Low‐frequency dynamics of a floating wind turbine in wave tank–scaled experiments with SiL hybrid method

The design of floating wind turbines requires both, simulation tools and scaled testing methods, accurately integrating the different phenomena involved in the system dynamics, such as the aerodynamic and hydrodynamic forces, the mooring lines dynamics and the control strategies. In particular, one of the technical challenges when testing a scaled floating wind turbine in a wave tank is the proper integration of the rotor aerodynamic thrust. The scaling of the model based on the Froude number produces equivalent hydrodynamic forces, but out of scale aerodynamic forces at the rotor, because the Reynolds number, that governs the aerodynamic forces, is not kept constant. Several approaches have been taken to solve this conflict, like using a tuned drag disk or redesigning the scaled rotor to provide the correct scaled thrust at low Reynolds numbers. This work proposes a hybrid method for the integration of the aerodynamic thrust during the scaled tests. The work also explores the agreement between the experimental measurements and the simulation results through the calibration and improvement of the numerical models. CENER has developed a hybrid testing method that replaces the rotor by a ducted fan at the model tower top. The fan can introduce a variable force which represents the total wind thrust by the rotor. This load is obtained from an aerodynamic simulation that is performed in synchrony with the test and it is fed in real time with the displacements of the platform provided by the acquisition system. Thus, the simulation considers the displacements of the turbine within the wind field and the relative wind speed on the rotor, including the effect of the aerodynamic damping on the tests. The method has been called “Software-in-the-Loop” (SiL). The method has been applied on a test campaign at the Ecole Centrale de Nantes wave tank of the OC4 semisubmersible 5MW wind turbine, with a scale factor of 1/45. The experimental results have been compared with equivalent numerical simulations of the floating wind turbine using the integrated code FAST. Simple cases as only steady wind and free decays with constant wind showed a good agreement with computations, demonstrating that the SiL method is able to successfully introduce the rotor scaled thrust and the effect of the aerodynamic damping on the global dynamics. Cases with turbulent wind and irregular waves showed better agreement with the simulations when mooring line dynamics and second order effects were included in the numerical models.

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OC6 Phase I: Investigating the underprediction of low-frequency hydrodynamic loads and responses of a floating wind turbine

Journal of Physics Conference Series ◽

10.1088/1742-6596/1618/3/032033 ◽

2020 ◽

Vol 1618 ◽

pp. 032033 ◽

Cited By ~ 1

Author(s):

A N Robertson ◽

S Gueydon ◽

E Bachynski ◽

L Wang ◽

J Jonkman ◽

...

Keyword(s):

Phase I ◽

Wind Turbine ◽

Low Frequency ◽

Hydrodynamic Loads ◽

Floating Wind Turbine

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On the Correlation Between Floating Wind Turbine Accelerations, Rotor and Tower Loads

Volume 9: Ocean Renewable Energy ◽

10.1115/omae2020-18610 ◽

2020 ◽

Author(s):

Thomas Choisnet ◽

Yohan Percher ◽

Raphaël Adam ◽

Mathieu Favré ◽

Robert Harries

Keyword(s):

Wind Turbine ◽

Low Frequency ◽

The Body ◽

Offshore Wind ◽

Main Shaft ◽

Offshore Wind Turbines ◽

Floating Wind Turbine ◽

Speed Range ◽

Main Driver ◽

Wave Conditions

Abstract Research in floating wind turbine resulted in the publication of a number of studies comparing wind turbine loads on a variety of floaters and fixed foundations. Some of them concluded that the blade and drivetrain loads would only be marginally increased -if increased at all — when a turbine would be installed on a floater [1] [2] and [3]. This paper proposes to evaluate how rotor and tower loads are correlated to nacelle acceleration, wind and wave conditions. It can somehow be considered as an extension to a barge-type floater and onsite measurements, of published work applied to fixed offshore wind turbines [4] and a spar-type floating wind turbine [5]. The body of data used for this exercise includes results from full-scale prototype measurements and simulations for a variety of turbine ratings. It can be concluded that in power production cases, blade and main shaft loads are only weakly correlated to nacelle low frequency accelerations and hence wave conditions, hence aerodynamic loads are still the main driver for rotor and tower top loads. In rotor-idling conditions, the situation is mainly dependent on the wind speed range, but aerodynamics are the largest contributor of blade and main shaft loads in severe wind conditions. These results can help understand where design uncertainties lie in floating wind turbine loads.

