Error running HydroDyn file

Dear Dr. Jonkman,

the system damping is set the same between the regular and irregular wave simulations.I agree with you that increasing wave height can induce more resonance in irregular wave.But I still have some doubts.firstly,Can I come to the conclusion that regular wave can cause tower top resonance and irregular wave can’t cause resonance under the environmental conditions mentioned above ? what’s more,I found that the resonance amplitude didn’t increase much under the conditions ii mentioned above,when I increased the simulation time from 800 seconds to 1500 seconds .Is the response of tower top motion at resonance frequency related to simulation time ?

Best regards

Dear Ka.Ching,

After the initial growth during the start-up transient, the resonant response will be limited by the damping in the system and not grow indefinitely. So, I would not expect the resonant response to be strongly dependent on the simulation length. And I wouldn’t say that regular waves can cause resonant response and irregular waves cannot cause resonance. Instead–as I implied in my prior post–you can increase the resonant response by adding energy at the resonant frequency or by decreasing the damping.

Best regards,

Dear Jason,

When I use OpenFAST to run 5MW_OC4Semi_WSt_WavesWN.fst, I set the environment to still air and regular waves, with a wave height of 7.56 m and a period of 10.6 s. The result of surge after running for 600 s is shown in the figure. The result seems to be wrong, because the result changes around 0 after the operation is stable, which seems incorrect. Could you please guide me where is the mistake? Thank you very much!
surge.png

Dear Yicheng,

I don’t really see anything unexpected with this response. The system appears to be initialized with zero initial conditions. And the initial platform velocities are zero. During the start-up transient, the system will want to pitch (due to the overhanging weight of the rotor), which induces surge motion. The periodic excitation from regular waves also gets applied at time zero. Eventually, the initial condition solution dies out and the periodic response from regular wave excitation dominates the response.

Best regards,

Dear Jason,

Thank you very much for your answers, what you said is correct, I set all initial conditions to 0. The correct result of this calculation is very useful to me. At the same time, I want to ask one more question. When I just only change the load environment to use P-M irregular waves (WaveMod = 2) and Kaimal turbulent wind, and then use the same files to calculate again, the results of the platform pitch and tower top fore-aft displacement make me feel very confused and doubt whether it is correct. Could you please answer my confusion? It would be greatly appreciated if you can guide me from which papers I can obtain the results and figures of floating wind turbines under the action of irregular waves or/and turbulent wind. Thank you in advance!

Best regards,TTDSPFA.pngpitch.png

Dear Yicheng,

Again, these results are expected. After the initial start-up transient, the structure is excited from the wind and waves. The mean pitch comes from the aerodynamic thrust force. As with any simulation, it is generally recommend to post-process the simulation results only after the start-up transient is over–which looks to be about 100 s in your case. You can minimize the start-up transient by setting proper initial conditions for the rotor speed, blade, pitch, platform surge, and platform pitch DOFs, based on their expected value for the given mean hub-height wind speed you are simulating.

Best regards,

Dear Jason,

Thanks a lot! Your suggestions are very helpful.

Best regards,

Hi Jason,

I am using OpenFAST v3.0 and I am using stiffness matrix at the bottom using the text file. I am getting the following error while running the OpenFAST. Could you please advise on this.

FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption1:FullOpt1_InputOutputSolve:HydroDyn_CalcO
utput:Morison_CalcOutput:The upper end-plate of a member must not cross the water plane. This is
not true for Member ID 10
Morison_CalcOutput:The lowest element of a Morison member has become partially submerged! This
is not allowed. Please review your model and create a discretization such that even with
displacements, the lowest element of a member does not become partially submerged.
HydroDyn_CalcOutput:Morison_CalcOutput:The upper end-plate of a member must not cross the water
plane. This is not true for Member ID 10
Morison_CalcOutput:The lowest element of a Morison member has become partially submerged! This
is not allowed. Please review your model and create a discretization such that even with
displacements, the lowest element of a member does not become partially submerged.

Regards,
Satish J

Dear @Satish.Jawalageri,

Due to the analytically exact treatment of its hydrodystatic solution, the HydroDyn module in OpenFAST v3.0 does not allow end plates of members (the cross section of a member at the joint) to cross the free surface. If you want hydrodynamic member to cross the free surface, one joint must remain below and one joint must remain above the free surface throughout the simulation or you’ll receive this error. Can you set up your model this way?

FYI: HydroDyn has been extensively upgraded in the following pull request, which is slated to be released in OpenFAST v4.0: Additional features for the new SeaState module and HydroDyn by luwang00 · Pull Request #1008 · OpenFAST/openfast · GitHub. With this upgrade, this limitation of HydroDyn has been addressed and the associated error has been eliminated from the source code (although there are likely inaccuracies in the solution if joints cross the free surface dynamically throughout a simulation).

Best regards,