Simulation going unstable in extreme conditions

Hi everyone,

I am trying to simulate a spar-type FOWT (my own model) in extreme wind and waves, with the rotor idle. I have been able to run the model in milder conditions but when I try to run it in wind speeds >20m/s and Hs >5m the simulation crashes within the first 2 minutes of simulation time, and I get the following error message:

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: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_TSF
FWind_CalcOutput [position=(29.048, -44.23, 5.4457) in wind-file coordinates]: FF wind array
boundaries violated: Grid too small in Y direction. Y=-44.23; Y boundaries = [-40, 40]
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 29.345 of 3660 seconds.
Simulation error level: FATAL ERROR

Aborting FAST.

I have turned off BEMT and unsteady aerodynamics, I have added a correction step, I have tried reducing DT and DT_UJac and I have tried turning off AeroDyn altogether but they have not helped. After playing around with the ElastoDyn settings I have found that the problematic degrees of freedom appear to be PtfmPitch and PtfmRoll, so I’m guessing that the problem is related to hydrostatics, but I am not sure how to solve the problem without altering the platform model significantly. Does anyone have any suggestions?

Thank you, any help will be greatly appreciated.

Best regards,
Rachael

Dear Rachael,

I agree with your assessment and with the steps you’ve taken to solve the problem so far. Indeed, it sounds like the system spar-type FOWT is hydrostatically unstable, or at least susceptible to excessive motion in large waves. Have you calculated the natural frequencies of the platform in FAST (e.g., via free-decay test simulations) and are the natural frequencies what you expect? Are the platform-pitch and roll natural frequencies sufficiently far away from the first-order wave frequencies / periods (0.05 - 0.5 Hz; 2-20 s)?

Best regards,

Dear Jason,

Thank you for your reply. Yes, I have done free decay tests in FAST and the pitch and roll natural frequencies are around 24 s, although I did have to increase C44 and C55 in the AddCLin matrix to achieve this desired value. I am using a Jonswap spectrum with a wave period Tp = 12.5 s.

Best regards,
Rachael

Dear Rachel,

Do you have second-order hydrodynamic terms enabled in HydroDyn? Perhaps the platform-pitch mode is being excited by second-order effects?

Best regards,

Dear Jason,

No, I have not enabled second order hydrodynamics. However, when I ran the model in BModes to obtain the tower mode shapes for FAST, the roll and pitch natural frequencies were 0.135 Hz, which doesn’t correspond to the natural periods from the free decay tests in FAST but might correspond with first order wave frequencies. I’ve checked my inputs in BModes and am fairly confident that they are correct, but do you know of any reason why there might be this difference?

Best regards,
Rachael

Dear Rachael,

I would expect that BModes and FAST would predict very similar natural frequencies. Have you properly accounted for the gravitational restoring in the BModes solution (because gravitational restoring is not intrinsically accounted for in BModes, you must manually add it through a stiffness matrix)?

Best regards,

Dear Jason

Yes, I input the gravitational restoring values into the stiffness matrix (hydro_K) in BModes. I calculated values of -77559000 for pitch and roll restoring based on a displaced volume of 389.89 m3 and a centre of buoyancy 19.79 m below the mean sea level, and 44531 for heave restoring based on a waterplane area of 4.43 m2.

Best regards,
Rachael

Dear Rachael,

I would expect that you’d specify a positive value for the pitch and roll stiffness. I agree that the buoyancy term you’ve calculated is negative-valued, but I would expect an equally large (or larger) positive value from the gravitational restoring associated with the full system weight and center of mass (-massgz_CM).

Best regards,

Dear Jason,

Thank you for the advice, I had not realised that gravitational restoring needed to be included in BModes. The natural frequencies from BModes now agree with those from FAST, around 0.04 Hz. I have corrected the tower mode shapes, but the FAST model is still going unstable. Sorry for the trouble, but do you have any more suggestions?

Best regards,
Rachael

Dear Rachael,

I guess I would start by computing the response amplitude operators (RAOs) via FAST. Perhaps large excitation is expected, even though the natural frequencies are outside the wave-excitation range? We’ve published a paper on how to compute RAOs via FAST–see: nrel.gov/docs/fy13osti/58098.pdf.

Best regards,

Dear Jason,

Thank you for your advice. I have attached the RAOs computed for roll and pitch. It looks like there is some excitation around 0.05 Hz, though I am not sure why.

Best regards,
Rachael


Dear Rachel,

Have you compared the FAST-generated RAOs with the RAOs generated via WAMIT? Do they both predict the response at 0.05 Hz?

Best regards,