Quantitative Risk Analysis (QRA)
A risk analysis comprises calculation/estimation of the probability of accidents as well as of the loads from the accidents. Such analyses are called QRAs – Quantitative Risk Analyses. A QRA could be comprehensive and take up to 6 months if the facility is large and when advanced models is required for flow, fire and explosion simulations. For small facilities or when only analytical models are used, the QRA could be completed within a period of 1-2 months.
There are several objectives of a QRA. The two most apparent are;
- Compare calculated risks with the acceptance criteria given by the Operator and applicable Safety Legislations. Risk criteria are often expressed as a maximum tolerable frequency value (probability) of fatalities, of asset damage and of environmental impact.
- To use the results for defining Design Accidental Load (DAL), i.e. to describe the accidental loads (e.g. blast pressures) for which the facility has to be designed to achieve an acceptable level of safety.
A QRA can be used for verification purposes only or as a tool to improve safety in the design and planning phase when changes have minor cost and weight impact.
A QRA is preceded by a HAZID and shall estimate risks associated with the hazards having the potential to develop into major accidental events. Computer codes used in QRAs are; ASAP, FLACS, KFX.
Example of Process Flow Diagram modelled in the risk analysis tool “ASAP”: