I want to study the behaviour of a floating wind turbine with mechanical dampers attached under seawater level and am looking for methods of how this could be done. I am new to FAST and a lot of this is difficult to understand:
- Is there any way of modelling a flexible floating platform in FAST? Is the reason for making the platform rigid because the flexible modes are only related to the tower and not the platform and tower as a whole?
- When looking at pictures in the documentation for FAST v8 it seems impossible to use the SubDyn module, as it is only available for fixed bottom structure. Is this also true for OpenFAST?
- I see that the ExtPtfm module is available in OpenFAST. Are there any documentation or (better) examples available at the present moment for how to use this module? Are the matrices found from the Craig-Bampton method?
Best regards,
Even Lund
Dear Even,
Here are my answers to your questions:
- Currently FAST/OpenFAST are limited to modeling floating platforms as a 6-degree of freedom rigid body. The flexible tower is cantilevered atop the rigid floater. We are actively trying to obtain funding for adding floating-platform flexibility and the associated member-level loads to OpenFAST.
- Yes, SubDyn is currently limited for use in fixed-bottom offshore or land-based wind turbines.
- We don’t really have documentation around the user module ExctPtfm. We were hoping that the user-defined module is quite straightforward. Basically, the ExctPtfm provides a simple way to specify 6x6 mass, damping, and stiffness matrices, as a well as a 6x1 load vector to create loads applied to the platform from the foundation e.g. from a Guyan reduction. Of course, the routine is user-specified and so the user can also implement nonlinear loads or include additional internal states (e.g. for a Craig-Bampton reduction).
Best regards,
Dear Jason,
Thank you for the answers!
Regarding the ExtPtfm module: In lack of examples I found an test inputfile at github “old-NWTC/ExtPtfm/test/SubStructureMatrices.dat” that can probably be used as a starting point. Are the six matrix rows/columns describing the six degrees of freedom for the platform? In which order are they stacked? Will the matrices be connected to the interface between the platform and the tower or could you specify this?
Best regards,
Even
Dear Even,
Yes, the sample ExtPtfm_MCKF.f90 file can be used to read in the matrices and load vector from the file you found, or you can custom-purpose the module for your own use.
Yes, the row/columns are associated with the platform DOFs and loads. The order is translational surge, sway, and heave, followed by rotational roll, pitch, and yaw.
The motions/loads are defined relative to the platform reference point, whose vertical location along the tower centerline for the undisplaced system is specified in the ElastoDyn primary input file.
Best regards,
Dear Jason,
I realized after trying to use ExtPtfm that it was not possible to use without disabling HydroDyn (“Set CompHydro = 0 or CompSub < 2”). Is this a bug? Is it intended to be used only with aerodynamic forces?
Best regards,
Even
Dear Even,
This is as intended. The user module ExctPtfm is meant to replace existing models of the substructure–including SubDyn, HydroDyn, moorings, etc.–with a user-defined module.
Best regards,