Unexpected heave response (rising to 200 m) in tidal turbine simulation

[YuYuJou]

Hello,

I am trying to simulate a tidal current turbine, but the results look very strange.
During the simulation, the heave displacement keeps increasing continuously and reaches about 200 meters.

The simulation runs without errors, but the output seems physically unrealistic.
I have attached screenshots of my settings for reference.
As additional context: I ignored the rotor and tower effects by setting their masses very small, and I also disabled them in ElastoDyn.

Besides,
I changed the current input (both speed and direction/angle), but the simulation results are exactly the same after those changes. Why might modifying the current have no effect on the results?

Could you please let me know if there is something wrong with my configuration, or which parameters I should double-check when such an unrealistic heave response occurs?

Thank you for your help!

[jjonkman]

Dear @YuYuJou,

In your floating tidal OpenFAST model, does the buoyancy balance with the full-system weight and vertical mooring pretension in the undisplaced position of the floater, as is generally recommend when setting up an OpenFAST model of a floating system?

Best regards,

[jjonkman]

Dear @YuYuJou,

Thanks for the additional information. I'm sure there is something set incorrectly in your model, but I still don't understand enough to offer specific suggestions.

Best regards,

[YuYuJou]

FKTTTT.zip
Thank you for your reply.

At the moment, my simulations only use ElastoDyn, MoorDyn, HydroDyn, and SeaState. From the WAMIT results, the responses of the submerged case appear to be much smaller than those of the surface case, which is why I find it very strange that the OpenFAST simulations show such a large heave response (up to 200 m) when submerged.

Could you please suggest which input files or additional details would be most useful for you to check, so that I can provide them for further clarification?

Best regards,

[jjonkman]

Dear @YuYuJou,

From my quick look at your OpenFAST model files, I see you have current defined in SeaState, but you don't have viscous drag enabled in HydroDyn and you don't have a rotor. The current does not impact the potential-flow solution of HydroDyn, which explains why current is not affecting your results. Moreoever, the anchor of your mooring system is far upstream, and with nothing keeping the entire system from rotating upward about its anchor, the small imbalance in buoyancy and weight will result in large heave displacement.

Best regards,

[YuYuJou]

Hello, I tried adding drag and mean drift and also increasing the system weight, but the underwater simulation still shows an excessively large heave motion.

I’d like to add that my underwater simulation is for an ocean current turbine — the entire system is submerged, with no part above the water surface. May I ask if this type of configuration can be simulated? If so, could you please let me know what specific aspects I should pay special attention to? Thank you.

[jjonkman]

Dear @YuYuJou,

Yes, OpenFAST should be able to model a fully submerged ocean current turbine. It is difficult to offer specific guidance, but as with any complicated dynamics problem, I would start with the simplest model first (limited functionality enabled in OpenFAST) and build up complexity in steps (enabling additional functionality), only after verifying that the simpler model is behaving as you'd expect. This way you should be isolate what feature, when enabled, is yielding the behavior you don't expect and allow you to identify why.

Best regards,