I am aiming at building a coupled model using FAST and a multibody model Orcaflex in which the floater is defined by several elements of type “vessels”.
I tried some simulations but they instantaneously failed. I guess the interface FAST-Orcaflex is adapted for only one vessel.
Do you know if using FAST-Orcaflex is possible with this kind of multibody model?
In this case, do you have an idea of the amount of work / level of difficulty necessary in changing the source code to achieve it?
If not, do you have some contacts from NREL or Orcina who could help me?
Thank you by advance.
From our experience, the current interface between OrcaFlex and FAST v8 (similar to the interface between OrcaFlex and FAST v7) requires that that the floating platform mass in OrcaFlex be large relative to the wind turbine mass in FAST (tower + nacelle + drivetrain + rotor). This is the case for many platforms, but if you attempt to build a multi-body model in OrcaFlex of the floating platform, my guess is that the actual portion of the floating platform mass directly involved in the interface between OrcaFlex and FAST is not large relative to the wind turbine mass. If you can introduce multiple bodies while still maintaining that the portion of the floating platform directly involved in the interface is large, than I would expect that to work; otherwise, your model could suffer a numerical instability.
Eliminating the limitation that the floating platform mass in OrcaFlex be large relative to the wind turbine mass in FAST would require a complete overhaul of the interface and coupling approach (a lot of work).
Thanks for your prompt reply and your return of experience.
Your guess is right, the mass involved in the interface is just a portion of the total mass of the floating platform.
We will try to involve a larger mass.
I am currently working on the coupling between OrcaFlex and Fast. We have an issue understanding how the prediction is done to solve the platform movement.
Which routine is doing the prediction? How is it done?
Can you clarify what you are asking? What prediction are you referring to?
As the solver is implicit, you have to predict the loads at the time step t + delta_t. I am wrong?
If you do so, were is this prediction done?
Thanks in advance for your help.
Actually, the version of OrcaFlex that FAST v8 calls (FASTLinkDLL.dll) is only called once per time step, even if corrections are added to the FAST solution. You can see this in routine OrcaFlexInterface.f90/Orca_CalcOutput() through the use of m%LastTimeStep,