I was just wondering what is the easiest way to implement a platform in HydroDyn when the platform/tower connection is above the mean free water surface. Is it possible to set the TwrDraft in the HydroDyn input file to for instance -3, if this is the deck clearence of the platform, or is the correct way to do it to set this parameter to 0 and then change the tower properties in the tower input file to also account for the part of the platform that is above the water surface?
To set the platform/tower connection above the still water level, you should set TwrDraft to a negative number. This will decrease the flexible tower length, such that the tower base is ABS(TwrDraft) above the still water level.
Is what you’re saying that the tower input file should not be changed because the code is programmed in such a way that setting TwrDraft to a negative number automatically cuts off a part of the tower? The way I did it was to also change the tower file and selecting a new number for TowerHt in the FST file. If the total length of the tower originally is 100 m (from MSL to hub height), and TwrDraft is -5 m, I also change the tower file such that tower length is 95 m. TowerHt in the FST file is set to 95 m. The mass of the platform that is above the MSL is then included in the platform mass matrix. I suspect that the approach I’ve been using might be wrong.
The tower in FAST is flexible between the tower base and tower top. The tower base is located a distance of TwrRBHt - TwrDraft above the MSL. The tower top is located a distance of TowerHt above the MSL. So, the flexible length of the tower equals TowerHt - TwrRBHt + TwrDraft. The distributed tower properties (mass per unit length, bending stiffness, etc.) in the tower input file are defined with regards to the nondimensional flexible tower length, with the tower base at 0.0 and the tower top at 1.0. (Please note that all of these distances are for the undeflected system; turbine deflections will change these throughout the simulation.)
I hope that clarifies things.