Possibilistic Analysis of Structural Systems Subjected to Blast Loads

The accidental release of the hydrocarbons and the possibility of resulting explosion have to be taken into account while designing the topside systems of the offshore structures. Determination of design explosion loads for the topside structures is a complex task since it involves several sources of uncertainty. Dimensioning of blast loads is important in achieving the desired safety level against the structural failure and related consequences. The design loads must incorporate uncertainties due to variability in the ignition point location, the type of ignition source, the volume of the gas released and the characteristics of the gas cloud etc. These uncertainties which are not statistical in nature may not be categorised as random or probabilistic but are cognitive and fuzzy in nature. The probabilistic framework for structural analysis subjected to blast loads could be quite cumbersome due to high number of uncertain variables and complex interdependency. The uncertainty in the load and corresponding uncertainty in the structural response can either be predicted from variations in the uncertain load parameters — a sensitivity evaluation or through a compact “possibilistic analysis”. The blast loads are usually defined as a triangular pulse through peak pressure, rise time and the blast pulse duration as the parameters. In the present investigation, the parameters in the triangular blast load description are assumed fuzzy. The peak pressure, rise time and blast pulse duration are defined using triangular fuzzy numbers. The possibilistic dynamic response of simple structural system — beam — used in the blast wall is obtained using single-degree of freedom approximation. It is shown that the possibilistic response provides rational decision making tool to arrive at desired safety level.