Fig. 9. Detail of Fukushima compact semi-submersible structure design. (The figures were reproduced according to the description provided in literature.)
conducted to show the technical feasibility of employing the braceless semi-submersible foundation to support offshore wind turbines.
4. Experimental studies
Owing to the high cost and inherent complexity in the construction of a full-scale offshore wind turbine, only a few semisubmersible prototype projects, such as WindFloat [50], and pilot wind farms have been launched. Given the promising future of floating wind turbines, a series of experimental studies, as complements to the full-scale prototype projects, have been conducted in wave basins to investigate the dynamical behaviors of the wind turbines and floating foundations [51]. Understandably, the wave basin test provides:
1. Data valuable for investigations on the dynamic responses ofthe floating foundation with specific features.
2. Validations of numerical simulations.
The present paper reviews the main three aspects concerning a wave basin test on the floating wind turbine, namely the generation of realistic wind-wave environment, the simulation of the rotor system and the simulation of the mooring system.
4.1. Wind and wave
Usually, the experiments concerning the dynamics of floating foundations are conducted in a wave basin embedded with a wind tunnel. Ishihara et al. [52] have carried out a wind-wave basin test to obtain the dynamic behavior of semi-submersible floating foundation in Japan, which provides valuable data source for benchmarking further investigations on the dynamic behavior of a
Fig. 10. Detail of Fukushima V-shaped semi-submersible structure design. (The figures were reproduced according to the description provided in literature).
semi-submersible floating foundation with a wind turbine installed. In their test [51], a flap type wave generator is attached to the end of the basin to simulate regular waves with the maximum wave height exceeding 0.3 m. The period of the generated wave spans across 0.6–4.0 s. An absorbing bench is installed at the end of the basin to eliminate undesired wave reflections.
In the cases where the sophisticated wind tunnel facility is not available, a matrix of simple fans is employed to generate the required wind field. For different types of the wave basin tests, the layout of the fan matrix is different:
1. In a wave basin test concerning the offshore structures, arrays offans are employed. In such a case, the center of the experiment zone is placed 3 m away from the fan arrays. In addition, the experiment zone should be placed at a distance of 2.4 m from the basin lateral walls.
2. In a wave basin test concerning the ship dynamics, the fanmatrix is arranged in a triangular shape to make the voids among fans minimal. The experiment zone is required to be placed within a distance of 1 m from the matrix of fans. Such an experiment configuration is supported by the studies focusing on the Hywind [53] and the WindFloat [54] projects.