The Prevention and Mitigation of Gas Explosions
Authors: Johnson D.M. , Vasey M.W. 
Published: December 2007

Research has shown that following an accidental release of gas into a process area, the severity of an explosion resulting from the ignition of the gas/air mixture formed is increased by the presence of closely packed plant and pipework within the gas cloud, particularly when the process area is partially or completely confined. Such conditions exist on many onshore petro-chemical sites, but are especially characteristic of offshore oil and gas platforms in the UK and Norwegian sectors of the North Sea. The potential for a gas explosion on such platforms to cause damage and further loss of containment of flammable material was tragically illustrated by the Piper Alpha disaster, in which an explosion initiated a sequence of events which rapidly led to the total destruction of the platform.

There are a range of factors which can influence the likelihood and severity of a gas explosion, such as the size of the gas cloud and the concentration of gas within it. Whilst it may not be possible to design a facility to withstand the "worst case explosion overpressures, the overall risk presented by gas explosions can be minimised by considering prevention, control and mitigation measures for explosions as early as possible in the design process. Such measures include ventilation, platform layout, explosion relief and the use of water sprays. This broad approach of attacking all of the factors which contribute to the risk from explosions can significantly reduce the probability of an accidental release of flammable gas leading to major structural collapse and potential loss of life.

Introduction

The delayed ignition of an accidental release of flammable gas or vapour can, under particular conditions, lead to an explosion which generates damaging overpressures. Further larger releases of fuel can then be caused with the potential for the total loss of a petro-chemical facility. Examples of such events in the UK are the explosions which occurred at Flixborough in 1974 and on the Piper Alpha platform in 1988.

There is a clear need to examine the possible effects of gas explosions on structures and to identify practicable means of reducing the potential consequences, in terms of risk to personnel and damage to structures. This paper highlights many of the important factors related to pressure generation in gas explosions and relates these, by the use of large scale experimental data, to the conditions present on many offshore oil and gas platforms in the UK and Norwegian sectors of the North Sea. Techniques for modelling gas explosions on such facilities are also considered, particularly in relation to their use in the design of new installations. Finally techniques which can be used for the management of explosion risks by prevention, control and mitigation are described with particular attention being given to recent work on the use of water sprays to mitigate gas explosions.

Generation of Overpressure in a Gas Explosion If an accidental release results in the build-up of a flammable mixture in a confined volume and ignition subsequently occurs, a flame will propagate through the gas cloud, Because the flame produces high temperature combustion products and the gas within the volume cannot expand to occupy a larger volume, the pressure within the confined volume rises, Explosions in largely empty, confined volumes have been the subject of study for a number of years, particularly with reference to domestic buildings and gas fired industrial and commercial plant. This type of explosion generally involves a flame propagating at a few metres per second within the confined volume and can theoretically generate a gradually increasing overpressure, up to a maximum of the order of 8 bar, if the structure remains intact.

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