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

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UPWIND WP4: Offshore Support Structures and Foundations The non-dimensional first-order excitation forces are given for the same modes and wave periods in Table 8.15. The sum- component is about twenty times smaller than the first-order excitation force and ten times larger than the difference component for surge and pitch modes. For heave the sum- component is ten times smaller than the first-order excitation force and about five times larger than the difference component. Table 8.15: Non-dimensional linear excitation forces for surge (1), heave (3) and pitch (3) modes at three monochromatic wave periods for the OC3-Hywind. T= 5s T=7s T=9s Mode Abs val Phase(deg) Abs val Phase Abs val phase 1 84.3 -102.6 106.0 -94.30 116.0 -91.79 3 8.85 -170.8 18.02 -177.93 23.95 -179.44 5 6912.43 -102.6 7937.18 -94.30 7740.44 -91.79 An example time series comparison between first- and second-order excitation forces associated with the OC3- Hywind platform with monochromatic waves is shown in Figure 8.12. The Figure shows the excitation forces in the surge mode for the largest of the three monochromatic waves considered, with period T=9s. The results are similar for the pitch and heave modes and for the smaller wave periods. The first-order excitation forces are dominant relative to second-order and only small differences between the total excitation force and the firstorder can be perceived for the steeper waves at the crests and troughs of the excitation force signals. Excitation Force (surge) [MN] 3 2 1 0 -1 -2 Monochromatic wave (T=9 s; H=4 m) -3 0 5 10 15 Time [s] 20 25 30 Figure 8.12: Comparison of first and second-order excitation forces in surge mode with monochromatic waves for the OC3-Hywind platform. An example time series comparison between first- and second-order unrestrained motions associated with the OC3-Hywind platform with monochromatic waves is shown in Figure 8.13. The Figure shows the unrestrained motions in the surge mode for the largest of the three monochromatic waves considered, with period T=9s. The results are similar for the pitch and heave modes and for the smaller wave periods. The time histories show that for the three incident waves studied, the unrestrained motions are small and the second-order effects are in turn very small when compared with the first-order effects. 104
UPWIND WP4: Offshore Support Structures and Foundations unrestrained motion (surge) [m] 0.02 0.015 0.01 0.005 0 -0.005 -0.01 Monochromatic wave (T=9 s; H=4 m) -0.015 0 5 10 15 Time [s] 20 25 30 Figure 8.13: Comparison of first and second-order unrestrained surge motion of the OC3-Hywind for three monochromatic waves. Second-order hydrodynamic loads and unrestrained motions for mixed seas This section describes the comparisons between first and second-order excitation forces and unrestrained motions obtained for the OC3-Hywind floating structure associated with three sea states described by a Pierson- Moskowitz distribution. This distribution is used to describe fully developed seas for when wind blows steadily over a large area of the ocean for a long time. The three unidirectional Pierson-Moskowitz spectra considered in this study are described with sixteen components with the parameters listed in Table 8.16. Given the input wave spectrum and the second-order sum- and difference-frequency force QTFs ( ), the time series of the second-order excitation force is directly calculated from: 105 [8-12] where the sum and difference-frequency force QTF satisfy the symmetry relations: and . Table 8.16: Parameters which define the three Pierson-Moskowitz spectra considered in this study. Hs [m] Tp [s] fmin [Hz] fmax [Hz] df [Hz] TR [s] 0.5 3.54 0.15 0.8 0.04 25 2.5 7.91 0.08 0.4 0.02 50 5.0 11.18 0.06 0.2 0.01 100 The number of components (N) for each spectrum is equal to sixteen. In the above table Hs is significant wave height; Tp is peak period; fmin and fmax are the minimum and maximum cutoff frequencies; df is equal to (fmax-fmin)/N; and TR is the repeat period. An example of the frequency components associated with the most severe of these spectra, with significant wave height equal to 5.0m, is shown in the top histogram of Figure 8.14. The bottom histogram shows the wave amplitude and periods associated with this spectrum.
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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
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Table 5.3 shows annual reliability
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Steel substructure for offshore win
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Table 5.16 shows the required FDF v
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A Fracture Mechanical (FM) modellin
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Figure 5.4: Annual reliability indi
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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
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
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Final report for WP4.3: Enhancement of design methods ... - Upwind

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