Sound of the underground

New acoustic tools could change the way that water companies repair and maintain the UK's biggest hidden infrastructural challenge. Ewan Group's technical director, Richard Long, reports

The UK's underground sewer system is 300,000km long and has a replacement value estimated at £104B. It is an ageing and decaying asset stock hidden from view, and represents one of the water industry's most daunting challenges.

In England and Wales, Ofwat imposes a legal duty on the privatised utility companies to maintain the condition and serviceability of their assets. And it is a statutory requirement for each water company to clearly demonstrate that services to customers can be maintained at the least cost, in terms of both capital maintenance and operating expenditure.

In Scotland, the Water Industry Commissioner fulfils a similar role. Unfortunately, at present the only techniques with which they can assess the condition of sewers are slow, expensive and subjective.

Water companies need enhanced information to tackle this overwhelming issue. This is needed in two key areas to manage efficiently both the day-to-day performance and long-term capital maintenance of sewers which relate strongly to the operational and structural conditions and rate of deterioration.

The condition of sewer systems changes over time due to silting, fatting, blockages, ageing and interference. These conditions are key and have a direct harmful effect on the performance of the system. In 2003, Ofwat identified 5,700 internal flooding incidents from sewers managed by water companies, and an additional 11,600 properties with a one-in-ten-year risk of flooding.

Authorising massive capital investment
Ofwat has required water companies to progressively reduce the number of properties at risk of flooding, and has authorised massive capital investment to make this possible. As flow capacity inadequacies are addressed, "other causes" such as sewer blockages and collapses become increasingly significant as a failure of service that impacts on customers and the Overall Performance Assessment score. Companies are now looking for new ways of reducing these incidents through measures such as cluster analysis to identify hot spots and CCTV to locate developing problems, followed by proactive sewer maintenance activities such as jetting.

Sewer collapses and blockages
In contrast to flooding due to hydraulic overload, however, "other causes" problems commonly exist on small-diameter local sewers. These include those previously designated under the now-repealed Section 24 of the Public Health Act 1936 which today have the same status as other public sewers, which make up a large part of the sewer network. No longer will it be sufficient to focus attention on the 20% or so, of so-called critical sewers.

Current technologies are limited by cost and time - new technologies are needed that will work quickly and economically, and provide information in a format that
will permit ongoing programmes of proactive maintenance to be carried out cost-effectively across the entire asset base.

Furthermore, the inspection and cleansing required is not a one-off activity. Having identified areas at risk, it is important to regularly check for progressive operational deterioration and intervene again at the right time, before service deteriorates to an unacceptable level. Traditional CCTV techniques are not the ideal means of doing this as they are relatively time-consuming, costly and subjective.

A far more effective alternative is required to provide objective measurement of current operational condition that will allow the operator to show that the condition is stable or whether a change in the ongoing level of maintenance is needed. This alternative will also need to be more cost-effective, quicker and be capable of being used in the smaller sewers.

In terms of structural condition and rate of deterioration, there is mounting pressure on water companies to improve their knowledge of sewer condition, asset lives and failure modes. In this respect, it should be noted that deterioration of condition and deterioration of serviceability do not occur at the same rate.

Diagram 1 illustrates how it is often the case that a sewer may be serviceable even when its condition has seriously decayed. The problem is that if attention
is focused on the sewer only when its serviceability has substantially decayed it
is probably too late to rehabilitate the sewer economically.

Expensive and disruptive renewal is then the only solution. A system that can measure the progress of the sewer along the condition curve so that rehabilitation can be carried out in the right place at the right time will ensure that whole-life costs are minimised and the utilisation of resources is optimised.

In January 2004, the National Audit Office carried out a review of how the 10 sewerage companies in England and Wales have fulfilled their statutory duties relating to the stewardship and performance of the sewer network and how Ofwat has fulfilled its regulatory duties in relation to the network.

The review concluded that assessments of past performance and condition give a limited view of the future performance of networks, and how much activity properly directed each company should carry out on maintaining its sewer networks. Similarly, the review directed that companies should develop a clearer understanding of the rate of deterioration of their sewerage network assets.

