Compared with onshore wind turbines, floating wind turbines work in a more complex marine environment: the upper wind turbine bears wind loads, and the floating support platform and mooring system are subject to the combined action of waves and ocean currents. Moreover, mutual coupling will occur between the various parts of the structure. On one hand, the aerodynamic load received by the upper wind turbine will be transmitted to the platform through the tower. The movement of the platform under this action is no longer a simple response under waves especially as the size of the wind turbine continues to increase. On the other hand, the motion response of the platform under the combined action of the wave and mooring system will be transmitted to the upper wind turbine through the tower, which will change the aerodynamic loads and output power of the wind turbine.

The numerical simulation of the aerodynamic performance of wind turbine and the hydrodynamic performance of floating platform provides a certain reference for the design and research of FOWT system. In order to simulate the performance of FOWT system more accurately under normal operating conditions, it is necessary to simulate the aerodynamic-hydrodynamic fully coupled numerical simulation of FOWTs.

With the further development of the research on aerodynamic and hydrodynamic performance analysis of FOWT and the advancement of computing technology, the full coupling analysis of the aerodynamic and hydrodynamic loads of FOWT is now possible. More and more scholars are focusing on coupled analysis of FOWT system.

At present, some software for coupled analysis of FOWTs is based on the existing onshore wind turbine simulation program, adding the simulation of floating platform and mooring system. Others add a wind turbine aerodynamic simulation module on the basis of original floating platform hydrodynamic simulation code. In order to analyze and compare the accuracy of different numerical calculation procedures, the International Energy Agency organized “wind energy task 30” [40] and used 24 different numerical calculation procedures to analyze the dynamic response of 5 MW semi-submersible FOWT in OC4 project, which has played a guiding role in the development of numerical calculation program of FOWT. Table 1 [40] shows several typical theoretical modeling codes and methods.

National Renewable Energy Laboratory (NREL) has carried out a lot of research work on FOWTs and made great contributions to the research. In 2006, Jonkman and Sclavounos [41] of MIT improved the calculation software FAST developed by NREL, enhancing its preprocessor Adams function and integrating it with swim-motion-lines software and WAMIT software developed by MIT to make it suitable for the numerical calculation of dynamic response of FOWTs. In 2007, Jonkman [42] introduced HydroDyn module and mooring module into FAST, making it a numerical full coupling analysis program of aerodynamic analysis, hydrodynamic analysis, servo control and elastic analysis.

Norwegian University of Science and Technology has also done a lot of works on the development of numerical calculation program for FOWT. Linde [43] used Simo to analyze the dynamic response of 5 MW FOWT in OC3 project. Thomas [44] integrated HydroD, DeepC and TDHMILL3D to conduct fully coupled numerical calculation for TLP, Spar and semi-submersible FOWTs.

The fully coupled numerical simulation of FOWTs is still in its infancy. Most studies are based on the above-mentioned methods of aerodynamic and hydrodynamic simulation. However, some traditional aerodynamic or hydrodynamic analysis methods have been found unreasonable to be directly used in coupled analysis. Roddier [45] pointed out that the empirical characteristics of Morison’s equation and potential flow theory do not help the design of a new supporting platform. Sebastian and Lackner [46] pointed out that traditional BEM plus some corrections (such as dynamic inflow, yaw model, etc.) cannot accurately describe the interaction between blades and wake. Moreover, the commonly used stall models such as the semi-empirical B-L model and their applicability to FOWTs have not been fully studied. Therefore, it is necessary to develop more advanced FOWT modeling methods and coupled analysis models.

Due to the complexity of the aerodynamic-hydrodynamic coupled analysis, the full-model and real-scale CFD numerical simulation will become the most ideal tool. For large-scale FOWTs, there are not only complicated structures but also multi-scale effects, which brings great challenges to the study of the fully coupled analysis of FOWTs using CFD methods. Nematbakhsh [47] used the CFD method to conduct a preliminary study on the coupled effect of FOWT. They simplified loads on blades to constant thrust and used the single-phase flow Navier–Stokes equation to study the dynamic response of a TLP FOWT system. Quallen [48] conducted a two-phase flow CFD simulation of FOWT system. They used overset technology to deal with the grid movement around the platform and blades.

In general, the current research based on CFD methods is still in its infancy. There are still many unresolved difficulties in CFD methods, such as the division of the calculation domain of the water and gas flow field, the selection of the time step, the time calculation of the random wave action and so on.