Holistic Modelling of Flexible Floating Offshore Wind Turbine Systems in Modelica
Mareike Leimeister, Philipp Thomas
Fraunhofer Institute for Wind Energy and Energy System Technology IWES, Bremerhaven, Germany
Designed floating wind turbine concepts need to be modelled, simulated, and analysed. As several modelling methodologies and simulation programmes exist, a floating wind turbine system can be implemented in different ways. For validation, those models and simulation results can be checked against each other, as done in the code-to-code comparisons of IEA Tasks 23 and 30 within the OC3- and OC4-projects.
In this work, the object-oriented and equation-based modelling language Modelica is used for implementing a fully flexible floating wind turbine system. The complex and nonlinear wind turbine system is structured hierarchically into subcomponents (for rotor, nacelle, operating control, support structure, wind, and waves) of less complexity, which are all based on a multibody approach and interconnected by floating frame of reference connectors to ensure correct represented fully coupled load analyses.
The implemented code is tested, based on two exemplary models: the spar-buoy of the OC3-project, and the DeepCwind semi-submersible of the OC4-project, both supporting the NREL 5MW wind turbine. Simulations are carried out in Dymola and the results are compared to the findings of the code-to-code comparisons in the OC-projects. Furthermore, the feasibility of easily adapting to different design concepts is shown on the basis of two bottom-fixed offshore wind turbine designs: the tripod of the OC3-project, and the jacket of the OC4-project.
The multibody-based wind turbine model uses anisotropic beams, to discretize the rotor blades and tower structure, and modal reduction techniques, to ensure efficient computation. Aerodynamic loads are calculated based on unsteady BEM or generalized dynamic wake, both including dynamic stall correction.
The operating control can be freely adapted by using either the generic DLL interface, or built-in supervisory control with pitch and generator torque control, and covers different operating phases.
The support structure (floater and tower up to RNA) is modelled by 3D-Bernoulli beam elements, for which wind drag loads, hydrodynamic forces (based on Morison's equation and MacCamy-Fuchs approximation), and time-varying buoyancy forces are computed. Furthermore, additional weights (ballast) and station-keeping system are included.
Wind and wave models allow modelling of different sea states.
In free decay simulations, the OC3-spar-buoy and the OC4-semi-submersible remain stable and floating. The models and components embedded in the OneWindŽ Modelica Library allow simulating the OC-load cases and adding the results to the code-to-code comparisons. Due to the component-based programming in Modelica, even bottom-fixed offshore wind turbine designs can be modelled, just by exchanging single components. Thus, models for the OC3-tripod and the more complex OC4-jacket, both carrying the NREL 5MW turbine, are set up relatively quick, by replacing the structure element data of the support structure, removing the station-keeping system, and adapting the connections (bottom contact instead of free motion) in the existing offshore wind turbine model.
All public funded research results obtained with the OneWindŽ Modelica Library are published, available to everybody, and not just a black box, as the Modelica language is plain, the set-up is coherent, well-structured, and complete, and the programming is equation-based. Consequently, the user can easily modify the source code and implement differential equations of motion. Furthermore, the multibody approach reduces the system complexity from the entire turbine structure to sub-components, and allows adapting or replacing of single components. Thus, cooperating industry and research institutes can attach new research models to a coupled simulation environment for load calculations, include new engineering models, as done in the German TANDEM or SeaLoWT projects, and develop and assess new concepts for floating offshore wind turbines or their single components.
The present work provides another possibility for modelling and simulating a fully flexible floating offshore wind turbine system, using the modelling language Modelica and the compatible simulation environment Dymola. The hierarchical programming structure allows breaking down the complex system into several components, while still ensuring a holistic representation of the floating device. This makes it feasible to adapt the existing wind turbine model to new turbine concepts (e.g. multi-rotor designs), different support structures (floating and bottom-fixed), or changed environmental conditions. The floating frame of reference formulation allows easy interfacing of any kind of model. The analysis of the overall system is fully coupled and includes aero-hydro-servo-elastic domain and coupling. Thus, completeness, transparency and high degree of flexibility make wind turbine modelling in Modelica practically applicable.