Browsing by Author "Lemmer, Frank"

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Open Access
Advances on reduced-order modeling of floating offshore wind turbines
(2021) Lemmer, Frank; Yu, Wei; Steinacker, Heiner; Skandali, Danai; Raach, Steffen
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 flapwise 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.
Open Access
Collective pitch feedforward control of floating wind turbines using lidar
(2015) Schlipf, David; Simley, Eric; Lemmer, Frank; Pao, Lucy; Cheng, Po Wen
In this work a collective pitch feedforward controller for floating wind turbines is presented. The feedforward controller provides a pitch rate update to a conventional feedback controller based on a wind speed preview. The controller is designed similar to the one for onshore turbines, which has proven its capability to improve wind turbine control performance in field tests. In a first design step, perfect wind preview and a calm sea is assumed. Under these assumptions the feedforward controller is able to compensate almost perfectly the effect of changing wind speed to the rotor speed of a full nonlinear model over the entire full load region. In a second step, a nacelle-based lidar is simulated scanning the same wind field which is used also for the aero-hydro-servo-elastic simulation. With model-based wind field reconstruction methods, the rotor effective wind speed is estimated from the raw lidar data and is used in the feedforward controller after filtering out the uncorrelated frequencies. Simulation results show that even with a more realistic wind preview, the feedforward controller is able to significantly reduce rotor speed and power variations. Furthermore, structural loads on the tower, rotor shaft, and blades are decreased. A comparison to a theoretical investigation shows that the reduction in rotor speed regulation is close to the optimum.
Open Access
Control co-design optimization of floating offshore wind turbines with tuned liquid multi-column dampers
(2024) Yu, Wei; Zhou, Sheng Tao; Lemmer, Frank; Cheng, Po Wen
The technical progress in the development and industrialization of floating offshore wind turbines (FOWTs) over the past decade has been significant. Yet, the higher levelized cost of energy (LCOE) of FOWTs compared to onshore wind turbines is still limiting the market share. One of the reasons for this is the larger motions and loads caused by the rough environmental excitations. Many prototype projects tend to employ more conservative substructure designs to meet the requirements for motion dynamics and structural safety. Another challenge lies in the multidisciplinary nature of a FOWT system, which consists of several strongly coupled subsystems. If these subsystems cannot work in synergy, the overall system performance may not be optimized. Previous research has shown that a well-designed blade pitch controller is able to reduce the motions and structural loads of FOWTs. Nevertheless, due to the negative aerodynamic damping effect, improvement in the performance by tuning the controller is limited. One of the solutions is adding tuned liquid multi-column dampers (TLMCDs), meaning that there is a structural solution to mitigate this limiting factor for the controller performance. It has been found that the additional damping, provided by TLMCDs, is able to improve the platform pitch stability, which allows a larger blade pitch controller bandwidth and thus a better dynamic response. However, if a TLMCD is not designed with the whole FOWT system dynamics taken into account, it may even deteriorate the overall performance. Essentially, an integrated optimization of these subsystems is needed. For this paper, we develop a control co-design optimization framework for FOWTs installed with TLMCDs. Using the multi-objective optimizer non-dominated sorting genetic algorithm II (NSGA-II), the objective is to optimize the platform, the blade pitch controller, and the TLMCD simultaneously. Five free variables characterizing these subsystems are selected, and the objective function includes the FOWT's volume of displaced water (displacement) and several motion and load indicators. Instead of searching for a unique optimal design, an optimal Pareto surface of the defined objectives is determined. It has been found that the optimization is able to improve the dynamic performance of the FOWT, which is quantified by motions and loads, when the displacement remains similar. On the other hand, if motions and loads are constant, the displacement of the FOWT can be reduced, which is an important indication of lower manufacturing, transportation, and installation costs. In conclusion, this work demonstrates the potential of advanced technologies such as TLMCDs to advance FOWTs for commercial competitiveness.
