(104c) When to Switch from LOPA to Focused QRA

Layer of Protection Analysis (LOPA) provides an intermediate level risk analysis that is more thorough than PHA, but not as time and cost intensive as a full quantitative risk analysis (QRA). In some cases, LOPA does not provide reasonable results and additional analysis using more quantitative means is desired. LOPA is subject to simplifications that do not allow enough nuance to obtain an accurate estimate of risk. The tools of QRA, such as fault tree analysis, event tree analysis, and dispersion/explosion modeling, can be applied to supplement or replace the original LOPA so that more reasonable recommendations can be implement.

This paper presents a listing of situations where LOPA should be considered to be supplemented with Focused QRA. The discussion begins with guidelines for determining when a LOPA provides results that are “unreasonable”. For each situation, the rationale for why LOPA is leading to an inaccurate risk assessment is explained along with presentation of the Focused QRA technique that should be used to address the shortcoming. Each situation will be described using process application examples.