CFD Modelling of the Dispersion and Burning of a Limited Oil Inventory from an Offshore Installation

Abstract

Abstract During management of an accident aboard an offshore platform, it may be advantageous to jettison flammable inventory in order to minimise the risk of escalation. This paper reports a study, using Computational Fluid Dynamics (CFD) techniques, of the dispersion of jettisoned oil, its subsequent burning following ignition, and the hazards posed to personnel and topside structures. The modelling allows the impact of such scenarios on the platform's emergency arrangements to be assessed. Preliminary indications are that jettison of flammable liquid inventories may be an acceptable response to a major accident. Quantified Risk Analysis (QRA) would indicate if this response is appropriate to a defined scenario. Introduction The accidental release and subsequent ignition of crude oil on an oil production platform can present hazards to the structure and the personnel on the platform from the following mechanisms:Direct flame impingement, or transfer of radiant heat to the structure which, if sufficiently weakened by the consequent rise in temperature, could result in progressive structural collapse. The same phenomena causing failure of equipment containing additional flammable materials may reduce the time to the onset of structural failure because of an increase in fire size. Such a structural failure would present an immediate and severe hazard to personnel on the platform even if they were unaffected by the initial incident.Crude oil fires, especially if these are oxygen limited to any significant degree, will produce large volumes of dense smoke. This smoke may present a hazard to personnel attempting to shelter from the direct effects of the fire or while effecting escape and evacuation from the platform. Methods employed to manage such hazards on offshore platforms include inventory depressuring systems and active and passive fire fighting systems. Automatic isolation and inventory depressuring systems attempt to limit the inventory of flammable materials released and thus the duration and intensity of any subsequent fire. The systems are particularly effective in the management of hazards associated with flammable gas releases but less so for those associated with flammable liquid releases. This is because the flammable liquid inventory on a typical oil production platform will typically be concentrated in a few large process vessels such as gas/liquid separators. If the release of flammable liquid is associated with one such vessel then these systems cannot limit the inventory released, but only the rate of release of the inventory. Active fire fighting systems, such as fixed water deluge or foam systems, can be extremely effective in rapidly bringing under control two-dimensional liquid fires, if they are designed and maintained to the requirements of an accepted code of practice such as NFPA 15 or NFPA 11. Fixed fire water deluge can under some circumstances extinguish such fires, but cannot be guaranteed to do so. An experimental programme is currently ongoing in the UK to determine the probability of success of this fire extinguishing method and to identify the factors which limit this probability. This project is sponsored by the UK Health and Safety Executive, British Gas plc, Elf Enterprise and Mobil North Sea Limited. Fixed foam fire fighting systems, if correctly designed and maintained, should be capable of extinguishing such fires before the onset of any significant structural damage. P. 377