressed from the output source,and the effectiveness of the two proposed jitter processing methods is verified experimentally.In order to evaluate the application of the research content of this topic in practice,the thesis is based on the proposed excitation field parameters of the test sensor,the developed detection system is extended on the main body of the pipeline inspection gauge,forming a balanced-field electromagnetic technique in-line inspection system,through dynamic pulling experiments in the detector form to verify the resistance to inter-channel interference and the suppression of spatial electromagnetic noise and jitter interference effect,crack length and The length of cracks and the depth of cracks on the inner and outer walls are quantified by fitting,and the detection capability and engineering applicability of balanced-field electromagnetic technique for surface and buried cracks are determined by combining the actual pipeline inspection data.