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

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UPWIND WP4: Offshore Support Structures and Foundations The computational grid used consists of the components blade, hub and background grid, leading to total number of about 10 million grid points. Refined grids around each blade (Figure 8.1, green grid) with a dimensionless grid distance y+-value between 0.1 and 2 are embedded in a cylindrical grid around the hub (Figure 8.1, red grid), which is rotating within a coarser background grid (Figure 8.1, blue grid). Overall, the grid has the in-plane dimension of 3 rotor diameters in each direction, centred at the hub, and an out of plane dimension of 3.5 diameters upwind and downwind respectively. FLOWer features the Chimera technique [109], allowing for arbitrary relative motion of aerodynamic bodies. Applying the Chimera technique, the previously described pitch motion was prescribed as a rigid body motion on the rotor and hub grids, which are moving within the background meshes. Chimera is also applied for modelling the rotor rotation. In this study, no elastic deformation of the blade is introduced, but fluid-structure coupled simulations are planned for next analyses. Further studies will also include the tower and nacelle, as already performed by Meister et. al. with FLOWer for a 5MW onshore turbine without prescribed floating motions [110]. The presented CFD results showed good numerical convergence, therefore it is assumed that the presented flowfield is realistically representing the actual aerodynamic conditions during such a motion. Nevertheless the presented results are only a preliminary study and need further validation from CFD and potential flow calculations, as well as, ultimately, experimental data. Figure 8.1: Prescribed rotor motion for CFD study and CFD mesh The pressure distribution over the blades is presented in Figure 8.2. Motion (1) corresponds to the first half of the downwind swing (c.f. Figure 8.1), i.e. 4/3 rotor rotations, motion (2) corresponds to the 2 nd half of the downwind swing and the first half of the upwind swing, i.e. from 5/3 to 4 rotor rotations, and motion (3) finally represents the last half of the upwind swing, i.e. 13/3 to 5 rotor rotations. During motion (1), the pressure is decreasing due to the backward motion and the increased turbulence in the wake. When the turbine is pitching upwind again (2), the pressure on the blades is increasing until the rotor is back in vertical position and then slightly decreasing until the end of move (2). The pressure increase is due to the increased inflow velocity, and possibly also due to rotor wake interactions. In the last part of the period (3), the pressure again is decreasing, but to a lesser extent than in the first downwind motion (1). Figure 8.2: Pressure contours of rotor during prescribed motion 92
UPWIND WP4: Offshore Support Structures and Foundations While no dedicated scientific research on FOWT-specific blade design has been published yet, the work package “Offshore Blades” of the European KIC (Knowledge & Innovation Community) InnoEnergy research project Offwindtech, led by University of Stuttgart and started beginning of 2011, will address this issue. 8.2 Hydrodynamic theories In order to take proper account of the influence of a floating body on the surrounding fluid, potential flow theory must be used. This is particularly important for floating wind turbine support structure designs which have large diameters and experience significant motion. In potential flow theory the fluid is considered to be incompressible, inviscid and surface tension effects are neglected. The flow is irrotational and so the velocity of the fluid ( ) at a certain point in a Cartesian coordinate system fixed in space and instant (t) is given by: The total velocity potential ( ) satisfies the Laplace equation in all fluid domain: and also at the boundary conditions at the air and solid interfaces that define the problem. The complete formulation of these boundary conditions is in general difficult to solve, and first or second-order approximations are typically used to define the respective hydrodynamic formulation. These are also referred to in the literature as the linear and weakly nonlinear formulations. Linear potential flow theory is used in many different offshore engineering problems. This theory considers, in addition to the potential flow assumptions described above, that the amplitudes of both the incident waves and the motions of the floating structure are small when compared with the incident wavelength. Second-order, weakly nonlinear hydrodynamic theory assumes (as in the first-order case), small amplitudes for the incident waves and motions in comparison with the wavelength and characteristic body dimensions. However, this theory takes into account a more detailed representation of the velocity potential ( ) and all derived variables by considering a second-order approximation through a Taylor expansion series about the mean positions. A full description of the second-order approximation can be found in [111]. The second order approximation more properly accounts for hydrodynamic loading on the wetted surface of the body for platforms which are subject to steep-sided or very large waves. Second order hydrodynamic loads are proportional to the square of the wave amplitude, and have frequencies equal to both the sum and the difference of the multiple incident wave frequencies. This means that although the natural frequencies of the structure are designed to be outside the wave energy spectrum, the second order forces will excite these frequencies, so despite the forces being small in magnitude the resonant effect can be important. Three examples of second order hydrodynamic forces are given below. • Mean drift forces. These forces result in a mean offset of the body relative to its undisplaced position, and are typically an order of magnitude lower than first order wave excitation forces. The mean drift force is a combination of second order hydrodynamic pressure due to first order waves and the interaction between first order motion and the first order wave field. The viscous drag contribution to this force is significantly increased when there is a current present. Since the mooring line tension is often related non-linearly to platform displacement, the mean drift forces can have an important effect. • Slowly varying drift forces. These forces have much longer periods than the main wave energy spectrum but are still within the range of horizontal platform motion. They result from non-linear interactions between multiple waves with different frequencies. Again the forces resulting from slowly varying drift are generally small compared to forces at the wave frequency, but they can cause large displacements in moored floating wind turbines which can in turn lead to high loads in the mooring lines. In addition these forces can excite the large amplitude resonant translational motion of the floating platform. • High frequency forces. These forces have a frequency which is higher than the wave frequency and are also generally small in amplitude. They arise from the same source as low frequency drift forces, i.e. interactions between multiple waves of varying frequency. The contribution from these forces can be particularly important when analysing ‘ringing’ behaviour for floating wind turbine configurations such as 93 [8-1] [8-2]
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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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damaging if the frequency of the ex
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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
Page 58 and 59: Guideline, DLC 1.5). Furthermore, t
Page 60 and 61: Table 5.28: Output locations for da
Page 62 and 63: From these results it can be clearl
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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UPWIND WP4: Offshore Support Struct
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Final report for WP4.3: Enhancement of design methods ... - Upwind

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