Dear Kevin,
The FAST v7 model of the NREL 5-MW baseline turbine atop the OC3-monopile with coupled springs (CS) foundation has been provided on the forum before, requiring:
- An updated tower input file, NRELOffshrBsline5MW_Tower_Monopile_CS.dat, available in the following forum topic: Simulating OC3 phase II coupled springs in FAST v7 - #2 by Jason.Jonkman.
- An updated platform input file, NRELOffshrBsline5MW_Platform_Monopile_CS.dat, available in the following forum topic: Simulating OC3 phase II coupled springs in FAST v7 - #2 by Jason.Jonkman.
- A recompilation of FAST v7 with the user-specified platform SUBROUTINE supporting the CS foundation, UserPtfmLd_CS.f90, available in the following forum topic: Turbine-soil interaction. Influence on the mode shapes. - #13 by Jason.Jonkman.
- A reduction of the time step to resolve the higher natural frequencies with the CS foundation; NREL used DT = 0.00125 s.
Are you using these input files, source code, and time step?
Best regards,