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This version of the article has been accepted for publication, after peer review (when applicable) and is subject to Springer Nature’s AM terms of use, but is not the Version of Record and does not reflect post-acceptance improvements, or any corrections. The Version of Record is available online at: https://doi.org/10.1007/s40722-023-00281-2, This study proposes a multi-connection cross-flow vertical axis wind turbine, an innovative device to supply electric power in aquaculture farms. The device is a new type floating offshore wind turbine consisting of two independent wind turbine floats and a mooring float set in a straight line. A single-point mooring system with tension is utilized at the mooring float, which allows the wind turbine floats to turn around the moored point. However, there are various challenges to this new concept for its practical application mainly related to turning motion about the moored point. Therefore, the focus of this study is to understand the turning mechanism of the proposed FOWT through dedicated water tank experiments and numerical simulations. As a concept demonstration, two cross-flow wind turbines were mounted on the wind turbine floats and turning motion characteristics about the moored point were observed. A prototype model was built with a model scale of 1/36 using Froude scaling assuming rough weather conditions at the aquaculture farm. Wind speed of 35m/s, wave height of 0.75m, and wave period of 5–7.5s are the assumed environmental conditions in the actual model. Free yawing tests were conducted in only-wind, only-wave and combined wind–wave conditions. Further, a numerical simulation considering the wind loads acting on the turbines is developed. It is found that the wind turbine floats turn to a position where the wind loads acting on the left and right sides of the moored point are balanced. The numerical simulation reproduced the turning motion within an error of 10.

In this study, a series of dedicated water tank tests were conducted in wind and waves to investigate the stability performance and turning motion of Floating Offshore Wind Turbine (FOWT) equipped with two vertical axis wind turbines (VAWT). The FOWT targeted in this study is called Multi-connection VAWT, which is a new type of FOWT moored by Single-Point-Mooring (SPM) system. We designed and manufactured two types of semi-submersible floating bodies. One is a type in which VAWTs are mounted in two places of a right-angled isosceles triangle (Type-A) on a single floater, and the other is two independent units equipped with VAWTs on two separate floaters centered on a moored body. This is a type in which two semi-submersible floating bodies are lined up in a straight line (Type-B). The experimental conditions were determined by scaling down to 1/100 using Froude’s scaling law based on a wind thrust load of 320 kN (rated wind speed of 12 m/s) assuming an actual machine. In the free yawing test in waves, Type-A turned downwards, while Type-B was barely affected by the waves. Furthermore, in the free yawing test in wind, both Type-A and Type-B turned leeward and stabilized at a final point where the wind load was balanced.

Single-Point Mooring (SPM) is a cost-effective and promising solution that can be utilized to station a Floating Offshore Wind Turbine (FOWT). However, it poses new challenges such as wind tracking performance and slow-drift motion, which are unexplored. In this study, an attempt is made to understand the wind tracking performance of a new-type FOWT ‘Optiflow’ moored by SPM system. Real-Time Hybrid Model (ReaTHM) testing is conducted on a 1/60 scale model and motion about the moored point is observed. It is found from experiments that the model follows the wind in rated, cut-out and storm conditions. Forced yawing test is also carried out on a 1/200 scale model to obtain a database of slow-drift damping (SDD) force coefficient. A time-history simulation code is developed to incorporate the slow-drift motion response following the wind. Two cases of simulation, one with slow-drift damping and another without slow-drift damping are carried out. Comparing the experimental and simulation results, it is revealed that the simulation with slow-drift damping coefficient predicts the wind tracking phenomenon better than the simulation without slow-drift damping coefficient. It is concluded that slow-drift damping is an important factor while simulating the turning motion of ‘Optiflow’ about the moored point.