HydroFzi exceptionally high when SubDyn is ENABLED, while the data in HD.sum remains normal and identical to the data while SubDyn is DISABLED

[yxy636363]

I am currently working on an OpenFAST(v5.0.0) simulation of the OC4 semi-submersible platform with the NREL 5MW wind turbine. My model is based on the r-test reference, but I am rebuilding it to enable SubDyn for flexible substructure analysis, because I need to compute the internal forces and bending moments at various locations within the substructure members (columns, pontoons, braces). I'd greatly appreciate any insights from the community.

When SubDyn is DISABLED (CompSub = 0 in the main .fst file), the model runs smoothly for 60 seconds with reasonable platform motion and visualization output.

However, when SubDyn is ENABLED (CompSub = 1), the simulation fails immediately at the first time step with convergence error NaN (or occasionally error=20+) and the following warning:

Small angle assumption violated in SUBROUTINE SmllRotTrans() due to a large Nodal rotation.
Angles in GetSmllRotAngs() are larger than 0.4 radians.
The solver fails to converge within the maximum number of iterations (currently 6), and MoorDyn subsequently reports NaN states.What I Have Already Tried (Based on Forum and Documentation)
Following recommendations from the OpenFAST documentation and NREL forum discussions , I have attempted:

1. Transferred all platform mass from ElastoDyn to SubDyn (set PtfmMass = 0 in ElastoDyn)

2. Added concentrated masses at column ends to simulate cylinder caps (following Roger Bergua's approach)

3. Enabled static improvement method (SttcSolve = True)

4. Reduced time step (DT = 0.01, 0.005, and even 0.001 s)

5. Increased correction iterations (NumCrctn = 3, 5)

6. Relaxed convergence tolerance (ConvTol = 1e-3)

7. Temporarily disabled ServoDyn and AeroDyn to isolate the issue

8. Tested Nmodes = 0 (Guyan reduction, treating platform as rigid) – still diverges

9. Adjusted quasi-rigid beam properties (diameter from 2.5m → 5.0m)

10. Enabled Guyan damping via AddBLin in HydroDyn (1% critical damping for heave DOF, as recommended in the modeling guide )

11. Extreme simplification of the SubDyn model – Following the guidance in the modeling documentation that virtual members should be characterized by "low self-mass and high stiffness" , I simplified the entire platform to a single concentrated mass at its center of gravity, connected by a single quasi-rigid beam to the tower interface node.

My core questions:

Mooring force transfer: How are mooring forces from MoorDyn applied to the platform when fairlead points are NOT explicitly modeled as joints in SubDyn? Does FAST's mesh-mapping utility automatically transfer these forces to the SubDyn structural mesh based on coordinates, or must I define corresponding joints at the fairlead locations? Could missing explicit fairlead joints cause the forces to be applied at incorrect locations or not applied at all?

Mass and force integration: When SubDyn is enabled, where should each component be defined, and how are they integrated?

Platform structural mass: SubDyn (beam density + concentrated masses) - ElastoDyn PtfmMass set to zero - is this correct?

Ballast water: HydroDyn FILLED MEMBERS - is this mass automatically added to the system's total mass matrix, or does SubDyn need to account for it separately? The HydroDyn summary shows negative internal buoyancy, indicating ballast weight is subtracted from buoyancy, but is the inertia also correctly accounted for?

Tower: Should remain in ElastoDyn with DOFs enabled, or move to SubDyn with tower DOFs disabled in ElastoDyn? (My tower is tubular, not lattice - documentation suggests ElastoDyn may be more accurate for tubular towers due to geometric nonlinearities)

Hydrodynamic loads: Transferred via mesh-mapping - confirmed by documentation

Given that even the extreme simplification (single point mass + one quasi-rigid beam) still diverges, this seems to rule out issues with complex mode shapes or member connectivity. Could this indicate a fundamental issue with how loads are integrated across modules when SubDyn is enabled, particularly the transfer of mooring forces or the handling of ballast mass?

I have attached the following files for reference:

Main input file
SubDyn input file and summary file
HydroDyn input file and summary file
ElastoDyn input file and summary file
error output

NREL_OC4_4body_sub_files.zip

Any insights or suggestions would be greatly appreciated. Thank you for your time and expertise.

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Recently, while checking the output files, I noticed a data inconsistency. When SubDyn is disabled, the HydroFzi output matches the net buoyancy calculated from the HydroDyn summary file (approximately 4.59e7 N). However, when SubDyn is enabled, the initial HydroFzi value becomes 1.3456e11 N—nearly 3000 times larger—even though the summary file shows identical values for the displaced volumes of the four WAMIT bodies and the net buoyancy (excluding WAMIT contributions) in both cases. I guess this could be the main cause of my problems, but still wondering how to solve it.

[jjonkman]

Dear @yxy636363,

I haven't reviewed your input files in great detail, but I do see one obvious problem that could explain why your model with SubDyn fails quickly. I see that you've initialized the platform degrees of freedom (DOF) in ElastoDyn to have an initial platform surge of 5 m and an initial platform pitch of 1.9degrees. However, your rigid body in SubDyn is initialized without any displacement. This mismatch would cause extreme deflection of the SubDyn Guyan modes and probable model failure.

