I have a question on the platform DOFs that I’m hoping to get some help on.

It’s discussed in the SubDyn User Manual that to avoid numerical issue, additional damping in the platform heave DOF may be needed in the HydroDyn input file by specifying the AddBLin(3,3) term which can be calculated by Eq.1.

I’m modeling a monopile-supported fixed bottom offshore wind turbine. The platform is defined in ElastoDyn as follows: PtfmMass=0, PtfmRIner=0, PtfmPIner=0, PtfmYIner=total rotational inertia of the undeflected tower about its centerline. The AddF0, AddCLin, AddBLin, and AddBQuad terms are all zero in the HydroDyn input file except for AddBLin(3,3) term representing zeta=0.01 damping for platform heave. The Guyan reduction is used in the SubDyn module.

Running OpenFAST v2.3.0 with a time step of 0.01s (fixed by the controller DLL), I observe the following:

- Platform heave still grows unbounded unless zeta is increased to around 0.05.
- Platform yaw also experiences instability. I do not know how to properly add damping for platform yaw, so as an attempt, I used Eq.1 and simply substituted M_SD_3,3 and K_SD_3,3 by M_SD_6,6 and K_SD_6,6. Using AddBLin(6,6) that corresponds to zeta=0.01, the platform yaw instability goes away.

My questions are:

- Can Eq.1 be extended to platform DOFs other than platform heave, or should different formulas be used?
- Do the diagonal terms of AddBLin, while useful for removing instability, impact the dynamics of the support structure? Is there a recommended threshold? Is the highest damping I’ve gone to i.e. 5% too big so as to significantly alter the behavior of the model away from the actual model?
- Following the above, if I’m limited both by the time step size and by a AddBLin threshold, are there additional ways to address platform instability?

Thank you in advance,

Jing