hydrodynamic stability and more structural stiffness to sustain the wave load. For the connections between columns, steel braces/bars, which increase the stiffness of the foundation, are widely used. A typical layout of the semi-submersible foundation system is shown in Fig. 3.
Comparing to the other two types of foundations, namely the spar and the TLP types, the semi-submersible foundation is advantageous because [29]:
1. The construction, assembly, outfitting and commissioning of the foundation and even the installation of wind turbines can be done on the dock. The finished foundation, with the wind turbine installed, can then be towed-out and deployed by towboat.
2. The cost associated with the installation of mooring system of asemi-submersible foundation is relatively lower than the other types of floating foundations.
3. The hydrodynamic behaviors of a semi-submersible foundationto the wind load excitations are better due to the deep draft when comparing to the other two types of floating foundation. The reason is apparent that the natural heave period is longer when the draft becomes larger [34]. In particular, the wave exciting force almost disappears at the equilibrium position during the heave motion.
4. The surge response of a semi-submersible foundation, which issimilar to the responses of a spar type, is better than TLP. The pitch motion of the semi-submersible foundation, on the other hand, is smaller than the spar type [35].
Although the semi-submersible foundation is advantageous for supporting offshore floating wind turbines, the design of a semisubmersible foundation is challenging due to the complex dynamic responses of a semi-submersible foundation induced by the wind-wave combined loads. The heave response of a semisubmersible foundation is the prime concern since the control of vertical motions is the most difficult task in the design of the semisubmersible foundation. Moreover, floating ices, sea currents, storm surges and tidal variations are also influence factors which are required to be considered in the design of a semi-submersible foundation.