Linearized eigenvectors including CB superelement


I would like to obtain linearized eigenvectors of the integrated structure (foundation + turbine) for different operational conditions:

  • Jacket foundation modelled in external software reduced to Craig-Bampton (CB) superelement (SE)
  • Linearized for a range of wind speeds [U_cutin U_cutout]

So far I am aware of the following functionalities of OpenFAST/FASTv8 which could help me in obtaining the results:


  • Can CB SE be imported in ExtPtfm (n x n) matrix, where n > 6. Any recommendations on input file / format to start with?
  • Can the linearization tool be run with ExtPtfm foundation where coupling between blades and internal foundation dynamic is included
  • Can linearized (averaged) eignevectors be obtained for various wind speeds
  • Where would you recommend to start with? Test14 extended with ExtPtfm foundation?

Thank you,

Dear Dawid,

Here are my answers to your questions:

The ExtPtfm module when it was first introduced into OpenFAST was only valid for superelements with n=6, i.e., a Guyan reduction. However, an updated ExtPtfm module has recently been developed that supports superelements with n>6, i.e., Guyan + Craig-Bampton reduction. This is currently awaiting review and merging into OpenFAST dev–see the following pull request:

The linearization functionality of OpenFAST does not yet support linearization with the ExtPtfm module enabled. That said, we are currently working an update to OpenFAST that does support linearization with the SubDyn module enabled.

Normally, the linearization process involves finding a periodic operating point at given mean wind speed (or rotor speed), linearizing an OpenFAST model at a number of azimuth steps about the operating point, applying MBC3 to transform the matrices with rotating states/inputs/outputs into the fixed frame, azimuth averaging the MBC3-transformed matrices, and performing an eigenanalysis on the averaged state matrix; this will result in full system eigenvalues and eigenvectors for a given wind speed. This process would be repeated at each desired wind speed (or rotor speed).

Because it is not yet possible to linearize with the ExtPtfm module enabled, direct eigenanalysis is not possible for models with ExtPtfm. Instead, the eigenfrequencies of such systems could be found through free-decay simulations or simulations with broadband (e.g., white noise) excitation.

Best regards,

Dear Jason,

thank you for your clarification.

It seems like ExtPtfm with CB SE and further existing linearization procedure is exactly what I am after.

Do you have any feeling for when the ExtPtfm with CB SE will be released? Alternatively, do you maybe have some beta version for tests available?

Thank you,

Dear Dawid,

You can access the updated ExtPtfm module through the links in the pull request: But because this has not yet been reviewed, changes may occur before merging into OpenFAST-dev. I’m not sure when the review and merging will occur.

Best regards,