OrcaFlex Manual - Orcina

System Modelling: Data and Results, General Data
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� If Python 2 is selected then either version 2.6 or 2.7 will be used. If both of these versions are present on your
system, then the most recent, i.e. 2.7 will be used.
� If Python 3 is selected then one of version 3.0, 3.1 or 3.2 will be used. Again, if more than one of these versions is
present on your system, then the most recent will be used.
Initial Value
When OrcaFlex calls the external function for the first time it passes this value to the function. The function can use
this value as its starting value if it so chooses. Alternatively it can ignore this value and substitute a different value.
Time Step
Note: This value is used by OrcaFlex when drawing the model in Reset state.
This specifies how frequently the external function is called to update the value of the variable data item. You can
use either the inner time step, the outer time step or a user-specified time step. If you choose a user-specified time
step then you input a target time step and the program uses the nearest whole multiple of the inner time step.
Note: When using implicit integration, this data is ignored and the external function is called on each
iteration of each time step.
External Function Parameters
The external function facility provides a general purpose method of passing parameters to external functions.
For an example of why this is desirable consider an external function which implements a PID controller used to
model the active control system for a heave compensating winch. A PID controller has a number of parameters and
these could be hard coded into the external function. However, in OrcaFlex you may want to experiment by varying
these parameters to see how they affect a simulation. If the parameters are hard coded then the external function
DLL will have to be recompiled each time you experiment by changing the parameters.
Instead OrcaFlex allows you to specify parameters for the external function as part of the OrcaFlex data. These
parameters are passed to the external function. The parameters are specified as a free form multi-line text field. This
means that the external function has to do some work to decode and interpret the text before it can access the
parameters. However, this also means that there is a great deal of flexibility in the parameter passing mechanism.
Whenever an OrcaFlex object (e.g. a Winch, a 6D Buoy, a Vessel, the Environment etc.) uses an external function then
a page titled External Functions is available on that object's data form. The parameters are specified in the text
field contained in this page.
For native code external functions, the parameters are passed to the external function in the lpObjectParameters
member of the TExternalFunctionInfo data structure. For Python external functions, the parameters are passed
in the ObjectParameters attribute of the info object.
For more details please refer to the OrcaFlex programming interface documentation. Also, the External Function
Examples illustrate the procedure involved in passing parameters to external functions and are a useful source.
Data which can be controlled by External Functions
The following data items can be controlled by External Functions:
� Vessel and 6D Buoy applied loads.
� Wing orientation angles.
� Winch tension and winch payout rate.
� Current reference speed and reference direction.
� Bend stiffness for Line Types.
� Vessel primary motion.
External Function Examples
A selection of example external functions are supplied on the OrcaFlex installation disc. Alternatively these example
functions can be downloaded from www.orcina.com/Support.