Design modelling to minimise the risk for offshore platforms

Safety cases must be produced by offshore operators to assess the risks posed to the personnel by potential accidents. On an offshore platform two of the major hazards are fires and explosions resulting from an accidental hydrocarbon release. The overpressures generated during an explosion can threaten the integrity of the platform structure. It is therefore important to be able to estimate the overpressures generated, should an explosion occur, and to predict the frequency of such an event. A methodology has been developed to predict the frequency of explosions of different magnitudes occurring in a module on an offshore platform. This methodology combines established risk assessment techniques, such as event tree analysis and fault tree analysis, with fluid flow modelling. Assumptions have been made in the methodology to simplify the calculation procedure. These assumptions relate to the conditions under which the leak occurs, the build up of gas in air concentration and the probability calculations. Frequency predictions are required to be as accurate as possible to enable the acceptability of the risk to be determined and reduced to a level which is as low as reasonably practicable. Hence each of the assumptions within the methodology has been addressed, to determine a more complete prediction tool. Once an accurate frequency for the explosion occurring has been determined, the risk to personnel must be minimised to an acceptable yet practical level. On existing designs it is impractical to alter the layout of the platform. However the nature of the safety systems may be changed. These safety features include isolation, blowdown, mitigation and detection systems. An optimisation study presents three schemes to identify the optimum configuration of the safety systems, in terms of the overpressures generated, as a means of reducing the risk to the platform