TMD weight in the barge platform frame

Wind & Water Computer-Aided Engineering Software Tools
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Hello @Jason.Jonkman ,

Hope you are doing well.

I am writing to see your opinion about the transformation to apply to get the TMD weight (mass x gravity) in the platform frame.

Here is what i have done:

– - - > since in OpenFAST, small rotation angles are assumed, the seqence of rotation does not matter right ?
— - - > the rotation of the platform is of type extrinsic meaning that all rotation degrees of freedom are defined with respect to the global/inertial frame right ?
– - – > for a TMD placed in the platform the frame attached to the TMD rotates in 3D as the platform does right ?

if you confirm, this means that the weight vector in the platfor frame is jst the multiplication of the weight vector expressed in the inertail/global frame by the 3d rotation matrix where the seqence does not mattter since OpenFAST assumes small angles of rotation. Do you confirm ?

Sorry for this basic question bt the 3D rotation is a bit confusing for me.

Thanks in advance,

Best Regards,

Riad

2

Dear @Riad.Elhamoud,

For older version of ElastoDyn (before OpenFAST v4.0), ElastoDyn used small/moderate angle assumptions for the platform roll, pitch, and yaw rotations. However, as of OpenFAST v4.0 and newer, ElastoDyn now uses an Euler yaw, followed by pitch, followed by roll rotation sequence for the platform motions, without small/moderate angle assumptions.

Best regard,

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ok thanks.
So if i get it, yo validate my statments except for the one of small rotation right ?

Sorry for asking too much, is there a reason for the seqence chosen or it is chosen to match teh standards ?

Thanks a lot for your help.

Best Regards,

Riad

4

Dear @Riad.Elhamoud,

Three of your points (small rotations, no rotation sequence, and global rotations) are impacted by the change to Euler angles.

The yaw-pitch-roll sequence was chosen because yaw is the one that is most critical to floating platforms; that is, we expect some floaters to have large yaw either by design (single point mooring) or by accident (loss of a mooring line), but pitch and roll we expect to remain limited.

Best regards,

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