Detection And Characterization Of Defects In Pipes Using Ultrasonic Guided Waves

Pipelines play an important role in the energy, chemical industry, city construction and other fields. Corrosion, deformation and other damage is a major security risk during pipeline operation. Therefore, regular inspection of the pipelines is a significant measure to guarantee the structural safety. Ultrasonic guided wave technology has been successfully applied to industrial pipeline inspection applications. This technology has the privileges such as long-distance propagation, time efficiency, and cost effectiveness. However, defect characterization in guided wave-based pipeline inspection remains extremely challenging because of the complexities of the interaction between guided waves and defects. The resolution of this problem is crucial for understanding pipeline health status, evaluation the pipeline residual life and performing pipeline maintenance and repair.The work in this thesis aims to further develop the key technologies which influence the defect characterization realization, and explore new strategies and methods for defect characterization. To this end, the following issues have been carried out in-depth research: optimal selection of guided wave inspection parameters, high-efficiency excitation method of guided waves in pipes and transducer design, effective feature extraction techniques and the interaction of guided waves with new types of damages.In this work, the dispersion characteristics of longitudinal guided waves in hollow cylinders and the selection of inspection frequencies are first investigated. According to the dispersion phenomenon, the quantitative relationship between defect echo resolvable distance and excitation signal parameters is established, and then the problem of frequency optimization selection is analyzed. A distance parameter caused by dispersion of guided waves is defined, and basis on this, the limiting frequencies of the longitudinal modes in non-dispersive region are calculated. It is shown that the optimized cycles of tone burst and frequencies can be selected by restricting and minimizing the defect echo resolvable distance. According to the dispersion characteristics of longitudinal modes changing with the pipe geometry, several important function relations between the limiting frequencies and the pipe diameter and wall thickness are obtained. These function relationships provide reference for quickly determining the frequencies of non-dispersive region in pipe inspection.The excitation of the desired guided wave mode with high signal-to-noise ratio (SNR) in pipes is critical to obtain satisfactory inspection results. The issue of the high-efficiency excitation of axisymmetric guided waves in pipes is investigated. Firstly, the loading models depending on different types of transducers are built up, and the quantitative relationship between applied boundary loadings and acoustic field is advanced using the normal mode expansion method (NME). On this basis, the mechanism of the effects of the loading direction and the circumferential loading distribution parameters on the excitation of guided waves are discussed, and the boundary loading conditions for the high-efficiency excitation of guided waves in pipes is proposed. For the non-axisymmetric surface loading occurred in the actual pipe inspection, a new loading compensation strategy is proposed and implemented. The results show that the compensation strategy can suppress the flexural modes effectively, and contribute to improve the noise floor of the guided wave signals.In guided wave-based pipeline inspection, the reflection from defects usually includes sufficient defects relevant information. Extraction and analysis this information is always helpful to characterize the features of defects. A study of the reflection of the fundamental torsional mode T(0,1) from axial non-continuous double defects has been carried out via finite element modeling and experimental validation. On this basis, an optimized dictionary of matching pursuit method is developed. This method not only efficiently decomposes the overlapped signal, but also conveniently extracts the parameters which relate to the geometric characteristics of defects. The study shows that the proposed method can accurately identify the axial continuous single defect and non-continuous double defects according to the phase change of each decomposed signals; the difference between the flight time of the decomposed reflections can characterizes the axial separation of the front and back defects directly and accurately; and the amplitude of the decomposed front-defect reflection can effectively evaluate the size of the front defect.Dents or deformation defect is one of the primary failure modes in pipelines. A study of the reflection characteristics of guided waves from dents of varying geometrical profile in pipes is investigated through experiments, and then the detection and identification of dents in pipes using guided waves is discussed. The results show that part of the energy of the incident L(0,2) mode will be reflected at the dented region due to the geometry changes along the pipe. Mode conversion occurs at the dents because of its non-axisymmetric characteristics and reflections of the F(1,3) mode are identified. It is shown that the L(0,2) reflection coefficients from dents increase monotonically with deformation rates at all selected frequencies. The amplitude of the reflected L(0,2) mode provides a reference for evaluation of the deformation extent of the dents, and the ratio between the L(0,2) and F(1,3) modes can be used to identify the geometrical symmetry of the dents. For the same dent, the L(0,2) reflection coefficients decrease markedly at lower frequencies while decrease slightly at relatively high frequencies. The tendency of the curve of the L(0,2) reflection coefficient with frequency is similar to the radial displacement of the L(0,2) mode versus frequency. These characteristics indicate that the wideband L(0,2) mode provides a viable method for detection and identification of dents in pipelines. The guided wave provides a potential technology for the rapid deformation detection of pipes.Considering the in-service pipeline inspection, a novel diagnostic system based on ultrasonic guided wave NDT technology is developed. The diagnostic system combines a transducer array, which employs piezoelectric ceramic as the sensitive element, guided wave excitation/receiving hardware subsystem and data analysis software. The holding device used modularized designing idea and all modules connected with each other by a hinge pin. This method makes the transducer array has a certain degree of flexibility and expandability. The experimental results show that a ring of transducers can be efficiently used to excite single T(0,1) mode; the diagnostic system can identify corrosion defects removing more than3%of the total cross-sectional area of the pipes effectively. A novel transducer array applicable to on-line inspection of tubes is developed. The transducer also employs shear piezoelectric ceramic as the sensitive element, and has the characteristics of flexible and lightweight. The flexible transducer array can be surface-mounted in a ring of any size tube’s outer wall, and then identify the state changes of tubes real-time.