How is Quantitative Risk Assessment (QRA) used in SIS design and justification?

QRA for SIS design: develop frequency-consequence curve for each hazard scenario using fault tree/event tree analysis, establish risk acceptance criteria (individual risk, societal risk, ALARP region), credit existing protection layers with justified probabilities, identify residual risk requiring SIF protection, determine required risk reduction (frequency reduction = existing risk / target risk), translate to SIL requirement using LOPA or direct calculation, and demonstrate ALARP through cost-benefit analysis. QRA provides quantitative basis for SIL selection beyond prescriptive LOPA. Requires substantial data (failure frequencies, consequence modeling, population exposure) and specialist expertise. Used for major hazard facilities, modifications to existing systems, or when simpler methods inadequate.