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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]

The unsteady aerodynamic characteristics and interference effects of floating offshore wind turbine (FOWT) are mainly affected by the pitch motion of the ocean platform. Based on computational fluid dynamics (CFD) and advanced overset grid technology, the aerodynamic performance and wake characteristics of a fully configured wind turbine with rotating blades, nacelle , and tower are studied in this paper. The effects of the amplitude and frequency of pitch motion on the wind turbine aerodynamic loads and flow field are investigated herein in detail. The power and thrust between numerical simulation and experiment are compared. The results show that the grid and simulation parameters used in this study can accurately capture the aerodynamic characteristics and the flow field around wind turbines. The influence of the pitch amplitude and frequency on the performance of wind turbines is discussed. The complex flow interaction between the tip vortex, tower shedding vortex, and the turbulent wake was observed. The present results indicate that the pitch motion amplitude and frequency have a great influence on the power, thrust, and wake characteristics. [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]

Tuned sloshing dampers have long been used to stabilize tall and thin buildings and are now being applied to marine structures. In this study, the effectiveness of two new passive control devices at stabilizing floating offshore wind turbine substructures was investigated: the bidirectional tuned liquid damper and bidirectional tuned liquid column damper. Numerical simulations were performed to consider the sloshing effect on the passive control devices and the pitch-induced external moment brought about by wave action. The effects of adding baffles and orifices to the passive control devices were explored to optimize the geometric design. The results showed that the optimal combination of substructure geometry and passive control device type could reduce the pitch motion by 20%–40%. [Display omitted] • A numerical model was developed to simulate a floating offshore wind turbine (FOWT) under a regular wave. • An integrated numerical model was developed for integrating various FOWT substructures with tuned liquid dampers (TLDs). • The optimum geometry and location of the TLDs were determined based on the minimum pitch motion of the FOWT substructure. • The effectiveness of the FOWT-TLD was assessed for regular waves with various wave periods and amplitudes. [ABSTRACT FROM AUTHOR]

This paper aimed to examine the effects of a bidirectional tuned liquid damper on the stability of a barge-type floating offshore wind turbine. This analysis considered the pitch-induced external moment resulting from wave action and the sloshing effect on a bidirectional tuned liquid damper. Bidirectional tuned liquid damper geometries with different baffle heights at the middle of the bidirectional tuned liquid damper were examined, and the pitch motion of the floating offshore wind turbine integrated with the bidirectional tuned liquid damper was considered to determine the optimal bidirectional tuned liquid damper configuration. Based on the wave period, optimizing the bidirectional tuned liquid damper may reduce the pitch motion of a barge-type floating offshore wind turbine by 10%–30%. [Display omitted] • A numerical model was developed to simulate a floating offshore wind turbine (FOWT) under the regular waves. • A bidirectional tuned liquid damper (BTLD) was simulated using a numerical model. • To simulate a barge-type FOWT substructure integrated with a BTLD, a numerical model was developed. • The effectiveness of a BTLD on the stability of a barge-type FOWT were investigated to determine the optimum geometry and location of the BTLD. • The optimum integrated FOWT-BTLD was effective for regular waves with various periods and amplitudes. [ABSTRACT FROM AUTHOR]

The aerodynamic performance of floating offshore wind turbines is extremely complex due to the motions of the floating platform. The surge and pitch motions are the most influential motions among the six degrees-of-freedom motions. In view of this, the aim of this study is to investigate the unsteady aerodynamic characteristics of a floating offshore wind turbine under single (surge or pitch) and combined motions using computational fluid dynamics simulations, In addition, the coupling technique of dynamic mesh and sliding mesh is employed, as well as the unsteady Reynolds averaged Navier-Stokes method. The numerical simulation method in this paper is first validated by comparison to the results of the blade element momentum method and the vortex method. Then, the aerodynamic characteristics of the floating offshore wind turbine under harmonic platform motions with different periods and amplitudes are investigated. The results show that the increase of amplitude and frequency aggravate the fluctuation of the overall aerodynamic performance of the wind turbine. In addition, the combined surge-pitch motion reduces the average power generation indicating that complex platform motions adversely affect the power generation of floating offshore wind turbines. • The effects of wind load were analyzed by comparing the platform motions under different amplitudes and frequencies. • Two representative platform motions and the combined motion were considered for analysis. • Analyze the aerodynamic characteristics of different airfoil sections to reveal the variations of overall performance. • The detailed analysis of the wake effects showed the three-dimensional flow under the platform motions. [ABSTRACT FROM AUTHOR]

