Abstract. The most expensive component of a water supply system is the distribution network. Deterioration due to aging and stress causes increased operation and maintenance costs, water losses, reduction in the quality of service, and reduction in the quality of water supplied. In this paper an approach is proposed in which the water distribution network economics and hydraulic capacity are analyzed simultaneously over a predefined analysis period while the deterioration over time of both the structural integrity and the hydraulic capacity of every pipe in the system is explicitly considered. The cost associated with each pipe in the network is calculated as the present value of an infinite stream of costs. In Kleiner et al. [this issue] a methodology is presented to implement this approach into a decision support system that facilitates the identification of an optimal rehabilitation strategy. IntroductionThe most expensive component of a water supply system is the distribution network. Aging water supply infrastructure, coupled with the continuous stress placed on pipes by operational and environmental conditions, has led to system deterioration which manifests itself in increased operation and maintenance costs, water losses, reduction in the quality of service, and reduction in the quality of water supplied. Given the reality of scarce capital resources, it is imperative that a comprehensive methodology be developed to assist planners and decision makers in selecting the most cost-effective rehabilitation policy that addresses the issues of safety, reliability, quality, and efficiency. [1988] proposed an alternative to the Woodburn et al. model in which some parameters are considered as random variables and the resulting probabilistic form is then converted to a deterministic form using a chance-constrained programming approach. The problem is then decomposed into a master problem which enumerates combinations of rehabilitated/replaced/intact pipes and a subproblem which uses a nonlinear programming solver 2039
This paper provides an overview of the authors' previous work in formulating a comprehensive approach to the important problem of water distribution network renewal planning, with a particular emphasis on the computing aspects involved. As pipes in a water distribution network age in service, they are characterized by increased frequency of breakage and decreased hydraulic capacity. The resulting service failures incur utility costs for the repair or rehabilitation of the pipe systems and consumer costs for degraded system performance. The challenge to the decision maker is to determine the most cost-effective plan in terms of what pipes in the network to rehabilitate, by which rehabilitation alternative and at what time in the planning horizon, subject to the constraints of service requirements (system reliability, service pressure, etc.) A dynamic programming approach, combined with partial and implicit enumeration schemes, was used to search the vast combinatorial solution space that this problem presents. A computer program was written to implement these concepts. A hydraulic network solver is used by the program to assure the network conformance to hydraulic constraints during the search for a solution. The outcome is a strategy that identifies, for each pipe in the network, the optimal rehabilitation/renewal alternative and its optimal time of implementation. The significance of this method is in its ability to identify an optimal rehabilitation strategy while considering the deterioration of both structural integrity and hydraulic capacity of the entire network. The best current heuristic method is limited in practical studies to a network of up to 15-20 pipe links. A more efficient heuristic method is required for implementing these principles in a larger-scale water distribution system and is the subject of current research.
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