HydroDyn strip theory implementation - can I specify surface piercing horizontal members?


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.

Thank you,

Dear Leah.Ewart,

The strip-theory solution in HydroDyn is not as advanced as you are assuming. The strip-theory solution in HydroDyn cannot really be used for horizontal members partially submerged at the free surface. The initial displaced volume of the strip-theory members assumes that the entire cylindrical volume between the start and end node (or the location where the member centerline crosses the still water line) is submerged. Thus, the strip-theory solution in HydroDyn should really only be used for members near the free surface that are (or are near) vertical.

To model a horizontal pontoon partially submerged at the free-surface, I would recommend using the potential-flow solution of HydroDyn instead.

Best regards,

Thanks Jason for your reply

I thought that was probably the case, but I just thought I would ask the question in case I was missing something.

I will look at the potential flow solution instead.

Thank you,