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

From these results it can be clearly seen that the loads on the jacket are dominated by the wind. This is
due to the small member diameters in the jacket at the water level, which results in a relatively low contribution
from the hydrodynamics. At the pile head the contribution from the waves is proportionally greater,
as expected, but even at this depth the relative contribution from the waves is still small.
The DEL histograms also show the effect of aerodynamic damping. The damage equivalent loading from
combined wind and waves is lower than the sum of the damage calculated from the wind and waves
separately. This is due to the damping of the wave-induced motion resulting from the rotor thrust.
Wind-wave misalignment
The baseline fatigue loads were calculated with codirectional wind and waves. In order to test the influence
of wind-wave misalignment on jacket structures, three additional cases were considered with the
wind-wave misalignment varied. The waves were considered to approach the structure at six different angles
from the wind, from -30° to 120° in 30° intervals. A different proportion of time was then assigned to
each wind-wave misalignment angle for the three cases. Case 3 models a uniform distribution between all
six misalignment angles, and Cases 1 and 2 are intermediate weightings. A graphical display of the cases
considered is shown in Figure 5.9.
Proportion of time spent at each
misalignment [-]
1.2
1
0.8
0.6
0.4
0.2
0
Codirectional Case 1 Case 2 Case 3
-30 0 30 60 90 120
Wind/w ave misalignment [deg]
Figure 5.9: Misalignment cases considered
Figure 5.10 presents normalized DELs at the chosen output locations on the jacket for the four different
cases. At the pile head the damage equivalent load increases by up to 2.5% when misalignment is taken
into account. This is due to the additional forcing from the waves approaching from a different angle.
However the DEL also increases by nearly 2% at the upper joint, which is located 15m above the water
level. This may be due to the fact that jackets are relatively soft in the torsional mode, which can be excited
by large misalignments. The reduced aerodynamic damping may also have an effect.
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