LOPA Is Obsolete! Move to Quantitative Bowtie Analysis

Layer of Protection Analysis (LOPA) is a ubiquitous part of the workflow for process hazards analysis and engineered safeguard design. Even so, from its inception LOPA has suffered from limitations that are rooted in the “one-cause, one-consequence” paradigm to the risk analysis the limits the analysis scope. This limitation can result in design errors where engineered safeguards that protect against a consequence with multiple causes can be under-designed by looking at one cause and a time. Also, safeguards that reduce the magnitude of consequences (i.e., mitigate) instead of preventing loss of containment cannot be addressed at all without ignoring the residual consequence that exists even if the safeguard activates successfully. While some extensions of LOPA that address the issue of multiple causes are commonly used, but mitigative safeguards are rarely appropriately addressed and designed. Finally, the textual nature of the process also makes results hard to communicate to non-practitioners.

Recent research into Unified Hazard Assessment that combines HAZOP, LOPA, and bowtie analysis has yielded techniques that elegantly address all the limitations of LOPA while also providing a graphical presentation that facilitates result communication. This paper will provide background on how Unified Hazard Assessment yielded the techniques of Quantitative Bowtie analysis. The paper will also describe in detail how to implement quantitative bowtie analysis along with the mathematical concepts used for quantification of risk for multiple causes and multiple consequences inside a single scenario. The concepts will be presented using example studies that include mitigative safeguards and multiple cause scenarios.