Numerical Simulation of Offshore Gas Explosions
Authors: van den Berg A.C. , van Wingerden C.J.M. 
Published: October 2008

Modern computational fluid dynamics makes it increasingly possible to simulate all aspects of the process of a gas explosion by computation. The TNO Prins Maurits Laboratory is developing software for this purpose. The authors demonstrate that this software is a useful tool in the evaluation of gas explosion safety of offshore installations.

INTRODUCTION

At about 10 p.m. on July 6, 1988 the offshore production platform Piper Alpha in the North Sea was struck by a gas explosion in the compressor module. The explosion crushed a fire wall and damaged a control room located behind it. As a consequence, vital control and communication functions failed. A fire started which, 20 minutes later, resulted in the failure of a high-pressure transport gas pipe line. In the resulting sea of flames, 167 people were killed and the installation was totally destroyed [1].

Even under the most preventive conditions, the possibility of gas explosions can never be fully ruled out. Therefore, such incidents should be anticipated in the design, for example, of offshore installations. The shape and the layout of the various compartments on board, for example, should be chosen in such a way that explosive, blast generating combustion cannot develop. In particular, vital facilities for people and installations such as life-saving, control and communication equipment should be placed in positions where they cannot be struck by the destructive power of a possible gas explosion. The dramatic events on the Piper Alpha have shown how, by a lack of awareness of this safety aspect, a small-scale gas explosion can escalate into a disaster of unprecedented proportions.

To enable a gas explosion-safe design, the characteristics of gas explosions should be known in detail. What pressure loadings are produced by a gas explosion on confining structures and what blast effects are to be expected?

Modern computational fluid dynamics makes it increasingly possible to simulate all aspects of the process of a gas explosion by computation. The 1NO Prins Maurits Laboratory is developing software for this purpose [2,3]. In this paper, the capabilities of this software are demonstrated.

GAS EXPLOSIONS

Basic mechanism

If a quiescent fuel-air mixture is ignited, it is consumed by a combustion process which initially propagates through the medium in the form of a thin laminar flame front. Laminar flame propagation is largely determined by molecular transport of heat and species. Therefore, laminar flame propagation is a slow process. The expanding combustion products, however, induce a flow field by which the flame front is convected. If the boundary conditions of this flow field are such that turbulence is generated, the combustion will interact with the turbulence. The turbulence increases the flame front area - the interface between reactants and combustion products - by which the combustion process intensifies. More reactants are converted into combustion products per unit of volume and time, resulting in stronger expansion flow. Higher flow velocities go hand in hand with more intense turbulence which, in turn, intensifies combustion etc., etc..