No abstract

The experimental proof of the floating wind turbine has been started off Goto Islands in Japan. Furthermore, the project of floating wind farm is afoot off Fukushima Prof. in north eastern part of Japan. It is essential for realization of the floating wind farm to comprehend its safety, electric generating property and motion in waves and wind. The scale model experiments are effective to catch the characteristic of floating wind turbines. Authors have mainly carried out scale model experiments with wind turbine models on SPAR buoy type floaters. The wind turbine models have blade-pitch control mechanism and authors focused attention on the effect of blade-pitch control on both the motion of floater and fluctuation of rotor speed. In this paper, the results of scale model experiments are discussed from the aspect of motion of floater and the effect of blade-pitch control.

Ministry of the Environment of Japan started the national demonstration project of Floating Offshore Wind Turbine (FOWT) in 2010 FY. In this project two SPARs with different scales are used. The first half-sized model, supporting a 100kW wind turbine, was installed in 2012 and is currently in operation. It is characteristic that the floater has a hybrid structure, i.e., the upper part is constructed of steel and the lower part of pre-stressed concrete and that the turbine is downwind type of horizontal axis. The FOWT is equipped with many sensors for measuring motions of 6 DOF, the strain of the steel / concrete parts and other factors. In this paper the measuring system is presented. In September 2012, the FOWT was attacked by a huge typhoon and it survived, although the measured wave height exceeded the designed value. The measured record of the floater in the storm is also presented.

Wind power has the primary potential among renewable energies. Because Japan consists of little flat land and little shallow coast, floating wind turbine must be developed to make wind farms in Japan. Therefore, Japanese national demonstration project of Floating Offshore Wind Turbine (FOWT) was started in 2010FY by Ministry of the Environment and a SPAR-type FOWT is under construction at present. The floater is planned to be hybrid, consists of upper part by steal and lower part by pre-stressed concrete. Four fins are attached around the floater to suppress yaw motion. The floater is moored by three catenary chains. In order to confirm the safety of the FOWT in storm condition, experiments of a scale of 1/34.5 model were carried out at Ocean Engineering Basin of National Maritime Research Institute (NMRI), Japan. The draft of SPAR, the height of hub above sea level and the diameter of rotor of the model are 1.07m, 0.68m and 0.64m, respectively. In all experiments, blades are fixed to the hub under feathering condition and the hub is irrotational and fixed to the tower because this wind turbine is assumed to be under the storm condition, but wind blows transversely to the nacelle to give the maximum wind force. Water depth of the basin is smaller than the planned sea area on a reduced scale of model, therefore, springs and wires were used instead of chains in order to correspond to characteristics of horizontal mooring tension. Environmental forces are wind, wave and current in 50-year return period. Tensions of the 3 moorings and the motion of the model are measured in condition of wind and/or wave and/or current. Three kinds of direction of wind are adopted. One is the same direction as the wave and current, another is perpendicular to the wave and current, and the other is against to the wave and current. Besides the intact conditions a mooring-line-cut experiment in a storm condition was also conducted. Moreover, the effect of vortex induced motion (VIM), which occurs in current, was discussed. The results of the model experiment are reported to show the sufficient safety of this FOWT.

The study of floating offshore wind turbines has recently been attractive to many research groups in the renewable energy. Because the area of shallow water along Japanese coast is limited, the development of floating base for wind turbine is inevitable for making large scale wind farms. There are some problems to be solved for floating offshore wind turbines. Besides the mechanical problems of turbines, the influence of the motion of the floater in wind and waves to the electric generation properties, the safeties of floating structures such as the fatigue of machines and structures or criteria of electric facilities should be studied. Several types of floating structures have been proposed such as SPAR, TLP, pontoon, and semi submersibles. The authors have focused on SPAR type because its simpler shape seems to have economical advantages. In this paper, the authors performed experiments in a wind tunnel and a water basin from the viewpoint of “wind turbines on a SPAR type floating structure”. Firstly, forced pitching experiments were operated in a wind tunnel, and the difference in two types of wind turbines, upwind type and downwind type, is discussed. The former type is very popular and the latter type is thought to be suitable for floating structure. Secondly, experiments which thought to be more relevant for a floating wind turbine were carried out in a water basin. The relationship between the location of the attachment point of mooring lines and the motion of the SPAR in waves, and the influence of pitching angle of turbine blades to the motion of the SPAR in waves were inspected. In these experiments it was used a mechanism to control the pitch angle of the blades of the scale model of wind turbine.