Large displacement

Dear Dr. Jonkman

I’m trying to do an equilibrium test on a floating scaled wind turbine on a triple spar platform but as soon as I turn on the roll and pitch DOFs in ElastoDyn the fast aborts with the following error:

FAST_Solution:FAST_AdvanceStates:ED_ABM4:ED_CalcContStateDeriv:SetCoordSy:Small
angle assumption violated in SUBROUTINE SmllRotTrans() due to a large platform displacement (Elas
toDyn SetCoordSy). The solution may be inaccurate. Simulation continuing, but future warnings from SmllRotTrans() will be suppressed.
Additional debugging message from SUBROUTINE SmllRotTrans(): 10.275 s
FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption1:ED_HD_InputOutputSolve
:HydroDyn_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4 radians.
ED_HD_InputOutputSolve:HydroDyn_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4 radians.

FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption1:ED_HD_InputOutputSolve
:HydroDyn_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4 radians.
ED_HD_InputOutputSolve:HydroDyn_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4 radians.

FAST encountered an error at simulation time 10.55 of 50 seconds.
Simulation error level: FATAL ERROR

Aborting FAST.

I checked all the weights and they are correct. The total weight of the platform and the turbine is almost equal to the buoyancy force. The AeroDyn is turned off. Could you please help me with finding the problem?

Best regards
Masih

Dear Masih,

Your model appears to be either hydrostatically or numerically unstable in one of the platform-rotation degrees of freedom (DOFs) e.g. roll, pitch, or yaw. Is the metacentric height for your system positive? I suggest that you eliminate DOFs to debug e.g. is your model hydrostatically stable with only the platform-surge and -heave DOFs? What happens when you add the platform-pitch DOF?

Best regards,