Floating Turbine in Extreme Events with SubDyn enabled

Wind & Water Computer-Aided Engineering Software Tools
10

Dear Jason,

Thank you for your kind help.

I replaced the virtual members by rigid links, but it unfortunately didn’t help. I think the root of the problem is somewhere else. So I went a few steps back to try and find the root of the instability problem.

For the moment I am interested only in the rigid body case ( Nmodes=0 ).
Starting from the certification case #25, I tried to add the SubDyn module to the simulation. I did so by taking the following steps:

In the main Fast file:

  • CompSub = 1
  • NumCrctn = 1
  • DT_UJac = 2.7027
  • UJacSclFact = 1.4E+07
  • DT = 0.0125

In the SubDyn file:

  • FEAMod = 3
  • NDiv = 1
  • CBMod = True
  • Nmodes = 0
  • JDampings = 1
  • GuyanDampMod = 0
  • SttcSolve = True
  • GuyanLoadCorrection = True

In the ElastoDyn file:

  • PtfmMass = 0
  • PtfmRIner = 0
  • PtfmPIner = 0
  • PtfmYIner = 1.638890E+06

Following your previous advise, I set the platform Y inertia equal to the tower rotational inertia about its centerline, which I calculated to be the value mentioned above. The structural members are defined as in the picture from my previous post, where the red ‘virtual’ members have been defined as rigid links with a mass density of 1 [kg/m]. When I run the model, I run into NaN states within 1 second:

Time: 0 of 1000 seconds.

FAST_Solution:FAST_AdvanceStates:SrvD_UpdateStates:DLL_controller_call:BladedInterface option was
designed for an explicit-loose coupling scheme. Using last calculated values from DLL on all
subsequent calls until time is advanced. Warning will not be displayed again.

FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption1:FullOpt1_InputOutputSolve: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():

Warning: SkewedWakeCorrection encountered a large value of chi (91.351 deg), so the yaw
correction will be limited. This warning will not be repeated though the condition may persist.
See the AD15 chi output channels, and consider turning off the Pitt/Peters skew model (set
SkewMod=1) if this condition persists.

NaN detected at time 0.729326923076923 in Line 1 in MoorDyn
.
NaN detected at time 0.729326923076923 in Line 2 in MoorDyn
.
NaN detected at time 0.729326923076923 in Line 3 in MoorDyn
.
NaN detected at time 0.72981 in MoorDyn.
NaN detected at time 0.72981 in MoorDyn.

FAST_Solution:FAST_AdvanceStates: NaN state detected.

OpenFAST encountered an error at simulation time 0.725 of 1000 seconds.
Simulation error level: FATAL ERROR

Aborting OpenFAST.

I have tried different approaches to resolve the instability and noticed the following behaviour:

  • when I set all the RNA and tower DOFs (except the generator) to False, it runs without problems. However, as I am interested in the internal loads in the platform, this is not a desired solution because they are likely inaccurate without turbine/tower flexibility
  • when I increase the platform inertia’s in ElastoDyn, the model runs without problems. However, this might lead to unrealistic platform motions as the total inertias are overestimated.

Please, could you share your insights on this matter? I could share the files in case you’d need to review them.

Lastly I was wondering if setting appropriate values for DT_UJac and UJacSclFact is absolutely necessary when Nmodes = 0, because it would save me some precious CPU time.

Your help is much appreciated!
Best,