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

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.
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