The development of risk criteria for high severity low frequency events

Quantitative risk assessments (QRAs) are used within the field of process safety to decide the allocation of resources and risk reduction investments. Typically risk assessments involve the evaluation of probabilistic measures that estimate the average expected value for the situation being considered across a range of potential outcomes. The resulting expected value is then used to determine if a situation represents an acceptable or unacceptable risk based on a threshold value allotted to the risk. This approach often gives guidance that is at odds with the thoughts and behaviors of some stakeholders as illustrated by the “but what if it does happen?” type of question. This inconsistency results from the inherent limitation associated with expected value approaches in that the methodology is based on whether or not a mean assessed risk represents an acceptable risk while overlooking the possibility that a single scenario could represent an intolerable event. This article looks at an adjustment to traditional QRAs so as to assess both the acceptability of risk and the tolerability of the associated consequences relative to risk criteria. These adjustments have been found to better represent stakeholder perceptions of risk, more closely relate risk tolerance to corporate values and resources, and to better justify the use of various risk transfer strategies. © 2008 American Institute of Chemical Engineers Process Saf Prog, 2009