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

are capturing the worst case loading. This significantly reduces the number of simulations required in the
preliminary extreme load analysis.
It is important to note, however, that for detailed design using the most conservative load will not necessarily
result in the most cost effective support structures. Rather, the load with a return period of 50 years
should be used. This implies that a statistical approach should be used as a basis for the rules in the
standard.
Tide height
The baseline extreme loads were calculated with the tide height at HSWL, equal to +3.29m. This is the
upper bound of the range specified by the IEC standard [69], which is generally considered to be conservative.
In order to test the influence of the tide height on the extreme jacket loading, three additional cases
were considered in addition to the baseline case with tide heights varying from LSWL to HSWL+2.13m.
Normalised axial force
1.1
1.05
1
0.95
0.9
0.85
0.8
LSWL HAT HSWL HSWL+2.13m
Mbr 1 End 1 Mbr 2 End 1 Mbr 3 End 1 Mbr 4 End 1
Figure 5.18: Normalised extreme loads at pile head, with variation in tide height
Figure 5.18 presents normalized extreme loads at the pile head for the four different cases. The results
confirm that higher water levels result in higher extreme loads, although the load increases are smaller in
magnitude than when the wave period is varied. Again, this is an important result because it enables designers
to perform extreme load calculations at a single water level with greater confidence that they are
capturing the worst case loading.
5.4.3 Summary and key findings
The fatigue loading on the structure is found to be dominated by the wind, with a relatively low contribution
from the hydrodynamics. This is reflected in the small changes in DEL when marine parameters are varied
(tide height, wind-wave misalignment) compared to the large changes in DEL when wind parameters are
varied (wind class). The small hydrodynamic influence is also shown in the fact that a decrease in
availability leads to a reduction in loading. The parameter which has the most effect on fatigue loading is
the structural natural frequency. This demonstrates the importance of placing the natural frequency in the
right range when designing a jacket support structure.
The parameter which has the most effect on the extreme loading on the structure is the wave period of
the 50 year maximum wave. Conservative load results are given when this parameter is set to the lower
bound of the range given in the standard.
69