Pipe detection and remote condition monitoring using an in-pipe excitation technique

Abstract

Finding the location of underground utilities, essentially plastic water pipes, can be difficult and disruptive, which is due to the lack of efficient techniques that can be used to map these buried network services. Vibro-acoustic techniques to determine the location of buried water pipes have been studied within the Institute of Sound and Vibration Research within (ISVR) in the past 10 years. One technique involves excitation of the water pipe where it is accessible, normally from hydrant, followed by measuring the ground surface vibration, at the vicinity of suspecting pipe location. From an earlier feasibility study it was indicated that the emitted ground borne waves from the waves that propagate along the pipe and its contained fluid can help manifest the pipe location. Because the attenuation of the waves is generally large for the plastic water pipe, the pipe ground borne waves cannot be sensed at large distances away from the exciter’s location, of order of a few tenths of a metre, or at high frequencies, above 100 Hz. This thesis aims at using an in-pipe excitation source, which can be deployed at any desired location along the pipe length, to overcome the attenuation problem. Although in-pipe sources can transfer energy to the pipe contained fluid and might allow tracking of the pipe at larger ranges, current acoustic exciters are not always appropriate, being cumbersome and too large to fit into a typical buried water pipe. In this thesis, two pneumatic devices were designed with the aim of generating high amplitude signals at low frequencies and with the ability of accessing pipes with a wide range of diameters, down to 1 cm. The devices are experimentally characterised by a series of laboratory tests in a water-filled plastic pipe section. A comparison of the acoustic pressure wave transmitted to a fluid filled pipe between a standard electroacoustic device, an electromagnetic shaker, and the pneumatic ones is made. From the previous work on characterization of wave propagation along plastic water filled pipes, it is known that at below 100 Hz, among all kinds of waves, the axisymmetric fluid borne wave can predominately drive the ground borne wave. Owing to iii the good coupling of the axisymmetric fluid and the shell borne wave they should be considered altogether. The wave speed and attenuation of both waves were measured experimentally, and the results were checked for consistency with the theory. An analytical simulation was developed to explain the reason of high wave attenuation variances obtained from the experimental measurement. Due to the dependency of the axisymmetric fluid and structural borne waves’ amplitude to the elastic properties of the pipe a simple experimental method was proposed to distinguish between the pipe wall displacement that arises from the two axisymmetric waves. Such a technique might help assess the condition of pipes through indicating the reduction in their elastic properties due to ageing. From the earlier work on detecting buried water pipes, it was indicated that in addition to the axisymmetric fluid borne wave, the exciter applied to the pipe directly/indirectly drives the ground borne waves with substantial amplitudes. To have a better understanding about the overall response of the ground surface, a simple analytical model was developed, taking into account a ground borne wave emitted from the pipe and the source respectively. Like numerous other digital image processing, in the vibro-acoustic technique, an arctangent operator is used to extract the phase or time delay information, as the assumed ground borne waves are emitted and travel to the measurement point. By virtue of the fact that the arctangent operator produces phase images wrapped between −π to π, an unwrapping operator is required to remove the phase discontinuities embedded within the image. Therefore, a novel phase unwrapping algorithm is developed with a low-cost computational requirement. Furthermore, different state-of-the-art two-dimensional unwrapping algorithms are reviewed and compared for their ability to remove the phase discontinuities, produced by the model. In addition, a drawback of applying one-dimensional unwrapping to the two-dimensional wrapped phase image, used in the previous study, is discussed. To benchmark the effectiveness of the developed pneumatic devices, different exciters such as standard mechanical and electroacoustical exciter are utilised to map a buried pipe. Applying mechanical excitation to the pipe is associated to the earlier feasibility study which repeated for the sake of benchmarking the in-pipe exciters’ results against it. Of the exciters, one of the designed pneumatic sources successfully mapped the total length (18 m) of the pipe. Because the aim of this thesis is deploying an in-pipe source to overcome the attenuation problem, the utilised acoustical exciters: the underwater speaker and the designed pneumatic sources are deployed at two more locations along the pipe. The results from each exciter are analysed in detail and compared to each other.

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