Risk modelling of personnel fatality at accidents on offshore installations

Risk to safety of personnel on offshore installations is normally modelled by application of event trees, where the risk is defined as a product of event frequency and its consequences. This method is steady state whilst the actual event is time dependent. For example, gas release is an event comprising the size of gas cloud being released, probabilities of ignition, fire or explosion, fatality, escalation to new releases and fire and/or explosion, and the probability of fatality, all varying with time. This paper brings new perspective, how the risk to safety of personnel could be evaluated in dynamic context. New approach is presented whereby the time-dependent events and the time-dependent probability of fatality are modelled by means of the analytical computation method based on modeling of different accident scenarios by use of the directed acyclic graph and Fault Tree Analysis method. Using these methods the modeled scenarios change with relevant probabilities at defined times to configurations with appropriate probabilities of fatalities. The paper uses a realistic example from the offshore industry, where different sizes of leak have different probability characteristics. Specifically small, medium and large leaks are evaluated.