The problem of Water Distribution System (WDS) rehabilitation is formulated here as a multiobjective optimisation problem under uncertain demands. The objectives are to minimise the total rehabilitation cost and to maximise the hydraulic reliability of the WDS. In this context, reliability is defined as a probability of simultaneously satisfying the minimum pressure head constraints at all nodes in the network. The total rehabilitation cost takes into account, not just the structural costs, but also the revenue lost due to the volume of water required by users but not supplied to them because of the structural inadequacy of the network. On this basis, safeguarding the hydraulic reliability of a WDS also implies a reasonable rate of return on the water company’s capital investment. A probabilistic approach is used to characterise demand uncertainties within the optimisation model. The relevant probability density functions and their parameters are estimated through an experimental study conducted on a real-life WDS. The rNSGAII optimisation algorithm is used to solve the optimisation problem. The lost revenue estimation is first performed as a part of the post-optimisation procedure, and then as an integral part of the optimisation process. The results obtained in the two approaches are compared showing the better performance of the integrated type approach. The methodology presented allows the identification of the specific optimal solution of the Pareto front that corresponds to the minimal total rehabilitation cost (structural cost plus lost revenue) yet corresponding to an high level of reliability, – the Economic Level of Reliability (ELR). This value could be assumed as the optimal solution or, allowing for socio-economic considerations (if higher WDS performance standards are required), as a lower threshold of reliability which restricts the range of technically feasible solutions that could be adopted in a rehabilitation programme. This way, the proposed methodology contributes to the definition of the reliability threshold values with which studying or designing a WDS.

Multiobjective optimal rehabilitation of hydraulic networks based on the cost of reliability

DE MARINIS, Giovanni;GARGANO, Rudy;TRICARICO, Carla
2007

Abstract

The problem of Water Distribution System (WDS) rehabilitation is formulated here as a multiobjective optimisation problem under uncertain demands. The objectives are to minimise the total rehabilitation cost and to maximise the hydraulic reliability of the WDS. In this context, reliability is defined as a probability of simultaneously satisfying the minimum pressure head constraints at all nodes in the network. The total rehabilitation cost takes into account, not just the structural costs, but also the revenue lost due to the volume of water required by users but not supplied to them because of the structural inadequacy of the network. On this basis, safeguarding the hydraulic reliability of a WDS also implies a reasonable rate of return on the water company’s capital investment. A probabilistic approach is used to characterise demand uncertainties within the optimisation model. The relevant probability density functions and their parameters are estimated through an experimental study conducted on a real-life WDS. The rNSGAII optimisation algorithm is used to solve the optimisation problem. The lost revenue estimation is first performed as a part of the post-optimisation procedure, and then as an integral part of the optimisation process. The results obtained in the two approaches are compared showing the better performance of the integrated type approach. The methodology presented allows the identification of the specific optimal solution of the Pareto front that corresponds to the minimal total rehabilitation cost (structural cost plus lost revenue) yet corresponding to an high level of reliability, – the Economic Level of Reliability (ELR). This value could be assumed as the optimal solution or, allowing for socio-economic considerations (if higher WDS performance standards are required), as a lower threshold of reliability which restricts the range of technically feasible solutions that could be adopted in a rehabilitation programme. This way, the proposed methodology contributes to the definition of the reliability threshold values with which studying or designing a WDS.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11580/3347
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