Simulation errors of MIT NREL TLP (Test 23)

Dear all,

I was trying to run the simulation using Test 23 files and got some problems.

I used the environmental condition used for model validation in nrel.gov/docs/fy10osti/45891.pdf (Denis’s PhD thesis): 11 m/s hub-height wind speed, 10-m significant wave height and Tp of 17.64s. Based on the inital files, I made the following modifications:

In ElastoDyn, I disabled all the other DoFs except for the last six for the platform motion. RotSpeed was set to zero and BlPitch(i) were set to 90 in order to model the parked turbine state.
In HydroDyn, I changed WaveHs to 10 and WaveTp tp 17.64 and WaveDir to 0.
In AeroDyn, I raised the first aerodynamic node to 10m above the SWL and set WakeMod to zero.
In ServoDyn, I turned off the generator.
Besides, I extend the simulation time in fst file and HydroDyn file to 10800 s (3h) and increased Dt to 0.25.

I got the following error messeges:
FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption1:ED_HD_InputOutputSolve:HydroDyn_CalcOutp
ut: 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:SolveOption2:InflowWind_CalcOutput:CalcOutput:IfW_Uni
formWind_CalcOutput:GetWindSpeed:Height must not be negative.
IfW_UniformWind_CalcOutput: Error calculating the wind speed at position (-4.31086E+09,
2.42859E+09, -4.99252E+09) in the wind-file coordinates
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 7.75 of 10800 seconds.
Simulation error level: FATAL ERROR

Aborting FAST.

I think FAST is capable of simulating the model under the environment I used since this has been done according to the previous published work. Could you give any suggestions about correcting the input files? I would appreciate your kind help.

Best regards,
Yajun

Dear Yajun,

Increasing the time step to DT=0.25 s is likely leading to a numerical instability in the FAST solution. While you have disabled structural DOFs, one or more of the remaining 6 DOFs platform frequencies is likely high enough that a smaller time step is required. My rule of thumb for selecting the time step for the structural module is given in the following forum topic: http://forums.nrel.gov/t/fast-integrator-step-size-precision/247/3.

Best regards,

Dear Dr Jonkman,

I am running the MIT/NREL TLP model with the input file relating to Test 23. Something I could not explain is that the heave motion of the platform under regular waves (without wind) shows two periods, one is wave period and another one is half of wave period (as shown in the attached figure). I am wondering if this is correct or not?

Best regards,
Yajun

Dear Yajun,

Can you clarify what wave period you are simulating and plot the surge motion as well?

Due to the taut tendons of TLPs, TLPs have unique behavior in that the heave motion will have twice the frequency as the surge motion when excited by regular waves. That is, positive surge motion will result in a downward heave motion and negative surge motion will also result in downward heave motion because of the taut tendons. Is that what you are referring to?

Best regards,

Dear Dr Jonkman,

Thank you very much for your timely help. The wave period was set to be 12 s and the surge motion is plotted in the figure attached.

Yes this is exactly what I was referring to and your explanation is convictive for me.
YajunMIT-NREL TLP surge.JPG

Best regards,