Acoustic and ultrasonic techniques for defect detection and condition monitoring in water and sewerage pipes: A review
Introduction
Buried infrastructure, in the form of networks of pipes, is important to urban life and forms a vital part of many engineering structures for transporting fluids and gases. In the UK alone there are over 600,000 km of sewer pipes [1]. The US Environmental Protection Agency estimates that water collection systems in the USA have a total replacement value between $1 and $2 trillion. The EU has a similar value of buried water pipe network. These networks are aging rapidly and becoming more heavily used due to population growth, increasing demand for water and climate change. With an increased use of pipe networks comes increased chance of faults occurring and when they do occur their impact is greater. Therefore, safe and reliable techniques for condition monitoring and fault detection are required for the maintenance and targeted replacement of the pipe infrastructure.
The underground water and wastewater/sewerage pipe networks are challenging environments for sensing. The requirement for sensors in this environment ranges from measurement of internal geometry and operational parameters (e.g. flow), to blockages, leaks and the structural integrity of the pipe itself. The pipes are made from a disparate selection of materials including, various polymers (e.g. high-density polyethylene (HDPE)), cast iron, ceramic, concrete and masonry [2], [3]. Pipes of a different age are often made from different materials, where some of the older materials are now in poor condition. The topology of this system is very complex. It is full of connections, inspection chambers, hydrants, valves and pumps (see an example shown in Fig. 1). Typically, some details of the networks are uncertain, particularly the location of discontinuities in the properties of the pipe. This uncertain and challenging environment often means that that no single sensor technology is suitable.
The most common sensors in use today in underground water and wastewater/sewerage pipe networks are ultrasonics, passive and active acoustics, but other technologies such as CCTV, laser profiling, Eddy Current Testing (ECT) and Magnetic Flux Leakage (MFL) are also used. This means that the pipe inspection engineer has a large toolbox of sensors and methods at their disposal to cover this wide range of needs [2], [3], [4], [5]. This paper reviews recent developments in acoustic and ultrasonic technologies and their application to the inspection of water and sewerage pipes. The review covers the use of accelerometers, hydrophones, fibre-optic sensors, bulk wave and guided wave sensors. These technologies have recently attracted a significant interest because of their high sensitivity, flexibility, speed and ability to cope with complex circumstances [2], [3], [4], [5]. We also explore the future potential of these technologies for use on autonomous robotic platforms.
A majority of acoustic sensors are still deployed and operated manually. Leak detection in water pipes is regularly performed by human inspectors who visit suspect regions to take manual measurements with listening sticks or to attach acoustic detectors to hydrants [6]. Blockages and structural damage in sewer pipes are often investigated by an operator working from a manhole with acoustic pulse reflectometry [7]. The need for human inspectors means that such measurements are expensive and time consuming. Typically, inspections are performed in response to a reported incident, such as a flooding, leakage or blockage meaning that only a tiny fraction of the network is covered by sensors at a given time. A consequence of this responsive approach is that the opportunity for automated and condition-based maintenance is missed. Furthermore, the manual nature of the inspections means that they are relatively slow, not sufficiently pervasive and often subjective. There is a strong drive for water utility companies and municipal/government departments to move from reactive maintenance to predictive assessment and maintenance that can be achieved with advanced autonomous robotic systems [8]. Robotic sensing systems working in buried pipes have the opportunity to capitalise on recent advances in acoustic and ultrasonic sensing techniques. Although there have been reviews of pipe inspection technologies (e.g. [2]), there is still a limited understanding how these technologies can be adapted for autonomous sensing. Therefore, the purpose of this paper is to review the state-of-the-art acoustic and ultrasonic sensor technologies for water mains and wastewater/sewerage pipe networks and discuss their potential for being deployed on autonomous robots used for pipe condition assessment.
The paper is organised in the following manner. Section 2 reviews the existing acoustic inspection methods. Section 3 reviews ultrasonic methods using bulk wave and guided waves. Section 4 is a summary of the applicability of the reviewed inspection methods and Section 5 is the conclusions.
Section snippets
Acoustic methods
A wide variety of acoustic techniques have been developed over the years for applications in the water and sewerage industries that include detection of leaks [9], blockages [7] and sediment depositions [10] as well as mapping the location of underground pipes [11]. These methods rely on sound waves with frequencies less than 20 kHz, i.e. waves generated in the audible frequency range. Acoustic sensing methods are non-invasive and allow inaccessible pipe sections to be inspected with minimal
Bulk wave
Bulk wave ultrasonic inspection for structures such as plates and pipes has traditionally been performed using single or multiple transducers [55], [56]. A typical configuration involves a single transducer (pulse-echo) or a pair of transducers (pitch-catch) that is usually attached to the outside of the pipe as shown in Fig. 6(a)-(b). Ultrasonic bulk waves are most commonly generated by piezoelectric ceramics or polymers that require contact or a liquid or solid couplant. However, non-contact
Accuracy of the methods to water and sewerage applications
For sewerage pipes, airborne acoustic waves have been extensively used to localise and characterise blockages with a stated accuracy of the order of several centimetres over a range of 100 m [7], [50], [128]. The accuracy and performance of this method mainly depends on the ability to measure the temperature and cope with a relatively high attenuation caused by the rough clay and concrete pipe walls which is typically 0.1–0.5 dB/m [10].
Accelerometers, hydrophones, and fibre-optic sensors have
Conclusions
In this paper acoustic and ultrasonic methods for condition monitoring of underground water and wastewater/sewerage pipe networks have been reviewed. Although traditionally these methods have been applied to pipes manually or installed on human-controlled robots, they are well suited for being used in combination with autonomous inspection robots for detection of the onset of in-pipe defects. Appendix A provides a critical summary of these methods in terms of their industrial applications to
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors are grateful to the UK’s Engineering and Physical Sciences Research Council (EPSRC Grant EP/S016813/1) for support of this work. The authors are also grateful to their industry partners for their advice about the commercial systems for the inspection of buried pipes.
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