Similar to the configuration proposed by the Dutch tri-floater, the WindFloat is also based on a three-column foundation. While the wind turbine is installed at the center of the triangle connecting the three floating columns, one column of the WindFloat is employed to support the wind turbine as shown in Fig. 5. A horizontal Water Entrapment Plate [38] is added to the base of each column to provide additional hydrodynamic inertia to the foundation. Because the plate displaces large volume of water, as reported by Roddier et al. [17], the horizontal dynamic response of the structure is improved.
A permanent water ballast system is installed at the bottom of each column, which lowers the foundation to reach the target draft. In order to reduce the rigid-body response induced by the mean wind loads acting on the wind turbine, there is an active water ballast system right above the permanent water ballast to move waters from column to column. When the mean wind speed or direction changes, the active water ballast system adjusts the distribution of waters among three active water ballasts to keep the axis of the wind turbine vertical, which in turn guarantees the performance of the wind turbine [39].
Six catenary lines, comprising common mooring system parts such as chain jack, shackles, drag-embedment anchors, fairleads and chains, moored the foundation to the seabed. Among the six mooring lines, four are connected to the turbine-supporting tower and the other two lines are moored to the columns without turbine.
The floating hull, tower and turbine in the WindFloat project are assembled onshore and the entire foundation is then towed to the target place and tied to a moored anchor system installed beforehand [37].
It is commonly recognized that the three-legged floating foundation introduced by the WindFloat project is the first meaningful semi-submersible foundation, which presents a feasible configuration allowing wind turbine to be sited in areas with the water depth ranging from 30 to 50 m [17]. The WindFloat project costs about 26 million U.S. dollars and produces energy to power 1300 homes per wind turbine [40].
Despite the unprecedented success, there are still shortcomings associated with the WindFloat project. The most significant shortcomings can be listed as:
1. The structure of the wind turbine should be modified before installing to sustain some of the floater induced motions. In other words, the structure commonly used for offshore wind turbine is not suitable for installing on a floating foundation [29].
2. The cost of the active water ballast system is still too high tomake the floating foundation massively producible.