(121a) Risk Criteria Selection and the Impacts on the LOPA Process

In the CCPS book Layer of Protection Analysis – Simplified Process Risk Assessment, Layers of Protection Analysis (LOPA) is initially described as the analysis of a single cause-consequence pairing. However, later in the book, there is the discussion of summing risk for multiple scenarios. In practice, several companies prefer to sum the frequencies of multiple causes leading to a single consequence when conducting LOPA. Summing the causes can be a useful tool in that it will ensure proper reliability of a safety function to address all of the causes for a single consequence and assist in the reduction of the numbers of IPLs necessary at a facility; however, caution must be taken in using this method, as there may be unrealized effects on LOPA results, and therefore unintended impacts to the entire safety lifecycle.  In contract, evaluating only a single cause-consequence pair also poses different concerns when relating the results to selected risk criteria.

This paper will seek to provide insight into the effects of each choice, including the pros and cons of each method.  Deeper examinations into the definitions of risk criteria and consequence will be explored.