In this paper the motions of a barge-type floating foundation installed with four moonpools and a VAWT are investigated through physical model tests and numerical calculations. The characteristics of motion responses and mooring tether tensions under various wave conditions are examined. The gyroscopic effects of turbine rotations are studied by varying the mass and the rotational speed of wind turbine. Linear potential calculations are carried out using WAMIT. It is found that the gyroscopic effect due to turbine rotations can be significant. The firstorder motions of the floating system are substantially reduced by the gyroscopic effect, while the second-order motions and tether tensions may be significantly increased. The viscous damping of water motions in moonpools is found not negligible to produce reasonable predictions.
To exploit ocean wind energy, varied types of FOWTs (Floating Offshore Wind Turbines) have been proposed over the past decades. In terms of floating foundation, there are mainly Spar, semi-submersible, TLP (Tension Leg Platform), and pontoon (or barge) types of FOWTs. Among these types, the pontoon type of FOWTs are simple to design, construct and install, and they are generally placed in shallow water. With regard to wind turbines, there are mainly two types: HAWTs (Horizontal Axis Wind Turbines) and VAWTs (Vertical Axis Wind Turbines). When multiple HAWTs are installed on a floater, it is necessary to consider the wake effects of air flow and the motions of the floating foundation due to the turbulence around the turbines. The distance between two HAWTs should be no less than one third of the diameter of a turbine rotor. Hence a large area is usually required for the floating systems installed with an array of HAWTs. So far, the concept of floating systems installed with multiple HAWTs has not been commercialized
By contrast, VAWTs have advantages over HAWTs in multi-turbine installed floating systems. For example, VAWT has no requirement for wind direction and its installation and maintenance costs are relatively low. The concept of VAWTs installed on a floating barge may go back to the report of Shigeo (2002). So far, various floating VAWT concepts have been proposed (e.g., Akimoto et al., 2011; Collu et al., 2012; Shires, 2013; Paulsen et al., 2015). The floating systems installed with multiple wind turbines have several advantages, such as reducing mooring tethers, causing less disturbance to sailing and fishery activities, improving the maintainability and workability of the facilities, and potentially enhancing power extraction (Jin et al., 2020). In addition to these advantages, for the development of the floating systems, it is necessary to consider the performance of floating systems in safety and stability.