Firstly, I have to appreciate you for the time you have spent on checking my model. The only thing that I have changed in my last model comparing to the model you just ran( is the last four joints Z coordinate in my SubDyn input file as they have taken values -76 (i.e. -50 + (-26) ). As a result, the KBBT and MBBT matrices are changed considerably as the apparent fixity length have been increased from 12.46 to 26 m.

Best regards,
***BTW here is my SubDyn input file.

I changed the the flp_iner and edge_iner values in tower section properties file from 0.0001 to 0.001 and it seems the problem has been solved. Now by looking at the BModes output the first four mode shapes can be considered as first and second S-S and F-A modes respectively. Would you please take a look at my result and let me know if it is correct or not ?

Very interesting! The natural frequencies calculated by BModes did not change by much, but increasing flp_iner and edge_iner a little definitely makes the S-S and F-A mode shapes more readily identifiable. I’m not sure why this sensitivity exists in BeamDyn (likely a numerical problem), but I’ll have to remember this sensitivity in the future.

I would interpret the modes as follows (same ordering as before):
Mode 1 - freq=0.286 Hz; 1st tower side-to-side
Mode 2 - freq=0.288 Hz; 1st tower fore-aft
Mode 6 - freq=2.033 Hz; 2nd tower side-to-side
Mode 7 - freq=2.336 Hz; 2nd tower fore-aft

Modes 3-5 show different coupling of the tower to the jacket, but they are not 2nd tower modes.

I really appreciate you for your attention. But would you please interpret that why you have chosen modes 6 and 7 instead modes 3 and 4 ? Is there any consideration except domination of displacements in order to decide which ones have to be selected ?

If you look at modes 3 and 4 in the ModeShapePolyFitting.xls spreadsheet, you’ll notice that they are simply repeats of modes 1 and 2 (but couple to the substructure differently) i.e. the curvature of the beam does not change sign. When you look at modes 6 and 7, you clearly see that they are second modes i.e. the curvature of the beam changes sign once along the beam. In ElastoDyn, the actual tower deformation will be formed by some linear combination of the modes, so, you’ll want to provide the first two mode shapes to have a solid basis for deformation.

In regard to poly fitting procedure, according to FAST guideline and as you mentioned earlier we must sum a2-a6 coefficients to one preceding to move them into Elastodyn_tower file as the structure mode shapes. But given that the picture I have attached here, which row have to be considered ? Does the projection method’s row (the highlighted one in the picture) is the target ?

As described in the “ReadMe” worksheet, I would normally recommend that you use the Normalized Improved Direct Method when you are deriving the modes for ElastoDyn from BModes. Make sure that you specify the slope at the bottom of the beam (as taken from the BModes output) in addition to the x and y data.

As I see in the forum, the stiffness and mass matrix are taken from .SD file which are inputted to BModes. But these stiffness and mass matrices are at the TP reference point (i.e, at the tower base right?). My doubt here is as we are taking these values at the TP reference point, the tower section (HtFract) (attached figure 1) should start from 0.27882 or is it 0.0 in BModes tower input file? If it is 0.0, does the BModes will consider from the TP reference point or from the mudline?

Does tower modes which we are getting from BModes will include the of mass of RNA and secondary steel mass or will it consider just the mass which below tower top elevation?

I have run the BModes by considering the values obtained form .SD of MW_OC3Mnpl_DLL_WTurb_WavesIrr.fst and considering the tower section properties which consist of 13 section (will this start from mudline or tower base) (attached figure 2), is the frequency obtained (attached figure 3) are correct?

And I see we need to use ModeshapePolyfitting.xls to get the polynomial coeffiecents which we should inputted for NRELOffshrBsline5MW_OC3Monopile_ElastoDyn_Tower.dat file, I couldn’t find any excel file, so could you please provide the excel file and related documentation to understand?

Based on your questions, I assume you are modeling the support structure in FAST v8 or OpenFAST with the tower (above the TP) in ElastoDyn and the substructure (below the TP) in SubDyn and that you are trying to use BModes to derive the tower mode shapes for ElastoDyn. My answers to your questions below are based on this understanding.

I gather that you are modeling the substructure in BModes as a lumped mass/stiffness and the tower in BModes as a finite elements. Thus, only the tower should be modeled with distributed mass and stiffness in BModes; i.e., draft = ref_msl = -10 m (10-m above MSL) and sec_loc = 0 in the distributed tower section file is at 10 m above MSL for the OC3 monopile.

The tower mode shapes will be impacted by the tower-top (RNA) mass and inertia specified in BModes.

It looks like these results from BModes include the full tower + monopile modeled in BModes.

ModeShapePolyFitting.xls is provided in the various archives of FAST, but I’ve also attached it here for your reference. The ReadMe section on the first worksheet provides the documentation.

Mass and stiffness matrices obtained in .SD file is related to monopile (till interface level) right?

If .SD is related to monopile (till interface level), then by setting ref_msl = -10m and considering the above matrices. Does that mean BModes will give overall structural natural frequency.

I am trying to match the coefficients in Elastodyn_tower.dat of 5MW turbine atop monopile. After calculating from BModes, I have copied the deflection and slope at the bottom. But I am getting these values (attached figure), I am not sure which values I need to take from this excel file. Could you please help me with this.