OrcaFlex Manual - Orcina

System Modelling: Data and Results, Vessels
For further details of the file format see Time History Files.
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w
Notes: If there is any wave-generated motion present in a vessel's time history motion then the OrcaFlex
wave data needs to match the wave that generated that motion. If you have suitable data for the
wave elevation then you can use that to specify the wave by time history. This can be done either in
a separate time history file for the wave or else in an extra column in the vessel's time history file.
The position and velocity specified by a time history file for the start of the simulation (i.e. for
SimulationTime = -BuildUpDuration) will not, in general, match the static state from which
OrcaFlex starts the simulation. To handle this OrcaFlex uses ramping during the build-up stage to
smooth the transition from the static state to the position and motion specified in the time history
file.
Externally Calculated Primary Motion
The externally calculated primary motion data only apply if the vessel's Primary Motion is set to Externally
Calculated. It enables you to impose a motion on the vessel that is calculated programmatically in your own
external function. Typically the calculation will be based on values of variables as the simulation proceeds –
otherwise it is preferable to use a time history to impose a pre-calculated motion.
To use externally-calculated primary motion:
� On the Calculation page of the vessel data form, set the Primary Motion to be Externally Calculated.
� In the External functions section of the Variable Data form, set up an variable data source that specifies the
details of your external function.
� On the Primary Motion page of the vessel data form, set the Externally calculated primary motion to the
external function variable data source that you have set up. Also set the Origin to the vessel axes coordinates of
the point on the vessel whose motion your external function specifies.
Unlike most standard external functions, the externally calculated primary motion external function returns
multiple values per call: position, orientation, velocity, angular velocity, acceleration and angular acceleration. These
values are returned in a TExternallyCalculatedPrimaryMotionStructValue struct – full details are given in
the OrcFxAPI help file.
Applied Loads
You can optionally include applied loads on a vessel.
You can apply to the vessel external Global Loads that do not rotate if the vessel rotates. These are specified by
giving the components of Applied Force and Applied Moment relative to global axes. These components can be
constant, vary with simulation time or be given by an external function. If the vessel rotates then the loads do not
rotate with it.
In addition, you can specify external Local Loads that do rotate with the vessel. These are specified by giving the
components of Applied Force and Applied Moment relative to vessel axes. Again these components can be constant,
vary with simulation time or be given by an external function. If the vessel rotates then the loads do rotate with it.
These are suitable for modelling thrusters, for example.
In both cases the Point of Application of the load is specified by giving its x,y,z coordinates relative to vessel axes.
Note: Applied loads will only affect vessel static position if the corresponding degree of freedom is
included in the static analysis, and will only affect the motion if the Primary Motion is set to one of
the calculated options which includes the degree of freedom.
Multiple Statics
The offsets for multiple statics calculations are specified here. Offsets are from the vessel's initial position and are
specified by giving a range of azimuth and offset values. For example: