Techniques for modelling the frequency of explosions on offshore platforms

This paper describes a scheme by which the explosion frequencies and resulting overpres- sures can be predicted for modules on an offshore platform. Given a leak of gas, the success and failure of the isolation, blow-down and deluge systems, which are designed to activate following a leak to mitigate the consequences, are considered. The frequency of each possible response to the leak is predicted using a conventional event tree and fault tree approach. Each scenario has a potential escape inventory, which together with the hole size distribution provides the escaping mass flowrate. Once the flowrate into the module is established, the wind conditions are used to determine the gas in air concentration build-up. The concentration profile with respect to time is linked to the ignition source probabilities, from which calculations can be made to assess the likelihood that an explosion will occur within a specific concentration range. The initial models assume that the gas concentration at any specific time is uniform within the locality of the leak. The work presented in this paper proposes an approach that determines the explosion probability dependent on the volume fraction of gas within the module and the likelihood of encountering an ignition source.