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Non-Linear 3D Hydrodynamics of Floating Wind Turbine Compared Against Wave Tank Tests

Volume 2: CFD and VIV ◽

10.1115/omae2016-55090 ◽

2016 ◽

Author(s):

Nicolai F. Heilskov ◽

Ole Svenstrup Petersen

Keyword(s):

Wind Turbine ◽

Degrees Of Freedom ◽

Cfd Simulation ◽

Wave Structure ◽

Point Of View ◽

Body Motion ◽

Irregular Waves ◽

Floating Body ◽

Wave Tank ◽

Floating Wind Turbine

Assessment of wave-structure interaction in terms of combined hydrodynamic stability and structural survivability is paramount in extreme wave conditions. Components of CFD methodologies needed for accurately capturing the detailed motion of a floating wind turbine (FWT) in survival sea-state is the focus of the study. Physical wave tank tests of a Tension Leg Platform (TLP) concept with four moorings are applied as a first validation, due to its simplicity from a CFD point of view. Two different codes have been objects of study, namely the open source code OpenFOAM® with a flexible mesh approach and the commercial CFD code StarCCM+ with the overset mesh method. The influence of the surface capturing algorithm (VOF method) and the two-way coupling of the six degrees-of-freedom body motion solver and the hydrodynamic solver have been identified as the crucial components in CFD simulation of the FWT. A major advantage of StarCCM+ was that it does not suffer from the same sensitivity as OpenFOAM to the fact that motion of the floating body is strongly coupled to the solution of the hydrodynamics (a stiff FSI problem) which led to instability of the numerical solution. The results obtained with StarCCM+ are comparable with the measured motion of and tension forces on the TLP in both in regular waves and irregular waves.

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Model tests of a 10 MW semi-submersible floating wind turbine under waves and wind using hybrid method to integrate the rotor thrust and moments

10.5194/wes-2021-148 ◽

2021 ◽

Author(s):

Felipe Vittori ◽

José Azcona ◽

Irene Eguinoa ◽

Oscar Pires ◽

Alberto Rodríguez ◽

...

Keyword(s):

Wind Turbine ◽

Natural Frequency ◽

Hybrid Method ◽

Deep Ocean ◽

Ocean Basin ◽

Experimental Results ◽

Floating Platform ◽

Scaled Model ◽

Floating Wind Turbine ◽

Tank Test

Abstract. This paper describes the results of a wave tank test campaign of a 1/49 scaled SATH 10MW INNWIND floating platform. The Software-in-the-Loop (SiL) hybrid method was used to include the wind turbine thrust and the in-plane rotor moments My – Mz. Experimental results are compared with a numerical model developed in OpenFAST of the floating wind turbine. The tank test campaign was carried out in the scaled model tested at the Deep Ocean Basin from the Lir National Ocean TF at Cork, Ireland. This floating substructure design was adapted by Saitec to support the 10MW INNWIND wind turbine within the ARCWIND project with the aim of withstanding the environmental conditions of the European Atlantic Area region. CENER provided the wind turbine controller specially designed for the SATH 10MW configuration. A description of the experimental set up, force actuator configuration and the numeric aerodynamic parameters are provided in this work. The most relevant experimental results under wind and wave loading are showed in time series and frequency domain. The influence of the submerged geometry variations in the pitch natural frequency is discussed. The paper shows the simulation of a case with rated wind speed, where the tilted geometry for the computation of the hydrostatic and hydrodynamic properties of the submerged substructure is considered. This case provides a better agreement of the pitch natural frequency with the experiments, than a equivalent simulation using the undisplaced geometry mesh for the computation of the hydrodynamic and hydrostatic properties.

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Experimental Investigation of the Coupling Between Aero- and Hydrodynamical Loads On A 12 MW Semi-Submersible Floating Wind Turbine

10.1115/omae2021-62980 ◽

2021 ◽

Author(s):

Maxime Thys ◽

Carlos Souza ◽

Thomas Sauder ◽

Nuno Fonseca ◽

Petter Andreas Berthelsen ◽

...

Keyword(s):

Wind Turbine ◽

Low Frequency ◽

Turbine Rotor ◽

Ocean Basin ◽

Sea State ◽

Turbulent Wind ◽

Aerodynamic Loads ◽

Wave Elevation ◽

Floating Wind Turbine ◽

Quantities Of Interest

Abstract Model tests were performed with a model of the INO WINDMOOR 12 MW floating wind turbine in the Ocean Basin at SINTEF Ocean. The tests were done at a scale of 1:40. RealTime Hybrid Model testing was used for the modelling of the wind turbine rotor and aerodynamic loads. A subset of the results is analysed to study the influence of the wind on the platform motions, the acceleration at tower top, the loads at base of tower and the relative wave elevation. The study is based on the comparison of the quantities of interest for different tests with the same moderate sea-state but with different wind modelling: no wind, constant thrust force, turbulent wind of 11.5 m/s and turbulent wind of 25 m/s. The wind modelling has a minimal influence on the platform surge and pitch response in the wave-frequency range. On the other hand, the aerodynamic loads, including wind turbine controller dynamics and turbulent wind, has an important impact on the low-frequency surge and pitch response. The aerodynamic loads are important for the loads at tower base due to the dominance of the tower-RNA induced gravitational loads at low-frequency. Maximum relative wave elevation was found to be mainly dependent on the thrust induced mean pitch angle.

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