Determining the Criteria of the Risk Acceptability of Accident Scenarios Using the LOPA Method

The semi-quantitative risk assessment method «Layer of Protection Analysis» (LOPA) is used, as a rule, to assess the risk acceptability of increased hazard scenarios identified during the qualitative hazard analysis (HAZOP) and to make the respective risk management decisions. The adopted risk management measures can include the introduction of additional protection layers to prevent or mitigate the consequences of potential scenarios or increasing the safety integrity level for safety instrumented functions of safety instrumented systems. When using the LOPA method for risk assessment, as well as other methods, the risk management decisions are made based on the comparison of the design risk value with the established criterion of acceptability. The absence of a unified approach to establishing scenario risk acceptability criteria can cause significant discrepancies in risk assessment results: in some cases, the existing hazard can be underestimated, and in other cases, overestimated. The divergence of opinions creates preconditions to establish the diversity of approaches to acceptability criteria. The study discusses various approaches in order to determine the scenario risk acceptability criterion when performing LOPA, such as ALARP, determining acceptable risk using the risk reduction matrix, and the concept of acceptable individual risk. The choice of approach based on the risk reduction matrix represented in IEC 61511-3 is substantiated. The limitations associated with the approach and potential ways to overcome them have been discussed. An example of the use of LOPA, where the risk acceptability is determined using the ALARP principle, and the risk reduction matrix has been provided.