In May 2004, The Public Accounts Committee of the House of Commons concluded that the way in which Ofwat and the water companies manage the structural condition of sewers should be improved. Two key recommendations were:
  • "Ofwat should require companies to include the same sewers in its regular five-year asset inventory assessments"
  • "Ofwat should develop measures which provide an indication of the future condition and performance of sewer networks"
Ofwat stated that: "Each company needs to demonstrate how the flow of services to customers can be maintained at least cost in terms of both capital maintenance and operating expenditure, recognising the trade off between cost and risk." Ofwat also emphasised the need for water companies to use economic and risk-based principles within their asset management systems.

The UKWIR report, Capital Maintenance Planning: A Common Framework, widely referred to as the Common Framework, provides a framework by which such assessments are to be made.

To properly apply the Common Framework, it is essential to have reliable information on asset deterioration and failure modes for each type of sewer so that the probability of failure and impact on service can be estimated with confidence. Unfortunately, up to now, there has been a dearth of suitable, forward-looking, asset observations relating to sewers from which such information can be determined, not least because current technology is insufficiently sensitive to determine the small changes that, over time, lead to decay and eventual failure.

In April this year, Ewan Group completed work on UKWIR's Deterioration Rates of Sewers project. This project showed how records of collapses and blockages in sewers, combined with asset attributes such as size, age, gradient and so on can be used to develop deterioration models that can be used to plan future operational and capital maintenance requirements.

Ewan and Anglian Water are now working to extend this method to a fully risked-based approach that takes account not only of the likelihood that any pipe may block or collapse, but also what the impact of that performance failure would be in terms of flooding of customers' property and pollution of the environment.

By incorporating these approaches in their day-to-day planning as well as their business planning for the regulatory cycle, Anglian Water will be able to demonstrate that the delivery of their services achieves the right balance of cost and risk.

Developing new tools for assessing sewer condition
Working with Ewan Group's Technical Director Richard Long, Professor Kirill Horoshenkov of Bradford University has secured funding from the Engineering and Physical Sciences Research Council (EPSRC) to carry out a two-year programme of research and development of these techniques.

The project is being supported by Yorkshire Water, Thames Water, Sheffield and Birmingham universities.

The aim of the project is to develop an acoustic-based system that will provide a step change in technology to provide information on sewer condition, in "real time" and at a network scale. This will require new insights in acoustic signal processing and the application of mathematical techniques to link signal responses to physical sewer condition.

The potential reduction in costs required to operate an automated acoustic-based sewer condition system, anticipated to be around an order of magnitude, offers for the first time the potential to directly assess the majority of a sewer network from the numerous small ex-S24 sewers up to the large trunk sewers at a time scale suitable for operational and capital planning purposes.

Diagram 2 shows some preliminary results of the acoustic signature in terms of spectrograms for the negative amplitude of the real part of the vector measured in a clean 20m-long ductile iron pipe of 400mm diameter. Within this pipe, partial blockages have been positioned in succession at various distances from the probe. These initial results demonstrate that the acoustic signature of the sewer can be used to detect the location and extent of a minor change in the cross-section of a large-diameter pipe.

Although instruments have already been developed to determine variations of the cross-section of narrow pipes using the method of acoustic reflectometry, there have been hardly any systematic studies devoted to the reconstruction of the cross-section profile from the multi-modal acoustic response of a pipe. This lack of theoretical basis and relevant experimental data is evident and can be explained by considerable mathematical challenges and by the nature of harsh, ever-changing environment that a sewer pipe represents. In this context, the results obtained are novel, challenging but highly promising and relevant to the needs of society.

It is intended to provide a digital fingerprint of a sewer length that is representative of the condition without necessarily containing full details of every defect. By repeating the measurements at intervals and comparing the fingerprints, it will become apparent whether or not significant change has occurred. Digital processing means that comparison will be both quick and objective and can be carried out at a network scale.

Fingerprints may be compared some time after the test, or immediately by accessing the results of previous tests which are stored either on portable equipment on site connected by cable or wireless, or at a remote location using wireless connection. The accurate estimation of sewer condition is crucial to managing flooding and pollution. Hence, the beneficiaries will be those managing sewer systems, and the customers they serve. Operators will be able to obtain information about sewer deterioration over time by carrying out repeat surveys, thus having for the first time sufficient data to assess the whole-life costs of interventions.

The results indicate that small acoustic-based sensors have the potential to measure pipe condition. It is our vision that, in the longer term, a range of acoustic-based sensors can be developed, either to be deployed permanently at a position so providing real-time continuous information on sewer condition, or used as hand-held portable devices. This project is the first step towards making this vision real.

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