Regarding FAST for soil-structure analysis

Wind & Water Computer-Aided Engineering Software Tools
1

Dear Sir,

I am really new to FAST. I am trying to do dynamic time-domain simulations for Soil-Structure Interaction in FE program USFOS for a fixed OWT. I have some very basic doubts regarding this.
What i have done is model a monopile with the NREL 5MW OWT, with respect to natural frequency considerations, using some realistic soil data. From my understanding of literature, i believe that the following steps should be followed.

  1. model an equivalent monopile/tower in FAST
  2. run FAST for the required wind and wave conditions and extract the responses (forces and moments)
  3. use the responses from FAST for the hydrodynamic simulations in USFOS

Sir, my doubts are as follows:

  1. which responses do i extract from FAST? is it the forces and moments at the tower-top, the tower-base or the mudline? Are there any specific guidelines in this regard?
  2. are the responses from FAST to be applied dynamically (as time-series) or statically (by taking extreme values)?

Sincerely

Abhinav

2

Dear Abhinav,

I don’t think your questions can be answered in general, but likely depend on your wants/needs.

One way you could couple the tools would be to do the coupled aero-hydro-servo-elastic resopnse simulations in FAST for the full rotor + drivetrain + nacelle + tower + substructure + foundation, using a simplified model of the foundation, such as an apparant fixity (AF) model. Then the outputs (time series, mins/maxes, or spectra–depending on your wants/needs) could be taken from FAST and passed to a code (such as USFOS) for detail soil-structure interaction modeling and pile design.

However, there a certainly other coupling approaches you could take.

Best regards,

3

Dear Sir

Thank you for the reply. I shall try to make use of the AF approach.

Abhinav

4

Dear Sir

Are the mudline forces and moments obtained from SubDyn representative of the overall loading acting on the OWT? If i am extracting the mudline forces/moments and the tower base forces/moments from FAST, and use both set of forces (at mudline and tower base) for analyzing an OWT in a code with geotechnical capabilities, like DNV-USFOS, will i be doubly defining the actual response from FAST?

Sincerely

5

Dear Abhinav,

Yes, that would be double booking the loads. The mudline forces and moments output from SubDyn are the total reactions at the seabed, including load contributions from the wind turbine, tower, and substructure.

Best regards,