Final report for WP4.3: Enhancement of design methods ... - Upwind

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UPWIND WP4: Offshore Support Structures and Foundations rigid body motion, are considered rigid. SIMPACK has the capability to include flexible FE bodies of arbitrary geometry with the Craig-Bampton (C-B) method into the MBS model to account for larger deflections. This option is used for modelling of the WT blades and tower. A FE blade model, consisting of Euler-Bernoulli or Timoshenko beam elements is reduced by the C-B method and is capable of considering bending in flap- and edgewise direction, torsional and tensional rigidity and the relevant coupling effects. The relevant geometric stiffening effects are included for the reduction, representing a non-linear model for medium displacements. The blade model can also be split into separate C-B reduced flexible bodies, which are connected with zero DOF, representing a non-linear blade model for large displacements. The flexible tower is modelled with the same approach. Single and multi-torsional drive train models can be implemented, accounting for flexibility of the bedplate and other components as well. Drive train models for specific analysis, mainly for frequency domain analysis, also can include models for tooth contacts. Aerodynamics The AeroDyn aerodynamic subroutine package [81] is used to calculate aerodynamic forces in SIMPACK, as described above. Hydrodynamics The hydrodynamic forces in SIMPACK are calculated by interfacing with the hydrodynamic subroutine package HydroDyn [82], as described above. Mooring lines SIMPACK can either model mooring lines by solving the mooring line tensions quasi-statically in a separate module and interfacing with the main code at each time step, or using an integrated non-linear MBS mooring line model, where each line is discretized into separate rigid or flexible bodies, connected by spring-damper elements. 7. Benchmarking of design tools The development of design tools capable of modelling floating platforms is an important step forward for the offshore wind turbine industry, but in order to give security to the industry the results obtained from these codes must be shown to be accurate and reliable. Comprehensive testing and validation is therefore crucial for giving sufficient confidence to developers and investors. The best way to achieve this kind of confidence is to take measurements from a real machine and compare the measured data with the results from numerical simulations (code-to-measurement comparisons). In the case of floating wind turbines there is limited measurement data available with which to validate the codes, so a second method is also employed, that of comparing the results of different codes with each other (code-to-code comparisons). 7.1 Code-to-measurement comparisons A number of studies have been performed by Hydro Oil & Energy for the development of the Hywind floating wind turbine concept [87]. The floating platform consists of a deep-water slender spar-buoy with three catenary mooring lines. The integrated SIMO/RIFLEX/HAWC2 design tool was used in [88] to model the structure, as described above. As part of the development of this concept model scale experiments were carried out at the Ocean Basin Laboratory at Marintek in Trondheim in order to validate the coupled wind and wave modelling of the Hywind concept. A variety of sea states, wind velocities and control algorithms were tested and a number of parameters measured for the purposes of comparison. The hub wind speed from the model scale experiments was measured and used as the basis for the turbulent wind field used in the simulations. The JONSWAP wave spectrum was applied for both simulations and model experiments. The results of these tests showed very good agreement between the responses of the scale model and the predictions from the simulation code. The results also showed a significant increase in the damping of the tower motion when active blade pitch damping was introduced. Another floating wind turbine code which has been validated with the use of measurements is TimeFloat, a time-domain design tool for coupled analysis of floating structures described above. The hydrodynamic calcula- 82
UPWIND WP4: Offshore Support Structures and Foundations tions within this code were validated using wave tank tests performed at the UC Berkeley ship model testing facility [83]. A scale model of the floating platform was fabricated at 1:105 scale, with a foam disk at the tower top to represent wind forces and an electrical motor to model the gyroscopic effect of the rotor. A 3-hour realization of the 100-year sea state was generated with and without steady wind, and the resulting platform motion measured using a digital video camera. The floating platform was also modelled in the TimeFloat software using a simplified model for aerodynamic forces acting on the rotor. The results from these numerical simulations were then compared with the measurements from the tank tests. The comparison between model test results and numerical simulations showed good agreement, with the TimeFloat software generally underpredicting platform motion slightly. Further measurement campaigns are being planned. The University of Maine DeepCwind Consortium in the U.S. has been awarded a $7.1m grant to develop floating offshore wind capacity [93]. One of the stated aims of this project is to validate the coupled aero-/hydro-elastic models developed by NREL, as part of a