The risk of pipe failure is one of the factors taken into account by drinking water utilities when they decide whether or not to replace water mains. They assess this risk on the basis of both the failure probability itself and of the impact a failure would have. To support their decision-making, the utilities use specific software (BOS), in which information about the main and the surrounding environment is aggregated into probability, effect and ultimate risk. But this approach does not assess the failure probability of an individual main, even though this is something that the drinking water utilities increasingly need. It is for instance expected that, in connection with the energy transition, there will be growing pressure to combine work on the network with third-parties.
One method that is indeed suited for a more specific interpretation of failure probabilities, involves the use of condition models. These are physical models that can calculate the condition of the main in relation to the relevant technical limits, on the basis of a parametric description of mains and the surrounding environment. The present project is examining how such a method can contribute to the decision-making process, such as through the pre-assessment of failure probabilities, the closer analysis of a prioritised main, or the support of a posterior analysis of the causes of a failure.
Estimating failure probability with Comsima
The Comsima condition model was developed to assess a main’s condition as accurately as possible on the basis of stress calculations. The calculations include pipe specifications, geo-information and knowledge-based rules for aging. Comsima was a product of the collection of insights into main conditions resulting from earlier research in the Joint Research Programme of KWR and the water utilities. Several drinking water utilities are now for the first time incorporating the Comsima model into their decision-support software. They use the calculated condition of their pipe network as an approximation of the failure probability. In order to model actual failure probabilities and future failure behaviour of groups of mains, research was done in 2019 on how Comsima could be combined with uncertainties in model parameters. The next step involves determining how, in this way, the (future) failure probability of individual mains can be effectively approximated, so as to assist the decision-making.
Comsima areas for improvement
The project will study whether the following areas within Comsima can be improved:
- probabilistic calculation of (future) failure probability for individual mains;
- the influence of temporary stresses, such as water hammer action and regular pressure fluctuations, and the work activities of third-parties.
This project will produce a description of how a condition model can support the decision-making process concerning individual mains, and of the relevant boundary conditions. To this end, a methodology will be provided for the adaptation of Comsima, enabling it to predict pipe failure probability and frequency for individual mains. Research into the different sources of dynamic loads will result in a report describing the importance of these sources, including the possibility of including them in existing condition models, and a specification of which of these models is most suitable in this regard.