TABLE OF CONTENTS In much risk assessment, it is not possible to calculate the magnitude of the consequences. For example, a risk engineer in a railroad company cannot calculate the number of people who would be killed if two trains collide. The consequences need to be estimated by experienced staff by examination of previous incidents.
In the process industries, it is possible to perform calculations of the impact at a distance of some kinds of incident. This is because the consequences are governed to some extent by physical and chemical laws. But it is important not to allow the rigor of these laws to blind us to the uncertainties about the circumstances of the incident, such as the location of a leak, the size of the hole, the presence or absence of ignition sources, the direction of the wind, the terrain over which a leaked burning liquid spreads, or the vulnerability of people to different levels of exposure. So, even where calculations are performed, there is a large element of estimation needed to convert the calculated answers into realistic conclusions about the likely consequences.
In particular, in the case of reliability, where the consequences are likely to be expressed in terms of the amount of production interruption (measured in days, or in physical quantity of a product), it is not possible to calculate the severity of the interruption, as it is determined by many factors such as the availability of spare parts, the extent of buffer storages, and...
