(44cc) Frequencies of Accidents As Functions of Design Variables

Risk is a function of two variables: the frequency of an occurrence and the consequences associated to said event. There are many methods that can be used to estimate the frequency of an accident, like failure and event trees, historical analysis, Monte Carlo simulations and many others. Many times, these methods produce values that can be used to know the frequency of an event in a single unit or piece of equipment, and not the frequency with which the event will occur in a whole installation or industrial site.

In installations in which there are similar or identical equipment, like storage parks, it would be useful to know the frequency with which certain accidents will occur in the installation, not per equipment, but in the totality of the site. There are studies like Quantitative Risk Assessment (QRA) in which this information could be used in order to reduce the time needed to complete the study. When performing QRAs in installations with similar equipment, sometimes only one scenario is defined for many equivalent units, and the frequency of the accident studied in said scenario is multiplied by the number of equipment in which it can occur, in order to take into account the fact that the accident will always be similar, but that it can happen in many units. However, this approach does not take into account the fact that the equipment may interact with each other after an accident in a domino effect, so that the possibility of a certain accident occurring in the installation is higher than only the possibility of it occurring in any and each of the units.

In this work, a model to produce possible accident sequences in a domino effect and calculate frequencies associated to the accidents in them, developed by some of the authors and published in the Process Safety and Environmental Protection journal has been used to calculate the number of times an accident will occur in a LPG storage installation, depending on the number of tanks used in the facility (the distance between them calculated by using NFPA standards) and the mass of product stored. The final results are equations that can be used to determine the frequencies of different accidents like BLEVEs, pool fires or jet fires in the installation, depending on the mass and number of tanks in the studied facility or in a similar site.