Floating offshore wind turbines, as large equipment for deep-sea wind energy utilization, must be carefully studied for their aerodynamic performances under platform motions. In experiments, the model wind turbine designed by the traditional thrust coefficient similarity is usually insufficient for unsteady aerodynamic reproductions of the prototype wind turbine. In this paper, a dynamic aerodynamic similarity criterion based on the mapping of the optimal tip speed ratio is introduced for wind tunnel model tests, and a model wind turbine is designed using the NREL 5-MW prototype wind turbine. In this case, to avoid the over-height model tower aroused in pitch motions, a method of equivalent surge motions instead of pitch motions is proposed and validated for feasibility. Then dynamic aerodynamic similarities under different pitch amplitudes and frequencies are validated by the Unsteady Blade Element Momentum theory. Results indicate that temporal and spatial characteristics and hysteresis effects of thrust and power coefficients all maintain better similarity with those of the prototype wind turbine than those of the model wind turbine designed by the traditional criterion. This paper provides a criterion with equivalent pitch motions for unsteady aerodynamic studies in wind tunnel model tests, especially the high-fidelity Hardware/Software-In-the-Loop test. • A new similarity criterion is proposed for FOWT wind tunnel model tests. • The feasibility of equivalent surge motions instead of pitch motions is verified. • The problem of over-height model tower is avoided by equivalent pitch motions. • Spatiotemporal and hysteresis characteristics of unsteady aerodynamics are analyzed. • Unsteady aerodynamic performances of the prototype can be reflected more accurately. [ABSTRACT FROM AUTHOR]

Floating horizontal-axis wind turbines (FHAWTs) experience six degrees of freedom motion in space with their floating platforms. Such dynamic process may induce complex dynamic flow and variable local load on the blades, however, still insufficiently understood from a perspective of unsteady aerodynamics. In this study, the local aerodynamic load in the dynamic process induced by the platforms' pitch motion is investigated by computational fluid dynamics. Our results indicate that the axial force of each blade section fluctuates significantly over time, with varying levels of fluctuation among the blade sections. The pressure distributions around the blade sections also vary significantly. Additionally, we observe the light dynamic stall phenomenon that cause stronger lift in upstroke and weaker lift in downstroke, and the flow separation and reattachment process during the dynamic stall demonstrates a delayed response relative to instantaneous changes in the angle of attack. The load variations due to the dynamic process and dynamic stall can lead to severe fatigue and extreme loads on the surface and structure of the blades. These findings suggest that quasi-steady analysis, which neglects the dynamic process, is not sufficient for accurately predicting the blade loads under pitch motion. Furthermore, we find that the fatigue-load properties vary with location on a blade section, with the leading and trailing edges being more susceptible to fatigue problems. These results highlight the importance of considering the potential harmful effects of pitch motion in the design of FHAWTs. • The local loads of the blades experience dynamic process due to FHAWTs' pitch motion. • CFD is employed to explore local loads of the blade sections of FHAWTs. • The local pressure distributions fluctuate significantly over time during the dynamic process. • Dynamic stall phenomenon occurs during the pitch motion of FHAWTs. • The dynamic process and dynamic stall may cause severe fatigue and extreme loads. The leading and trailing edges are more prone to the fatigue problem during the dynamic process. [ABSTRACT FROM AUTHOR]

Many commercial engineering software programmes used to design floating tidal turbines neglect the added mass and damping because most of the code considers floating wind turbine designs, which is acceptable since air is significantly less dense than seawater. However, added mass and damping forces are important parameters that need to be included in the design of a floating tidal turbine. The increase in instantaneous time-dependent loading and changes in the natural frequency of a floating tidal turbine make these hydrodynamic forces non-negligible, especially for large turbines. The present study describes the construction of matrices of added mass and damping that can be used as inputs to simpler engineering models. These matrices were constructed by conducting three-dimensional blade-resolved CFD simulations for a floating tidal turbine operating under a prescribed pitch motion (i.e., pendulum-like motion) under various motion amplitudes and frequencies. The added mass and damping were also empirically extracted from the fluctuating thrust force, the empirical model derivation of which is given in this paper. Higher motion amplitude and frequency increase the pressure on rotor blades, which increases the amplitude of loading variations. A floating tidal turbine's mean power and thrust deviate from that of a stationary turbine due to suboptimal rotor operating conditions caused by the change in tip speed ratio (which is due to the apparent velocity variation). Formulations can be constructed (as functions of motion amplitude and frequency) to calculate added mass and damping forces based on the data of hydrodynamic forces provided in this paper, which can be applied to a conventional turbine design model. The added mass and damping forces increase the overall mass and damping of the floating rotor, respectively, which affects the motion and loading of the device. • Added mass and damping forces on a floating tidal turbine undergoing the pendulum motion. • Development of generalized hydrodynamic coefficients of floating tidal turbines oscillating in pendulum motion. • Shifts in the natural frequency of the floating turbine due to hydrodynamic forces. [ABSTRACT FROM AUTHOR]

