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Erskine A.,MWH UK Ltd. | Watson T.,ICS Consulting Ltd. | O'Hagan A.,University of Sheffield | Ledgar S.,Yorkshire Water Services | Redfearn D.,Yorkshire Water Services
Journal of Infrastructure Systems | Year: 2014

The replacement and maintenance of subsurface assets, such as water and wastewater pipes, is of great interest to water utilities because these infrastructure networks require large amounts of investment over time. Each asset requires investment relatively rarely, but the number of assets is so enormous that the flow of money is large. Therefore the accurate estimation of the deterioration and aging process of these assets is critical to the efficient and sustainable allocation of investment spend. The development of failure models is difficult for various reasons: short spans of data (very little longitudinal data), very sparse failure rates, inaccuracy of observational data, and accuracy and availability of potential predictor data. Technical difficulties also arise such as variability and noise, censoring effects, overdispersion, and, throughout the exercise, the large volume of data usually involved. In this paper, a new regression approach is formulated that maintains a rigorous statistical approach while still being practical and easy to apply. In addition, the formulation involved permits the individual pipe history to be used in an elegantly simple Bayesian update. The example provided refers to work done for Yorkshire Water Services in estimating probability of blockage failure for all their sewerage assets. © 2013 American Society of Civil Engineers.

Mauelshagen C.W.,Cranfield University | Pollard S.J.T.,Cranfield University | Owen D.,Yorkshire Water Services | Herndlhofer S.,Yorkshire Water Services | And 4 more authors.
Environment Systems and Decisions | Year: 2014

We present a risk-based contingency fund management methodology to mitigate the impact of external risks on asset value and performance. Many asset intensive industries, such as water and energy utilities, are significantly affected by external risks such as extreme weather events. We put the case for a centrally held risk-based contingency fund that would mitigate against ‘medium’ impact ‘medium’ probability events that fall outside of large losses covered by insurance and smaller ‘normal’ operating losses. Our risk-based contingency approach is appropriate for short-term business planning (1–5 years) and would complement longer term planning, for example climate change adaptation and mitigation strategies. Our approach offers a risk-based methodology to manage contingency that is explicit and defensible. Critically, our methodology allows contingency to be managed dynamically as risk probabilities and impacts change, creating a mechanism for contingency funds to be periodically released if risk exposure reduces. The long-term benefit of dynamic, risk-based contingency is to reduce the impact of external risks and support long-term sustainability. © 2014, Springer Science+Business Media New York.

Cook D.M.,Yorkshire Water Services | Boxall J.B.,University of Sheffield
Journal of Pipeline Systems Engineering and Practice | Year: 2011

Despite a significant ongoing investment in asset renewal and rehabilitation and high compliance rates for drinking water quality standards, water companies continue to receive customer contacts relating to water quality, dominated by discoloration. This paper investigates the accumulation of material responsible for causing such discoloration in drinking water distribution systems from new field data. Engineering experience suggests that factors influencing this accumulation might include local pipe properties such as age, material, or diameter. On the basis of recent research, it also seems reasonable to expect hydraulic conditions and bulk water quality to be important. Results of extensive repeated flushing field trials in two representative distribution management areas (DMAs) over a two and a half year period are presented. Initial flushing suggested that the maximum thickness of discoloration material accumulated within the pipes was tentatively inversely proportional to the daily conditioning shear stress, in agreement with previous research, but independent of pipe material and/or diameter. An analysis of discoloration material from different strength layers, facilitated through stepped flushing, showed that the metal composition was uniform, and the process of accumulation simultaneously occurs across all strength characteristics. In plastic pipes, a limiting layer strength of 0:7 N=m2 was observed, above which significant additional discoloration material was not mobilized. Repeat flushing at different return intervals showed the accumulation rate of discoloration material in both DMAs to be consistent, within field-based experimental error, with an average of 0:0057 mm=month (95% confidence level of 0.0015). This rate was consistent across the different pipe materials, diameter, age, and hydraulic conditions within each DMA and between the two DMAs. Results from repeat flushing at increasing time intervals show that accumulation is a linear function of time; although, because of the uncontrolled nature of field work, this assertion is only definitive for five pipe lengths. From this, it is suggested that accumulation is primarily dependent on factors external to the DMA, and it is likely that this is dominated by the supplied water quality. This accumulation rate might seem small; however, after only 43 days of accumulation, the mobilization of the total accumulated volume of material into one pipe volume of a 140 mm pipe would be sufficient to exceed UK regulatory limits of 4 NTU for turbidity. © 2011 American Society of Civil Engineers.

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