Modeling a spar in FAST (ElastoDyn & HydroDyn)

Hello everybody :slight_smile:
I have tried to model a spar in AQWA which I would now like to model in FAST to have a coupled analysis.
I have gone through HydroDyn manual and there are few questions in my mind:

  1. if my spar is ballasted, does the variable PtfmMass in ElastoDyn include the mass of ballast or is it just the mass of the hollow spar? If it includes the mass of the ballast, should I leave the field ‘Filled Members’ in HydroDyn input file empty?
  2. A question regarding ‘PtfmCMzt’. Do I take into account a moored scenario or an unmoored structure while describing the vertical distance of the spar CM from MSL? (an unmoored scenario will have a lower negative value)

thank you for your help.
Best regrads,

Hi Tiasa,

Here are my answers to your questions:

  1. You can place the ballast in either ElastoDyn or HydroDyn. If you place the ballast in HydroDyn, you should do so through the filled members of the strip-theory; otherwise, set NFillGroups = 0. However you choose to model the ballast will impact how you set the platform mass, center of mass, and inertias in ElastoDyn.

  2. PtfmCMzt, as well as other platform properties, is specified in the body-fixed coordinate system, so, it shouldn’t matter whether it is moored or unmoored. In general, though, I recommend setting up the model such that the vertical equilibrium is maintained, i.e., the FOWT weight balances with the static buoyancy and vertical mooring pretension.

Best regards,