OrcaFlex Manual - Orcina

Fatigue Analysis, Load Cases Data for Spectral Analysis
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� If Torsethaugen is selected then you must specify Hs and Tp for each load case. You can also specify fm but since
Tp and fm are tied together then setting either one changes the other to match. For details see Data for
Torsethaugen Spectrum.
Setting up load cases for Spectral Analysis
When performing a spectral fatigue analysis you will typically have a wave scatter table describing the relative
probability of storm occurrence. This determines a number of wave classes, e.g. storms defined by Hs,Tz pairs.
The load cases data should be setup to match load cases with wave classes. For example, suppose that you were
working with the following (truncated) wave scatter table:
4-5 9 3
3-4 6 18 6
Hs 2-3 22 132 117
1-2 3 57 201 249
0-1 15 48 69 45
4-5 5-6 6-7 7-8
Tz
The values in the table represent joint probabilities in parts per thousand, so that a value of 201 represents a
probability of 0.201.
This wave scatter table gives 16 wave classes and so the fatigue analysis data in OrcaFlex would be setup with 16
corresponding load cases with appropriate Hs and Tz values.
Simulation files for spectral fatigue analysis load cases
The simulation files used to represent a load case for spectral fatigue analysis should model all aspects of the system
and environment other than the wave spectrum. So you must specify vessel offset, current profile and direction,
wave direction and so on which are appropriate for the load case being analysed.
The wave type for the load case simulation file must be response calculation. This effectively calculates system
responses (i.e. RAOs) for a range of wave frequencies. The spectral fatigue analysis then combines these RAOs with
the load case wave spectra (i.e. the Hs,Tz pairs) to produce fatigue damage estimates for the load case.
Choice of Hs for response calculation simulation files
The Spectral Response Analysis method which is used to calculate system responses (RAOs) includes non-linear
effects such as hydrodynamic drag. In order for these non-linear effects to be well modelled the choice of Hs for the
response calculation simulation files is important. Essentially the RAOs can be considered as being dependent on
wave height. How significant this dependence is will vary from case to case. Certain systems are dominated by linear
physical effects and the RAOs may not in fact be dependent on wave height. To determine how significant this effect
is we would recommend sensitivity studies.
In the example above we might choose to run a response calculation simulation for each row of the wave scatter
table (assuming that the system had significant non-linearities). This would give 5 simulation files for Hs ranges 0-1,
1-2, 2-3, 3-4 and 4-5. There are 4 wave classes corresponding to the 0-1 Hs range. The load case corresponding to
each of these wave classes would then be represented by the same simulation file. The other Hs ranges are dealt
with similarly and so the load cases table would look as below: