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Consequence Modelling and QRA

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Page revision date - 21 November 2019

COMAH Consulting Ltd -Consequence modelling and Quantitative risk assessment (QRA)

Fig 1: Consequence Phenomena and their Interrelationship.  Copyright and source acknowledged as The International Association of Oil and Gas Producers.  Report No. 434-7, “Consequence Modelling”, March 2010.

Flow diagram extract from - Consequence Phenomena and their Interrelationship. Copyright and source acknowledged as The International Association of Oil and Gas Producers. Report No. 434-7, “Consequence Modelling”, March 2010.

Step 2 of the risk assessment process ( - “Decide who might be harmed and how” can often be a complex stage for major accident hazards.  For example, the same loss of containment incident at an installation could easily foreseeably have a multitude of different outcomes depending upon factors such as: the weather at the time, the time of day, whether ignition occurs and when; and whether the leak is impeded or not. 

Consequence modelling is the analysis that establishes how far reaching the hazardous event can be, how many people could foreseeably be affected, and to what degree.  This work helps to establish 'SEVERITY' and 'EXTENT'. (see also 

For any given scenario the calculations for explosion over-pressure, thermal radiation levels, toxic dose etc… are very specialised and very complex. 

The real skill in consequence modelling is selecting the scenario to model and then interpreting the result.  A chemical release from a vessel could actually be represented by an almost infinite number of potential cause / consequence scenario combinations (see fig 1 below).  Analysis of the full range of theoretically possible consequence scenarios is unlikely to be proportionate or practical, and so the analyst must select a number of representative scenarios to capture the breadth and range of theoretical scenarios.  If the selection of scenario is inappropriate then the resulting risk assessment will either be inadequately optimistic or overly conservative leading to excessive spending on safeguarding arrangements.  Further, the resulting assessment findings are likely to be insufficient in making the COMAH demonstrations. 

Fig 1 Below shows the potential harm pathways for a release of hazardous material.