I am recently working on developing my own code to simulate the OFWT. Since I am in the initial stage, I am considering to pre-calculate the aerodynamic loads using FAST, and then translate these loads to the platform CG so that I can merely deal with the platform itself and consider the wind-induced loads in the RNA and tower as external loads in the platform.
Therefore, here is my question: when I set the FAST simulation in a non-platform mode, can I use the output “Tower Base Loads” in 6 DOF as the direct loads translated from all the upper structure apart from the platform itself ?
I also have another question now. Using FAST, I see that the 6 DOFs of the platform can be chosen to be ON or OFF. I wonder what is the rationale behind the limitation of the DOFs of the motion. Is there some external forces loaded in the system to limit the certain modes of motion? Or do you just simplify the equations to omit certain modes of motion?
I would appreciate much if you can explain this to me. Thanks.
Dear Yu Ma,
“Yes,” the tower-base load outputs from FAST (TwrBsFxt through TwrBsMzt) include all of the loads applied to the platform that are transmitted from the wind turbine and tower. These will include the applied aerodynamic loads, as well as the inertial and gravity loads of the wind turbine and tower.
Disabling a degree-of-freedom (DOF) in FAST really means that the acceleration will be zero, velocity will be constant, and displacement will vary linearly (if the constant velocity is nonzero). Inside FAST, the equation of motion for the disabled DOFs are not formulated, resulting in no acceleration of those DOF. For the platform DOFs, unless you’ve changed the source code, the initial velocities are always zero, so, if one disables a platform DOF, then the displacement of that DOF is fixed. When modeleling a floating wind turbine, one may only want to disable platform DOFs for model-debugging purposes.
Please note, however, that the direct aero-elastic interaction of the floating wind turbine and aerodynamic loads may be quite important. The uncoupled modeling approach that you are proposing may not give you accurate results.