research program which will include tank testing, deployment of prototypes and field validation. The EOLIA project, led by Acciona [94] has also included some code-to-measurement tests. The objective of the project is to develop solutions for the design and implementation of deep water offshore wind farms. As part of the project the capabilities of FAST with AeroDyn and HydroDyn have been extended and applied to the analysis of three floating concepts (spar buoy, TLP and semi-submersible), alongside comparisons with the Simo-Riflex code. Tank tests have also been performed for each of the concepts at 1:40 scale, in order to verify the models. The HiPRwind project [95] also aims to develop and test new solutions for offshore wind farms at a large scale. One of the main aims of HiPRwind is to install a 1:10 scale model of a future commercial-size floating wind turbine installation. The model will be deployed in real sea conditions, and will be used to monitor and assess the important operational parameters. The resulting measurement data will be an important contribution to overcoming the gap between small scale tank testing and full scale offshore deployment. 7.2 Code-to-code comparisons In addition to the validation of codes using measurements, an important way to verify the predictive accuracy of numerical simulation tools is through code-to-code comparisons. Most of the codes used for the analysis of floating wind turbines have been validated in this way. One example is the FAST code, the aero-elastic features of which have been verified through comparisons with ADAMS, described in [96]. Another example is the SIMO/RIFLEX code used to model the Hywind floating wind turbine concept, which was validated in part through comparisons with HywindSim, a relatively simple Matlab/Simulink code developed for the purposes of such comparison [87]. The methods used to validate the hydrodynamic calculation module HydroDyn used in the FAST code are described in [82]. These methods included comparisons between the output from WAMIT and results from a different numerical solver, comparisons between the WAMIT frequency to time conversion and HydroDyn calculations, using a benchmark problem to test the accuracy of the quasi-static mooring line calculations, comparing the mooring line force-displacement relationship calculated by the quasi-static method with that calculated by another code, and comparisons of time-domain results with frequency-domain results. The most extensive code-to-code comparison work in the offshore wind industry has been performed as part of the Offshore Code Comparison Collaboration (OC3) project within IEA Wind Task 23 [15]. In this project a number of participants used different aero-elastic codes to model the coupled dynamic response of the same wind turbine and support structure, with the same environmental conditions. The results were then compared in order to verify the accuracy and correctness of the modelling capabilities of the participant codes, and to improve the predictions. 7.2.1 Offshore Code Comparison Collaboration Phase IV In Phase IV of the OC3 project a floating offshore wind turbine was modelled [17]. The turbine model used was the publicly available 5MW baseline wind turbine developed by NREL, and the floating platform was a modification of the Hywind spar-buoy developed by Statoil of Norway. The turbulent wind fields and irregular wave 83
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Project UpWind Contract No.: 019945
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Acknowledgement The presented work
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Summary The overall aim of the UpWi
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Table of Contents Acknowledgement..
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Task 4.1 Integration of support str
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Sequential coupling The sequential
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analysis. In the new multibody code
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Figure 2.4: 2nd global bending mode
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3.2 Comparison of deterministic loa
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Figure 3.5: Time series of axial fo
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• Time domain simulation in ADCoS
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• A supplementary load vector for
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The following results are found: Fi
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Figure 4.5: Output positions in the
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It is directly visible that there i
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Page 42 and 43: Table 5.3 shows annual reliability
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Page 48 and 49: A Fracture Mechanical (FM) modellin
Page 50 and 51: Figure 5.4: Annual reliability indi
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Page 54 and 55: Rainflow evaluation The load cases
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Page 66 and 67: Figure 5.14 presents normalized DEL
Page 68 and 69: Figure 5.16: Output locations for e
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UPWIND WP4: Offshore Support Struct
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Final report for WP4.3: Enhancement of design methods ... - Upwind

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