Integrated unavailability analysis including test degradation and efficiency, components ageing and common cause failures

Abstract

The article proposes an integrated model for the unavailability of standby components and systems, considering the impacts of ageing, test degradation and test efficiency on independent and common cause failures (CCF) probabilities. The results showed a considerable impact of test efficiency on systems reliability and risk.The CCF model was expanded to incorporate the additional contribution derived from the repair time of identical and redundant components. Two application cases, of system unavailability and Probabilistic Safety Assessment (PSA), were evaluated for several combinations of test efficiency, degradation and ageing, using the traditional and the expanded CCF models. Ignoring the contribution to CCFs derived from the repair time leads to significant risk underestimations, even when ageing and test degradation are not included.Great uncertainties of the model parameters represent an important restriction for the introduction of adverse test effects in risk analysis, but they are important contributors that must be considered explicitly.