Establishing risk tolerance criteria for incident scenarios in LOPA

Layer of Protection Analysis (LOPA) is widely used in process industries as a semi-quantitative method for assessing risks of incident scenarios and evaluating their tolerability. This paper examines a key challenge in LOPA implementation: defining risk tolerance criteria for individual scenarios. It analyzes several approaches to addressing this challenge, including the direct assignment of risk targets to individual scenarios and the allocation of either individual or societal risk criteria to individual scenarios. The strengths, limitations, and suitable applications of each approach are discussed. The paper provides guidelines for establishing risk tolerance criteria for individual scenarios and calculating scenario frequencies consistent with these criteria. These guidelines combine recommendations extracted and systematized from relevant publications with original contributions.