Regarding Apparent Fixity model

Dear Abhinav,

We have plans to include linearization functionality in FAST v8 that will enable direct calculations of full-system mode shapes and natural frequencies via eigenanalysis. Until that feature becomes available, one can identify the natural frequencies by the classical method of post-processing time-domain simulations e.g., by computing power-spectral densities (PSDs) of response time series.

You should rederive the mode shapes of the tower in ElastoDyn (changing only the ElastoDyn tower file) whenever the substructure mass or stiffness has been changed in SubDyn. The SubDyn ReadMe file provides some guidance on how to use BModes to derive the mode shapes needed by ElastoDyn when FAST is used with SubDyn.

Best regards,

Dear Jason

Thanks for the reply. I have read the SubDyn Readme file. It is mentioned that one should substitute the hydro_M and hydro_K matrices from obtained from SubDyn, in the BModes file. I haven’t understood a couple of things.

Are these two matrices obtained from SubDyn by running it as a standalone code, with the modified support structure?

It is also said that i should modify the tower properties accordingly, to get eigenmodes compatible with FAST. Is there a way in which i can validate the correctness of the mode shapes i derive for the tower?

Sincerely

Abhinav

Dear Abhinav,

The equivalent substructure stiffness and mass matrices derived by SubDyn are available in the SubDyn summary file created when input parameter SSSub is set to True in the SubDyn input file, both when SubDyn is run in standalone mode and when SubDyn is run coupled to FAST v8.

FAST is not capable of judging whether the specified mode shapes are correct or not. You could verify that the derived mode shapes are correct by verifying the solution against the results of another code with similar capabilities.

Best regards,

Dear Jason

Thanks for the clarification.

Sincerely

Abhinav

Dear Sir

Is wind.nrel.gov/public/jjonkman/BModes/ the latest version of BModes available? Does the OWT tower transition uniformly in diameter and thickness, over the length of 87.6 m?

Sincerely

Abhinav

Dear Abhinav,

Regarding BeamDyn, “yes”, the version located at wind.nrel.gov/public/jjonkman/BModes/ is the newest version. See the following forum topic for an explanation of the differences between the two versions: http://forums.nrel.gov/t/run-bmodes-with-hydrodynamic-effect/690/1.

There are several versions of the tower for the NREL 5-MW turbine, depending on the substructure type (monopile, tripod, jacket, floating platforms), but in most cases the diameter and thickness vary uniformly along the tower length.

Best regards,

Dear Sir

Bmodes require 7 properties at each section of the tower, for calculating the mode shapes. For the FAST8, Monopile supported tower, only the mass densities and the FA and SS stiffness along the tower are specified in the ElastoDyn_Tower file. How do i calculate the other properties?

I tried to compute the SS and FA mass moments of inertia’s using standard formulae, but got wrong values (i first checked it for the bottom section of the CS_monopile_tower_secs.dat file). I used the formula for a hollow cylinder:

I = 1/2 m ( ri^2 + ro^2)

also, i tried modifying the equation you mentioned in the post http://forums.nrel.gov/t/bmodes-flp-iner/335/1, for a hollow cylinder, but got nowhere near the correct value. Please help me out.

Sincerely

Abhinav

Dear Abhinav,

FAST’s tower model only takes into account fore-aft and side-to-side bending (with no axial, torsional, or shear DOFs) for a straight beam of isotropic material with no mass or elastic offsets. You are using BModes to derive mode shapes for FAST, so, you want the BModes model to be consistent with FAST. I suggest the following values in BModes when deriving tower mode shapes for FAST:

str_tw = 0
tw_iner = 0
flp_iner = very small number (you can’t specify exactly zero for this input in BModes)
edge_iner = very small number
tor_stff = very high number (BModes won’t allow infinite; just set it high enough that it doesn’t impact the BModes results for the modes you care about)
axial_stff = very high number
cg_offst = 0
sc_offst = 0
tc_offst = 0

Best regards,

Dear Jason

Thanks for the reply. Sorry for disturbing you again!

BModes seems to give me no answers. I have changed the tower properties, replaced the hydrodynamic stiffness and mass matrices with those from the SubDyn sum file and changed the boundary condition to ‘free-free’. But i am getting all eigenvalues as 0, when i run BModes. It seems that BModes expects values for ‘flp_iner’ and ‘edge_iner’. I had used ‘0’ for these parameters, as you suggested. Could you please take a look at my files?

