(104n) Quantifying LOPA Uncertainty

Layer of Protection Analysis (LOPA) is a semi-quantitative risk evaluation tool.  The methodology is widely used in setting Safety Integrity Level (SIL) targets for safety instrumented systems (SIS).  The methodology is often used to evaluate the adequacy of existing protective systems against corporate risk targets.  The LOPA method is based on the use of conservative but representative estimates of the failure probability of protective systems.  The results of these studies are presented to management for concurrence with the conclusions and for funding of recommendations.  Management sometimes asks questions as:

 What is the uncertainty in the results?

How sensitive are the results to the underlying data?

How conservative is the LOPA methodology?

Should additional safety factors be included in the LOPA calculations?

This paper presents a framework to answer these and related questions.  Guidance is presented on methods to incorporate an uncertainty analysis into a LOPA or QRA study.