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.
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?