(41c) Beyond a PHA: Using QRA and FTA to Evaluate Complex Hazards

Process Hazard Analysis (PHA) and Hazard and Operability Studies (HAZOP) are foundational tools for identifying and addressing common hazards within industrial processes. However, for complex hazards involving rare events, intricate process or instrumentation interactions, human factors, and uncertainty, a more comprehensive and quantitative approach becomes essential for proper risk assessment and management.

This paper presents the experience of AcuTech’s global consulting practice. It explores limitations of PHA and HAZOP in assessing complex hazards and identifies categories of complex hazards better suited for Quantitative Risk Assessment (QRA) and Fault Tree Analysis (FTA). It delves into real-world situations where risk assessments should go beyond traditional PHA/HAZOP methods to advanced techniques, specifically Quantitative Risk Assessment (QRA) and Fault Tree Analysis (FTA), to effectively manage a diverse range of complex hazards. Two case studies are presented to demonstrate the utility of QRA and FTA where HAZOP was inadequate to accurately assess and manage the risks of a complex process.

Key Topics:

1. PHA/HAZOP Limitations in Complex Hazard Assessment: Limitations of PHA and HAZOP in assessing complex hazards are explored, emphasizing the need for a broader risk management strategy. This includes challenges related to scope, cognitive biases, and the potential for incomplete hazard identification.
2. When to Progress Beyond PHA/HAZOP: Critical criteria should be considered when deciding to move beyond traditional methodologies like PHA/HAZOP and LOPA to a more detailed, quantitative approach. It includes factors such as risk assessment objectives, system complexity, consequences, and the balance between regulatory compliance and holistic risk management.
3. Complex Hazards Suited for QRA/FTA: This segment highlights categories of situations and hazard types that are better suited for QRA and FTA. Hazards that typically benefit from a QRA or FTA include rare events with severe consequences, processes that present escalation risk, and chemical compatibility risks. When the risk assessment needs to evaluate risk holistically, such as when designing safeguards or assessing safeguard efficacy, QRA or FTA can be used to assess the aggregate risk as well as consider uncertainty and sensitivities in the analysis.
4. Real-World Case Studies: Two case studies are presented to showcase the application of QRA and FTA:

• Event Tree Analysis for a Hazardous Manufacturing Facility: This cast study shows how PHA, LOPA, and FMEA fell short in capturing the complexity and likelihood of ignition scenarios at the facility. A QRA was initiated, and it was identified that Event Tree Analys is was the best tool to understand the ignition mechanisms and frequency of impact to business operations and personnel, thus enabling the development of practical risk mitigation strategies.
• Fault Tree Analysis for Overloading a Flare: When a HAZOP revealed the potential for liquid accumulation in the flare system, a concern arose that a relief event could send liquids to the flare, cause flammable liquid rainout from the flare, and cause the flare system to become overloaded. The study then implemented a fault tree analysis to estimate the frequency of significant relief events that could send liquids to the flare.

In summary, this paper emphasizes the need for a transition from qualitative approaches like PHA and HAZOP to the more advanced and quantitative methodologies of QRA and FTA when addressing complex hazards. It provides practical insights into the criteria for choosing the appropriate approach, with case studies demonstrating the real-world utility of these advanced risk assessment techniques.