Currently, explosion loads can only be specified realistically towards the end of detailed design. This has led to late design modifications and project delays. To date, little work has been done on using the available experimental data to generate better design guidance. At an early project phase, there is a need for some quantitative guidance on the characteristic magnitudes of the explosion loads to be used in any given situation, ideally without recourse to detailed Computational Fluid Dynamics (CFD) simulation. Previous work has been performed to produce 'Nominal Overpressures' as an early indication of the magnitude of the explosion hazard, but the variability of the loading and the dynamic characteristics of target structures makes the derivation and use of valid nominal overpressures difficult. It has been observed that the Fourier transforms (or spectra) of explosion pressure traces have a very similar form for a given release/dispersion/ignition scenario during the development of the explosion and throughout the explosion region. This similarity of form has also been observed between different scenarios and between test results and CFD simulations. The work described in this paper is ongoing and aims to drive 'envelope' or generic response spectra which may be used for design and can be interpreted as representing equivalent static loads for a wide variety of explosion situations and target structures. This approach is similar to that already used in earthquake response analysis.
The response spectrum approach takes into account the variations in response of structural elements resulting from their differing natural periods and enables the reserves of strength released when elements are allowed to deform plastically to be taken into account. The response spectrum approach has been in use for decades in the earthquake response context and was in fact pioneered in the Second World War to calculate ground motion effects and structural response from explosions . Figure 1 at the end of this paper shows the application of blast response spectra in determining a static design pressure. The severity of the blast loading is determined from local conditions by the use of nominal overpressures, previous experience, risk classification, simulations or experiment. The structural element to be assessed may be a panel, a deck, module or a whole topside if they can be idealized as a one degree of system oscillator. This process will be familiar to designers who use the Biggs response method  and is in routine use. The structural element is represented by its natural period and resistance at effective yield. A further important parameter is the allowable ductility of the element which is a measure of the amount of deformation an element can sustain before rupture or when its performance standards cease to be satisfied. This is usually expressed as a multiple of the peak elastic displacement. The allowance of local plastic deformation is an essential part of efficient blast resistant design.