Projection risk assessment

Projection risk assessment is an essential part of any hazard study, and of a number of engineering projects where facilities are required to withstand exceptional events. However, it appears that this type of risk is rarely studied quantitatively, even if it is likely to lead to a major dreaded event.

Indeed, even if it is generally accepted that a projectile does not create a hazard zone on its own, it is nonetheless particularly dangerous in terms of the domino effects potentially induced, as an initiating phenomenon. In certain situations, where the risk cannot be excluded by treating the causes appropriately and reducing the probability of occurrence to an acceptable level, it may be necessary to carry out additional analyses to measure the effects in the environment, or even on potential critical targets.

The first step is to define with sufficient precision the projectiles likely to be generated, depending on their origin, whether accidental or intentional. This first step is based, on the one hand, on accident experience and, on the other hand, on mechanical models using more or less complex preferred failure mechanisms, which are often defined on a case-by-case basis. It can be used to characterize essential variables such as the shape, mass, velocity and number of projectiles emitted.

The next step is to study the trajectory of projectiles formed in this way over time, based on simple aerodynamic data, in order to assess their range and impact characteristics. In view of the many uncertainties relating to the assumptions used for modelling, and the specific nature of the associated consequences, a probabilistic approach to projection risk may be envisaged to demonstrate its acceptability in relation to the issues at stake.

If, on the other hand, the risk is unacceptable, then additional risk reduction measures will have to be implemented to limit the effects. Different strategies can be adopted, depending on whether the aim is to retain projectiles at source or to protect critical targets directly. These protective barriers can either be directly integrated by appropriately sizing the targets for this very particular accidental load case, or they can take the form of a separating screen between the sources and potential targets, thus protecting the latter from any impact.

The concepts introduced in this document will gradually familiarize the reader with the problem of projections, both in terms of characterizing them on the basis of an initiating event, and assessing their consequences using models commonly used in industrial safety.