What is quantitative risk assessment (QRA)?
Quantitative risk assessment (QRA) puts a number on the risk from a facility by combining how often hazardous events happen with how severe they are. The output is not a single figure but a set of risk measures — individual risk contours, societal risk F-N curves, and risk to specific occupied buildings or escape routes.
The QRA workflow
- Define the scope: study boundary, inventories, isolatable sections and receptors.
- Build the failure case list: hole sizes per equipment type, usually a small, medium, large and full-bore set.
- Assign frequencies from parts count and generic leak frequency data.
- Model source terms for each case — release rate and duration from the isolatable inventory.
- Model consequences: dispersion, jet and pool fire radiation, flash fire envelopes, explosion overpressure.
- Apply event trees for ignition, immediate versus delayed, and explosion versus flash fire.
- Apply vulnerability (probit) models to convert physical effects into fatality probability.
- Sum over all cases, weather classes and wind directions to produce risk results.
Frequency: the parts count
Frequency comes from counting equipment — flanges, valves, instrument connections, pipe length by diameter, vessels, pumps — and multiplying by generic leak frequencies per hole size band. The parts count is the single most audit-visible input in a QRA, and small errors there scale the whole risk result linearly.
Consequence and vulnerability
Consequence modelling produces physical effects: concentration, thermal radiation flux, overpressure. Vulnerability models turn those into harm. Thermal fatality is usually a dose-based probit combining radiation intensity and exposure time; toxic fatality uses a concentration-time dose; overpressure fatality is usually tied to building damage rather than direct blast injury.
Reading the results
Individual risk per annum (IRPA) or location-specific individual risk is plotted as contours across the site — the risk to a hypothetical person present continuously at each point. Societal risk plots cumulative frequency against fatality count, and is compared against a tolerability criterion band.
The value of a QRA is rarely the absolute number, which carries large uncertainty. It is the ranking: which scenarios, which equipment groups and which locations dominate the risk, and therefore where risk reduction money should go.
Frequently asked questions
What is the difference between QRA and LOPA?
LOPA is a scenario-by-scenario, order-of-magnitude check that enough independent protection layers exist to meet a target frequency. QRA is a facility-wide summation across many scenarios producing spatial risk measures. LOPA sizes safeguards; QRA answers whether overall risk is tolerable.
What is ALARP?
As Low As Reasonably Practicable — the principle that risk must be reduced until the cost of further reduction is grossly disproportionate to the benefit. In practice risk between the intolerable and broadly acceptable bands must be demonstrated ALARP with documented option assessment.
How accurate is a QRA?
Absolute risk numbers are typically uncertain by an order of magnitude because leak frequency and ignition probability data carry wide bounds. Comparative use — ranking options and locations — is far more reliable than the absolute value.
Calculators for this topic
- Gas Discharge — Gas discharge rate calculator for pressurised releases
- Gas Dispersion — Atmospheric gas dispersion calculator
- Flare Radiation — Flare thermal radiation calculator