Hello!!!

I would like to ask for help. I am modeling a floating offshore wind turbine. I use MoorDyn for the dynamic of mooring line (hybrid). My mooring line composes of several sections. Two materials used in mooring line are rope and chain. For the part of rope, the mass is very light and the density of the rope is a little higher than the sea water density. There are also clump weight parts. For the clump weight part I model it as lumped mass model as well by imputing the mass density, stiffness and so on.

Now, I am checking balance of the wind turbine. I started with heave balance. I found that my simulation goes unstable and have the error shown in the attached image. I checked the mooring line position at the line very close to anchor and found some strange movement (see the attached image for detail). I am not sure how to solve it.

*Could you please kindly guide me solving this problem?*

Dear @Danupon.Subanapong,

It looks like your OpenFAST models has gone unstable. It is unclear to me if this is because of a problem in the MoorDyn input, or that is just the error that finally causes the abort. I would suggest simplifying the model to debug, e.g., if you replace MoorDyn with an equivalent mooring preload and stiffness matrix, which you can enter into HydroDyn via inputs `AddF0`

and `AddCLin`

, is the model still unstable? Or does the model run stably with a subset of platform DOFs in ElastoDyn enabled?

Best regards,

Dear Dr. Jason,

Thank you so much for your reply and suggestion. I don’t know the Mooring line restoring stiffness. However, I think I will check heave static equilibrium first. I think that Mooring stiffness in heave direction is minor. **Am I right??** Therefor, I can only input the vertical loads from mooring tension (preload) as AddF0 and check the equilibrium. **Do I understand correctly??**

Dear @Danupon.Subanapong,

The importance of the mooring stiffness in the heave direction depends on what type of floater you are simulating. Certainly it is important for a tension-leg platform or floater with taut moorings. The mooring heave stiffness is likely negligible for a semisubmersible.

Best regards,

Dear Dr. Jonkman,

Thank you so much for the reply and clarification. My case is catenary mooring. I understood that for this type the mooring stiffness in heave direction should be minor. Can you please confirm if I understand correctly.

Best regards,

Dear @Danupon.Subanapong,

Correct.

Best regards,

Dear Dr. Jonkman,

Thank you so much for the confirmation.

Let me give some more detail of my model before discussing about the instability problem. In my model, I also have the SubDyn for the dynamic of the floater. Regarding SubDyn module, all joints are cantilever joint. For the mooring line, there are three mooring line. It is catenary system with unsymmetrical layout (the line length of these lines are equal but the angular spacing between lines are uneven).

I simplified model by replacing MoorDyn with equivalent mooring preload and run static equilibrium (only heave DoF being considered). I am not sure about the mooring line stiffness; therefore, I chose to check only the heave motion. The result shows that the mean heave motion is close to zero (Please see the image named “Heave_with_SubDyn_No_MoorDyn.png”). **However, the heave motion fluctuates largely throughout 300 sec simulation.** I think this is strange. Therefore, I simplified the model more by modeling the floater as rigid body in ElastoDyn instead of SubDyn and ran the simulation for the static equilibrium analysis. The heave displacement of the model without SubDyn does not fluctuate much (Please see the image named “Heave_without_SubDyn_No_MoorDyn.PNG”). It looks reasonable to me. I think the instability might be from the SubDyn but I have no idea which part can cause this problem. Can you please kindly help me fix the problem?

Dear @Danupon.Subanapong,

I don’t see a numerical instability in your heave response with SubDyn enabled, but there certainly is an undamped mode. In your model with SubDyn enabled, are you following the guidance I’ve outlined here: FeaCoupling of an External floater (platform+mooring) with Openfast #791 · Discussion #801 · OpenFAST/openfast · GitHub?

Best regards,