Risk assessments use frequency data mostly from generic sources, typically from the North Sea offshore environment or from process facility records mainly from Europe. As QRA's are being applied increasingly in North America and in the developing world, novel techniques are necessary to establish whether modification to frequencies are warranted, especially if comparison to formal risk criteria is an objective. Older modification techniques were based on management systems alone. This paper describes an approach based on the bow tie barrier approach that counts and maps prevention and mitigation controls across a range of hazards and establishes a means to compare a local facility controls to a typical North Sea facility.
A total of 6 hazards were reviewed in detail with an average of 5-8 threats. Controls were identified and the effectiveness of controls compared to similar controls in the base data set. This allowed establishing an equivalency of controls. A second weighting applied to the risk contribution so that higher risk threats were weighted more than lower risk threats. A total weighted modification factor was developed based on the number of controls deployed, their effectiveness, and the risk importance of each threat. When applied to a refinery, this gave a modification factor of 2, thus local frequencies should be multiplied by 2 compared to the base data set. In practice since North Sea leak data is improving, this should be interpreted that onshore refineries tend to be 6-8 years behind the North Sea in generic leak data.
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