Experimental and CFD analysis of roll damping of a wind turbine installation vessel

The global transition to renewable energy has put pressure on wind turbine installation vessels (WTIV), thus creating an urgent demand for optimizing their operation. This includes more accurate predictions of the ship motions at sea, at which roll motion is of particular interest. For accurate prediction of the roll motion, roll damping is important to consider and is commonly found from empirical methods if experimental data are not available. Since the hull geometry and loading conditions of WTIVs are significantly different from conventional ships the validity of existing empirical methods has not been justified. This paper studies an alternative approach of determining the roll damping, by utilizing CFD to simulate free roll decay of a WTIV, from which the roll damping can be extracted. A WTIV is simulated under different loading conditions, varying the vertical center of gravity and adding a bilge keel to study the influence on the roll damping. Model tests are carried out to validate the CFD simulations. CFD simulations are performed in both model scale and full scale. The implied extreme roll motions in irregular waves are compared with empirical methods. It is found that CFD can predict roll damping accurately and that roll damping decreases for high centers of gravity. The viscous scaling effects for a 2 m long model is significant, causing too large damping in model scale. The investigated bilge keel reduced roll motions significantly. The empirical methods consistently overestimated the roll damping, especially near the resonant frequency.