The additional ship resistance due to water oscillations in an open moonpool as constructed in drillships can be obtained through model tests. The oscillation has a dominant frequency and character, and the amplitude increases with forward speed. The resonant oscillation mode can be piston or sloshing. Recent measurements show that both types of oscillation can increase ship resistance to the same magnitude. The sloshing mode dominates in longer moonpools, while piston mode oscillations are dominant in shorter moonpools. Based on two model tests series carried out at Delft University of Technology, a resistance prediction model is constructed for piston type oscillations. The model was verified with a resistance measurement performed at MARIN in the past, showing fair agreement between the predicted and measured resistance increase.

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