Open Access
Control design methods for floating wind turbines for optimal disturbance rejection
(2016) Lemmer, Frank; Schlipf, David; Cheng, Po Wen
An analysis of the floating wind turbine as a multi-input-multi-output system investigating the effect of the control inputs on the system outputs is shown. These effects are compared to the ones of the disturbances from wind and waves in order to give insights for the selection of the control layout. The frequencies with the largest impact on the outputs due to the limited effect of the controlled variables are identified. Finally, an optimal controller is designed as a benchmark and compared to a conventional~PI-controller using only the rotor speed as input. Here, the previously found system properties, especially the difficulties to damp responses to wave excitation, are confirmed and verified through a spectral analysis with realistic environmental conditions. This comparison also assesses the quality of the employed simplified linear simulation model compared to the nonlinear model and shows that such an efficient frequency-domain evaluation for control design is feasible.
Open Access
FlexDyn - a new OpenFAST structural dynamics module for a general, user defined wind turbine topology
(2021) Lemmer, Frank; Pérez Brovia, Santiago; Skandali, Danai; Raach, Steffen
In the present work, FlexDyn, a new structural dynamics module for the OpenFAST framework is developed. FlexDyn can generate structural equations of motion through a formalism, given user-defined rigid and elastic bodies and associated Degrees of Freedom (DOFs). The Newton-Euler formalism uses beam models with shape functions for a reduced-order representation in the same way as ElastoDyn of OpenFAST. The equations of motion are formulated in minimal coordinates, equally to ElastoDyn. FlexDyn is fully integrated into the OpenFAST framework with a coupling to AeroDyn and the new SubDyn module for FE representations of floating substructures (Jonkman, et al., 2020), among others. The formalism was previously implemented and verified in the low-order aero-hydro-servo-elastic code SLOW (Lemmer, et al., 2020). The objective of the presentation is to show the methodology of the formalized generation of equations of motion and first results of the new FlexDyn module for OpenFAST. The use case is an improved aero-elastic model, which includes the torsional DOF of the blades. The torsional DOF is not included in the ElastoDyn module but can potentially contribute to the motion and load response of the blades. The fidelity level of this use case of FlexDyn is higher than that of ElastoDyn but still below that of BeamDyn, which is a full FE representation of the blades. For this reason, the computational performance is still in the range of ElastoDyn, taking advantage of the order reduction.
Open Access
Low-order modeling, controller design and optimization of floating offshore wind turbines
(2018) Lemmer, Frank; Cheng, Po Wen (Prof. Dr.)
Ongoing demonstration projects prove the technical feasibility of offshore wind turbines on floating foundations, moored by anchor lines. The present work starts with the derivation of a tailored numerical model, which represents not more than the most relevant effects to simulate the system dynamics, observed from experiments, including aerodynamics, hydrodynamics, structural dynamics and the control system. This provides the basis for an integrated design study to take floating wind turbines to the next stage of development. It is shown that the integrated design yields systems with a smooth dynamic behavior in harsh wind and wave conditions.
Open Access
Multi-variable feedforward control for floating wind turbines using lidar
(2020) Schlipf, David; Lemmer, Frank; Raach, Steffen
In this work a multi-variable feedforward controller for floating wind turbines is presented. The feedforward controller provides a pitch rate and a torque update to a conventional feedback controller based on a wind speed preview. A 10 MW reference wind turbine is used on a semi submersible floating platform to study the potential of the controller. An open-source simulation tool is extended with an realistic lidar simulator and the lidar data processing, feedforward controller, and feedback controller are implemented in modular setup. The lidar measurements are fully motion compensated and combined to provide a preview of the rotor-effective wind speed to the controller. The feedforward controller is designed to minimize structural loads and to decrease the platform pitch motion. In verification and simulation studies the concept is demonstrated and the multi-variable feedforward controller shows a promising improvement in speed regulation and load reduction on the floating wind turbine.
