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PT Notes

Allocation of Risk Tolerance Criteria

PT Notes is a series of topical technical notes on process safety provided periodically by Primatech for your benefit. Please feel free to provide feedback.

Process hazard analysis and layers of protection analysis evaluate the risk of individual hazard scenarios. Therefore, in order to judge the need for risk reduction measures, risk tolerance criteria are needed for individual scenarios. However, it is not possible to assign a risk tolerance criterion directly to a hazard scenario. Such criteria can only be assigned with reference to overall facility risk tolerance criteria, which are the only truly meaningful risk tolerance criteria. Consequently, overall facility risk tolerance criteria must be allocated or apportioned to individual hazard scenarios.

Both individual and group risk are important for people. Individual risk criteria protect any single individual from bearing too large a share of the risk. Group risk criteria protect populations of people from bearing undue risk. Allocated risk criteria are needed for both types of risk. Individual and group risk are important for people both on-site and off-site.

For individual risk, the overall facility individual fatality risk tolerance criterion is allocated to hazard scenarios by estimating the maximum number of scenarios that could result in the fatality of any one particular person and dividing the overall criterion by that number. The resulting value is the maximum tolerable frequency of an individual fatality from a hazard scenario. Such allocated criteria are used to assess the tolerability of the risk for individual scenarios by comparing their estimated risks with the criteria.

Several pitfalls exist in the allocation and use of risk criteria. For example, the maximum number of scenarios that could produce the fatality of any one particular person includes not only single fatality scenarios but also multiple fatality scenarios that impact the same person. Consequently, both types of scenarios must be included in the number count. It is challenging for practitioners to estimate the maximum number of scenarios that may cause the fatality of one person, but it is even more challenging to estimate the number of multiple-fatality events that could cause the fatality of the same person, particularly as such events often will originate not just in the process in which the individual works but also in other processes at the facility.

Furthermore, the maximum number of events or scenarios that result in the fatality of one particular person must not be confused with the number of events or scenarios that can produce a fatality. For example, it may be estimated that there are no more than 100 scenarios that may result in the fatality of one particular person but there may be 1,000 scenarios that can result in the fatality of any person in the facility. Thus, the overall facility individual risk tolerance criterion should be divided by 100 to obtain a scenario risk tolerance criterion because in no case will there be more than 100 scenarios that result in the fatality of one particular person. Of course, the maximum number of scenarios to which a person might be exposed may well vary from one facility to another and risk allocation must be performed for each facility.

The allocation of group risk tolerance criteria to scenarios is more challenging than for individual risk tolerance criteria due to the larger number of scenarios that may contribute to the risk and the need to address multiple levels of consequences. Group risk is often expressed in the form of F-N limit lines which display the cumulative frequencies of N or more casualties. Allocation of group criteria directly from F-N limit lines to hazard scenarios is problematic because reference points taken from F-N limit lines represent cumulative risks summed over different consequence severities. An alternative approach is to use reference points for specific consequence severities by converting F-N limit lines to f-N form which display the frequencies of all events that result in N casualties. The number of scenarios for each number of fatalities will vary and this must be factored into the allocation process.

These and other issues encountered in allocating facility risk tolerance criteria, including the lack of equivalence and uniqueness of scenarios, are discussed in the article:

Allocation of risk tolerance criteria, Process Safety Progress, Vol. 33, Issue 3, pages 227–230, September 2014.

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