Tower parameter TwrRBHt

Dear all,
I need to simulate with FAST a wtg tower with a positive offset from ground level (say flexible tower length 10m, tower base 1m above ground level, rigid base between 0-1m agl).
My approach is:

  1. Tower input file with struct properties (and modes) normalized for 10m tower (to match the actual tower flexible length)
  2. FAST main input file with following parameters:
    • TowerHt = 10 (the help on this line says “height of tower above ground level”…so my doubt comes from here! )
    • TwrRBHt = 1
  3. In AeroDyn input file param HH must be consistent with the real hub height :
    HH = TwrRBHt + TowerHt + Twr2Shft + |OverHang|*sin(ShftTilt)

The TwrBs output loads will be given in the ref frame with origin 1m above ground level, accordingly to the FAST manual.

Is this right? I ask this question basically as a confirmation of what I’m doing, because I do not find param TwrRBHt depicted in the Fig.16 of the manual

Thank you very much for you help.


Dear Stefano,

Assuming that you don’t have a platform input file (with additional input TwrDraft), TowerHt should be specified as the distance from the ground to the top of the tower (to the yaw bearing). TwrRBHt should be specified as the distance from the ground to the base of the tower (the length of the rigid portion); yes, the tower-base loads are specified at this height. The flexible tower length is the difference between these distances, TwrFlexL = TowerHt - TwrDraft. So, if TowerHt = 10 and TwrRBHt = 1, than TwrFlexL = 9. I’m not sure where you got your equation for HH, but the AeroDyn input file I checked says HH = TowerHt + Twr2Shft + OverHang*SIN( ShftTilt ), which is correct.

I hope that clarifies things.

Best regards,

Dear Jason,
I have no platform file so TwrDraft = 0. The equation you gave for HH is correct (mine was just an assumption)
At this point, you can confirm that the length of tower I have to use to produce normalized Tower input file (and also for BModes and calculation of tower modes) must be exactly
= TwrHt - TwrRBHt (and not simply TwrHt)
FAST will then consider the tower cantilevered at 1m agl, right?

Thank you very much.


Dear Stefano,

Yes, your understanding is correct.

Best regards,


Thank you Jason

Hi Jason

I am currently simulating the model of “NRELOffshrBsline5MW_Monopile_RF”.
According to this post, the flexible tower length is 87.5+20=107.5m by default settings, which is also summarized in the .fsm file.
I am wondering whether we should treat the monopile structure as part of the tower in the analysis.
The material properties of tower and monopile are probably quite different, and the tower-foundation joint is above MSL.
Is it typical to just simply see them as a whole flexible tower?
Thanks you~

Best regards

Dear Yulin,

When modeling fixed-bottom offshore wind turbines, it is common to model the substructure as a flexible body with hydro-elastic effects.

In FAST v7, we’ve modeled the NREL 5-MW atop a monopile with the FAST “tower” model representing the flexibility of the combined tower + substructure.

In FAST v8 (to be released soon), we are introducing a new structural dynamics module (called “SubDyn”) for multi-member fixed-bottom substructures, which will be applicable to monopiles, tripods, jackets, etc. In this case, the substructure will be modeled within SubDyn and the tower (above the transition piece) would be modeled with the existing FAST tower model (with 2 bending modes each in the fore-aft and side-to-side directions).

Best regards,

Thanks Jason. Looking forward to the new version~