An Approach to QRA Using Systems Modelling to Develop Event Frequencies

The traditional QRA in the Oil & Gas industry relies on historical data to account for the frequency of the loss of containment. Such approaches tend to emphasize mitigation over prevention and do not take into account process/cultural/procedural contributors specific to each operation which have important effects into the overall risk profile, and therefore are usually oversimplified in the traditional QRA approach.

 For over four (4) decades a variety of industries including, but not limited to, nuclear, aerospace, and DOE chemical facilities have used an approach to QRA that allows for modeling of systems and human actions that may contribute to loss of containment and therefore emphasize on “prevention” as well as “mitigation.” This approach still relies on historical data to develop and quantify the model for loss of containment. This is done for a variety of reasons that include:

1. Allows for modeling of complex, as well as simple, systems with significant dependencies.
2. Allows for modeling of human interface for actions that are intended to prevent or mitigate the accident as well as those actions that may initiate and/or accelerate accident progression.
3. Provides insights into “prevention” as well as “mitigation” of risk.

 In this approach the accident is studied in two phases: pre-loss of containment and post-loss of containment.

• Pre-loss of containment – Involves definition of the initiating events leading to the loss of containment, including “accident escalation” (or a.k.a. “accident progression”) scenarios. The initiating event could be a disturbance to the system which could include any deviation from the normal safe operating envelope. The disturbance may be caused by internal (pump failure, mal-operation of equipment, etc.) or external factors (weather, seismic, etc.). Depending on the complexity of the system, the ramification of the pre-loss of containment scenarios can be modeled using fault trees or simpler tools such as FMEAs. The complexity of the systems is usually assessed based on the amount of components involved in the containment system and the number of functional dependencies between these components and the human interface (i.e. the more complex the system becomes in terms of the dependencies or human interface).For simpler systems, other techniques such as FMEA can be used. The main inputs of the pre-loss of containment model comes from the component reliability data, and human reliability data.

• Post-loss of containment - The final consequence following the progression of each event derived from the initial loss of containment is modeled using an event tree. The frequency each consequence is calculated by multiplying the frequency of the initial loss of containment by each outcome frequency.
  

Such approach not only provides estimate and insights into risk, it is also a powerful tool in improving operability and protecting assets through better design, operation and preventive maintenance. This has been demonstrated in a large number of industries instituting formal risk management practices that have been accepted and encouraged by authorities having jurisdiction.The paper will present applications of this approach to process industries.