Developing a Fire Model For Offshore QRA

An offshore installation normally houses complex facilities for a range of production processes. Space limitation leads to closely packed equipment and pipework and, subsequently, the likelihood of escalation from a relatively small fire is much higher compared to onshore installations. A riser failure occurred during the Piper Alpha incident in 1988 was thought to have contributed to the platform collapse and a major loss of life.

CFD models are the tool to accurately predict fire dynamics in offshore installations. They solve the fundamental equations of combustion with explicit account for the influence of boundaries (e.g. walls, decks, large equipment items) on the development of a fire.  However, due to the high demand on resources (e.g. cost, manpower and time) and the large number of scenarios to be considered in a QRA, CFD based analysis becomes impractical for Quantitative Risk Assessment (QRA). Instead, simple fire models developed for onshore scenarios, such as the jet fire model by Chamberlain et al & the pool fire models reviewed by Mudan et al, are normally used. Whilst the simple models offer quick solutions to very practical problems, they also make the analysis rather subjective, inconsistent and less accurate.

Modfire is a new empirical fire model developed specifically for offshore QRA. It extends the application of simple fire models mentioned above to the offshore environment. Using discharge results as the input, Modfire predicts flame shapes with consideration of the interaction between flames and boundaries, without the disadvantages which have prevented CFD models for being used routinely for QRA.  The predicted fire characteristics are dependent on release conditions, boundary geometries and ventilations. The model also considers escalation of boundaries by fires and fire escalation into the surrounding areas.

This paper presents the modeling approach of Modfire and validation work undertaken. Current validation against CFD results of a medium-size offshore platform has been satisfactory in the event of jet and pool fires.