Mooring line failure for OC4 semi-submersible

Wind & Water Computer-Aided Engineering Software Tools
1

Dear All,

I am trying to simulate the mooring failure condition for NREL 5MW supported on OC4 semi-submersible. Failure of mooring line 1 (ML1), as shown in figure below, is modelled in OpenFAST.

ML

Simulation is performed for 50-year extreme wind and wave condition (Wind speed = 47.5 m/s, significant wave height = 10.70 m, peak period = 14.2 s). The simulation is aborting with the below error message:

FAST_Solution:FAST_AdvanceStates:ED_ABM4:ED_CalcContStateDeriv:SetCoordSy:Small angle assumption violated in SUBROUTINE SmllRotTrans() due to a large platform displacement (ElastoDyn SetCoordSy). The solution may be inaccurate. Simulation continuing, but future warnings from SmllRotTrans() will be suppressed.

Additional debugging message from SUBROUTINE SmllRotTrans(): 580.89 s

FAST_Solution: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.

As parked condition is simulated, the BEM and the unsteady aerodynamics is disabled in the simulation. I have tried running the simulation with two different time steps of 0.0125 s and 0.005 s. I have also tried running the simulation by removing the second order wave forces but still similar errors are appearing. From the related post for similar error addressed in the forum I understand that there is some numerical instability in the substructure. The natural frequencies of the OC4 semi-submersible are not close to the peak wave period. I would like to mention that simulation ran without any issues when the failure of mooring line 2 (ML2) is considered. Does anyone have any suggestions to resolve this error?

Regards
Ramya

2

Dear @Ramya.Niranjan,

This error can be generated both by a numerical instability, as is often the case, and by physical conditions such as a physical instability (negative damping) or extreme situations (loads that exceed the resistance). I’m guessing it is latter (extreme situation) that is driving the error your case. You can confirm this by plotting the time series of platform rotation (instabilities are characterized by an exponential increase in response over time). If that is the case, it could mean that this condition is just too extreme for OpenFAST to model accurately.

Best regards,