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

UPWIND WP4: Offshore Support Structures and Foundations
unrestrained motion (surge) [m]
0.05
0.04
0.03
0.02
0.01
0
-0.01
-0.02
Monochromatic wave (T=5 s; H=1 m)
-0.03
0 5 10 15
Time [s]
unrestrained motion (surge) [m]
0.06
0.04
0.02
0
-0.02
-0.04
-0.06
Monochromatic wave (T=7 s; H=2 m)
-0.08
0 5 10 15 20 25
Time [s]
113
unrestrained motion (surge) [m]
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
Monochromatic wave (T=9 s; H=4 m)
-0.8
0 5 10 15
Time [s]
20 25 30
Figure 8.27: Comparison of first and second-order unrestrained surge motion for the semi-submersible platform for three
monochromatic waves.
unrestrained motion (pitch) [deg]
0.03
0.02
0.01
0
-0.01
Monochromatic wave (T=5 s; H=1 m)
-0.02
0 5 10 15
Time [s]
unrestrained motion (pitch) [deg]
0.1
0.05
0
-0.05
-0.1
Monochromatic wave (T=7 s; H=2 m)
-0.15
0 5 10 15 20 25
Time [s]
unrestrained motion (pitch) [deg]
0.5
0
Monochromatic wave (T=9 s; H=4 m)
-0.5
0 5 10 15
Time [s]
20 25 30
Figure 8.28: Comparison of first and second-order unrestrained pitch motion for the semi-submersible platform for three
monochromatic waves.
Second-order hydrodynamic loads and unrestrained motions for mixed seas
This section presents comparisons between first and second-order excitation forces and unrestrained motions
for the semi-submersible platform, for three sea states described by Pierson-Moskowitz distributions with the
parameters listed in Table 8.16. The absolute values of the sum- and difference- frequency force QTFs ( ,
) for the semi-submersible platform in the six modes of motion can be found in Appendix B of [111].
Pierson-Moskowitz spectrum with Hs=0.5 m
The comparisons between first and second-order excitation forces in the six modes of motion (surge, sway,
heave, roll, pitch and yaw) for the semi-submersible platform associated with this spectrum are presented in
Figure 8.29. For this spectrum and for all modes, the second-order excitation forces are dominant relative to
the first-order excitation forces.
The comparisons of the first and second-order unrestrained motions in the six modes for this spectrum are
shown in Figure 8.30. The unrestrained motions are small in all modes. Slow drift motions can be identified in
all modes except heave. For surge, pitch and yaw, the second-order component of the unrestrained motions is
of the same order of importance as the first-order. For the other modes (sway, heave and roll) the second-order
component is dominant over the first-order.