Tower Eigenfrequencies of NREL 5MW Turbine

Hi, I just have a few questions about Bmodes and how to operate it correctly. Im switching from modes to Bmodes because it is now advised. But also because im interested in modelling turbine soil interaction, i had been assuming that the foundation and turbine were decoupled and in that way generate the mode shapes using modes however in Bmodes I see there is an option to include the foundation stiffness in a CS model form.

I have been trying to use Bmodes to get the tower mode shapes for both a fixed base and flexible conditions.

Fixed base, for offshore wind turbine, I used the ‘CS_monopile.bmi’ modal and Bmodes.exe from wind.nrel.gov/public/jjonkman/BModes/. and simply put the tension to zero and the mooring stiffness to zero and hub_conn=1 (for a cantilever support) my first five modal frequencies were,

Mode 1= 0.273744E+00 Hz (s-s) Mode 2= 0.276006E+00 Hz (f-a) Mode 3= 0.132889E+01 Hz (s-s) Mode 4= 0.158898E+01 Hz (s-s) Mode 5= 0.186703E+01 Hz (f-a)

Modes 1 and 2 are the fore-aft and side-to-side 1st tower frequencies however modes 3 and 4 both seem to be side-to-side as they contain only s-s disp, does this seem correct? I then placed these modes shape (1,2,3,5) into the excel sheet and then into FAST (using the offshore monopile FAST files) and did a linearization run to calculate the modal frequencies I got back (for the tower modal frequencies) 0.277 Hz 0.278 Hz 2.2835 Hz 2.7394 Hz

The second tower modal of 2.7 seem a bit high.

The flexible foundation, for offshore wind turbine, I used the ‘CS_monopile.bmi’ modal and simply put the tension to zero and in the mooring stiffness matrix I put in my own coupled spring stiffness for a pile foundation

I assuming that, mass_pform is just the mass of the platform and Platform mass inertia 3X3 matrix just correspond to the terms PtfmPIner, PtfmPIner and PtfmYIner in the platform file in FAST, ( do I have the matrix in the correct order) [PtfmPIner 0 0 0 PtfmPIner 0 0 0 PtfmYIner]

n_secs_k_distr, I not sure what this term does (its not in the user manual) I just have it set to zero

When calculating the mode shape using the provided excel sheet Im not quite sure about the ‘Scaling factor of y’ I’ve been leaving the first one at the suggest value the second scaling factor it states should be ‘a user specified ratio of the max deflection to beam length for a deflected beam’ for this value im simply taking the biggest absolute value from column y (the deflection) and dividing by 107.6 (87.6+20) is this correct? and should the sign of the scaling factor of y always be positive.

I’ve have being taking polynomial coefficients from the Normalized Projection Method when the foundation is included.

I was also wondering does anyone have a link to this document, Memorandum: Derivation and Description of the Soil-Pile-Interaction Models, OC3-Derivation and Description of the Soil-Pile-Interaction Models.pdf,

Thanks in advance for any advice. Michael