structures. The economic cost associated with the installation and operation of thousands of offshore platforms, however, can be potentially too high to make a large-scale offshore wind farm feasible. Because the cost associated with the offshore wind turbine itself is not significantly higher than the bottom-fixed counterpart, the high cost delaying the development of floating wind turbines is mainly due to the floating foundation. The work of Musial et al. [13], however, showed, using a simple static cost statistic model, that the cost of floating foundation could be brought into a reasonable range. In fact, Castro-Santos and Diaz-Casas [14] introduced a method to calculate the life-cycle costs in terms of developing a semi-submersible floating wind farm composed of 21 offshore wind turbines. The installed capacity of the proposed wind farm is 107 MW. Their investigation showed that the total cost varies from 395 million U.S. dollars to 1021 million U.S. dollars according to the installation location, the water depth and the distance from the shore. Besides the high cost associated with the installation and operation of floating wind turbines, the dynamics of the floating foundation introduce a number of new challenges comparing with the bottom-fixed offshore wind turbine, including [7]:
1. How to keep the wave-induced motion of the platform to acontrollable level?
2. How to couple the foundation dynamically with the wind turbine at the design stage when sustaining wind-wave combined loads?