Questions regarding floating platforms


For the last few weeks I have been trying to create a really simple model of a floating platform in OpenFAST. Nevertheless, I have been having quite a lot of issues with its buoyancy and mooring system.

I just want to check the effect that sea currents have on the yaw rotation of the structure, without time series, without waves, just a constant/static wind and current speed in different directions (without even considering different current speeds based on the water depth). Thus, I was wondering if it would be possible to create a model of the structure without having to worry about its buoyancy and not having to import a MoorDyn or MAP file, (while keeping the PotMod variable set to 0). For example, to block the movement of the lowest nodes of the structure on certain directions (through the SubDyn file) while allowing its yaw rotation? Please check the attached picture to understand this concept better.

For instance, I could lock all the displacements and rotations except the rotZ of the lowest node at the point 1. And block only the displacement in the Z direction of all the other nodes (2, 3 and 4) to simulate that the platform is “floating” and that it can rotate around the point 1.

On another note, I was wondering if it is possible to import different aerodynamic and hydrodynamic porperties (lift and drag coefficients mainly) to certain members in the HydroDyn file to simulate that they have a non cylindrical shape?

Thanks, I hope you understood my questions,

King regards.

Dear @Pere.Palacin,

You mention SubDyn, but I expect the degrees of freedom (DOFs) you’ll want to change are in ElastoDyn. When modeling floating systems in OpenFAST with SubDyn enabled, the rigid-body modes are solved in ElastoDyn (via the 6 platform DOFs). In SubDyn, the Guyan modes correspond to the rigid-body modes (again solved in ElastoDyn) and the Craig-Bampton modes represent the elasticity of the structure relative to the rigid-body motion.

If you want to eliminate hydrostatics, that means to me that you should eliminate the heave, roll, and pitch DOFs, i.e., by setting PtfmHvDOF = PtfmRDOF = PtfmPDOF = False in ElastoDyn.

It is not currently possible to model noncylindrical members in HydroDyn, although, we do have plans to address this limitation by introducing support for regular cross sections in a future release of OpenFAST.

Best regards,