ExtPtfm for OC4 Jacket

[KausikDalui]

Dear @jjonkman @ebranlard
I am attempting to perform a Craig-Bampton reduction of the OC4 Jacket to model it as a dynamic superelement in OpenFAST. I am using SACS software to generate the superelement of the OC4 Jacket and referring to the paper here (https://iopscience.iop.org/article/10.1088/1742-6596/1452/1/012033/pdf).

After simulating a case with the ExtPtfm module, I noticed deviations when comparing the results with the corresponding coupled case. Upon closer inspection of the Craig-Bampton reduction, I found that the Guyan part of the mass matrix and stiffness matrix does not match the ExtPtfm.dat file in the glue code. However, if I replace the Guyan part (mass and stiffness matrix) of my file with the Guyan part from ExtPtfm, the results closely match.

I believe that for the OC4 Jacket's geometry, if all member properties are accurately modeled, the Guyan part of the mass matrix and stiffness matrix should be the same. I have observed that the Guyan part varies mainly due to how I connect the top eight joints of the jacket to the transition piece. I have tried different configurations, including a series of rigid links between the TP interface points and these eight joints, but I am still unable to match the Guyan mass matrix and stiffness matrix.

I suspect I am missing some crucial details in this connection. It would be extremely helpful if I could obtain the SACS file used to generate the superelement file in the referenced paper or receive detailed information about the TP connection. Any assistance would be greatly appreciated.

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Feel free to adjust any part of this if needed!