Quantitative Risk Assessment (QRA) Tools
QRA utilizes a number of tools that provide a more detailed estimation of the expected frequency and consequences of potential accidents associated with a facility or operation. QRA is the next level of risk analysis, and it can require a significant amount of additional effort for more accurate hazard modeling and statistical estimation of scenario frequencies. However, it does yield the most insights. For this reason, a complete QRA (note that focused applications are fairly common) is typically applied only to the highest-hazard processes.
Fault Tree Analysis (FTA) is a good general-purpose tool for detailed failure modeling, assignment of failure probabilities, and scenario frequency derivation. As depicted in the figure below, the top of the fault tree identified the undesired event and combinations of “and” and “or” gates (as well as other more complex logic functions) are applied to develop a model of how the system can fail and lead to the undesired, top event. Frequencies and probabilities are assigned to “basic events”, and once parsed by the computer, these binary logic relationships yield the resultant frequency of the top event.
Common mode failures, testing/maintenance frequencies, covert and overt failures, as well as other complex system characteristics are readily assimilated into the fault tree. Event Tree Analysis (ETA) is less common, but may be useful for some situations that involved many layers of protection. It, too, yields scenario frequencies, and can be useful for depicting and modeling system-level interdependencies.
Many consequence modeling tools are available that support the modeling of vapor cloud, fire and explosion hazards to provide an accurate, numerical indication of the magnitudes that must be assigned to each consequence category. Given the large numbers of scenarios that may be applied for a complex QRA, Risk Assembly software may be utilized to link the consequence bins with the associated scenarios and their frequencies, generate societal and individual risk values, and perform various graphing functions. Risk Management Professionals personnel have developed detailed QRA for refinery alkylation units and nuclear power plants.