Development and Verification of an Aero-Hydro-Servo-Elastic Coupled Model of Dynamics for FOWT, Based on the MoWiT Library

Abstract

The complexity of floating offshore wind turbine (FOWT) systems, with their coupled motions, aero-hydro-servo-elastic dynamics, as well as non-linear system behavior and components, makes modeling and simulation indispensable. To ensure the correct implementation of the multi-physics, the engineering models and codes have to be verified and, subsequently, validated for proving the realistic representation of the real system behavior. Within the IEA (International Energy Agency) Wind Task 23 Subtask 2 offshore code-to-code comparisons have been performed. Based on these studies, using the OC3 phase IV spar-buoy FOWT system, the Modelica for Wind Turbines (MoWiT) library, developed at Fraunhofer IWES, is verified. MoWiT is capable of fully-coupled aero-hydro-servo-elastic simulations of wind turbine systems, onshore, offshore bottom-fixed, or even offshore floating. The hierarchical programing and multibody approach in the object-oriented and equation-based modeling language Modelica have the advantage (over some other simulation tools) of component-based modeling and, hence, easily modifying the implemented system model. The code-to-code comparisons with the results from the OC3 studies show, apart from expected differences due to required assumptions in consequence of missing data and incomplete information, good agreement and, consequently, substantiate the capability of MoWiT for fully-coupled aero-hydro-servo-elastic simulations of FOWT systems.

Development of a Framework for Wind Turbine Design and Optimization

“…MoWiT is developed by engineers at Fraunhofer IWES. This allows continuous enhancement and extension of the library, including also verification and validation of the code [35,[38][39][40][41]. Thus, different theories and approaches are implemented to represent the aero-hydro-servo-elastic dynamics of a wind turbine system and the degree of detail is refined on and on.…”

“…Thus, different theories and approaches are implemented to represent the aero-hydro-servo-elastic dynamics of a wind turbine system and the degree of detail is refined on and on. The current capability of the in-house tool MoWiT is as follows [35][36][37]40]:…”

Design optimization of the OC3 phase IV floating spar-buoy, based on global limit states

“…Finally, as (local) structural integrity checks are not yet performed at this stage, the wall thickness of the floater remains unchanged at its original value of 0.0314 m, which was determined within a floater model verification process, done in a separate study [28]. Furthermore, the same stiffness provided by the mooring system is used throughout the optimization, as the mooring system design would require a separate in depth optimization approach, which is not yet included in this study.…”

“…The full capabilities of the MoWiT library are described in [25][26][27], however, this library is constantly further enhanced and extended. The OC3 spar-buoy FOWT system, used in this work for the design optimization, has been implemented in MoWiT and verified in a separate study [28]. The simulation engine for executing simulations with this FOWT system model is Dymola 3 .…”

“…Thus, in [23,24], a modular framework for automated simulation and optimization is developed and presented. This framework utilizes the MoWiT (Modelica for Wind Turbines) library 1 , developed and continuously enhanced at Fraunhofer IWES (Institute for Wind Energy Systems) [25][26][27][28], for modeling the entire wind turbine system including the environmental conditions and representing the fully-coupled aero-hydro-servo-elastic dynamics. The modeling happens component-based, which brings high flexibility in modeling of any state-of-the-art onshore or offshore bottom-fixed or even floating wind turbine system.…”