OC4 semi submersible

Mustafa.Vardaroglu · February 15, 2019, 3:20pm

Sai.Chaitanya:

Dear Mr. Jonkman,

Thanks for your reply. I have done the corrections. Now I am getting the following error.

Generating incident wave kinematics and current time history.
Reading in WAMIT output with root name “.\5MW_Baseline/HydroData/SSB”.
Computing radiation impulse response functions and wave diffraction forces.

MAP++ environment properties (set externally)…
Gravity constant [m/s^2] : 9.81
Sea density [kg/m^3] : 1025.00
Water depth [m] : 200.00
Vessel reference position [m] : 0.00 , 0.00 , 0.00

Timestep: 0 of 3600 seconds.

FAST_Solution0:CalcOutputs_And_SolveForInputs:SolveOption2:SrvD_CalcOutput:Running with torque
and pitch control of the NREL offshore 5MW baseline wind turbine from DISCON.dll as written by J.
Jonkman of NREL/NWTC for use in the IEA Annex XXIII OC3 studies. Includes modifications for the
Hywind spar.
Timestep: 8 of 3600 seconds. Estimated final completion at 15:18:07.

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 (-7.95252E-02, -22.145,
-8.80057E-02) in the wind-file coordinates

FAST encountered an error at simulation time 8.725 of 3600 seconds.
Simulation error level: FATAL ERROR

Aborting FAST.

I am unable to figure out which exact parameter should I change to increase the elevation of the first tower aerodynamic node in the AeroDyn file.
Please help.

Thank you.

Dear Sai Chaitanya,

I’ve got your mail. I am typing here, just to share with everyone.
AeroDyn.dat file contains a parameter “TwrElev”. TwrElev is the parameter that stores tower node elevations. InflowWind calculates the wind speed at TwrElev nodes.
In case of MIT NREL TLP, first tower node is defined at z=0 mt. When heave motion occurs, InflowWind can not calculate the wind speed in negative elevations so FAST aborts.
So Jason Jonkman advised me to slightly uplift the first tower node. And that worked with me. Here: Error calculating the wind speed at position

But with the Semi-submersible, Tower base node is defined at +10 m. (TwrElev =0, AeroDyn.dat file) So your case is different here. But your FAST abort message is the same with my case that indicates a submerged tower base. I think at time step 8, heave is more than 10 m so tower base is below the still water level where InflowWind alerts you and FAST aborts.

This discussion might be helpful where there is also a excessive heave motion.

I am also new to FAST and the community. I hope what I’ve experienced helps.

Sincerely,

Mustafa