scholarly journals Iterative Frequency-Domain Response of Floating Offshore Wind Turbines with Parametric Drag

2018 ◽  
Vol 6 (4) ◽  
pp. 118 ◽  
Author(s):  
Frank Lemmer ◽  
Wei Yu ◽  
Po Cheng
Keyword(s):  
System Dynamics ◽  
Frequency Domain ◽  
Wind Turbines ◽  
Domain Model ◽  
Offshore Wind ◽  
Viscous Drag ◽  
Drag Coefficients ◽  
Reduced Order ◽  
Offshore Wind Turbines ◽  
Floating Offshore Wind Turbines

Methods for coupled aero-hydro-servo-elastic time-domain simulations of Floating Offshore Wind Turbines (FOWTs) have been successfully developed. One of the present challenges is a realistic approximation of the viscous drag of the wetted members of the floating platform. This paper presents a method for an iterative response calculation with a reduced-order frequency-domain model. It has heave plate drag coefficients, which are parameterized functions of literature data. The reduced-order model does not represent more than the most relevant effects on the FOWT system dynamics. It includes first-order and second-order wave forces, coupled with the wind turbine structural dynamics, aerodynamics and control system dynamics. So far, the viscous drag coefficients are usually defined as constants, independent of the load cases. With the computationally efficient frequency-domain model, it is possible to iterate the drag, such that it fits to the obtained amplitudes of oscillation of the different members. The results show that the drag coefficients vary significantly across operational load conditions. The viscous drag coefficients converge quickly and the method is applicable for concept-level design studies of FOWTs with load case-dependent drag.

scholarly journals An efficient frequency-domain model for quick load analysis of floating offshore wind turbines

2018 ◽  
Vol 3 (2) ◽  
pp. 693-712 ◽  
Author(s):  
Antonio Pegalajar-Jurado ◽  
Michael Borg ◽  
Henrik Bredmose
Keyword(s):  
Wind Speed ◽  
Frequency Domain ◽  
Wind Turbines ◽  
Degrees Of Freedom ◽  
Bending Moment ◽  
Domain Model ◽  
Offshore Wind ◽  
Hydrodynamic Damping ◽  
Floating Offshore Wind Turbines ◽  
Load Analysis

Abstract. A model for Quick Load Analysis of Floating wind turbines (QuLAF) is presented and validated here. The model is a linear, frequency-domain, efficient tool with four planar degrees of freedom: floater surge, heave, pitch and first tower modal deflection. The model relies on state-of-the-art tools from which hydrodynamic, aerodynamic and mooring loads are extracted and cascaded into QuLAF. Hydrodynamic and aerodynamic loads are pre-computed in WAMIT and FAST, respectively, while the mooring system is linearized around the equilibrium position for each wind speed using MoorDyn. An approximate approach to viscous hydrodynamic damping is developed, and the aerodynamic damping is extracted from decay tests specific for each degree of freedom. Without any calibration, the model predicts the motions of the system in stochastic wind and waves with good accuracy when compared to FAST. The damage-equivalent bending moment at the tower base is estimated with errors between 0.2 % and 11.3 % for all the load cases considered. The largest errors are associated with the most severe wave climates for wave-only conditions and with turbine operation around rated wind speed for combined wind and waves. The computational speed of the model is between 1300 and 2700 times faster than real time.

scholarly journals Advances on Reduced-Order Modeling of Floating Offshore Wind Turbines

2021 ◽  
Author(s):  
Frank Lemmer ◽  
Wei Yu ◽  
Heiner Steinacker ◽  
Danai Skandali ◽  
Steffen Raach
Keyword(s):  
Wind Turbines ◽  
State Feedback ◽  
The State ◽  
Offshore Wind ◽  
Weight Functions ◽  
Elastic Model ◽  
Reduced Order ◽  
Offshore Wind Turbines ◽  
Modeling Uncertainties ◽  
Floating Offshore Wind Turbines