Open Access
Multibody modeling for concept-level floating offshore wind turbine design
(2020) Lemmer, Frank; Yu, Wei; Luhmann, Birger; Schlipf, David; Cheng, Po Wen
Existing Floating Offshore Wind Turbine (FOWT) platforms are usually designed using static or rigid-body models for the concept stage and, subsequently, sophisticated integrated aero-hydro-servo-elastic models, applicable for design certification. For the new technology of FOWTs, a comprehensive understanding of the system dynamics at the concept phase is crucial to save costs in later design phases. This requires low- and medium-fidelity models. The proposed modeling approach aims at representing no more than the relevant physical effects for the system dynamics. It consists, in its core, of a flexible multibody system. The applied Newton-Euler algorithm is independent of the multibody layout and avoids constraint equations. From the nonlinear model a linearized counterpart is derived. First, to be used for controller design and second, for an efficient calculation of the response to stochastic load spectra in the frequency-domain. From these spectra the fatigue damage is calculated with Dirlik’s method and short-term extremes by assuming a normal distribution of the response. The set of degrees of freedom is reduced, with a response calculated only in the two-dimensional plane, in which the aligned wind and wave forces act. The aerodynamic model is a quasistatic actuator disk model. The hydrodynamic model includes a simplified radiation model, based on potential flow-derived added mass coefficients and nodal viscous drag coefficients with an approximate representation of the second-order slow-drift forces. The verification through a comparison of the nonlinear and the linearized model against a higher-fidelity model and experiments shows that even with the simplifications, the system response magnitude at the system eigenfrequencies and the forced response magnitude to wind and wave forces can be well predicted. One-hour simulations complete in about 25 seconds and even less in the case of the frequency-domain model. Hence, large sensitivity studies and even multidisciplinary optimizations for systems engineering approaches are possible.
Open Access
Optimization of floating offshore wind turbine platforms with a self-tuning controller
(2017) Lemmer, Frank; Müller, Kolja; Yu, Wei; Schlipf, David; Cheng, Po Wen
The dynamic response of floating offshore wind turbines is complex and requires numerous design iterations in order to converge at a cost-efficient hull shape with reduced responses to wind and waves. In this article, a framework is presented, which allows the optimization of design parameters with respect to user-defined criteria such as load reduction and material costs. The optimization uses a simplified nonlinear model of the floating wind turbine and a self-tuning model-based controller. The results are shown for a concrete three-column semi-submersible and a 10MW wind turbine, for which a reduction of the fluctuating wind and wave loads is possible through the optimization. However, this happens at increased material costs for the platform due to voluminous heave plates or increased column spacing.
Open Access
Prospects of linear model predictive control on a 10MW floating wind turbine
(2015) Lemmer, Frank; Raach, Steffen; Schlipf, David; Cheng, Po Wen
The presented research has the objective of supporting the integrated conceptual design of floating offshore wind turbines (FOWT). The dynamics of the multidisciplinary coupled system with the aerodynamics, hydrodynamics, structural dynamics, the catenary mooring lines and the controller shall be represented in simulation models adapted to the current design stage. Here, a linear model-predictive controller (MPC) as an optimal multiple input-multiple output (MIMO) controller is designed for a novel concept of the floating foundation for a 10MW wind turbine. The performance of this controller is easily adjustable by a cost function with multiple objectives. Therefore, the MPC can be seen as a benchmark controller in the concept phase, based on a simplified coupled simulation model with only approximate model information. The linear model is verified against its nonlinear counterpart and the performance of the MPC compared to a SISO PI-controller, which is also designed in this work. The developed models show to be well suited and the linear MPC shows a reduction of the rotor speed overshoot and tower bending from a deterministic gust.
Open Access
The TripleSpar campaign: validation of a reduced-order simulation model for floating wind turbines
(2018) Lemmer, Frank; Yu, Wei; Cheng, Po Wen; Pegalajar-Jurado, Antonio; Borg, Michael; Mikkelsen, Robert F.; Bredmose, Henrik
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 semisubmersible 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.
Open Access
Validation of INNWIND.EU scaled model tests of a semisubmersible floating wind turbine
(2016) Koch, Christian; Lemmer, Frank; Borisade, Friedemann; Matha, Denis; Cheng, Po Wen
The subject of this study is the verification and the validation of existing numerical codes for floating offshore wind turbine structures using wave tank model tests as part of the INNWIND.EU project. A model of the OC4-DeepCwind semisubmersible platform, together with a Froude scaled rotor model with low-Reynolds airfoils is tested in a combined wind-and-wave basin. The simulation environment comprises the multibody software SIMPACK with the HydroDyn module for the hydrodynamic loads, MAP++ for the mooring line forces and AeroDyn for the aerodynamic loads. The focus of this paper is the validation of the hydrodynamics of a modified model hull shape, which compensates for the excess mass of the nacelle. Furthermore also first steady wind simulations without wave excitation have been carried out. The results show that the model is validated and gives the basis for further research based on the conducted experiments.