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

More documents

Recommendations

Info

UPWIND WP4: Offshore Support Structures and Foundations as TLP concepts, which typically have high natural frequencies in heave, roll and pitch. 8.2.1 Comparison of first and second-order hydrodynamics A detailed comparison has been performed between the solution of first (linear) and second-order potential flow hydrodynamic models with regard to the characterisation of the wave induced loading and motion response of typical floating offshore wind energy converters, under both regular and irregular waves [111]. Two floating offshore wind structures are considered. The first case study is a spar-buoy originally developed by StatoilHydro and later modified to accommodate a NREL-5MW offshore wind turbine. This concept is called “OC3-Hywind” and is described in detail in [112]. Figure 8.3(a) shows schematically this concept and Table 8.1 lists the main properties for this structure. The second case study is a semi-submersible platform with geometric dimensions similar to the WindFloat platform concept [83]. This structure is shown in Figure 8.3(b) and comprises three equidistant columns and a wind turbine centred in one of the columns. The stabilisation of the position of this structure is achieved through an active water ballast system which transfers water between columns to compensate for changes in the mean wind loading of the turbine. The hexagonal water-entrapment plates at the bottom of each column are designed to provide high heave added-mass and viscous damping to decrease the motions in this mode. Table 8.2 gives an overview of the main properties associated with this structure. The geometry for this structure is based on the dimensions reported in [83] and the mass is uniformly distributed. Figure 8.3: Offshore wind floating structures: (a) OC3-Hywind (b) WindFloat Table 8.1: Main properties for the OC3-Hywind platform [112]. OC3-Hywind Platform Total draft (below SWL) [m] 120 Centre of mass below SWL [m] 89.9155 Tower base above SWL [m] 10 Mass including ballast [kg] 7 466 330 Platform diameter above taper [m] 6.5 Inertia (I11 = I22) 4 229 230 000 Platform diameter below taper [m] 9.4 Inertia (I33) [kg m2] 164 230 000 Depth top to bottom taper bellow SWL [m] 4 - 12 Water depth (z0) [m] 320 Water density [kg m3] 1025 Table 8.2: Main properties for the semi-submersible platform (Geometry from [83] with mass uniformly distributed). 94
UPWIND WP4: Offshore Support Structures and Foundations WindFloat Platform Total draft (below SWL) [m] 22.9 Total mass [kg] 6 523 059 Diameter of each column [m] 10.7 Centre of mass below SWL 11.70 Diameter of hexagonal plates [m] 27.4 I11=I22 4525602948.75 Volume bellow SWL [m3] 6363.96 I33 6742642572.74 Water depth (z0) [m] 320 The hydrodynamic quantities compared in this exercise are: • The first and second-order excitation force. • The first and second-order Response Amplitude Operator (RAO) for unconstrained motions. The comparisons between linear and second-order quantities are performed for three distinct monochromatic waves and three unidirectional Pierson-Moskowitz spectra with parameters listed in Table 8.3. Both first and second-order hydrodynamic quantities were computed with the commercial software WAMIT (v6.1s). Table 8.3: List of monochromatic incident waves and Pierson-Moskowitz spectra used for the comparisons between linear and second-order hydrodynamic quantities. Monochromatic waves 1 H = 1.0 m T = 5.0 s 2 H = 2.0 m T = 7.0 s 3 H = 4.0 m T = 9.0 s Pierson Moskowitz spectra 1 Hs = 0.5 m (Tp = 3.54 s) 2 Hs = 2.5 m (Tp = 7.9057 s) 3 Hs = 5 m (Tp = 11.1803 s) Convergence tests Prior to the evaluation of any hydrodynamic quantity it is necessary to assess the degree of accuracy of the numerical solution with regard to a certain discretisation of the geometry. Finer discretisations should represent more accurately the geometry and therefore evaluate more accurately the velocity potential. However the computational effort increases with finer discretisations and so this exercise is required to understand and find the right balance between the required accuracy and computational effort. The methodology presented in this paragraph follows closely the methods presented by Cruz in [113] which applied standard convergence procedures based on Richard extrapolation method to the computations performed with WAMIT higher order method [114]. The exact value of a certain quantity ( ) is estimated by evaluating its value at three different discretisations of the geometry ( ). The error associated with the finer discretisations is given by: where ( ) are the values of the quantity being evaluated and hi is the grid cell size associated with the discretisation i. The subscript 1 refers to a finer discretisation than 2 and 3 to a coarser discretisation than 2. The order of convergence (p) is a quantity which depends on the implementation of the code itself and in general is not known. The value for this quantity is straightforward to compute for a constant refinement ratio, i.e. h3/h2 = h2/h1= r = const. Assuming also that the asymptotic range has been reached, p is given by: 95 [8-3]
Page 1 and 2:
Project UpWind Contract No.: 019945
Page 3 and 4:
Acknowledgement The presented work
Page 5 and 6:
Summary The overall aim of the UpWi
Page 7:
Table of Contents Acknowledgement..
Page 10 and 11:
Task 4.1 Integration of support str
Page 12 and 13:
Sequential coupling The sequential
Page 14 and 15:
analysis. In the new multibody code
Page 16 and 17:
Figure 2.4: 2nd global bending mode
Page 18 and 19:
3.2 Comparison of deterministic loa
Page 20 and 21:
Figure 3.5: Time series of axial fo
Page 22 and 23:
• Time domain simulation in ADCoS
Page 24 and 25:
• A supplementary load vector for
Page 26 and 27:
The following results are found: Fi
Page 28 and 29:
Figure 4.5: Output positions in the
Page 30 and 31:
It is directly visible that there i
Page 32 and 33:
ated eigenfrequency is shifted into
Page 34 and 35:
Water levels For the calculation of
Page 36 and 37:
guidelines for the inclusion of cli
Page 38 and 39:
damaging if the frequency of the ex
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
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
Page 70 and 71: UPWIND WP4: Offshore Support Struct
Page 144 and 145:
UPWIND WP4: Offshore Support Struct
show all

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

Create successful ePaper yourself

Delete template?

Save as template?