The role of safety barrier performance depletion in the escalation of Natech scenarios

Natural hazards can cause severe damages to chemical and process facilities, triggering technological scenarios involving hazardous materials. The risk related to this type of cascading events, defined Natech accidents, is expected to grow in the foreseeable future due to the enhanced severity of some categories of natural phenomena brought by climate change. A critical feature of Natech events is that the safety systems implemented might undergo some extent of depletion and performance reduction due to the natural event, and this might heavily influence the likelihood and the features of accident escalation. While methodologies have been proposed to perform a quantitative assessment of Natech risk, the role of the concurrent depletion of the safety systems has been only recently investigated and has not been addressed systematically yet. Hence, a comprehensive framework to assess the risk related to the escalation of Natech scenarios and to possible domino effects due to concurrent safety barrier depletion is presented. A specific three-level approach was conceived to evaluate barrier performance according to system complexity and uncertainty in the impact of natural events. A straightforward analysis (L0) based on a Boolean approach is applied for simple barriers when their missing action can be assessed with a low uncertainty. A more detailed analysis (L1) leveraging specific performance modification factors to express the likelihood that similar reference barriers will fail is applied in case of relevant uncertainty. For the analysis of complex barriers and situations when system architecture differs from reference configurations, a further level (L2) based on fault tree analysis is introduced to consider barrier subsystem failure during natural events and to update the overall unavailability of the system. A dedicated event tree approach is then used to embed barrier performance into the quantitative risk assessment of Natech scenarios. The methodology was applied to a test case demonstrating that the quantification of the updated performance of the considered set of safety barriers during natural hazards leads to a relevant increase in overall Natech risk figures.