The guided wave method is one of the non-destructive testing methods with the ability to inspect long length of pipeline. The presence of defects and pipe features can be indicated by analyzing the reflected echoes and the mode conversion phenomena of guided waves. To provide the profile of defects, the signals of guided wave are usually processed to form a B-scan or C-scan image. As for the characteristics of multimodes and dispersion, the C-scan image of defect shows a poor resolution both in the axial and cricumferential directions. Therefore, this study uses the synthetic aperture focusing technique (SAFT) to improve the resolution of the C-scan image. The propagation and scattering of the fundamental torsional mode T(0,1) in pipes are analyzed by the transient analysis of finite element method. Furthermore, the reflected signals are processed by SAFT to form a C-scan image with better resolution. Three types of defects including circumferential crack, axial crack and pitting were discussed in this study both by finite element method and experimental method. In the numerical study, the modification of the C-scan image of the circumferential crack showed a better consequent than the axial crack and pitting. The SAFT was also used to separate the images of two circumferential cracks with different axial location on the pipe succesfully. In the experimental study, the signals of the axisymmetric weld and non-axisymmetric defects were processed by SAFT to form the C-scan image. The results showed a nice resolution of circumferential cracks and the circumferential distribution of the weld in the modified C-scan image at the same time. The application of SAFT to guided wave on a pipe is valid to overcome the effects of guide wave multimode and dispersion characteristics, it provides a new way to defects identification.

Abstract In a petrochemical plant, to exert economic efficiency and spacing convenience for transporting fluid or gas, the pipelines used in the plant are often buried along the road. The buried pipelines are usually wrapped in the soil that only the guided wave method is a convenient technique to perform the nondestructive testing for the pipelines. However, the viscosity of soil causes the attenuation of the guided wave during the test, the accuracy and the detection distance will then be affected. Thus, the objectives of this thesis are to study the characteristics, such as the detection distance and the refraction signal, of the T(0,1) guided wave when propagating along pipelines wrapped in the soil at different depths. The thesis would be divided into two parts: experiment and numerical simulation. Four different depths, 0.5, 1.0, 1.5 and 2.0 m, are used in the experiment to evaluate the characteristics of reflected signals and its attenuation. Wavelet transform, which would enhance the capability of distinguishing guided wave defect, is used to improve the attenuation of defected refraction signal caused by soil. In the numerical simulation, this research applies the transient simulation by finite element method to analyze the wave propagation behavior of T(0,1) mode guided wave of buried pipeline, which is incorporated with Two-dimensional Fourier transform for modal identification. The result of experiment shows that the attenuation of the guided wave is caused by the leakage and the viscosity of the soil. The decay rate is proportional to the depth and due to the viscosity of the soil is proportional to the excitation frequency. This phenomenon is more obvious when the pipeline is buried deeper. The reflected signal amplitude of each characteristic would decrease along with the increasing soil depth, but the overall trends did not changed. The result of wavelet transform shows that the capability of distinguishing of the guided wave detection defect of buried pipeline, which attenuation of refraction signal caused by soil would be improved. The result of the numerical simulation indicates that the T(0,1) mode would not cause mode conversion and dispersion due to its propagation through the buried pipeline with different depths of soil. The soil caused leakage of the T(0,1) mode in the form of shear waves. The attenuation rate of guided wave and its detection distance in the study could be the reference of site selection for detection and defect refraction signal determination, which could effectively raise the efficiency of on-site detection.