Environmental Engineering Reference
In-Depth Information
Fig. 2.50 Ratios of pitch moments over wave period
The sensitivity analysis carried out on the simple cylinder and the calculation of the
hydrodynamic loads on the realistic example show that the differences between the
Morison formula and diffraction theory regarding the ensuing wave loads are small for
large wave periods and accordingly larger corresponding wavelengths.
It is sufficient to use the Morison formula within the scope of a preliminary design
or when only a rough estimate is needed of the maximum loads acting on a large-
volume compact offshore structure due to a design wave with correspondingly
large wavelength. If diffraction effects are ignored, the calculated forces increase
and therefore must be considered as being on the safe side. A modified inertia
coefficient c
M
canbeemployedtoimprovetheresultsgivenbytheMorison
approach.
At the fatigue limit state, environmental influences, for example wind and wave loads,
are described by a collective load. The collective wave load contains waves with
various wave heights and wave periods, normally represented by extreme value
distributions. In such distributions the mean value of the wave period for North Sea
conditions lies between 4 and 7 s. Therefore, considering diffraction effects is relevant
when checking fatigue. Applying the Morison formula here leads to an accumulation of
damage which would exceed the permissible limits. Only diffraction theory supplies
reliable results in this case.
The drag force is neglected here but, if necessary, can be calculated in an additional step
with the Morison formula (see also Section 2.6.4).
2.7 Thermal actions
According to Section 6.4.5 of the DIBt guideline [9], the following applies:
1. Deformations due to asymmetric solar irradiation are not considered (combination
coefficient
c
0,T
!
0 for combinations with wind loads).
