6. Conclusions
This paper aims to highlight the development and progresses made recently concerning the design, the experimental investigation and the numerical simulations on the semi-submersible foundation for offshore wind turbines. It begins with the review of classifications of the foundations supporting the offshore wind turbines. As for the design of the semi-submersible foundation, it is reviewed according to the shape of the foundation. More specifically, the three-legged, ring-shaped and V-shaped semi-submersible foundations are reviewed in sequence, and the projects exemplifying different designs of semi-submersible foundations are also included in the review. In addition, the advantages and shortcomings associated with each semi-submersible foundation design are discussed.
For the purpose of improving the design of semi-submersible foundation, it is necessary to investigate the dynamic responses of the foundation under the excitation of the wind-wave combined loads. Due to the cost and complexity to conduct a full-scale test on the real sea, the wave basin test and the numerical simulation are therefore the two main approaches for the scholars to research the dynamic responses of the floating wind turbines. Through surveying literatures about the wave basin test on floating wind turbines, it is clear that, among the two fan matrix set-ups, the ship towing basin configuration is recommended to be used in a test concerning the semi-submersible foundation. In addition, the wind tunnel simulation technique can be employed to generate the required, realistic wind field. As regards the rotor model, three simplifications are reviewed. Moreover, since the catenary mooring lines are widely employed in the semi-submersible foundations, they can be simulated with a set of springs in the basin tests. As for the numerical simulations, the aerodynamics, mooring line dynamics and hydrodynamics are usually simulated by different models. For the aerodynamic simulation, the FVW model is found outperform the conventional blade moment theory. Three mooring line models, force–displacement relationship method, quasistatic catenary method and dynamic cable method, are mainly discussed in this paper. For the hydrodynamic simulation, alternative methods such as Morison’s equation, first-order hydrodynamic theory and second-order hydrodynamic theory reveal different applications. With regards the simulation tools, the mainstream codes are listed and compared in the Table 7. A novel tool coupling a three-dimensional flow solver EllipSys3D with FLEX5 to calculate the wind combined force acting on the hub is highlighted.
Acknowledgements
The authors are particularly grateful for the valuable comments and suggestions from editors and reviewers. The work described in this paper was supported by the Economy, Trade and Information Commission of Shenzhen Municipality (Project No. 201510150880 and SZHY2014-B01-001).
References