I am in the process of designing a V-shaped tripod semi-submersible wind turbine with the tower located on the first side column. I currently have some confusion regarding the reference points and the center of gravity in both HydroDyn and ElastoDyn.
Following your previous responses, I have set the WAMIT hydrodynamic reference point and the platform center of gravity to the origin (0,0,0) in my HydroDyn modeling. The intersection of the tower centerline with the horizontal plane is also at point 0. I adjusted the COG in ElastoDyn to (51.9, 0, 2.21) and adjusted the COB in HydroDyn, and the mooring line position in MoorDyn is set to match the HydroDyn model. However, after making these adjustments, I encountered the following errors. Could you please advise on how to correct them? Attached are the relevant configuration screenshots for your reference.
Best regards,
FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption1:ED_HD_InputOutputSolve:HydroDyn_CalcOutp
ut: Angles in GetSmllRotAngs() are larger than 0.4 radians.
HydroDyn_CalcOutput:HDOut_MapOutputs: Angles in GetSmllRotAngs() are larger than 0.4 radians.
ED_HD_InputOutputSolve:HydroDyn_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4
radians.
HydroDyn_CalcOutput:HDOut_MapOutputs: Angles in GetSmllRotAngs() are larger than 0.4 radians.
NaN detected at time 77.0775000000000 in Line 3 in MoorDyn
.
NaN detected at time 77.0775000000000 in Line 4 in MoorDyn
.
NaN detected at time 77.078 in MoorDyn.
NaN detected at time 77.078 in MoorDyn.
FAST_Solution:FAST_AdvanceStates: NaN state detected.
OpenFAST encountered an error at simulation time 77.075 of 600 seconds.
Simulation error level: FATAL ERROR
Well, your model is numerically or hydrostatically unstable, but you haven’t provided enough information for me to understand why. If you are not already, I would suggest simplifying the model to debug, e.g., disable all structural degrees of freedom (DOFs) in ElastoDyn except for the 6 platform DOFs, disable aerodynamics, and use still water in HydroDyn. What happens then?
Perhaps the following forum topics are of use to you?:
Dear Dr. Jason.
I listen to all your advice, I disable all structural degrees of freedom (DOFs) in ElastoDyn except for the 6 platform DOFs, disable aerodynamics, and use still water in HydroDyn. But there are still some questions:
(1) I find that the platform itself is not balanced, the result converges on the pitch Angle, but diverges on the roll and yaw, what causes this?
(2) After opening moordyn, the platform became more unstable. Perhaps the position of the mooring line needs to be shifted?
I ran twice, turning moordyn on and off. Here are my Settings, results and schematic diagram of platform and mooring line locations. Can you tell me what the problem is?
Best regards
elastodyn
144.386 TowerHt - Height of tower above ground level [onshore] or MSL [offshore] (meters)
15 TowerBsHt - Height of tower base above ground level [onshore] or MSL [offshore] (meters)
52.26 PtfmCMxt - Downwind distance from the ground level [onshore] or MSL [offshore] to the platform CM (meters)
0 PtfmCMyt - Lateral distance from the ground level [onshore] or MSL [offshore] to the platform CM (meters)
2.21 PtfmCMzt - Vertical distance from the ground level [onshore] or MSL [offshore] to the platform CM (meters)
0 PtfmRefzt - Vertical distance from the ground level [onshore] or MSL [offshore] to the platform reference point (meters)
hydrodyn
1 WAMITULEN - Characteristic body length scale used to redimensionalize WAMIT output (meters) [only used when PotMod=1]
0 PtfmRefxt - The xt offset of the body reference point(s) from (0,0,0) (meters) [1 to NBody] [only used when PotMod=1]
0 PtfmRefyt - The yt offset of the body reference point(s) from (0,0,0) (meters) [1 to NBody] [only used when PotMod=1]
0 PtfmRefzt - The zt offset of the body reference point(s) from (0,0,0) (meters) [1 to NBody] [only used when PotMod=1. If NBodyMod=2,PtfmRefzt=0.0]
0.0 PtfmRefztRot - The rotation about zt of the body reference frame(s) from xt/yt (degrees) [1 to NBody] [only used when PotMod=1]
22223 PtfmVol0 - Displaced volume of water when the platform is in its undisplaced position (m^3) [only used when PotMod=1; USE THE SAME VALUE COMPUTED BY WAMIT AS OUTPUT IN THE .OUT FILE!]
51.96 PtfmCOBxt - The xt offset of the center of buoyancy (COB) from the platform reference point (meters) [only used when PotMod=1]
0 PtfmCOByt - The yt offset of the center of buoyancy (COB) from the platform reference point (meters) [only used when PotMod=1]
moordyn
----------------------- LINE TYPES ------------------------------------------
Name Diam MassDen EA BA/-zeta EI Cd Ca CdAx CaAx
(-) (m) (kg/m) (N) (N-s/-) (-) (-) (-) (-) (-)
main 0.2 596.00 3.46E+08 -1.0 0 2 0 0.4 0.27
---------------------- POINTS --------------------------------
ID Type X Y Z M V CdA CA
(-) (-) (m) (m) (m) (kg) (m^3) (m^2) (-)
1 Vessel 0.00 0.00 -8.00 0 0 0 0
2 Fixed -400.00 0.00 -50.00 0 0 0 0
3 Vessel 77.94 45.00 -8.00 0 0 0 0
4 Fixed 277.90 391.41 -50.00 0 0 0 0
5 Vessel 77.94 -45.00 -8.00 0 0 0 0
6 Fixed 277.90 -391.41 -50.00 0 0 0 0
FYI: I’m not sure this is a problem, but I noticed that the coordinate system you drew is not a right-handed coordinate system (y is pointed in the opposite direction).
I changed the coordinate system to right-handed coordinate system and followed general modeling guidance, but the problem still remains. I used aqwa for modeling, the platform center of gravity and reference point are (0,0,0), here is the hst file I output using aqwa. I had to set all the non-diagonal elements to 0 in order for the model to work properly, otherwise it would produce a very large pitch Angle. Is there a problem with how I set up the aqwa model? When I turn off moordyn and all wind waves and set all non-diagonal elements to 0, roll converges when PtfmYDOF is turned off. Similarly, yaw converges when PtfmRDOF is turned off. During this time, the pitch can always be convergent. What do I need to adjust to make my roll and yaw converge?
I’m not really understanding. You mention that non-diagonal elements of the .hst file are zeroed, but I don’t see that they are zeroed in the data you shared.
It looks like the x-z plane is a plane of symmetry, which means I would expect the (3,4), (4,3), (4,5), (5,4) and (5,6) elements of the .hst to be zero, but that is not the case in your data. I would also expect the (3,5) and (5,3) elements of the .hst to be negative considering that most of your structure lies along positive x, but these are positive in your .hst file. Perhaps these point to the issue?
