Abstract
As a consequence of the ignition of a flammable cloud restrained in a confined and/or congested region, an explosion could occur. Explosion studies are carried out in order to identify, for a specific plant, all the credible accidental events that could lead to flammable gas dispersion able to reach congested areas where ignition sources could be present. The frequencies, the consequences and the risk ranking of the explosion scenarios are then assessed for each identified congested area, in order to select credible explosions able to affect buildings or critical equipment.
The definition of the building blast requirements and/or the need of layout modifications can be performed adopting a Consequence-Based Approach, that takes into account the worst credible event, or a Risk-Based Approach, that considers both the consequence and the frequency values. However, sometimes Peak Overpressures derived from the Consequence-Based Approach can be far too large to be accommodated by the plant structures. The worst case scenario is however usually associated to very low probabilities, and this can result in overdesigning the structures, unless the probability of events is “implicitly” considered in the analysis by selecting a worst case according to some ‘credibility’ criterion. The Risk-Based method, on the contrary, considers the probability of occurrence of all the possible scenarios and allows designing the plant against an explicitly declared risk criterion.
The most common Risk-Based methods include Overpressure Probability Contours and Overpressure Exceedance Curves. The former shows the spatial location of the probability of being exposed to a specified overpressure value and it is useful when a specific target overpressure threshold value for the buildings is provided. Overpressure Exceedance Curves represent the probability of exceeding any overpressure value for a given location.
In this paper, both the Risk-Based and the Consequence-Based approaches will be applied to a realistic case study, in order to highlight the benefits and the disadvantages related to each methodological choice.
