Just offshore from the Dutch town Egmond aan Zee, a wind farm has recently become operational. As part of the project, a monitoring and evaluation programme will be carried out during the construction phase and the first years of operation. The monitoring programme includes the assessment of the effects of spills caused by ship collisions with the wind farm. But how can collisions of ships with the wind farm, and the consequences of possible oil spills resulting from these incidents, be monitored when the risk of these events is very small? Modelling the frequency and consequences of these events is the only way of making a quantitative assessment of the possible increase in risk resulting from the presence of the wind farm, and assessing its significance.