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Explosion Hazard Assessment


A wide variety of types of explosion may occur on offshore installations. These include unconfined explosions (overpressure generated by presence of obstacles), confined explosions (overpressure generated through a combination of confinement and obstacles), external explosions (a phenomenon associated with confined, vented explosions), internal explosions (e.g. within a flare stack), physical explosions (e.g. a failing pressure vessel), solid phase explosions (e.g. associated with use of well completion explosives), mist explosions and BLEVE's.

Explosions represent a significant component of the topsides fire and explosion risk on most installations. Over the 25 year period 1973-97 there were 10 significant (> 0.2 bar) explosions on offshore installations in the North Sea, 8 of which occurred in the UK sector, most during the 1980s (Vinnem, 1998). More recent data for the period 1992-99 shows that there have been 10 explosion incidents on UK installations, most internal explosions associated with gas turbines or flare systems.

Current position

Large Scale Experimental Data

There is now a considerable body of experimental data from large-scale tests that has greatly improved our understanding of how gas explosions behave in offshore installations. New phenomena have been observed which were not predictable by the models available at the time, for example the significant enhancement of the explosion pressure by the small-scale obstacles. In many cases, however, no in-depth analysis of the results has been undertaken and as a result the opportunity to gain even further insight into the explosion mechanism remains. Data from the large scale test is largely confined to measurements of overpressure and flame arrival time, however more detailed information (e.g. on flame speeds) is required for rigorous model evaluation and development.

Realistic Release Cases

The large-scale tests undertaken in Phase 2 and 3a of the JIP were concerned with quiescent, stoichiometric clouds filling the entire volume of the module. However there remains a question as to whether a realistic release, involving significant turbulence, could give rise to greater explosions overpressures than those observed in the tests. Phase 3b of the JIP addressed realistic release cases and the initial findings from this indicate that in some cases rapid filling of the module could occur with gas at near-stoichiometric concentrations. While lower overpressures generally occurred in the realistic release tests uncertainty still remains concerning whether this is always the case.

Mist Explosions

For equipment containing volatile liquids at elevated pressure, two-phase or 'mist' releases are possible. There has been limited experimental study of mist explosions and the mechanisms operating in mist explosions remain poorly understood. Because of the difficulty in characterising the initial conditions, their study is also experimentally challenging. In particular there is uncertainty in whether mists for certain drop sizes could generate explosions more violent than that of an equivalent vapour concentration.

Modelling capabilities

The explosion models currently available may be categorised as follows:

A critical review of modelling and suggestions for future areas of research and development up to 2000 is given in HSE (2004).

The empirical models have a limited range of applicability, cannot deal with complex geometries and have simplified the physics considerably. Nevertheless, these methods are useful for quick order-of-magnitude calculations and for screening of scenarios warranting further investigation with more sophisticated tools.

The phenomenological models are slightly more complex than the empirical models. They have a less limited range of applicability than empirical models, are essentially fits to experimental data and have a lower level of uncertainty than empirical models. They do not attempt to model the actual scenario geometry but instead represent it in a simplified manner, e.g. as boxes connected by corridors. The models are relatively easy to use, with modest computational requirements and therefore are suitable for use where large numbers of calculation runs must be made, as in exceedance curve generation.

CFD models can be divided into two groups; simple and advanced models. The distinction between the two groups, albeit somewhat arbitrary, is that the advanced models will attempt a more complete description of the physical and chemical processes involved, including better representation of the geometry and the accuracy of numerical schemes. To illustrate this difference, consider one of the key findings of Phase 2 of the JIP, i.e. the importance of taking into account small scale obstacles. The simple CFD models make use of porosity/distributed resistance (PDR) models in order to avoid having to resolve the smaller scales obstacles, while the advanced CFD models attempt the resolve the objects using adaptive mesh refinement. Compared to the phenomenological and empirical models, CFD offers the prospect of greater accuracy and flexibility, however computational run times are long and the scope for errors is greater. Particular areas of uncertainty are:

Industry practice

Explosion hazard assessments undertaken for offshore installations may vary widely from simple assessments using empirical models to complex analyses using multiple CFD simulations. Key issues identified from a review of industry practice in this area are as follows:

Strategy development issues

Understanding of explosion phenomena

Experimental data

Realistic releases

Mist explosions

Explosion modelling

Short term

Longer term

Updated 2012-12-13