Every summer, the chemical plant of Janssen Pharmaceutica in Geel is idle for 2 to 3 weeks for a shutdown. Although they have been doing this for years, they nevertheless decided to call in PDM to find out the ideal moment for carrying it out. Account had to be taken of the various parameters influencing strategy. It was a job that seemed easy at first sight but was considerably more complex in practice. PDM started by defining the various shutdown parameters and made an inventory of the exact extent of each parameter’s impact on the shutdown strategy. Then, based on this result, a model was created in the form of a flow diagram with parameters, so that the diagram can be reused every year. By using this model to go through a future shutdown step by step, the client can see exactly how long each phase takes and also the total duration of the shutdown.