Blast injury to people may comprise either direct effects (e.g. ear drum rupture) or indirect effects (injury due to flying debris). Blast damage to equipment of structures can result from either loading (applicable to large objects, e.g. walls) or drag loading (applicable to objects of narrow cross-section, e.g. pipework or primary steelwork) or a combination of the two. The extent of damage is dependent not only on the peak overpressure, but also the blast wave duration, impulse and rise time.
Of the 10 significant (> 0.2 bar) explosions which have occurred on offshore installations in the North Sea over the 25 year period 1973-97, all have resulted in significant damage to the installation, whilst 5 have caused injuries or fatalities (Vinnem, 1998).
Our understanding of the effects of explosions on people is, to a large extent, based on the observed far-field effects of conventional or nuclear weapons, which is of limited relevance to gas explosions on offshore installations. Our understanding of the effects of gas explosions on equipment and structures has advanced through the various phases of the JIP (Phase 2, Phase 3a and Phase 3b) each of which included some structural response experiments. However recent interpretation of some of this data has shown the importance of considering the response characteristics of a structure, as well as the explosion overpressure, in determining the loading imposed on such structures.
Current models for assessment of blast injury to people are based on the far-field effects of condensed phase explosions (as noted above) and therefore their application to gas explosions on offshore installations is highly uncertain. The prediction of explosion loading on equipment and structures (and their response) is likewise highly uncertain. This has been shown through the various model evaluation exercises undertaken as part of the JIP. Other aspects of concern include the prediction of localised explosion loads, e.g. on objects of narrow cross-section such as primary steelwork, and the uncertain validation of blast wave codes. The modelling of escalation (e.g. through blast or missile effects) is also an area of significant uncertainty, with little detailed guidance available.
Safety Case assessments of blast injury to persons are largely judgmental, making use of what little data there is of relevance to the near-field effects of gas explosions. Assessments of the effect of blast on equipment and structures is sophisticated for new installations (e.g. using CFD for explosion prediction and non-linear finite element analysis for structural response) but highly variable for existing installations, ranging from purely qualitative analysis to the use of advanced computational models or experimental techniques. Escalation modelling is performed to widely-varying degrees of rigour.