A model for calculating the frequency of collision between passing vessels and offshore platforms has been developed. The model basically considers two factors: The expected number of vessels on a collision course with a platform; The probability that the vessel does not take any actions to avoid collision. The probability that a vessel is on collision course with a platform, is primarily dependent on the traffic pattern in the area. It is a basic assumption that it is possible to define a lane pattern, where a lane represents the most commonly used route between two ports. The lane is characterized by the position of centerline of the lane, the number of vessels using the lane, and the lateral distribution of the vessels across the centerline. The probability that a vessel does not take action to avoid the platform has been modeled with a fault tree. The fault tree is based on existing models and data, but it has been modified to accommodate the other developments in the model. The data for the most important terminal events have also been updated. An important part of the work has been evaluation of the uncertainties associated with the collision frequency calculations. The uncertainties associated with all the parameters, as well as the models, have been considered and probability distributions have been established. The uncertainties have been propagated through the model with Monte Carlo simulation, partly to give estimates of the uncertainties in each of the main parameters in the model, but more importantly to obtain an estimate for the overall uncertainty in the calculated collision frequency.
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