Error running HydroDyn file

[Ka.Chang]

Dear Dr. Jonkman,

Thank you very much!

I revised it as you suggested and recalculated the PtfmHeave value.I think the simulation result should be correct.Do you think it is right？

As you mentioned，the expected mean values of rotor speed and blade-pitch angle as a function of mean wind speed are given in Figure 9-1 of the NREL 5-MW specifications report.I only modified the rotor speed and blade pitch angle，However，I noticed that there are other parameters(TTDspFA,TTDspSS，OoPDefl，IPDefl) that need to be modified in the ElastoDyn.

Best regards,

[Jason.Jonkman]

Dear Ka.Ching,

Indeed, YawBrFzp is looking much better now. A very small adjustment to the initial PtfmHeave could likely eliminate the small lingering ringing.

The initial rotor speed and blade-pitch angle (and platform heave when using SubDyn) are the most critical initial conditions to get correct for a fixed-bottom offshore wind simulation. You could set the intial blade and tower-top deflections to their expected mean values as well, which will further reduce the start-up transients, but not by much.

Best regards,

[Ka.Chang]

Dear Dr. Jonkman,

That's very useful guidance on how to set the initial conditions in ElastoDyn.Thank you once again for your help.

Best regards,

[Ka.Chang]

Dear Dr. Jonkman,

I'm sorry to trouble you again.I found RAO of the tower top(YawBrTDxp) under the unit regular wave(Hs=2m).I can get the resonant frequency from RAO.I study the motion response of the tower top under wave（still air:set CompInflow=0,CompAero = 2 and WakeMod = 0）.condition i：regular wave（Hs=0.5m Tp=3.2258s/Resonance period），condition ii：Pierson-Moskowitz spectrum（Hs=0.5m Tp=3.2258s/Resonance period）.however，I found that regular wave wave can cause tower top resonance and irregular wave can't cause resonance.Is the result reasonable？

I would be very happy if you could give me some guidance.
Best regards

[Jason.Jonkman]

Dear Ka.Chang,

A resonance response will be impacted by the amplitude of excitation at the resonance frequency and the damping of the system. I would assume the system damping is set the same between the regular and irregular wave simulations. However, the amplitude of excitation quite different in the two cases. In the regular wave case, all of the energy is concentrated at the resonant peak. In the irregular wave case, the energy is spread across a range of frequencies, with less energy at the resonant peak. I'm sure you can induce more resonance in irregular seas if you increase the wave energy / signficant wave height.

Best regards,

[Ka.Chang]

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

[Jason.Jonkman]

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,

[Yicheng.Fan]

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!

[Jason.Jonkman]

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,

[Yicheng.Fan]

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.png (20.9 KiB) Viewed 296 times

pitch.png (17.9 KiB) Viewed 296 times

[Jason.Jonkman]

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,

[Yicheng.Fan]

Dear Jason,

Thanks a lot! Your suggestions are very helpful.

Best regards,