Modelling of tower atop asymmetric floating platform

Hi Everyone,

I’ld like to derive the modal parameters of a tower mounted on a floating platform using Bmodes. The platform is asymmetric and has x-offset, very small y-offset and z-offset of CoM.

I understand that cm_pform under “the Properties of tower support subsystem” section of the Bmodes input file represents z-offset, the vertical location of platform CoM below the ML. I’m wondering if there is any possibility in Bmodes to account for x-offset of the platform CoM?

Moreover, I’ve a very basic question related to HydroDyn model for this platform. As the platform is asymmetric, I set the location of CoM (PtfmCMxt, PtfmCMyt, PtfmCMzt) in ElastoDyn input file to the values I obtained from 3D model. Upon taking a look at the source codes of ElastoDyn module (ElastoDyn.f90 line 6641) and HydroDyn modules (WAMIT.f90 lines 1450 to 1462), I understand that the current OpenFast version considers the moment induced by x-, and y-offset of CoM both in ElastoDyn and HydroDyn. Could anyone please confirm me if my understanding is correct?

While preparing potential flow model based on WAMIT output in HydroDyn, I shall set the location of CoB (PtfmCOBxt and PtfmCOByt) to the values obtained from Aqwa model. So, these values should also be based on the simulation output of the entire system (RNA, tower, platform and the mooring system). Is that correct?

Thank in advance for your help!

Best regards,
Moe Moe Aye

Dear Moe Moe Aye,

BModes does not support the modeling of platform body mass that includes nonzero offsets of the mass center in x and y. But instead, you could zero out the body mass values (cm_pform = mass_pform = i_matrix_pform = 0) and specify the fully hydrodynamic plus body mass matrix in the 6x6 hydrodynamic mass matrix input (hydro_M).

Regarding ElastoDyn and HydroDyn, from what I can tell your understanding is correct. Both ElastoDyn and HydroDyn support the modeling of asymmetric platforms (allowing for nonzero offsets of the center of mass and center of buoyancy in the x and y directions).

But I’m not sure I understand your specific question about PtfmCOBxt and PtfmCOByt. These values can be determined solely by the displaced volume (wetted geometry) of the floating substructure, which is not dependent on the entire system (RNA, tower, mooring system).

Best regards,

Dear Jason,

Thank a lot for your help. It is really great to know that I can specify the body mass values in Bmodes in this way.

Sorry for not describing my question clearly regarding the two parameters PtfmCoBxt and PtfmCoByt.
But I got a grasp on this from your reply already.

Actually, I’ld like to check if the entire system is in static equilibrium position. For this purpose, I set PtfmCOBxt and PtfmCOByt to the values obtained from hydrostatic results of the AQWA model which was without the mooring system. I adjusted the displaced volume of water PtfmVol0 until the equilibrium condition stated in section 6.8.1 of Hydrodyn manual is satisfied. This is why I come up with this question and wonder if I accurately define PtfmCOBxt and PtfmCOByt in HydroDyn input file. Now I understand well how to determine these values. Thank you again!

Best regards,
Moe Moe Aye