OrcaFlex Manual - Orcina

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System Modelling: Data and Results, General Data
and reports most such instabilities; the time steps can then be reduced and the simulation retried. However, it is
generally worth repeating important simulations with smaller step sizes to ensure that no significant loss of
accuracy has occurred.
Note: High values of Seabed stiffness may shorten the natural period of parts of the system lying on it.
This in turn leads to shorter inner time steps being required. Beware that the effects of seabed
stiffness will not be accounted for if touchdown does not occur until dynamics are run.
Recommendations for setting time steps
Both time steps must be short enough to give stable and accurate simulation. Experience indicates that the inner
step should not exceed 1/10th to 1/20th of the shortest natural nodal period of motion for any degree of freedom in
the model. The shortest natural nodal period is reported in the Full Results for statics.
The outer step can usually be set to 10 times the inner time step; this gives a good saving in computing time without
risking instability. In addition, the outer time step should generally not be more than 1/40th of the wave period (or
1/40th of the zero crossing period for a wave spectrum). If you are using a Wake Oscillator VIV model then we
recommend that the outer time step is no more than 1/200th of the minimum Wake Oscillator Strouhal Period.
Always use recommended time steps
OrcaFlex helps you set the simulation time steps using the above criteria. There are 2 modes of operation:
If Always use recommended time steps is checked (the default setting) then OrcaFlex will calculate recommended
time steps and use these values directly. This means that any values that you specify will be overwritten.
The main advantage of using this option comes during the design phase. If you make changes to the model's
properties which alter the shortest natural nodal period then OrcaFlex automatically modifies the time steps
accordingly. This is particularly useful when you make a change that allows the use of longer time steps. When not
using this option it is all too easy to forget to lengthen the time steps and suffer unnecessarily long simulation run
times.
The other significant benefit of this mode of operation comes when preparing a large number of similar simulations
using batch script methods. Suppose that the different models involved have differing shortest natural nodal
periods. The Always use recommended time steps option allows you to use appropriate time steps for each
individual model without having to set them manually.
Alternatively, if Always use recommended time steps is not checked then OrcaFlex first of all calculates the
recommended time steps. If these are shorter than the values specified on the General Data form then OrcaFlex
issues a warning and gives you the option of using the recommended values. You are free to disregard the warnings
if desired, but if either time step (though especially the inner step size) is set too large there is danger of instability
or inaccuracy in the simulation.
The main situation where this mode of operation is to be preferred is when OrcaFlex recommended time steps are
too long and lead to an unstable simulation. Sometimes the only solution is to set the time steps manually and this
option gives you that flexibility.
Recommended time step settings
Inner time step (fraction of shortest natural period)
Our experience is that for most cases the inner time step can safely be set to 1/10th of the shortest natural nodal
period. However, for some models you may find you need to use a shorter time step to achieve a stable simulation.
By changing this data item you can control what fraction of the shortest natural nodal period OrcaFlex uses to
calculate the recommended inner time step. The default value is 10, which equates to a recommended inner time
step of 1/10th of the shortest natural nodal period. A value of 20 would give a recommended inner time step of
1/20th of the shortest natural nodal period, and so on.
Outer time step (multiple of inner time step)
The recommended outer time step will be no greater than this value times the inner time step.
Outer time step (fraction of wave period or Tz)
The recommended outer time step will be no greater than T divided by this value, where T is either the wave period
(for regular waves) or Tz (for random waves).