QUANTITATIVE RISK ASSESSMENT STUDY

Quantitative Risk Assessment (QRA) is a formalized specialist method for calculating numerical individual, societal, (employee and public) risk level values for comparison with regulatory risk criteria.

Objective

The objectives of a QRA study are as follows

• Hazard identification and selection of failure case scenarios
• Calculation of physical effects of failure case scenarios which include Estimation of Jet Fire heat radiation distances, Flammable and Toxic gas dispersion distances, and Vapor cloud explosion distances.
• Estimation of Failure Frequency.
• Individual risk quantification and contour mapping.
• Societal risk quantification and projecting FN curve.
• Perform a risk assessment to confirm that risk can be reduced consistent with the ALARP principle as per the project specific risk acceptance criteria.
• Recommend risk reducing measures to ensure that all risks are in ALARP or in Acceptable region.

Methodology

The initial key step is the identification of the release scenarios, which is based on Hazard Identification process usually carried out by internal review of the process design flow diagrams and instrumentation drawings, and layout configurations. Process conditions such as operating pressure, temperature, density, molecular weight, specific heat ratio and stream compositions shall be taken from Heat and Material Balance sheet. Once the scenarios are defined, then these are evaluated further for their potential frequency of occurrence and consequence hazard zone.

The frequency analysis is based on the facility equipment count paired with historical frequency data. Failure rate data is generally derived from generic sources, if insufficient case specific data is available. It is important for the estimation of the frequency of releases of various sizes (and the consequence assessment) to take account of the full range of pipework sizes and equipment that will be present. Component counts, based on the P&IDs supplied by the client, can be used for this purpose.

The consequence modelling takes account of the potential harm to people using defined criteria. Consequence analysis involves modeling at different stages of a release from outflow through a hole or from a pipe end, through atmospheric dispersion, rain-out and re-evaporation of liquid, to thermal radiation from fires, explosion overpressures and toxic lethality.

The risk result is estimated by the combination of the scenario frequency and consequence with potential impact to the facility and personnel. The risk calculations take each release in turn and combine the dispersion and fire modelling results with the frequency, population and meteorological data to calculate risks to people.

Risk may be presented in many formats: Individual Risk LSIR contours, IRPA values, Societal Risk FN curves, Risk Ranking tables etc.

Assessment of risk levels: Once the overall risk level is known, it can be compared against risk acceptability criteria to determine whether the risk is acceptable.

If the risk is not acceptable, additional risk reduction measures may be required to reduce the risk level. The study will identify and highlight any potential risk reduction measures but will not fully assess their effect on the risk.