Regarding step 10 of steps 1-11 you've tried, this recommendation for AddBLin only applies to fixed-bottom substructures modeled in SubDyn, not floating substructures. Your other steps make sense to me, but I expect the mismatched initial conditions between ElastoDyn and SubDyn are still the cause of the error.

To answer your core questions:

• Mooring force transfer: Indeed, OpenFAST's internal mesh-to-mesh transfer allows the fairleads in MoorDyn to be offset from joints in SubDyn. Essentially, the point-to-point mesh mapping within OpenFAST will use a nearest neighbor mapping, and preserve rigid-body kinematics and kinetics in the process.
• Mass and force integration: I'm not sure what you mean by "each component".
• Platform structural mass: I agree that PtfmMass in ElastoDyn can be zero if the full floating substructure is modeled in SubDyn.
• Ballast water: If the ballast water is considered in HydroDyn, it should not also be considered in the structural modules (ElastoDyn or SubDyn) to avoid double-counting the effect. In HydroDyn, the full mass and inertias of the ballast water are considered.
• Tower: I agree that tubular towers are better considered ElastoDyn than SubDyn, but you could choose to model the full support structure in SubDyn. For the latter, geometric nonlinearities and tower aerodynamic influence and loading are neglected (although we are currently working to address the latter by upgrades to AeroDyn and by enabling a coupling between AeroDyn and SubDyn).
• Hydrodynamic loads: Correct.

Best regards,

[luwang00]

In the SD summary file, you can see the orientation matrix of each member under the heading #Direction Cosine Matrices for all Members. In there, you can see Member 5 is indeed yawed 90 degrees, so its local x-axis aligns with the global Y-axis, and the local y-axis is in the direction of global -X direction. This is because the X-coordinates of Joint 3 and Joint 9 differ in the last digit, so the member is not perfectly vertical.

[jjonkman]

Thanks for clarifying, @luwang00! That makes sense!

[yxy636363]

Dear @jjonkman and @luwang00,

Thank you for pointing out the issue! I have corrected the corresponding node coordinates. In the updated SD.sum file, the local coordinate directions of the columns are now aligned with the global coordinate system, meaning the direction cosine matrices are identity matrices.

To better understand the forces at the same point, I attempted to temporarily turn off the wind and wave effects (and also removed the initial displacements) by setting CompInflow from 1 to 0 and CompAero from 2 to 0 in the .fst file. Is this operation sufficient? I ran the simulation for 60 seconds with these settings and output the platform motions and reaction forces (I only output the reactions for two columns of interest: M1 and M2, which correspond to Upper Column 2 and Base Column 2). However, the motion still shows significant oscillations. I am not sure if this is due to some remaining wind/wave effects that were not turned off.

Regarding the force plots, after correcting the directions, the shapes of the plots are essentially the same, but the numerical values seem to be shifted up or down. Based on my understanding of reaction forces, shouldn't the plots be mirror-symmetric with respect to the time axis? I hope to get your clarification on this. Thank you very much!

[luwang00]

• You can also remove the waves in SeaState by setting WaveMod to 0, but you will still see some oscillations at the start either way because, at equilibrium, the structure experiences elastic deflection, but OpenFAST initializes the structure as undeflected.

• As @jjonkman already explained, there are some subtleties to the loads reported by SubDyn. When SubDyn discretizes a beam into elements, the mass of each element is split between the two end nodes of that element, and the element itself is modeled as massless (it only has stiffness). Therefore, the difference between the loads on either side of a node is the net structural force/moment on that node. In your case, each member is only divided into two elements, so the node in question has 1) a quarter of the mass of the base column, 2) a quarter of the mass of the upper column, plus 3) whatever additional lumped mass you assigned to that node for the endcap. That's a lot of mass, so you would naturally see large differences between the loads on either side of that node, both from its weight and inertia (if accelerating). If you increase NDiv, you should see the difference in loads on either side of the node shrink, but it will never go to zero because of the lumped mass you put there.

• In your results, both M1N1FKZe and M2N2FKZe are positive. This doesn't look correct as the weight of the structure should be pushing down. I think this is because your hydrodynamic model is not correct. You are modeling the four columns (upper+base column) as four potential-flow bodies. In this case, the potential-flow hydrodynamic and hydrostatic loads on each column are mapped to a single point in SubDyn. To get the correct load distribution to map to the structural model, you would need to replace the potential-flow model in HydroDyn with a strip-theory-only model that provides distributed hydrodynamic and hydrostatic loads.

[yxy636363]

Dear @jjonkman and @luwang00,

The gravity from the concentrated masses and ballast water is about 1.06e7 N. Under still water conditions, the difference between the two Z-direction forces is about 1.19e7 N. If we also consider part of the weight of the columns themselves, I think the difference in the Z-direction forces can be attributed to the gravity from these masses.

NDiv=4, wind&wave off NDiv=4, wind&wave on

Since the focus of my next study is the force from the beams on the upper column, the issue of the force difference between the upper column and the base column is likely not critical. I plan to put it aside for now and not investigate it further.

Thank you very much for your help!

Best wishes