Dear all,
I am running a subset of formal DLC simulations on the OC4 DeepCWind semisubmersible platform with the 5MW offshore reference turbine, including computation of the sub-structural dynamics with SubDyn. More precisely, I am running DLC’s 1.2, 1.6, 6.1 and 6.3 for the Buchan Deep site. Most of them run without any problems, but for a specific extreme event modeled in 6.3, the simulation is aborted.
Firstly, after some previous inquiries on a different forum topic on how to model the floating platform in SubDyn, I followed the guidelines and verified the SubDyn implementation against the OC4 certification test #25 and got a very close match between cases with and w/out SubDyn enabled (see picture below, blue line is w/out SubDyn, orange line is w/ SubDyn), therefore I would suspect my implementation in SubDyn is correct.
Now, moving on to my problem, I am trying to simulate DLC 6.3 with the following conditions:
- Turbsim EWM 1yr (mean Vhub = 40m/s)
- JONSWAP sea with Hs = 7.8 m, Tp = 12.4 s, peak shape factor = 2.24
- Aligned wind and waves, no currents
- Fixed yaw angle of ± 20 deg
After consulting posts related to simulating a parked rotor on this forum, I set the following (initial) conditions for the simulation:
- elastodyn[‘RotSpeed’] = 0 # [rpm] parked rotor
- elastodyn[‘BlPitch(1)’] = 90 # [deg] pitch-to-vane
- elastodyn[‘BlPitch(2)’] = 90 # [deg] pitch-to-vane
- elastodyn[‘BlPitch(3)’] = 90 # [deg] pitch-to-vane
- elastodyn[‘GenDOF’] = False
- elastodyn[‘YawDOF’] = False
- elastodyn[‘NacYaw’] = -20 # [deg] fixed nacelle yaw
- elastodyn[‘PtfmSurge’] = 5.0 # [m] initial platform surge
- elastodyn[‘PtfmSway’] = -5.0 # [m] initial platform sway
- elastodyn[‘PtfmPitch’] = 2.0 # [deg] initial platform pitch
- aerodyn[‘WakeMod’] = 0
- aerodyn[‘AFAeroMod’] = 1
- servodyn[‘PCMode’] = 0
I am running this simulation twice, with different wind- and wave seeds for each simulation. The first simulation (sim0) runs without errors (see below). Could you please confirm that the platform motions and turbine performance indicators are as expected for DLC 6.3?
The second simulation (sim1) however, stops just after 40 seconds:
The error message from OpenFAST reads the following:
FAST_Solution:CalcOutputs_And_SolveForInputs:SD_CalcOutput:Small angle assumption violated in
SUBROUTINE SmllRotTrans() due to a large UR_bar input angles. The solution may be inaccurate.
Simulation continuing, but future warnings from SmllRotTrans() will be suppressed.
Additional debugging message from SUBROUTINE SmllRotTrans():NaN detected at time 40.1966346153846 in Line 3 in MoorDyn
.
NaN detected at time 40.197 in MoorDyn.
NaN detected at time 40.197 in MoorDyn.FAST_Solution:FAST_AdvanceStates: NaN state detected.
OpenFAST encountered an error at simulation time 40.188 of 1000 seconds.
Simulation error level: FATAL ERRORAborting OpenFAST.
Indeed, when I plot the results from t=38 to t=40, an instability is observed, which as it seems to me originates from the fairlead tension:
I am puzzled as to how this could occur. The other 51 simulations ran without errors. I have tried decreasing the time step for this simulation from 0.0125 to 0.0025 but this didn’t solve the problem. When I re-run the same simulation (sim1) with SubDyn disabled, it does terminate normally:
Does this mean that the SubDyn module is set up incorrectly?
I would truly appreciate your help, as I have been unable to find a solution by my own efforts.
Please let me know in case you need me to share the OpenFAST files to verify the inputs.