External driven floater positions using MoorDyn

[Elaine.Liu]

Hi,
I wanted to check my understanding of how coupled bodies/points in MoorDyn works. If I wanted to load center of gravity coordinates of a floater from a physical experiment into MoorDyn through a driver script, would I specify “Coupled” body in the “Bodies” section and thus “Body1” connection for my fairlead, as per the screenshot below? This is how WEC-Sim uses coupled bodies so I thought it was most analogous to my situation except the floater is physically modeled. The information I would then pass to my driver script is coordinates of the floater, and the script would then return tension at the fairlead.

Or, would I remove the bodies section entirely, and just specify “Coupled” attachment for the fairlead? The information I would then pass to the driver script is, upon transformations, coordinates of the fairlead, and the script would return corresponding tension.

Are these scenarios equivalent? Or is does one make more sense and is preferred? Any insight would help, thanks!

Best,
Elaine

[Ryan.Davies]

Hi Elaine,

Your approach looks correct, just note that the point XYZ are defined relative to the body position if the attachment is body#. This approach will return the 6 DOF forces and moments on the body center. Alternatively, you can do the other approach you suggested. They will be equivalent if you have the fairleads rigidly attached to the device (as MoorDyn treats the body as a rigid object). The reason we set it up the way we did was that it allows any custom mooring system to be built with WECSim+MoorDyn, and allows for multiple WECs with shared moorings to be simulated.