Research On Ultrasonic Simulation And Inverse Time Migration Imaging Of Special Pressure Equipment And Its Application Based On Cylindrical Coordinates

In recent years,with the rapid development of China's national economy,the demand for special pressure equipment is increasing.Devices that transmit high temperature and high pressure,low temperature and low pressure or toxic and harmful radioactive media in the production process are collectively referred to as pressure bearing special equipment,including pressure vessels,boilers,pressure bearing pipelines,etc.Pressure bearing pipeline is widely used in oil,natural gas,electric power,metallurgy and other fields.Pressure bearing pipeline refers to the tubular equipment with an average outer diameter greater than 25 mm and a wall thickness greater than 20 mm,which uses a certain pressure to transport gas or liquid.Due to the long-term and continuous high temperature,high pressure and corrosive conditions of this thick wall pipeline,the service performance of the equipment is affected by the external environment and other factors,resulting in cracks,sand holes,thinning and other defects,which makes the equipment invalid.Once an accident occurs,it will cause unusually heavy casualties and economic losses.Therefore,the omni-directional and high-precision detection of pressure pipeline has very important economic and social significance.Because of its special properties,such as wide variety,large slenderness ratio and pipe wall thickness,the traditional detection technology is difficult to apply.Ultrasonic nondestructive testing provides the possibility to detect the shape and structure of internal defects of thick wall pipeline without damaging and affecting the performance and internal structure of pressure bearing pipeline.At present,the research on ultrasonic nondestructive testing of pressure pipeline mainly has the following three aspects,which are worthy of further discussion:(1)firstly,for the forward simulation of pressure pipeline,the rectangular coordinate system is usually used,but it is often difficult to deal with the arc boundary of pipeline space with rectangular grid,which is not conducive to obtain the wave field response characteristics of pressure pipeline in line with the reality;(2)Then,the traditional imaging methods for nondestructive testing of pressure pipelines have shortcomings in the boundary imaging and transverse continuity of defects;(3)Finally,at present,the research on inverse time migration imaging technology of pressure pipeline based on ultrasonic is limited to twodimensional conditions,and the three-dimensional structure in the whole pipeline can not be obtained.The research on three-dimensional inverse time migration imaging of cylindrical structure such as pressure pipeline has not been reported.To solve the above problems,this thesis studies the ultrasonic simulation and imaging technology of pressure pipeline,establishes the discrete format of three-dimensional high-order staggered grid finite difference in cylindrical coordinate system,realizes the three-dimensional ultrasonic field simulation of pressure pipeline under the condition of double arc free boundary treatment,and analyzes the three-dimensional wave field response characteristics of common defects in ultrasonic testing,The ultrasonic response mechanism of common defects is revealed.Based on the inverse time migration theory in cylindrical coordinates,the imaging of common defects in ultrasonic testing is realized,the main factors affecting the imaging accuracy of inverse time migration are analyzed,the observation system meeting the requirements of imaging accuracy is obtained,and the inverse time migration imaging technology for ultrasonic nondestructive testing of pressure pipeline is formed.The three-dimensional results of common defects in the pipe are obtained through inverse time migration imaging,It provides theoretical guidance for ultrasonic nondestructive testing method and sensor design.The main research results of this thesis are summarized as follows:(1)Firstly,according to the physical parameters and morphological characteristics of cylindrical structures such as pressure pipeline,the geophysical model of pressure pipeline in cylindrical coordinate system is constructed to avoid the false diffraction wave generated when simulating arc interface in rectangular coordinate system.Then,based on the stress-strain relationship and displacement strain relationship under the condition of three-dimensional wave equation,the discrete scheme of high-order staggered grid finite difference in cylindrical coordinate system is established.Through the trial calculation of three-dimensional uniform medium model,the superiority of SPML(Split Perfectly Matched Layer)boundary condition is proved.The correctness of the algorithm is proved by analytical solution.Finally,for the cylindrical structure with air medium inside and outside the pressure pipe,based on AEA(Acoustic-Elastic boundary Approach)free boundary conditions,a radial(i.e.r-direction)double free boundary treatment method in cylindrical coordinate system and its implementation scheme are formed.(2)Firstly,aiming at the source in actual ultrasonic detection,different source loading modes are studied.Taking homogeneous isotropic medium as an example,the source loading results of elastic wave and acoustic wave simulation are compared and analyzed.Taking the slag inclusion and sand hole defect combination model as an example,the numerical simulation results of single force source and surface source(axial Z and radial R)under stress and velocity terms are discussed in detail,and the wave field characteristics are compared and analyzed.The results show that the stress term is better than the velocity term when radial force and axial force are used.In order to comply with the reality and facilitate the subsequent signal extraction,the best scheme is to load the radial force surface in the stress term.Then,according to the structures of different common defects in ultrasonic testing,the typical defect models such as sand hole,slag inclusion and delamination are studied,and the ultrasonic numerical forward simulation of pressure pipeline is realized in cylindrical coordinate system.The analysis of wave field characteristics of different defect models provides a theoretical basis for processing and interpretation of ultrasonic testing data(3)By using the fast marching algorithm to calculate the three-dimensional travel time of the uniform medium model,it is analyzed that the second-order difference scheme and the selection of 0.2mm grid can meet the accuracy requirements and calculation speed requirements of inverse time migration imaging in this thesis.The stability and effectiveness of the fast advancing algorithm are verified by using vertical fractures,horizontal fractures and local abnormal bodies.(4)Through the layered slag inclusion model experiment,the main factors affecting the imaging accuracy of inverse time migration are analyzed and discussed.The results show that the migration imaging effect of cross-correlation is better than that of excitation time imaging,the imaging effect of signal processing of wave field recording is better,and the imaging effect of radial force loading is better.(5)The causes of false images in the process of inverse time migration imaging are analyzed.The combination of source illumination compensation and Laplace denoising method is proposed to suppress the noise in the imaging results,and the denoising effect of the three-dimensional layered slag inclusion anomaly model in cylindrical coordinate system is analyzed and verified.(6)Based on the layered defect model,the effects of ultrasonic prestack inverse time migration imaging under different observation systems,different excitation point positions and multi-point excitation superposition are discussed in detail.This thesis believes that at least 10 survey lines and 5 shot superposition are needed to obtain reliable imaging results.The best observation system is used to carry out ultrasonic inverse time migration imaging of slag inclusion,sand hole and layered slag inclusion model in cylindrical coordinate system.The imaging results show that the method used in this thesis can accurately image the sand hole with diameter of 2.4mm,locate the surrounding interface for the slag inclusion defect with vertical thickness of 1.0mm,and image the whole defect for layered slag inclusion.The three-dimensional imaging is carried out with noisy data to verify the anti noise of the method.Finally,an example is given to prove the correctness and practicability of the method.The main innovations of this paper are as follows:(1)according to the structural characteristics of cylindrical pressure pipeline,based on the theoretical method of ultrasonic numerical simulation in cylindrical coordinate system,the three-dimensional cylindrical coordinate wave field simulation with double free boundaries is realized,the accurate wave field records are obtained,and the three-dimensional wave field characteristics of common defects(slag inclusion,sand hole and delamination)in ultrasonic nondestructive testing are analyzed.(2)Based on the inverse time migration theory in cylindrical coordinates,the ultrasonic inverse time migration imaging of pressure pipeline is realized,the main factors affecting the imaging accuracy are analyzed,and an observation system meeting the requirements of imaging accuracy is established.The migration imaging results of experimental data verify the feasibility of the method.