We often look for ways to simplify most aspects of our daily activities, while still trying to achieve the goals from our expended efforts. Minimizing effort and maximizing outcomes can prove to be advantageous from an efficiency stand point.
However, making things too simple can prove to be harmful and the more tedious a task the more we look for ways to simplify the task in order to make it manageable. This same approach is too often applied when conducting facility siting studies (FSS) and/or quantitative risk assessments (QRA). The ultimate goal for conducting an FSS or QRA is to identify and assess the hazards, how do they effect personnel, and how do I manage them in order to minimise their impact?
Keeping these goals in mind we must first examine what a thorough FSS/QRA entails. A thorough and comprehensive QRA or FSS consists of assessing several hundred thousand potential hazards in terms of explosion, fire and toxic releases. This process is achieved by carefully following the process streams from plant Process Flow Diagrams (PFD) and creating a source any instance where the pressure, temperature or stream composition changes significantly. This ensures that the maximum number of plausible scenarios is assessed both of high and low probability and high and low consequence. Depending on the software or method used, the results can then be graphed into what are called Frequency-Fatality (FN) curves; the curves can be graphed for building, societal, individual, process or even hazardous specific risk (fire, toxic or explosion). A multitude of decisions can be made from assessing hundreds of thousands of outcomes; from efficient escape route selection to cost effective building upgrades. Additionally, when looking for mitigation options, source risk can be used in identifying with reasonable accuracy what sources or equipment are causing the most severe consequences.
When this approach is not used, for example, filling a unit with flammable mixture in assuming that this is the worst scenario no particular source of equipment can be attributed to this and thus never be mitigated. In addition the event is either overly conservative creating inordinately high blast loads and requiring unnecessarily strong and costly buildings or underestimating the potential for a catastrophic release which may cover more than one unit and thus underestimating the blast loads. When FSS/QRA’s are conducted you have to consider the multiple dangerous and costly pitfalls that are encountered when safety studies aim at simplification in order to achieve cost savings and/or conservative results.
