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The semi-submersible floating offshore wind turbines (FOWTs) are widely used for deep-sea power generation and the pitch motions usually affect the operation of semi-submersible FOWTs. As a damping device, gyro stabilizer has effectively suppressed the roll motion of ship. Hence, two passive gyro stabilizers are installed on an adapted semi-submersible FOWT in this paper. The effect of gyro stabilizer on the pitch motions of the wind turbine with different structure configurations is investigated by model tests under different wind and wave conditions. The pitch reduction by the gyro stabilizer is found to be highly correlated with the pitch angular velocity of the wind turbine, and the maximum pitch amplitude suppression rate is about 40%. The reduction amount of pitch motion significantly increases with the wind thrusts while the pitch reduction rate decreases under the rated wind speed. The reduction of spacing between column and deck center and the increase in the mass of the superstructure reduce the suppression effect of the pitch motion with regular waves, and the maximum difference between different models can be up to 86%. The heightening of the tower amplifies the contribution of wind loads to the pitch angular velocity, which in turn enhances the pitch suppression effect. • Investigate the effect of a passive gyro stabilizer on the pitch motion of a semi-submersible wind turbine by mode tests. • The effect of the gyro stabilizer has a great relationship with the pitch angular velocity of wind turbine. • Under irregular wave, the pitch suppression by the gyro stabilizer occurs at wave frequency and negative damping is found at low frequency. • The wind increases the pitch suppression effect of the gyro stabilizer. • The smaller spacing between column and deck center and larger ballast mass decreases the wave-frequency pitch suppression effect. [ABSTRACT FROM AUTHOR]

Floating offshore wind turbines suffer severe aerodynamics under platform motions. Novel control methods are urgently needed to improve power and load performances. A mechanical cyclic pitch control strategy is introduced in this paper, and the load reduction effect on the NREL 5-MW wind turbine is investigated under dominant pitch motions. The simulation is performed by the standalone Aerodyn_Driver code. Results show that the relative airflow under pitch motions can be equated to a linear sheared flow and is only related to wind velocity and pitch angular velocity. Cyclic pitch control hardly affects power and thrust but significantly changes aerodynamic moments. The resultant moment and moment axis azimuth are defined, and they are controlled respectively by pitch amplitude and phase. The optimal pitch parameter equations to minimize pitching and yawing moments are derived from fitting and derivation, and the standard deviations of aerodynamic moments are reduced by over 70 % under the sinusoidal pitch motion and steady inflow below rated operation. This paper provides a new idea for the individual pitch control structure, and the mechanism study for aerodynamic control is important and instructive to fill the technological gap in mature pitch control strategies for floating offshore wind turbines. • Cyclic pitch control is used to reduce aerodynamic loads under pitch motions. • The linear sheared airflow is only related to wind and pitch angular velocity. • Power and thrust are almost unaffected in the limited pitch angle range. • Load magnitude and direction are adjusted respectively by pitch amplitude and phase. • Standard deviations of moments are reduced by over 70 % with the optimal control. [ABSTRACT FROM AUTHOR]

This paper employs the simplified quadrupedal passive dynamic model to analyze the underlying property of the transverse gallop. First, the simplified sagittal quadrupedal planar model of the transverse gallop, the trot, and searching methods for achieving the periodic motion are introduced. Next, we explore the dynamic performance of the transverse galloping pattern on the simplified quadrupedal passive dynamic model at different forward speeds and different stiffnesses of legs in a range of initial angular velocities at which the periodic motion is achieved, and there exists a particular property that is defined as the pitch angular velocity dynamics (PAVD) property. If the forward speed and the stiffness of legs are adequate in the transverse galloping pattern, the PAVD emerges and the forces (e.g. peak ground reaction force and average vertical force) that the model undergoes during locomotion are sharply reduced. The Froude numbers of the minimum forward speed where PAVD holds at a fixed stiffness of legs for different samples are virtually the same. Moreover, a comparison between the trot and the transverse gallop illustrates that a trot is more favorable in the range of forward speeds where the PAVD property is not achieved, while a transverse gallop is better after the property emerges. [ABSTRACT FROM AUTHOR]