How is Layer of Protection Analysis (LOPA) performed?

LOPA methodology: identify hazard scenario from HAZOP or other analysis, determine initiating event frequency (process upset, equipment failure, human error), identify consequence severity category (fatality, injury, environmental, economic), identify Independent Protection Layers (IPLs) with credited PFDs (BPCS, alarms, mechanical devices - each must be independent, auditable, and have demonstrated effectiveness), calculate unmitigated and mitigated event frequency, compare to tolerable risk criteria, and determine if additional SIF is needed and its required SIL. Document assumptions, IPL credits (typically PFD 0.1 for each), and basis for frequencies. LOPA is more quantitative than risk graph methods.