For production, process or storage facilities which have potential major hazards, it is necessary to demonstrate that the risks to the public, staff, plant and the environment are as low as reasonably practicable (ALARP). Assessment of risks often benefits from detailed numerical analysis using quantified risk assessment (QRA). A QRA model can help identify the hazards that are driving the risk and simplify the evaluation of options for risk reduction. The model must have a clear audit trail to minimise the work for future updates and to simplify checking, both in-house and externally by organisations such as regulatory bodies or insurance assessors.

Woodhill has developed an integrated QRA model which has a transparent audit trail from the source data to the risk summary. Each potential major accident is modelled using event trees to determine the possible outcomes. The risks, in terms of fatality rates or critical system failure rates, are then estimated for each event outcome in assessment tables. A summary of risks for all accidental events is given in tabular and graphical format. The model is quantified using event and reliability data from industry accepted published sources.

The integration in the model enables multiple sensitivity studies and cost benefit analyses to be completed within the tight schedules that often apply to construction projects. For existing facilities, the integration assists the ranking of risk reduction against expenditure, to obtain maximum improvement for the facility within budget constraints, and to demonstrate ALARP.