Quantitative Risk Assessment (QRA)
We model risk numerically so the result withstands HSE, COMAH, MoEFCC and underwriter scrutiny
Quantitative Risk
Assessment (QRA)
Quantitative risk assessment puts numbers on the risk to individuals through measures such as location specific individual risk and individual risk per annum, to communities through societal risk and the F to N curve, and to the environment from the credible major accident set. Done well it is the analytical bridge between the scenarios that HAZID and HAZOP identify and the risk based decisions your team makes on siting, layout, occupancy, emergency response and demonstration of ALARP. Our modern QRAs rely on validated frequency banks such as OGP 434, the HSE failure rate data and the RIVM Purple Book, on consequence modelling in PHAST, SAFETI and FLACS, on ignition probability drawn from the Cox and Lees correlation, on careful atmospheric stability handling and on meteorological time step weighting. The challenges that distinguish a defensible QRA, which include realistic congestion and confinement for vapour cloud explosion modelling, sound indoor receptor and toxic load criteria, and honest justification of high impact low frequency cases, are exactly where we judge our own execution quality.
How the study is executed
A structured, facilitated process that runs from scope definition through close out and produces defensible, actionable outputs.
Compile major accident hazard inventory from HAZID/HAZOP, define equipment envelope (MAWP, temperature, phase, inventory mass), select credible loss of containment scenarios (rupture, leak, BLEVE, toxic release) for the QRA scenario register.
Derive leak frequencies from OGP 434 / HSE FRED / IOGP failure data, calculate source terms (orifice, fragmentation, two phase flash, cryogenic) for each representative scenario class, document frequency basis and uncertainty band.
Model thermal radiation (pool fire, jet fire), explosion overpressure (Multi Energy / BST / CFD), BLEVE fireball, and toxic dispersion (AEGL, ERPG, IDLH) in PHAST / SAFETI / FLACS, apply meteorological time step weighting.
Apply Cox Lees ignition probability model with congestion and confinement uplift, calculate conditional modifiers (presence factor, escape probability, weather fraction, time at risk) per scenario, derive final event frequency set.
Calculate Individual Risk (LSIR / IRPA) with GIS integrated contours, compute Societal Risk F N curves with national tolerability overlay, aggregate PLL by major accident hazard for dominant contributor identification.
Compare risk results against HSE R2P2 / CCPS tolerability criteria, run risk reduction option appraisal with capex / risk delta CBA, produce ALARP demonstration, sensitivity / Monte Carlo analysis, and safety case ready documentation package.
What the study covers in full
Outcomes of Quantitative Risk Assessment (QRA)
- We quantify harm distance for the credible major accident set both onshore and offshore
- We surface the dominant risk contributors that qualitative methods miss
- We anchor realistic emergency planning zones and your shelter in place strategy
- We support the structured decision dialogue that the CCPS risk information network approach encourages
- We make the work defensible under HSE PADHI consultation and COMAH safety case examination
- We deliver EPA RMP off site consequence analysis with regulator grade rigour
- We meet the MoEFCC, Factories Act and Seveso III risk demonstration
- We supply the IEC 61511 safety instrumented function target risk reduction factor inputs
- We identify the small fraction of equipment that dominates your site risk
- We drive inspection, isolation and detection design to where it pays off most
- We inform the siting of occupied buildings, control rooms and muster points
- We enable risk based MOC review against an objective baseline
- We target your capex to demonstrable risk reduction rather than perceived compliance
- We improve your underwriter pricing through objective risk evidence
- We defend you against gross disproportion challenges in ALARP arguments
- We help you avoid costly retrofits driven by late discovery of dominant scenarios
Codes & standards we work to
Triggers that signal the need
Where Quantitative Risk Assessment (QRA) applies
Wellheads, separators, gas compression, FPSO topsides, produced water systems.
Distillation columns, reactors, heat exchangers, storage spheres, LPG handling.
Cryogenic exchangers, liquefaction trains, BOG compressors, storage and sendout.
Reactive systems, batch reactors, solvent handling, runaway reaction scenarios.
Boilers, HRSGs, steam headers, hydrogen systems, ammonia SCR units.
Sterile vessels, CIP/SIP, pressure fermenters, solvent recovery, spray dryers.
Tangible deliverables
- QRA report with methodology, assumptions and an uncertainty register
- Frequency basis document with leak bank derivation
- Consequence modelling pack with PHAST, SAFETI and FLACS outputs
- Location specific individual risk and individual risk per annum contour set in a GIS compatible format
- F to N curve with a national or site specific tolerability overlay
- Potential loss of life summary by major accident hazard
- Risk reduction option appraisal with capex and risk delta
- ALARP demonstration and gross disproportion calculation
- Sensitivity and Monte Carlo analysis
- Safety case input pack with traceable references
Ready to start your project?
Speak with our team to scope an engagement tailored to your facility, regulatory context, and lifecycle stage.