FAST Seismic with OC3 Phase 2 Coupled Springs

Dear @Jason.Jonkman Sir,

The seismic archive of FAST v7 contains files with the OC3 monopile and tower for OC3 phase 2 models. Please confirm the following:

  1. The coupled springs can be used in conjunction with the seismic module? Is there an executable that has the FAST 7 compiled with the seismic and coupled spring module?

  2. Am I correct in assuming that its just the mode shapes of the needs to be updated in FAST v7 Seismic input files to include the effect of coupled spring?

  3. What shall be done to derive the forces on the pile head using the FAST Seismic module?

  4. The oscillator at the bottom in seismic code optimally transfers the motions to the tower base, we set the platform mass very high. Does this mean, I won’t be able to get the pile displacements and rotations due to a 3D seismic motion despite using the coupled springs along with the seismic motions?

  5. Please find the files attached. They are for the tower and monopile for the coupled springs. Can this be utilized in the seismic module without any recompilation?


Dear @Kashyap.Subham,

Actually, the Seismic archive of FAST v7 only contains examples with a land-based NREL 5-MW baseline wind turbine (not atop the OC3 monopile).

Here are my answers to your questions:

  1. The Seismic version of FAST v7 includes a customized UserPtfmLd routine specific for implementing seismic excitation of the foundation. The coupled springs version of FAST v7 includes a customized UserPtfmLd routine for implementing coupled springs of the foundation (without seismic excitation). These versions of the UserPtfmLd routine are independent have not been combined by NREL.
  2. If you use the coupled springs version of FAST v7, the tower mode shapes should be derived with the effect of the coupled springs boundary condition defined.
  3. The Seismic version of FAST v7 includes documentation that explains how the forces are derived. I’ve also summarized that in your related forum post: Instruction for the Seismic fast version - #24 by Jason.Jonkman.
  4. The Seismic documentation recommends values of mass, stiffness, and damping that result in a wide bandwidth so that the specified ground motion gets directly transferred to the platform motion. The coupled springs model does not following the same recommendation. So, if you change the Seismic version of FAST v7 to use coupled springs and follow the coupled springs guidance, than the bandwidth may not be optimal and the specified ground motion may not be fully transferred to the platform motion.
  5. I don’t see any attachments.

Best regards,