Tower modes

Dear Jason

 I'm using FAST v8 to analyze a new kind of floating offshore wind turbine. In preparation for the EalstoDyn input file, file containing tower properties is needed. To get the tower modes, Bmodes is adopted. But in the Bmodes main input file, I have to input the properties of the tower support subsystem, such as the platform hydrodynamic 6X6 matrix, the platform hydrodynamic 6X6 stiffness martix and the mooring-system 6X6 stiffness matrix. How can I get these properties? Could you give me some suggestions?

Best regards

Dear Yu Lei,

For hydro_M, you could use the hydrodynamic added mass from WAMIT at a given frequency e.g. the zero-frequency limit.

For hydro_K, you can use the hydrostatic restoring from WAMIT, augmented with system weight e.g. as discussed in the following forum topic: http://forums.nrel.gov/t/bmodes-input-parameters-about-tower-support-subsystem/604/2.

For mooring_K, you can use a linearized form of the mooring matrix e.g. as calculated by MAP++ as discussed in the following forum topic: http://forums.nrel.gov/t/force-displacement-relationship-of-oc4-deepcwind-mooring-sys/964/6.

Best regards,

Dear Jason

I have downloaded BModes that could be used for  offshore floating wind turbines from [url]http://wind.nrel.gov/public/jjonkman/BModes/[/url]. I have known how to calculate hydro_K, hydro_M and mooring_K, but I don't know how do define the properities of the distributed (hydrodynamic) added-mass and distributed elastic stiffness per unit length along a flexible portion of the tower length. Could you tell me how to get these parameters. 

Besides, I’m still confused about the use of the ModeShapePolyFitting.xlsx, even though I have read ReadMe. Taking a part of results of OC3Hywind.bmi as an exmaple, could you tell me how to use the OC3Hywind.out data (calculated from the OC3Hywind.bmi) in ModeShapePolyFitting.xlsx.

Best regards


Dear Jason

I have another question. The hydro_K can be get from the [i]spar.hst[/i] file, but in[i] spar.hst[/i] file  k(3,3) is 3.312247E+01, k(4,4)=k(5,5)=-4.973414E+05, these are different from the values in  Eq. (4-3) of the OC3-Hywind report. In Eq.(4-3), C(3,3)=332941, C(4,4)=C(5,5)=-4999180000.

I also checked the spar.1 file and the hydro_M, I found that the elements in hydro_M were also different from the valves (at zero-frequency) in spar.1 file. The elements in hydro_M are almost 1000 times larger than those in spar.1 file (at zero-frequency).

Could you tell me why they are different? If I have my own WAMIT.hst file and WAMIT.1 file, how should I deal with the data?

Best regards

Dear Yu.Lei,

Based on your various questions, presumably you are deriving the tower mode shapes for a wind turbine atop a floating platform. In this case, you can set the distributed (hydrodynamic) added-mass and distributed elastic sitffness per unit length along a flexible portion of the tower length to zero because all of the hydrodynamic loads are applied to the platform rather than the tower.

Regarding the use of ModeShapePolyFitting.xlsx, you must identify the tower modes from the BModes output file. You can then copy the “span_loc” into the “x” column of the spreadsheet and either the “s-s disp” or “f-a disp” into the “y” column for the side-to-side and fore-aft modes, respectively. For the “Improved Direct Method” and “Projection Method”, you can get the “slope (i.e. dy/dx) at bottom” from the “s-s slope” or “f-a slope” outputs from BModes.

Regarding the hydro_K and hydro_M matrices derived from WAMIT output, the WAMIT is nondimensional whereas hydro_K and hydro_M must be dimensionalized. See my post dated Jul 01, 2011 in the following forum post for more information: http://forums.nrel.gov/t/wamit/373/4.

I hope that helps.

Best regards,

Dear Jason

Thanks for your reply. It’s really helpful for me.

Best regards