(54ak) Selecting Release Scenarios and Credible Events for Siting Studies and QRAs

In Consequence and Risk Assessments, including quantitative Facility Siting Studies, there are infinite potential release scenarios which could be considered; however, practitioners are required to develop a quantifiable list of scenarios to represent the impacts which may occur. Depending on the study, practitioners may create this list by selecting Worst-Case Scenarios (WCSs), Maximum Credible Events (MCEs), or a Range of Hole sizes.

Generally, consequence-based studies evaluate WCSs or MCEs scenario, however, there is no generally accepted method for their selection, and the study results are highly dependent on the scenarios selected. Selecting MCEs as opposed to WCSs implies that some level of risk assessment has been applied on the front end of the study, as a basis to determine which scenarios will be considered credible, and which will be ignored. A study using the WCSs will simply select the largest potential release scenario possible (eg. a piping or vessel rupture); however, the impacts of such a scenario may be too great to develop practicable and actionable results. Additionally, the results may be out of line with historical incident data as such WCSs are extremely remote events. For studies using MCEs for the release scenarios, a hole size smaller than a rupture is generally selected. Hole sizes may be selected based on equipment size, or estimated frequency of occurrence. For example, in the design and permitting of LNG export terminals under 49CFR Part 193, PHMSA guidance has varied over the years in the selection criteria for accidental release scenarios. Previously, guidance required selection of the largest hole size exceeding an estimated frequency of occurrence of 3x10^-5 yr^-1 based on frequency analysis which considered the amount of piping and process equipment onsite. Recently however, the guidance was revised to define the hole size based on piping diameters, disregarding the amount of piping at the site.

Risk practitioners should have defined methods for selecting MCEs, and regardless of the method must consider that the study turns a blind eye to the potential for impacts from smaller release scenarios; which may be equally hazardous.