Quantitative Risk Analysis (QRA)

Flare used in Quantitative Risk Analysis

Quantitative Risk Analysis (QRA) provides a numerical measure of the risk a facility or operation poses to the public, to site workers, or to onsite or offsite equipment. The study begins with the identification of potential hazardous events.  The typical hazards considered include:

  • Toxic vapor clouds
  • Flammable vapor clouds (flash fire)
  • Vapor cloud explosions (overpressure)
  • Torch or jet fires (radiation)
  • Pool fires (radiation)
  • BLEVE fireballs (radiation)

With the above hazard types, failure case scenarios are selected to cover the potential impacts of concern, within the scope of the study.  Each scenario is modeled using the CANARY software to arrive at a set of consequences that describe the potential hazards.  Consequence modeling for hazard extents is conducted for many variations of the failure case:

  • Multiple accident outcomes (the various hazards)
  • Site-specific meteorological variations
  • Flammable fluid ignition (immediate, delayed, or none)
  • Vapor cloud explosion (dependent on a flammable cloud being ignited when within a confined or congested space)

For each variation of the failure case, a probability assessment is conducted. Beginning with the process equipment group that is associated with a failure case, the probability of each unique outcome is a product of:

  • Equipment failure rates (failures that result in a hazardous fluid release)
  • Conditional probabilities for flammable cloud ignition (immediate or delayed)
  • Conditional probability of weather parameters (wind speed, atmospheric stability, wind direction)
  • Conditional probabilities associated with equipment use or occupancy

Once the consequences and probabilities are defined, they can be combined into measures of risk associated with the failure cases included in the study.  These measures of risk are further influenced by the endpoints chosen to define the hazards and the population density and distribution assigned to the subject(s) of the risk analysis.  Quest can present the results of a QRA in a concise manner, using risk presentation methods including:

  • Location-specific individual risk (risk contours, or LSIR)
  • Individual risk per annum (IRPA)
  • f-N curves (societal risk)
  • Other measures such as risk tables or PLL comparisons

The last step in a quantitative risk analysis is comparison of the results to a company’s risk criteria, or to acceptable risk criteria/guidelines published by governmental organizations.  In addition, we have experience and expertise presenting these results in a regulatory setting to various agencies around the world.