type, a spar type and a Tension leg platform (TLP) type. A series of comparisons among the three above-mentioned types have been shown in Table 3. In a spar-type foundation, a large cylindrical buoy is used to stabilize the spar structure. The heavy lower part of the buoy, i.e. the ballast, and light upper part are combined to lower the center of buoyancy, which leads to a recovering moment when the floating cylinder declines from the preset axis [26]. The TLP, on the other hand, is stabilized by the mooring system. In detail, the TLP is moored onto the seabed through a set of tension legs. Since the leg is pre-tensed before the foundation is fully functional, the preset location and posture of the foundation are recovered by the adjustment of tensions among legs. Combining the two designs of the spar and the TLP, a semi-submersible foundation is introduced to achieve the static stability. The detailed design of a semi-submersible foundation was introduced respectively by Zambrano et al. [27] and Fulton et al. [28] while the former came up with a Mini-Float design, the latter introduced a WindFloat design. After the concept of semi-submersible floating foundation was introduced, the technological feasibility of installing wind turbines on the semi-submersible foundation has been demonstrated by two prototype projects [15,29,30].
1. Fukushima-FORWARD project Phase 1: a 2 MW turbine wasmounted on a semi-submersible floating foundation in Japan.
2. Windfloat project: a 2 MW turbine was mounted on a semisubmersible floating foundation deployed in the Atlantic Ocean.