Dear Jason,
Thank you for clarifying.
I have one follow-up question on the flexible model, and one minor question related to the physical platform properties:
- I started testing the flexible model. I have tried different values for
Nmodes, but my findings are thatNmodes = 5is the maximum number that I can choose to avoid the simulation becoming numerically unstable. From 6 or higher, the simulation crashes after about 0.5 seconds, with a number of error messages (see below). I suspect (although I’m not exactly sure) they originate again from the moordyn module, as in some cases, NaN states are detected (which is the problem I experienced previously)
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():
Warning: SkewedWakeCorrection encountered a large value of chi (93.482 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.FAST_Solution:FAST_AdvanceStates:ED_ABM4:ED_CalcContStateDeriv:SetCoordSy:Small angle assumption violated in SUBROUTINE SmllRotTrans() due to a large blade deflection (ElastoDyn SetCoordSy). The solution may be inaccurate. Simulation continuing, but future warnings from SmllRotTrans() will be suppressed.
Additional debugging message from SUBROUTINE SmllRotTrans(): 0.56375 s
The BEM solution is being turned off due to low TSR. (TSR = -1.5559). This warning will not be
repeated though the condition may persist. (See GeomPhi output channel.)FAST_Solution:FAST_AdvanceStates:AD_UpdateStates:BEMT_UpdateStates(node 18, blade 1):UA_UpdateStates:UA_UpdateDiscOtherState:ComputeKelvinChain:Mach number exceeds 1.0. Equations cannot be evaluated.
OpenFAST encountered an error at simulation time 0.56375 of 100 seconds.
Simulation error level: FATAL ERROR
Aborting OpenFAST.
The plots of the platform motion and turbine KPI’s show the unstable behavior:
These plots are for DLC 1.2 with a mean wind speed of 11 m/s, but similar errors/behaviour is observed across the whole range of wind speeds / wave heights.
I have tried reducing the time steps of both OpenFAST and MoorDyn, without success. In a previous reply to a different post, you mentioned that Nmodes should be set “large enough to capture the lowest modes of the substructure, say up to 5-10 Hz, as calculated by SubDyn. This is typically less than 10-20 for most floating substructures.” When I inspect the Craig-Bampton modes in the JSON output file, the 5th mode has a frequency of about 2.4 Hz, which is apparently still too low. Do you have any suggestions as to how can I increase the number of modes in SubDyn while keeping the simulation stable?
Currently in SubDyn I use JDampings = 1 and NDiv = 1
- I am trying to compute the submerged volume of the floating platform based on the geometry specification in HydroDyn (Members, Member Joints and Member Cross-Section Properties). I do this simply by computing the volume of the submerged part of each member (discarding anything above SWL), and then taking the sum of all the volumes to get the total displaced volume of the platform. However, in doing so I end up with a total displacement volume of 13873 m3, which differs from the value reported in the OC4 definition report, which is 13917 m3. This is somewhat problematic, as I need the displacement volume to compute the vertical mooring pre-tension, and in the hydrostatic equilibrium equation, the difference in this buoyancy volume results in a considerable difference in the computed mooring pre-tension. If my understanding is correct, the displacement volume reported in the technical report is an output of the WAMIT simulation. This leaves me wondering what the mesh geometry has been for the WAMIT simulation, and if it is any different from the geometry specified in HydroDyn. When I inspect the geometry defined in HydroDyn for example (see first two pictures below), the connection points of the cross braces are different compared to figure 1-1 of the report (also shown below). Please, could you clarify if there is any discrepancy between the displacement volume reported in the OC4 definition document and the geometry defined in HydroDyn?
