Risk Ranking For PHA, LOPA , and Facility Siting Part II

Risk-ranking is a common methodology for making risk-based decisions without conducting quantitative risk analysis. The basis for risk ranking is the risk matrix that has both a consequence and frequency axis. The product of consequence and frequency provides a measure of risk. Each consequence / frequency pair on the risk matrix is assigned a risk ranking that includes risk levels that are tolerable and others that are intolerable. The intolerable risk levels may be further divided into higher and lower risks to prioritize mitigation actions.

The process for developing a risk matrix is to start with the ranges of consequences of concern and then to determine the tolerability level for each. Generally the most severe consequence range includes one or more fatalities. However, some companies like to define multiple fatality event as the most severe range that typically is limited to a single fatality. Some companies also treat offsite or public impact as more severe than onsite impacts. One argument for the latter approach is that onsite employees are more prepared and protected against an incident than the public and therefore have a lower chance of realizing the impact given an exposure event. Another argument is that offsite impacts have more far reaching implications in terms of the Business Case for Process Safety and the License to Operate.

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