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

UPWIND WP4: Offshore Support Structures and Foundations
The comparisons of first and second-order unrestrained motions in surge, heave and pitch for the OC3-Hywind
for a Pierson-Moskowitz spectrum with Hs=0.5m are shown Figure 8.16. For this spectrum, the unrestrained
motions are very small. However it should be pointed out the relative importance of the second-order components
of the motion.
unrestrained motion (surge) [m]
0.5
0
-0.5
x 10-3
1
-1
0 5 10 15 20 25
Time [s]
unrestrained motion (heave) [m]
0.5
0
-0.5
x 10-3
1
-1
-1.5
0 5 10 15 20 25
Time [s]
107
unrestrained motion (pitch) [deg]
0.015
0.01
0.005
0
-0.005
-0.01
-0.015
0 5 10 15 20 25
Time [s]
Figure 8.16: Comparisons between first and second-order unrestrained motions in surge, heave and pitch modes for the
OC3-Hywind for a Pierson-Moskowitz spectrum with Hs=0.5 m (Tp=3.54 s).
Pierson-Moskowitz spectrum with Hs=2.5 m
The first and second-order excitation forces in surge, heave and pitch modes for the OC3-Hywind associated
with the Pierson-Moskowitz spectrum with Hs=2.5m (Tp=7.9s) are shown in Figure 8.17. As in the previous
case (Hs=0.5m) the influence of the second-order components is more important in surge and pitch modes.
The presence of higher frequency components in surge and pitch are due to the predominance of the sumfrequency
force QTFs for these modes.
Excitation Force in surge [MN]
2
1.5
1
0.5
0
-0.5
-1
-1.5
0 10 20 30 40 50
Time [s]
Excitation Force in heave [MN]
0.3
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
0 10 20 30 40 50
Time [s]
Excitation Force (pitch) [MN.m]
150
100
50
0
-50
-100
0 10 20 30 40 50
Time [s]
Figure 8.17: First and second-order excitation forces in surge, heave and pitch modes of the OC3-Hywind for a Pierson-
Moskowitz spectrum with Hs=2.5 m (Tp = 7.9 s).
The comparisons of first and second-order unrestrained motions in surge, heave and pitch for the OC3-Hywind
for a Pierson Moskowitz spectrum with Hs=2.5 m (Tp=7.9 s) are shown in Figure 8.18. The unrestrained motions
of this structure are very small. The second-order component is dominant in surge, in heave has very little importance
and in pitch is of the same importance as the first-order component.