The flow-field around the offshore floating vertical axis wind turbines (OF-VAWTs) is affected by the six-degrees of freedom (6-DOF) movement of the platforms. Understanding the impact of a certain DOF motion on the aerodynamics is beneficial to the design of wind turbines. In this paper, the aerodynamics and performance of a scale OF-VAWT in pitch motion are investigated. The computational fluid dynamics (CFD) method with the turbulence model of improved delayed detached eddy simulation (IDDES) and the overset mesh technique are employed to analyze the characteristics of pitch motion of wind turbines. The CFD model is verified by the experimental data from available literature when the turbine has no-pitch motion. Then, the aerodynamic forces, power, and wake of an OF-VAWT in periodical pitch motion are analyzed. Because of the varying wave loads, an unsteady aerodynamic analysis considering the pitch motion with different periods and amplitudes is performed. The results show that the pitch motion can improve the power output of the OF-VAWTs, and enlarge the variation ranges of aerodynamic force coefficients. Meanwhile, the graphs of the vortex structure show that the complex flow interaction emerges around the rotor blades. Additionally, the power coefficient and the instantaneous aerodynamic forces coefficients sensitively change with different pitching periods and pitching amplitudes. [ABSTRACT FROM AUTHOR]

The floating vertical axis wind turbine (VAWT) is considered as a competitive device in the utilization of offshore wind energy. However, the platform pitch motion would affect its aerodynamic behavior. In this paper, the aerodynamic performance of a floating φ-type VAWT under pitch motion is investigated by using the Improved Delayed Detached Eddy Simulation SST k − ω turbulence model. After verifying the feasibility of the numerical model, the effects of pitch motion amplitude and period on the aerodynamic characteristics were evaluated, and the impacts of these observations were elucidated. The results showed that the averaged net power coefficient increment of about 1.5 % –15 % could be obtained under platform pitch motions, and the fluctuation of aerodynamic loads was found to increase. Besides, the pitch motion pattern could be regarded as the combination of surge and heave motions, which explained the similarity of their effects on the wind turbine aerodynamics. Furthermore, it was found that the frequency of the peak torque coefficient would change under different periods of pitch motion, which should be noticed in the design of floating wind turbine. Finally, it was concluded that the current study provided additional information about the effect of pitch motion on wind turbine aerodynamics. • The aerodynamic performance of ϕ-type VAWT under pitch motion is investigated. • The similarity of aerodynamic changes under pitch and surge motions is explained. • The aerodynamic changes of ϕ-type and H-type VAWTs under pitch motion are compared. • The influence of pitch motion on aerodynamic spectrum of wind turbine is studied. [ABSTRACT FROM AUTHOR]

Compared with the conventional floating offshore platforms, the FOWTs (floating offshore wind turbines) are subjected to larger wind heeling moments to cause significant pitch motions which bring challenges to the power generation efficiency and structure safety, due to the existence of wind turbines. In addition, the dynamic cable connected with FOWT is subject to a load that is greatly affected by horizontal motions of FOWT. Therefore, the inhibitions of horizontal and pitch motions are vital to FOWT. However, conventional mooring systems mainly inhibit the horizontal motions of floating structures. In this paper, an innovative type of mooring system is proposed to inhibit both the horizontal and pitch motions of FOWT. The analytical results based on the innovative mooring system are compared with those of conventional mooring system. In addition, the mechanism of restoring forces and moments of mooring systems are analyzed and discussed. • An innovative type of mooring system is proposed to inhibit the horizontal and pitch motions of FOWT. • The horizontal and pitch motions of FOWT with the innovative mooring system are significantly reduced. • The restoring stiffnesses of innovative mooring system in surge and pitch DOFs are significantly enlarged. • The damping contributions of innovative mooring system in surge and pitch DOFs are significantly enlarged. • The pitch motions will improve the horizontal motion inhibition performance of innovative mooring system. [ABSTRACT FROM AUTHOR]

The aerodynamic performance of wind turbines is essential to evaluate their electricity-generating capability. Compared with the wind turbines with traditional fixed structures, the aerodynamic performance of floating offshore wind turbines (FOWTs) is affected by the additional platform motion, especially for the pitch motion. In view of this, the computational fluid dynamics (CFD) method with the improved delayed detached eddy simulation (IDDES) is applied. Before the numerical simulation, the CFD model of the redesigned 1:50 scale rotor from NREL 5 MW wind turbine is verified with the available experimental data and numerical comparison. Then, the aerodynamics of the FOWT under the harmonic pitch motion with different periods and amplitudes is investigated. It is shown that the aerodynamic performance of the FOWT is sensitive to these parameters of the pitch motion. First, amplitudes of the rotor thrust and torque decrease with the increment of the pitch period. In particular, the wake interference phenomenon is the most evident when the pitch period is short and the pitch amplitude is high, which may compensate some energy to the rotor. In addition, amplitudes of the rotor thrust and torque increase with the increment of the pitch amplitude. Also, the stall phenomenon happens when the pitch amplitude is high, which may impact FOWTs' aerodynamic performance adversely. Finally, the rotor power increases under the periodic pitch motion, especially by decreasing the period or increasing the amplitude. It is concluded that the pitch motion of the platform will change the aerodynamic performance of a wind turbine and should be taken into consideration during design procedure. • IDDES model is employed to analyze the aerodynamics of a FOWT under pitch motion. • Effects of pitch periods are studied, and wake interference phenomenon is observed. • Effects of pitch amplitudes are investigated, and stall phenomenon is revealed. • The rotor power under periodic pitch motion is investigated. [ABSTRACT FROM AUTHOR]