Internal pipe imaging for defect detection using acoustic waves

Abstract

Urban water supply systems operate inefficiently and even unsafely due to the presence of numerous defects, such as leaks and blockages which result in significant water and energy losses and an increased potential for contamination. In this thesis, a new acoustic-based method for detecting and characterizing single and multiple defects in pipes is developed. First, an extended blockage detection method, which assumes undamped one-dimensional wave propagation is developed by making use of the time reversal symmetry and linearity properties of the undamped wave equation. The proposed method uses measurement at one end of the pipe and does not require knowledge of the boundary condition at the other end, yet it detects multiple blockages of irregular geometries. Second, a relation between the undamped acoustic waves and the waves which are damped by the pipe wall visco-elasticity and unsteady friction is derived. The relation is applied to transform the measured damped signal into an idealized undamped signal to be used in conjunction with the proposed defect detection method. Laboratory experiments in visco-elastic pipes containing blockages of different severity are conducted to validate the proposed methods. It is shown that the location and size estimates of the blockages are accurate, and are further improved after compensating for the damping using the transformation proposed in this thesis. Third, an extension of the blockage detection method to detect and characterize defects of mixed-types is proposed, by analyzing the differences and similarities between the wave scattering imposed by the defects. The method uses two measurements, one at each end of the pipe, and can detect and characterize leaks, and discrete and extended blockages. The proposed algorithm is implemented for real-time defect detection in two laboratory systems in Hong Kong. The results show that, for more than 75% of the tests, the error of defect localization is lower than half the smallest wavelength of the probing wave. This is notable given that the diffraction limit is half a wavelength. Finally, the propagation of waves with wavelengths smaller than the pipe diameter, which invalidates the one-dimensional assumption, is investigated. It is shown that at this regime, the wave propagation properties in visco-elastic and in elastic pipes tend to be the same, simplifying the diminishing role of visco-elasticity at high frequency.

Date of Award	2019
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology