QRA(Quantitative Risk Assessment) is a method used to systematically calculate the risks from hazardous events. It involves predicting the size of consequences associated with a hazard, and the frequency at which a release of the hazard may be expected to occur. These aspects are combined to obtain numerical values for risk – usually risk of fatality. In order to quantify the overall risk levels, consideration of all identified hazardous events is important. Similar hazardous events are often grouped and assessed together as bounding events.
The traditional QRA approach shall be followed, which comprises the following steps:
The Quantitative Risk Assessment methodology is summarized as following
Hazard Identification is the primary step in Risk Analysis. Many techniques are available for hazard identification depending upon the objective of the study. The most relevant technique of risk assessment is the review of release sources of hazardous materials.
The isolatable sections are identified for the process under study. For each isolatable section, failure scenarios are developed depending upon the failure mode causing loss of containment, and for each failure scenario a set of consequences chains is developed showing the hazard outcomes.
Effects and Consequence Calculations are carried out to determine the potential for injury to people from the identified failure scenarios. The hazard outcomes are analysed using release rate, dispersion, and fire and explosion models. Appropriate models available in the software package of SAFETI OFFSHORE developed by DNV-GL shall be used for calculation of physical effects.
Key assumptions made in the calculations will be highlighted in the Assumption Register.
Likelihood of leak associated with each failure scenario is estimated based on generic leak frequencies from historical databases which include the TNO Purple Book, UK HSE, and UKOPA Pipeline Fault Database or OREDA Database.
Once the failure frequency of the failure scenario has been estimated, event trees are developed to represent the sequence of events from the onset of the initial failure. Using probabilities for each event branch, the event tree eventually determines the frequency of the final hazard outcomes.
Risk Summation combines the consequences and likelihood of all incident outcomes from all selected incidents to provide a measure of risk. SAFETI OFFSHORE developed by DNV-GL shall be used for Risk Summation. Based on the consequence and likelihood of each outcome and the associated fatality (given in the Assumption Register), the likelihood of the outcome resulting in a fatality is determined within the software and the overall risks are generated. The results of the Risk Quantification or Summation are presented in the form of Location Specific Individual Risk & Individual Risk Per Annum.
The following activities shall be undergone once risk estimation is completed.