Abstract Aero-hydro-servo-elastic modeling of Floating Offshore Wind Turbines (FOWTs) is a key component in the design process of various components of the system. Different approaches to order reduction have been investigated with the aim of improving structural design, manufacturing, transport and installation, but also the dynamic behavior, which is largely affected by the blade pitch controller. The present work builds on previous works on the SLOW (Simplified Low-Order Wind Turbine) code, which has already been used for the above purposes, including controller design. While the previous rigid rotor model gives good controllers in most cases, we investigate in the present work the question if aero-elastic effects in the design model can improve advanced controllers. The SLOW model is extended for the flap-wise bending and coupled to NREL’s AeroDyn, linearized and verified with the OlavOlsen OO-Star Wind Floater Semi 10MW public FOWT model. The results show that the nonlinear and linear reduced-order SLOW models agree well against OpenFAST. The state-feedback Linear Quadratic Regulator (LQR) applied with the same weight functions to both models, the old actuator disk, and the new aero-elastic model shows that the LQR becomes more sensitive to nonlinear excitation and that the state feedback matrix is significantly different, which has an effect on the performance and potentially also on the robustness. Thus modeling uncertainties might even be more critical for the LQR of the higher-fidelity model.

Effects of Platform Mounting Orientations on the Long-Term Performance of a Semisubmersible Wind Turbine

2019 ◽  
Author(s):  
Shengtao Zhou ◽  
Chao Li ◽  
Yiqing Xiao ◽  
Frank Lemmer ◽  
Wei Yu ◽  
...  
Keyword(s):  
Wind Turbine ◽  
Frequency Domain ◽  
Wind Turbines ◽  
Wind Wave ◽  
Domain Model ◽  
Offshore Wind ◽  
Wave Load ◽  
Symmetrical Configuration ◽  
Floating Offshore Wind Turbines

Abstract Due to the non-fully-symmetrical configuration, the platform laying angle of semi-submersible floating offshore wind turbines relative to wind/wave load directions has a noticeable influence on the dynamics characteristics of the whole structure, which indicates that the platform mounting orientation should be carefully considered before installation at sea. The directionality effects of short-term wind/wave loads had been discussed in previous studies, which are, however, insufficient to make a full understanding of the directionality impacts. In our study, based on a 25-year met-ocean database, long-term analysis is carried out by means of an efficient frequency-domain model with eight degrees of freedom. The nonlinear quantities such as aerodynamic loads, aerodynamic damping and mooring stiffness are derived from the time-domain simulation tool FAST, serving as a preprocessing database for the frequency-domain model. A case study is carried out by comparing the long-term responses of a Y-shape semi-submersible floating wind turbine in four mounting orientations. Significant differences can be seen. The platform mounted in the most unfavorable orientation tends to suffer from larger peak nacelle acceleration, which would increase the loads and cause higher tower base fatigue damage. These findings highlight the importance of platform mounting orientations and can serve as a basis for the installation of semi-submersible floating wind turbines.

A Formulation for the Unsteady Aerodynamics of Floating Wind Turbines, With Focus on the Global System Dynamics

2017 ◽  
Author(s):  
Ilmas Bayati ◽  
Marco Belloli ◽  
Luca Bernini ◽  
Alberto Zasso
Keyword(s):  
System Dynamics ◽  
Wind Turbines ◽  
Unsteady Aerodynamics ◽  
Offshore Wind ◽  
Scale Model ◽  
Wind Tunnel Experiments ◽  
Global System ◽  
Offshore Wind Turbines ◽  
Floating Offshore Wind Turbines ◽  
Thrust And Torque

This paper proposes a formulation for the assessment of the unsteady aerodynamics of floating offshore wind turbines, based on wind tunnel experiments through surge and pitch imposed motions on the 1/75 DTU 10 MW scale model. Rotor thrust and torque were analysed out of a set of different combinations of amplitudes and frequencies of the imposed mono-harmonic motion, following the idea of splitting these forces into steady and unsteady contributions, respectively through steady and unsteady aerodynamic coefficients. The latter were analysed, for different tip-speed ratios, both experimentally and numerically, with respect to a newly introduced parameter, the “wake reduced velocity”, which turned out to be effective in the description of the unsteady regime. Experimental results have shown good consistency of the formulation and put the basis for further studies on this topic, for the comprehension of this phenomenon and for the development of reduced-order models for control purposes, with the focus of the global system dynamics.

scholarly journals An efficient frequency-domain model for quick load analysis of floating offshore wind turbines

