A modelling approach to optimal imperfect maintenance of repairable equipment with multiple failure modes

Most of the existing works on optimal imperfect maintenance activities of a repairable equipment with independent components consider a single model for equipment behaviour. In addition, it is assumed that all the components of the equipment share the same model and the same maintenance intervals and that effectiveness of maintenance is known. In this paper we take a different approach. In order to formalize the uncertainty on the occurrence of failures and on the effect of maintenance activities, we consider, for each component, a class of candidate models. These models are obtained by combining failure rate models with imperfect maintenance models. The best model, that might be different for the different components, is then selected. All the parameters are assumed to be unknown and are jointly estimated via maximum likelihood. Model selection is performed, separately for each component, using standard selection criteria that take the problem of over-parametrization into account. The selected models are used to derive the cost per time unit and the average reliability of the equipment, the objective functions of a Multi-Objective Optimization Problem with maintenance intervals of each single component as decision variables. The proposed procedure is illustrated using a real data example.This work was a collaborative effort and was partly supported by the Conselleria d’Educació, Investigació, Cultura i Sport (Generalitat de la Comunitat Valenciana, Spain) under grant GV/2017/015

Prognosis of wear-out effect on of safety equipment reliability for nuclear power plants long-term safe operation

To reach a Net Zero Emission scenario, on 1st January 2022 the European Union (EU) declared nuclear and gas as transitional activities under strict safety conditions. A central challenge is that many nuclear reactors in operation are close to or have reached their design life, so that, it is required to demonstrate the influence of equipment ageing on plant reliability will be kept under control in the plan extended lifetime. In this work a three-step methodology is proposed to obtain the time instants at which the failure rate behaviour changes (break points) and the most appropriate age-dependant reliability model to explicitly include the effects of ageing and maintenance. The methodology requires the reliability parameters estimation at each phase of the plant equipment lifetime, what is carried out by using the available Nuclear Power Plant (NPP) historical data, which is quite scarce. The methodology is applied to a motor operated valve of a NPP safety system. The results demonstrate the capability of the approach proposed to estimate and predict the component reliability in the plant extended lifetime depending on the maintenance policy implemented, being necessary to estimate an accurate age-dependant reliability model to support the decision-making process on equipment ageing management.Grant PID2019-110590RB-I00 funded by MCIN/AEI/10.13039/501100011033 “ERDF A way of making Europe”