Assessment of safety barriers and mitigation of domino scenarios

Safety barriers play an important role in accident prevention and mitigation. The severity of domino scenarios usually calls for the displacement of a number of specific safety barriers to manage and reduce the risk due to escalation. However, the quantitative effect of safety barrier performance in managing the risk induced by domino scenarios is seldom addressed, at least in most of the methodologies proposed for domino effect risk assessment. A specific quantitative approach, based on a modified event tree analysis (ETA), is introduced for the probabilistic assessment of escalation accounting for the presence of safety barriers. The procedure considers the quantitative performance of safety barriers by the assessment of two parameters: the probability of failure on demand and the effectiveness of the barrier. The ETA approach and the assessment of barrier parameters are also discussed. In particular, specific gates are proposed for reference target equipment items, to support the ETA. Suggested approaches to barrier probability of failure on demand and effectiveness are presented, and a repository of baseline data for the performance parameters of these barriers is reported. Overall, tools and procedures to support the implementation of quantitative safety barrier performance assessment aimed at including the effect of safety barrier performance in the quantitative assessment of risk due to domino scenarios are presented.