Supporting LOPA with Fault Tree Analysis

Risk management tools like layers of protection analysis (LOPA) can oversimplify risk analyses. More advanced tools such as fault tree analysis can expand these simple studies and make them more accurate.

Layers of protection analysis (LOPA) is an important tool for risk analysis, but its limitations can result in unrealistic results, especially when the result of the analysis is used to develop safety integrity levels (SIL). LOPA relies on conservative assumptions that are not always appropriate, including the independence between initiating events (IE) and protection layers. When LOPA fails to yield reasonable results, analysts can use more sophisticated techniques such as a fault tree analysis. Fault tree analysis allows for elegant handling of shared components, which can yield more accurate risk analysis and appropriate credit for dependent systems.

This article presents situations in which LOPA yields inaccurate results and demonstrates improvement using fault tree analysis. A case study of a butane sphere batch filling operation is presented in which single pieces of equipment are used for multiple purposes, including a dual-level transmitter system that is used as part of the control system, alarms, and safety instrumented system (SIS). LOPA would not allow these shared components to be credited, but fault tree analysis elegantly models their effectiveness. Following up LOPA with fault tree analysis can result in more accurate analysis and better safeguarding of equipment designs...