Investigation of Gas Dispersion and Explosions in Offshore Modules

Abstract

Abstract There has been considerable progress over the last decade in validating models to predict the loading resulting from an explosion in an offshore module. However, much of the experimental work that provided the validation of the models has involved using idealized gas clouds, generally natural gas or methane at a single concentration throughout the whole volume. This paper describes an experimental project and modeling work undertaken to provide a more realistic representation of the risks posed by gas explosions in offshore platforms. The project involved laboratory, medium and large scale experiments, examining the effects of realistic fuel mixtures, realistic gas release conditions and nonhomogeneous gas clouds filling only a portion of the module. Comparison with gas dispersion and explosion models was also carried out. Background The Blast & Fire Engineering Project for Topside Structures [1] was initiated in May 1990 to study the special explosion and fire hazards facing offshore operators. The objective of Phase 1 of this joint industry project (JIP) was to provide interim guidance to designers and operators to reduce the potential for a major disaster offshore as happened on the Piper alpha platform in 1988. In addition to providing this interim guidance, this project recognised that there was a need to obtain data on these hazards from full-scale experiments. Therefore, as part of the follow-on Phase 2 project, a series of experiments was undertaken to obtain quantitative data on the explosion hazard in full-scale geometries truly representative of the offshore environment [2]. The results of this project indicated that high explosion overpressures could be generated and that water sprays activated prior to ignition could significantly reduce these overpressures. As a consequence of these findings, the UK Health & Safety Executive (HSE) commissioned a further project, known as Phase 3A [3], in which experiments were undertaken to identify methods of reducing the severity of explosions. The Phase 2 and Phase 3A projects showed that it is generally not practical to design an offshore platform against the ‘worst case’ explosion. Therefore, a risk based approach for design against explosion hazards is needed in which the risks are reduced to as low as reasonably practicable. In taking a risk-based approach, it is important that the full spectrum of possibilities is adequately understood. This includes scenarios involving ignition of a non-homogeneous gas cloud occupying only part of the module. The Phase 2 and 3A projects had, however, been conducted with homogeneous gas clouds throughout the full module, and in most cases the mixture had been at a stoichiometric concentration. This was appropriate at the time, as it was necessary to understand of the scope of the explosion hazard and to provide well defined experiments against which explosion models could be validated. However, there was a clear need to investigate more realistic conditions.