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.
Originally, semi-submersible-floating principle was adopted for the offshore oil and gas exploitation when the drill deck is required to be large [31]. Thanks to the ‘wave cancellation effect’, which could improve wave-induced dynamic responses of the offshore structure, the semi-submersible foundation is suggested for supporting offshore wind turbines (see Table 4). The ‘wave cancellation effect’ refers to the phenomenon that the wave forces acting on the submerged objects with different phases canceled each other due to the phase shift [32]. Usually, the semisubmersible foundation is composed of three or four slender columns that are connected to each other through braces [33]. The wetting surface area of a single column, the height of the buoyance center and the distances between two columns affect the forces acting on the floating foundation, which recover the original location and posture of the floating foundation. Moreover, the increase of wetting surface provides more