2018 ◽  
Author(s):  
Antonio Pegalajar-Jurado ◽  
Michael Borg ◽  
Henrik Bredmose
Keyword(s):  
Wind Speed ◽  
Frequency Domain ◽  
Wind Turbines ◽  
Degrees Of Freedom ◽  
Bending Moment ◽  
Domain Model ◽  
Offshore Wind ◽  
Hydrodynamic Damping ◽  
Floating Offshore Wind Turbines ◽  
Load Analysis

Abstract. A model for Quick Load Analysis of Floating wind turbines, QuLAF, is presented and validated here. The model is a linear, frequency-domain, efficient tool with four planar degrees of freedom: platform surge, heave, pitch and tower modal deflection. The model relies on state-of-the-art tools from which hydrodynamic, aerodynamic and mooring loads are extracted and cascaded into QuLAF. Hydrodynamic and aerodynamic loads are precomputed in WAMIT and FAST respectively, while the mooring system is linearized around the equilibrium position for each wind speed using MoorDyn. An approximate approach to viscous hydrodynamic damping is developed, and the aerodynamic damping is extracted from decay tests specific for each degree of freedom. Without any calibration, the model predicts the motions of the system in stochastic wind and waves with good accuracy when compared to FAST. The damage-equivalent bending moment at the tower bottom is estimated with errors between 0.2 % and 11.3 % for all the load cases considered. The largest errors are associated with the most severe wave climates for wave-only conditions and with turbine operation around rated wind speed for combined wind and waves. The computational speed of the model is between 1300 and 2700 times faster than real-time.

Control of Floating Offshore Wind Turbines: Reduced-Order Modeling and Real-Time Implementation for Wind Tunnel Tests

2018 ◽  
Author(s):  
Alessandro Fontanella ◽  
Ilmas Bayati ◽  
Marco Belloli
Keyword(s):  
Wind Tunnel ◽  
Wind Turbine ◽  
Wind Turbines ◽  
Offshore Wind ◽  
Scale Model ◽  
System Response ◽  
Wind Tunnel Tests ◽  
Reduced Order ◽  
Offshore Wind Turbines ◽  
Floating Offshore Wind Turbines

The present work deals with the implementation of a variable-speed variable-pitch control strategy on a wind turbine scale model for hybrid/HIL wind tunnel tests on floating offshore wind turbines. The effects that scaling issues, due to low-Reynolds aerodynamics and rotor structural properties, have in combination with the HIL technique developed by the authors are studied through a dedicated reduced-order linear coupled model. The model is used to tune the original pitch controller gains so to be able to reproduce the system response of the full-scale floating wind turbine during HIL tests.

scholarly journals The TripleSpar Campaign: Validation of a Reduced-Order Simulation Model for Floating Wind Turbines

2018 ◽  
Author(s):  
Frank Lemmer (né Sandner) ◽  
Wei Yu ◽  
Po Wen Cheng ◽  
Antonio Pegalajar-Jurado ◽  
Michael Borg ◽  
...  
Keyword(s):  
Simulation Model ◽  
Wind Turbines ◽  
Degrees Of Freedom ◽  
Coupled System ◽  
Offshore Wind ◽  
Drag Coefficients ◽  
Reduced Order ◽  
Irregular Wave ◽  
Floating Offshore Wind Turbines ◽  
Decay Tests

Different research groups have recently tested scaled floating offshore wind turbines including blade pitch control. A test conducted by the University of Stuttgart (Germany), DTU (Denmark) and CENER (Spain) at the Danish Hydraulic Institute (DHI) in 2016 successfully demonstrated a real-time blade pitch controller on the public 10MW TripleSpar semi-submersible concept at a scale of 1/60. In the presented work a reduced-order simulation model including control is compared against the model tests. The model has only five degrees of freedom and is formulated either in the time-domain or in the frequency-domain. In a first step the Morison drag coefficients are identified from decay tests as well as irregular wave cases. The identified drag coefficients depend clearly on the sea state, with the highest ones for the decay tests and small sea states. This is an important finding, for example for the design of a robust controller, which depends on the system damping. It is shown that the simplified model can well represent the dominant physical effects of the coupled system with a substantially reduced simulation time, compared to state-of-the-art models.

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