Facility Siting: Risk-Based Solutions and Benefits

The Benefits of QRA

Controlling Risk Sources

One large benefit of a QRA is that it identifies the controlling risk sources. Since scenarios are developed across the process, risk results are ranked by scenario to indicate which processes provide the highest risk and are good candidates for risk reduction via process controls and hazard mitigation. We provide individual risk for each scenario and aggregate risk (in the form of risk indices) by scenario and hazard type. Targeted mitigation has a significant risk reduction impact across the facility using these results. Examples of these mitigation measures include:

• Leak isolation via LEL detection and isolation
• More frequent and extensive mechanical inspections to identify degrading equipment
• Replacement of high leak frequency equipment with low-frequency alternatives (welded pipe for flanges; double seal pumps)
• Installation of explosion suppression water deluge systems
• Building isolation and sealing to prevent toxic infiltration
• Evacuation PPE for toxic and fire hazards

Building Occupancy

Building occupancy has a direct impact on calculated risks. The most hours worked by any individual determines the time fraction used for individual risks. Therefore, reducing the time spent in the building directly reduces the individual risk. Over a week, the total number of people in the building impacts the aggregate risk. In some cases, moving people to lower-risk buildings can be an acceptable risk mitigation strategy. QRA requires a company to have quantitative risk criteria for individual and aggregate risk. Quantitative criteria development requires understanding the different risk criteria, the best practices for developing the criteria and the industry risk criteria precedents.

Since we wrote the book on developing and applying quantitative risk criteria, entitled Guidelines for Developing Quantitative Safety Risk Criteria, published by CCPS, we are well-positioned to help companies navigate the quantitative risk criteria development process. 

Interpreting the risk results when compared to the criteria is often a point of confusion. Individual risk is straightforward since most companies define a maximum criteria value above which risk reduction is required. Aggregate risk illustrated as FN curves are more complex, consisting of an intolerable upper region, an ALARP middle region and a broadly acceptable lower region. The intolerable region requires risk reduction, while the ALARP region requires more risk assessment and risk reductions when practicable. Risk reductions are viable when the effort and cost are not disproportionate to the potential benefits achieved.

Companies must decide how current to keep their risk studies. PSM defines a 5-year revalidation cycle for facility siting in the US. PSM also requires the Management Of Change (MOC) program to evaluate the impact of significant facility changes. It is not reasonable or practical to update the QRA for every process change; however, some level of risk impact should be performed for significant process unit changes or the addition of new occupied buildings. Minor changes like replacing a pump can be documented and addressed during the next QRA revalidation cycle.