Research On Characterization Method Of Internal Defects In Pipeline Based On Laser Ultrasound

With the continuous construction of oil and gas pipelines,the safety of oil and gas pipelines is the top priority.In recent years,with the national attention to the field of safety,the inspection of oil and gas pipelines is not only aimed at the micro-crack defects on the inner and outer surfaces of the pipeline,but also on the inside of the pipeline.More and more attention is paid to the detection of defects,but there are still technical problems such as "inaccurate detection","undetectable" and "abstract" of 3D imaging,which puts forward new requirements for laser ultrasonic nondestructive testing technology.In view of the above problems,the key issues of quantitative characterization of internal defects based on laser ultrasound are studied,focusing on pulse reflection method and laser ultrasonic signal time domain feature analysis,shear wave parallel slope method and equidistant synchronous movement method angle characterization,wavelet analysis and volume defect analysis The research on scatter imaging methods,etc.,the specific research contents are as follows:Aiming at the problem that the specific action process and mechanism of laser ultrasonic wave and internal defect mode transition are not clear,the specific process of laser ultrasonic wave and internal defect mode transition is obtained by theoretical analysis combined with numerical simulation.A theoretical model of laser ultrasonic thermoelastic excitation and propagation in homogeneous isotropic materials is established,and the numerical simulation results are analyzed.The ultrasonic field distribution and ultrasonic signal waveform of laser ultrasonic waves in homogeneous isotropic materials are obtained.The process of mode transition at internal defects provides a theoretical basis for experimental research.Aiming at the problem of "inaccurate detection" of parameters such as position,depth and length of internal defects in pipelines,the method of numerical simulation combined with experimental research is used to propose the detection principle of position,depth and length of internal defects.The pulse reflection method and the body wave signal time domain analysis method are used to characterize the depth.The simulation results and physical experiment results show that the relative error does not exceed 5%.Comparing the pulse reflection method and the transmission method to quantitatively characterize the length of internal defects,and analyze the extracted bulk wave signal in time domain,the results show that the relative error of the pulse reflection method is within4%,and the relative error of the transmission method is within 5%.Considering the selection of pulse reflection method to characterize the length comprehensively,the goal of "accurate detection" is achieved.Aiming at the problem of "undetectable" defect angle inside the pipeline,a laser thermoelastic excited volume wave internal defect angle detection model is established,and the internal defect angle detection principle is proposed.The shear wave parallel slope method and the equidistant synchronous movement method are used to quantitatively characterize the angle of internal defects.The shear wave parallel slope method has high detection efficiency but low detection accuracy.The equidistant synchronous movement method has high detection accuracy but low detection efficiency,which can be selected according to actual needs.suitable method.The experimental results are consistent with the simulation results,which proves the accuracy of constructing the physical model and achieves the goal of "measurement".Aiming at the "abstract" problem of 3D imaging of internal defects in pipelines,a scatter 3D imaging method for defects in pipelines is proposed.In order to improve the accuracy of signal characteristic parameters,wavelet analysis and wavelet packet analysis are used to obtain ultrasonic signals with high signal-to-noise ratio by clarifying variables such as wavelet decomposition function,decomposition layer number and threshold selection through signal-to-noise ratio.In order to accurately describe the internal defects of the pipeline,the internal defects are decomposed into continuous points,and the conversion of time domain signals to three-dimensional coordinates is realized,which solves the technical problems of three-dimensional imaging of internal defects in A-scan,B-scan and C-scan pipelines,and realizes three-dimensional imaging."Intuitive" goals.The research work shows that the quantitative characterization and three-dimensional imaging of defects in the pipeline are realized,which lays a foundation for the engineering application of laser ultrasonic testing technology in the detection of internal defects in pipelines.