Browsing by Author "Sandner, Frank"

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Open Access
Conceptual design of floating wind turbines
(2013) Sandner, Frank; Cheng, Po Wen
The need for different numerical models with varying degrees of simplification for the conceptual design of a floating offshore wind turbine is the focus of this paper. While parts on the component level can be designed apart from the others the overall dynamics on the system level have to be assessed from the beginning. Starting with very simple models and identifying the significant contributions to the system behavior while going step by step to more detailed ones makes a successful dimensioning possible. The significant effect of the blade pitch controller on the system dynamics is analysed and preliminarily designed with a simple 1-degree of freedom (dof) model. Further on the section forces at tower base and the distributed platform loads are calculated with a 9-dof multibody system with simplified aerodynamics and Morison equation allowing a pre-dimensioning of the structure.
Open Access
Improved tank test procedures for scaled floating offshore wind turbines
(2014) Müller, Kolja; Sandner, Frank; Bredmose, Henrik; Azcona, José; Manjock, Andreas; Pereira, Ricardo
This study collects issues from previous tank test campaigns of scaled Floating Offshore Wind Turbines (FOWT), compares the different scaling methodologies, points out critical aspects and shows possible alternatives and recommendations for future tests depending on the specific objective. Furthermore, it gives practical recommendations for the modeling and construction of scaled rotors. The presented scaling procedure will be applied in tank tests within the EU Seventh Framework Program InnWind (ENERGY.2012.2.3.1 "Innovative wind conversion systems (10-20MW) for offshore applications").
Open Access
Integrated optimization of floating wind turbine systems
(2014) Sandner, Frank; Schlipf, David; Matha, Denis; Cheng, Po Wen
An exemplary methodology is shown for the integrated conceptioning of a floating wind turbine system with focus on the spar-type hull and the wind turbine blade-pitch-to-feather controller. It is a special interest to use a standard controller, which is easily implementable, even at early design stages. The optimization of the system is done with adapted static and dynamic models through a stepwise narrowing of the design space according to the requirements of floating wind turbines. After selecting three spar-type hull geometries with variable draft a simplified nonlinear simulation model with four degrees of freedom is set up and then linearized including the aerodynamics with the blade pitch controller in the closed-loop. The linear system allows conventional procedures for SISO controller design giving a theoretically suitable range of controller gains. Subsequently, the nonlinear model is used to find the optimal controller gains for each platform. Finally, a nonlinear coupled model with nine degrees of freedom gives the optimal solution under realistic wind and wave loads.
Open Access
Nonlinear model predictive control of floating wind turbines
(2013) Schlipf, David; Sandner, Frank; Raach, Steffen; Matha, Denis; Cheng, Po Wen
In this work a nonlinear model predictive control method for a floating wind turbine is presented. A reduced nonlinear model including disturbance preview of wind and waves is derived and implemented to compute optimal input trajectories for collective pitch and the generator torque. A cost functional is introduced which fulfills all desired constraints and controller goals for above rated wind conditions. The controller is tested for extreme and fatigue load cases and a significant reduction of the power and rotor speed deviations is obtained. Furthermore, ultimate tower loads and damage equivalent loads on shaft and blades are decreased. Although more detailed testing is necessary, this preliminary results show the advantages of nonlinear model predictive control for floating wind turbines.
Open Access
Nonlinear model predictive control of floating wind turbines with individual pitch control
(2014) Raach, Steffen; Schlipf, David; Sandner, Frank; Matha, Denis; Cheng, Po Wen
In this work a nonlinear model predictive controller with individual pitch control for a floating offshore wind turbine is presented. An aerodynamic model of the collective pitch control approach is extended by describing pitching and yawing moments based on rotor disk theory. This extension is implemented in a reduced nonlinear model of the floating wind turbine including disturbance preview of wind speed, linear vertical and horizontal wind shear, and wave height to compute optimal input trajectories for the individual pitch control inputs and the generator torque. An extended cost functional for individual pitch control is proposed based on the collective pitch control approach. The controller is evaluated in aero-servo-hydro-elastic simulations of a 5MW reference wind turbine disturbed by a three-dimensional stochastic turbulent wind field. Results show a significant blade fatigue load reduction compared to a baseline controller through minimizing yawing and pitching moments on the rotor hub while maintaining the advantages of the model predictive control approach with collective pitch control.
Open Access
Reduced model design of a floating wind turbine
(2012) Sandner, Frank
Floating platform concepts offer the prospect of harvesting offshore wind energy at deep water locations for countries with a limited number of suitable shal- low water locations for bottom-mounted offshore wind turbines. The floating spar-buoy concept has shown promising experimental and theoretical results. Al- though various codes for a detailed simulation exist the purpose of this work is to elaborate a reduced Floating Offshore Wind Turbine (FOWT) model that mainly reproduces the overall nonlinear low-frequency behaviour of the system with a significant saving in simulation time. One objective is to extend the model predictive control algorithm that has previously been developed for onshore wind turbines to the FOWT for motion control and load reductions. Another objective is a fast dynamic assessment of new concepts during design phase with respect to load cases defined by the International Electrotechnical Commission (IEC). The platform and wind turbine structure is modelled as a three-dimensional multibody system consisting of four rigid bodies with nine degrees of freedom. That is, unconstrained platform motion, tower bending in two directions and variable rotor speed. The coupled nonlinear system of equations of motion is calculated symbolically using the Newton-Euler formalism that takes Coriolis- and centrifugal forces into account. Complex disturbances on the system arising from aerodynamics and hydrodynamics are simplified along with the model as efficiently and accurately as possible. Wind loads are predicted by reducing the three-dimensional turbulent wind field to a scalar rotor-effective wind speed also considering restoring torques resulting from oblique inflow. Linear wave theory provides the wave kinematics and wave loads are calculated using the relative formulation of Morison’s Equation. An approach is presented to estimate wave loads on the floating structure based only on real-time wave height measurements. This allows also for an analytical calculation of wave loads in time-domain with- out iterative or recursive algorithms so that a significant saving in computational time is achieved. The presented disturbance reduction to simple and measurable inputs for wind and waves is a precondition for the implementation of an opti- mal control algorithm. The reduced nonlinear model is compared to the certified aero-hydro-servo-elastic FAST model in time and frequency domain. The results are promising as there is good agreement in static and dynamic response.
Open Access
Reduced nonlinear model of a spar-mounted floating wind turbine
(2012) Sandner, Frank; Schlipf, David; Matha, Denis; Seifried, Robert; Cheng, Po Wen
Floating offshore wind turbines (FOWTs) are complex dynamic systems requiring a thorough design for optimal operating performance and stability. Advanced control strategies, like model predictive control, are part of the integrated development of new concepts. This paper presents a simplified and computationally efficient model of the spar-mounted OC3-Hywind FOWT. Applications are, e.g., the real-time integration within the controller or an assessment during conceptual design, possibly within an optimization algorithm. Symbolic equations of motion of a multibody system are available as a set of ordinary differential equations. Aerodynamic forces are computed based on a rotor effective wind speed at hub height using data tables for thrust and torque coefficients. Hydrodynamic impacts on the floating body are modeled in a way that only the wave height serves as the disturbance signal. This estimation is based on potential flow theory and Morison’s formula for slender cylinders. The reduced model code is fully compiled and has a real-time factor of approximately 100. Various simulations of common load cases with a comparison to the certified FAST code have shown to be promising.