Hi Jason,

Recently I am working the openfast with the IEA-15MW wind turbine atop with monopile foundation, and the version of openfast is 3.1.0, the IEA-15MW wind turbine is version 1.1.2. when I read the ElastoDyn file, I realized both the DrTrDOF and YawDOF are disable by default. Since turbulent winds are essential for the following study, I turned YawDOF on for a simple test. hence, I turn YawDOF on and set the step size to 0.0025 or even smaller, however, some error occured and the message in the command as followings:

**Running InflowWind.**

**Running HydroDyn.**

** Generating incident wave kinematics and current time history.**

**Running SubDyn.**

** Fixed bottom case detected**

** Performing Craig-Bampton reduction 60 DOFs → 0 modes + 6 DOFs**

** Using static improvement method for gravity and ext. loads**

** Calculating Full System Modes for output files**

** Exporting Summary file**

**Running ServoDyn.**

**Running ServoDyn Interface for Bladed Controllers (using Intel Visual Fortran for Windows, ).**

**Using legacy Bladed DLL interface.**

**------------------------------------------------------------------------------**

**Running ROSCO-v2.5.0**

**A wind turbine controller framework for public use in the scientific field**

**Developed in collaboration: National Renewable Energy Laboratory**

** Delft University of Technology, The Netherlands**

**------------------------------------------------------------------------------**

**FAST_Solution:FAST_AdvanceStates:ED_ABM4:ED_AB4:ED_RK4: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(): 2.50000E-03 s**

**FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption1:FullOpt1_InputOutputSolve:SD_CalcOutput:**

**Angles in GetSmllRotAngs() are larger than 0.4 radians.**

**SD_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4 radians.**

**FullOpt1_InputOutputSolve:HydroDyn_CalcOutput:HDOut_MapOutputs: Angles in GetSmllRotAngs() are**

**larger than 0.4 radians.**

**FullOpt1_InputOutputSolve:SD_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4 radians.**

**SD_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4 radians.**

**FullOpt1_InputOutputSolve:HydroDyn_CalcOutput:HDOut_MapOutputs: Angles in GetSmllRotAngs() are**

**larger than 0.**

**Warning: SkewedWakeCorrection encountered a large value of chi (-161.47 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.**

**The BEM solution is being turned off due to low TSR. (TSR = -3.89171E-02). This warning will not**

**be repeated though the condition may persist. (See GeomPhi output channel.)**

**FAST_Solution:FAST_AdvanceStates:AD_UpdateStates:BEMT_UpdateStates(node 3, blade**

**1):UA_UpdateStates:UA_UpdateDiscOtherState:ComputeKelvinChain:Mach number exceeds 1.0. Equations**

**cannot be evaluated.**

** OpenFAST encountered an error at simulation time 2.50000E-03 of 300 seconds.**

** Simulation error level: FATAL ERROR**

** Aborting OpenFAST.**

I made a lot of attempts to address the above errors. At first, I looked it up and found that it needed to set PtfmYIner to a specific value if enable YawDOF. So I set an initial value to PtfmYIner, and the error did disappear. However, https://github.com/IEAWindTask37/IEA-15-240-RWT/blob/master/ReleaseNotes.md says that PtfmYIner zeroed out because transition piece is accounted for in SubDyn. It seems that it is not necessary to set a value to the PtfmYIner.

I was confused,and I can’t solve that by myself, hoping you could give me some suggestion.

Best regard,

Yiqi