Partially Confined Explosions Under Nonoptimal Explosion Conditions
Authors: Allen D.J. 
Published: October 2008

ABSTRACT
Gas explosions present a particular problem on offshore platforms because of the threat of widespread damage and the difficulties of mitigation and protection. Whereas jet fires may be controlled by segregation, deluge and blow-down, the only recognized defences against an explosion are venting and containment.
To have confidence in the results frequency and consequence predictions it is necessary to extend the definition of an explosion beyond idealized experimental conditions. This paper examines the influence on the consequences of partially confined explosion of various conditions which have not been conclusively investigated by experiment.

INTRODUCTION
Nobody involved in hazard assessments on offshore oil and gas installations will be unaware of the problems in assessing the scale of partially-confined explosions and their consequences. Any trustworthy predictive method must take account of the underlying physical factors which govern the development and behaviour of a rapidly burning gas cloud interacting with its surroundings, and must give results which both agree with experimental evidence and which ?make sense'. One of the problems in developing suitable prediction tools is that no satisfactory scaling laws exist to enable the complexities of a real installation to be represented by scale models in the same way, for example, that wind tunnels are used. Without this facility any experimentally-based predictions must overcome not only scaling problems but also be flexible enough to apply to untested layouts and configurations.
Apart from scale considerations, the actual consequences of an explosion will also be governed by factors such as the composition, location and size of a gas cloud. ?These factors are unquestionably significant, but very little information is available to the Risk Analyst. Most experimental testing has considered 100% module fills, usually perfectly mixed and at or close to the most explosive initial mixture. Even under these idealised initial conditions the repeatability of explosion tests is said to be relatively poor, usually of the order of ±25 % for peak overpressures. Considering the expense involved in large scale testing, it is probable that there will never be comprehensive experimental evidence for conditions such as fractional module fills or non-homogeneous gas clouds. In the absence of suitable experimental data any predictions must rely either on case-history evidence or theoretical modelling.
Findings presented in this paper arise either from a review of published data or from calculations by the author using a 2-D explosion modelling program. It has been attempted to deal with the various factors affecting explosions in a logical sequence and to account for inherited wisdom and prior information as well as explosion modelling results. The paper deals with the following subject areas in the order shown below:Review.Predictive methods.Gas composition.Ignition location and sources.Confinement factors.

REVIEW
The following paragraphs summarise some of the findings from prior analysis into partially confined explosions. This information is presented to underpin the explosion overpressures discussed in this paper, including those predicted by the author.