I am trying to model a floating wind turbine with a platform consisting of four circular horizontal members which sit at the water surface. A portion of each member is below the water surface, and a portion is above the water surface, as illustrated below. Therefore the initial submerged volume of the platform is less than the overall structure volume.
This is an initial model, so I am just using the strip theory method, with no Potential Flow input. The model runs without errors, however it appears that HydroDyn assumes that the whole of member is either submerged (in which case the submerged volume = structure volume and my buoyancy forces are too high) or out of the water (in which case the submerged volume = 0), depending on whether zi is below or above the still water surface. This means that my initial static buoyancy forces do not balance.
I note that when using the potential flow solver you can specify the initial displaced volume of the water (PtfmVol0), but am unable to find anything similar for the strip-theory member definiition. Is it possible to specify that the horizontal members are surface piercing, or set the initial buoyancy force to give the correct initial platform position? Or would I have to apply some fill / ballast load in order to counteract the fact the external buoyancy force is too high? I have used values of undisplaced waterplace area (A0) and undisplaced volume of the platform (V0) in the definition of the hydrostatic restoring matrix, but this does not appear to make a difference to the initial static buoyancy.