Risk Graphs and LOPA Viewed through a Lens of Application

Two of the most common methodologies used for SIL selection are the Risk Graph (RG) approach and Layer of Protection Analysis (LOPA). These methodologies are generally recognized as semi-quantitative approaches; both being quite suitable for SIL selection and elucidation of complex scenarios. While being very similar in utility, differences do in fact exist in their operation. These differences may not be readily apparent to a routine observer. Moreover, the results obtained from these two approaches may be in disagreement. Application of the methodologies, however, provides greater insight into the areas of divergence.

This article discusses a project in which nearly 100 hazard scenarios were studied and evaluated using both the RG and LOPA instruments. Each instrument was calibrated to the same risk criteria prior to the studies. Following the completion of both studies, the results were evaluated against one another to identify points of variance. The results arising from the application of both methodologies are summarized and explained. Key differences between the two methodologies are also described, which lead to a number of conclusions and recommendations. These conclusions have practical value for practitioners involved in the use of Risk Graphs or LOPA for SIL selection.