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

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UPWIND WP4: Offshore Support Structures and Foundations • Multi-megawatt, offshore, 3-blade, monopile tower, downwind, variable speed, pitch regulated • < 500kW, onshore, 3-blade, monopile tower, upwind, fixed speed, stall regulated • ~1 Megawatt, onshore, 2-blade, tethered tower, upwind, variable speed, pitch regulated Figure 7.5 and Figure 7.6 present comparisons of selected loads from one of the above load sets. In general good agreement was found between the previous versions of Bladed and Bladed v4.0. The main difference is in the edgewise fatigue loads, which have reduced a little. This is due to the use of individual blade modes rather than rotor modes, which tends to increase the damping in the edgewise direction due to the additional component of aerodynamic damping. The use of blade modes rather than rotor modes is one of the major advantages of the new structural model as it allows for the correct multidirectional mode shapes as well as the correct modelling of individual blade modes for transient load cases. FlapwiseM [%] 100 80 60 40 20 0 DLC 1.0 1.0 1.1 1.1 1.3 1.3 1.5 1.5 1.6 1.6 1.7 1.7 1.8 1.8 1.9 1.9 88 2.1 2.1 2.2 2.2 3.2 3.2 3.3 3.3 4.1 4.1 4.2 4.2 5.1 5.1 6.0 6.0 6.1 6.1 Figure 7.5: Blade root flapwise bending moment: extreme loads [98] Datum MB Cumulative cycles [.] Figure 7.6: Blade root edgewise bending moment: cumulative cycles [98] The final level of testing involved a code-to-measurement comparison. The CART2 research turbine at NREL in Colorado was used to obtain measurements, primarily for the purpose of testing advanced control features [99]. A Bladed model was set up based on data supplied by NREL, with a number of assumptions where data was not available (e.g. shaft torsional damping, teeter brake friction, pitch actuator model, rotor imbalance). Four measured datasets were used to compare against simulations. Initial simulations were carried out using the above-mentioned assumptions, as a result of which some adjustments were made to the assumptions before the final runs. The measured and simulated results were compared by means of spectral analysis, and also using rainflow cycle counting to give an indication of fatigue loading. Figure 7.7 presents an example of tower top load spectra for the four datasets. In general a very good level of agreement is demonstrated, with the spectral peaks representing structural resonances corresponding very closely with the predicted frequencies of the coupled system modes. There are some discrepancies, but in most cases this is likely to be due to the uncertainties in modelling the wind field. The fatigue loading is sensitive to the turbulence model used in the simulations, and since the detailed structure and coherence of the actual wind field could not be known it was not possible to produce an exact fit. Differences are also present due to noise on the measured signals, and imbalances in the real turbine. 6.2 6.2 6.3 6.3 Datum Multibody 7.1 7.1 8.1 8.1
UPWIND WP4: Offshore Support Structures and Foundations Figure 7.7: Example of Bladed v4.0 validation against measurements: spectra for tower top loads [98] The floating capabilities of the Bladed code have also undergone testing. Prior to the incorporation of multibody dynamics into Bladed a version of the code, modified to include large rigid body motions and global rotations of the support structure, was used to carry out simulations for Phase IV of the IEA Offshore Code Comparison Collaboration [17]. Good agreement was found between Bladed and the other codes in the calculation of the surge, sway, heave, roll, pitch and yaw natural frequencies of the platform and turbine. The Bladed v4.0 code will also participate in the IEA Task 30 (Offshore Code Comparison Collaboration Continuation) phase 2 which will involve the modelling of a floating offshore wind turbine. The type of floating platform has yet to be finalised. More recently, Bladed v4.0 was used to model a tension leg floating platform with the purpose of assessing the suitability of these structures for supporting wind turbines in the North Sea [97]. The natural periods and buoyancy of the structure calculated by Bladed were validated against analytical calculations based on the tension leg properties and platform mass and inertia. 8. Advanced modelling approaches Current modelling techniques applied for fixed-bottom offshore wind turbines, for instance blade element momentum (BEM) method for rotor aerodynamics and Morison’s equation for hydrodynamics, are insufficient to accurately describe the large rotor and platform motions and the usage of non-slender support-structures for FOWTs. Advanced modelling methods and techniques are therefore required in order to effectively design and analyse wind turbines on floating offshore platforms. Regarding aerodynamics, the large low-frequency platform motions experienced by FOWTs, including significant pitch and surge motions, lead to a change in the interaction between the rotor and wake. Dynamic stall and yawed inflow models also have increased importance. Section 3.1 discusses the limitations of BEM and the capabilities of CFD and potential flow (PFM) methods for FOWT simulations. Regarding hydrodynamics, for modelling of non-slender floating platforms potential theory is required in order to correctly determine the local pressure force and global wave loads due to diffraction and radiation. Section 3.2 presents the results of detailed comparisons between first and second-order hydrodynamic models for simple floating body configurations. The development of non-linear potential flow based methods is outlined, and the importance of vortex induced vibrations for FOWT simulations is discussed. Regarding mooring line dynamics, different techniques for the representation of mooring lines, including quasistatic, look-up table, FEM and MBS methods, and their impact on global system loads are investigated in Section 3.3. 89
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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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ated eigenfrequency is shifted into
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Water levels For the calculation of
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guidelines for the inclusion of cli
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Page 40 and 41: uncertain parameters carefully. Thr
Page 42 and 43: Table 5.3 shows annual reliability
Page 44 and 45: Steel substructure for offshore win
Page 46 and 47: Table 5.16 shows the required FDF v
Page 48 and 49: A Fracture Mechanical (FM) modellin
Page 50 and 51: Figure 5.4: Annual reliability indi
Page 52 and 53: Further, the effect of possible ins
Page 54 and 55: Rainflow evaluation The load cases
Page 56 and 57: For DLC 7.2 it has to be considered
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Page 60 and 61: Table 5.28: Output locations for da
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Page 64 and 65: Figure 5.11: Normalised DELs with v
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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