Sincerely

Abhinav

Dear Abhinav,

I forgot that BModes doesn’t like flp_iner and edge_iner to be exactly zero; BModes won’t complain, but it won’t deliver the correct results either. I’ve modified my forum post above to clarify this. A similar discussion took place in the following forum topic: http://forums.nrel.gov/t/tower-eigenfrequencies-of-nrel-5mw-turbine/517/1.

I also noticed that you have tower draft (draft and ref_msl) set to 20 m in your BModes input file. If its not, you want this value to be equivalent to the platform reference point in FAST v7 (or in ElastoDyn module of FAST v8).

Best regards,

Dear Jason

Thanks for the reply. Yes, my tower draft value was a wrong one. I have replaced it with the platform reference point value (10 m) from FAST8. I have added very small values for flp_iner and edge_iner and BModes is working now. Based on my understanding of the FAST8 files, i have drawn a schematic diagram of the monopile OWT. Please see the attached drawing.

The tower begins at 10 m above the MSL and ends at 87.6 m. So, in effect, the tower height is 77.6 m? Jason, if i am adding a few nodes below the mudline in SubDyn (to model the apparent fixity), should i add the same nodes in HydroDyn too? Or is the first point below mudline enough, in HydroDyn? I have tried both approaches and the answers don’t differ.

Sincerely

Abhinav

Dear Abhinav,

I’m glad you got BModes to work. Yes, a tower that begins at 10 m above MSL and ends at 87.6 m will be 77.6-m long.

HydroDyn needs at least one joint below the seabed to avoid having a static pressure load applied on the bottom of the structure. But there is no need to add members that are fully below the seabed, as HydroDyn won’t compute hydrodynamic loads below the seabed. Nodes need not match between HydroDyn and SubDyn. See Section 6.5 of the HydroDyn User’s Guide and Theory Manual for more information.

Best regards,

Dear Jason

Thanks for the really quick reply. It has helped me a lot.

sincerely

Abhinav.

Dear all,

I am trying to develop an “apparent fixity model” using FAST v.8 and SubDyn. The AF model has the following characteristics:

• AF length = 16.964m
• Cross-section diameter = 6m
• Cross-section thickness = 0.03457m

Following the instructions presented above in this topic, I modified the existing SubDyn dat file of the 5MW Baseline turbine in order to add the apparent fixity model and I made no changes in any other input file. However, FAST didn’t run and I received a notification of a logical error in SubDyn.

The modified dat file is attached below. Any advice would be most appreciated.
NRELOffshrBsline5MW_Monopile_RF_SubDyn.txt (7.06 KB)

Thank you in advance.

Best regards,
Georgia

Dear Gia,

I quickly skimmed your input file and it looks OK to me, although I haven’t run it myself. What is the exact error message you are getting?

Best regards,

Dear Jason,

Thank you for your prompt reply.

The error message is the following:
"Invalid logical input for file ‘’.\5MW_Baseline/NRELOffshrBsline5MW_Monopile_RF_SubDyn.dat’’ occured while trying to read SSSum.’’

Best regards,
Gia

Dear Gia,

Ah, I now see that you added a line to the MEMBER X-SECTION PROPERTY DATA table without increasing NPropSets from 4 to 5. The remaining rows in the file are not being read properly.

If this happens again, you can debug an input file by reviewing the echo file created with Echo is set to TRUE.

Best regards,

Dear Jason,

Thank you very much for your help. It finally worked!

Best regards,

Gia

Dear Sir

For the same wind and wave conditions, one would expect lower response from the OWT with increasing apparent fixity depth. However, for a HH wind speed of 30mps (idling rotor), I find that the FATBM is steadily increasing with increasing AF depth. My Hs and Tp values are 10m and 14s, respectively.

I have modelled the idling rotor in FASTv8, using the recommendations in the post Extreme events. I have used the default monopile OWT in FASTv8. Please help me diagnose the error.

Sincerely

Abhinav
FAST8_idle.zip (16.1 KB)

Dear Abhinav,

Can you confirm/clarify a few things:

  • By FATBM, you are referring to the TwrBsMyt output from ElastoDyn (the tower base is located 10-m above MSL for each model of different apparent fixity depth)?
  • You’ve modeled the apparent fixity by introducing another joint and member in SubDyn?
  • The cross-sectional properties of the apparent fixity beam are identical with each depth?
  • Using BModes, you update the tower mode shapes in FAST with each change to the apparent fixity depth in SubDyn?

When you increase the apparent fixity depth, you are increasing the length of the beam, which increases its flexibility if the cross-sectional properties of the beam are unchanged between depths. If the pile deflection is substantial, I can foresee an increase in FATBM with increasing depth/flexibility.

Best regards,