Problems on OC3-hywind Spar mooring line tension calculation

Wind & Water Computer-Aided Engineering Software Tools
1

Dear Jason,
I am a FAST user and I met some problems when I used FAST to do some analysis on OC3-Hywind Spar, hoping to get some useful help and suggestions from you.
I used FAST to calculate the coupled dynamic responses of mooring line tension in time domain under the wind and wave load. JONSWAP spectrum and IEC Kaimai model which is calculated in Turbsim are adopted for simulating the random sea state. In the simulation the wind and waves are always from the same direction. I choose some sea state as example as following table list.
Sea state No. Wind speed Significant wave height Peak-spectral period Spectral bandwidth
22 6 1 6 0.999734
36 8 1 6 0.999722
48 10 1 6 0.999560
60 12 1 6 0.998924
69 12 7 14 0.998649
94 18 7 12 0.997909
because of the limited attachments of pictures,I add a word file to express my calculation and problems.
attachment of calculation .docx (479 KB)
After comparing the results, I have some problems.

  1. No matter how I change the wind and wave load in different sea state, the maximum PSD value occur in almost the same frequency which is at the range of 0.04-0.05rad/s. So I am no sure if the results I calculated are correct and reasonable?
  2. The maximum PSD value occur in low frequency which ranged in 0.04-0.05rad/s. The frequency is closed to neither wind frequency or wave frequency. So I feel puzzled by the results and I want you can give me some suggestions.
  3. The spectral bandwidth are all very closed to 1 which means it is the wideband, but for each sea state the value of spectral bandwidth is almost the same.so I am not sure about the results.
  4. Currently, I have used FAST to calculate totally 100 sea states which are based on the joint distribution model of wind and waves in the Northern North Sea to do a long-term analysis on mooring line tension. In each sea state, I just change the wind file in AD file and wave parameter in HD file, I am not sure if it is correct or not during my calculation. Should I change another parameter ?
    Thank you for your kind advice.
    Best regards,
    Yanjian Wu
2

Dear Yanjian,

Here are my answers to your questions:
1/2) It looks like the PSDs of your line tension peak at around 0.05 rad/s = 0.008 Hz, which is the natural frequency of the platform in surge/sway, so, it looks like the line tensions are dominated by excitation of the surge/sway natural frequency (likely from wind, low-frequency first-order waves, difference-frequency second-order waves, or start-up transients).
3) I’m not sure I understand what you mean by spectral bandwidth. Have you defined some threshold for where there is energy in the JONSWAP wave spectrum? You don’t seem to have set any high or low cut-off frequencies in the wave spectrum within HydoDyn, so, you don’t seem to be referring to those.
4) When changing the wind speed, you should set appropriate initial conditions to minimize the effect of start-up transients on the results; section 6.8.2 of the HydroDyn User’s Guide and Theory manual provides some guidance for choosing proper initial conditions to minimize start-up transients for floating wind systems. You’d likely also want to change the WaveSeed between different simulations, as mention in section 6.1 of the HydroDyn User’s Guide and Theory manual. (wind.nrel.gov/nwtc/docs/HydroDyn_Manual.pdf).

A few other comments:

  • You are using WindType = 3 in InflowWind, so, you only need to change the Filename in the WindType = 3 section of the InflowWind input file.
  • You are using FEAMooring, but you showed the MoorDyn input file.
  • You say the simulation is 7200 s in length; however, you’ve only set 3630 s of wave data, so, the waves will repeat after 3630 s, which you may not want.

Best regards,

3

Dear Jason
Thank you for your answers and comments! I’m so sorry to reply late for your reply because I have lots of issues to deal with at the end of the semester in our school.
Your answers to the questions help me a lot. Under your comment and suggestions I also do some analysis on the result of fore-aft shear forces at the base of the tower(TwrBsFxt in FAST), side-to-side bending moments at the base of the tower(TwrBsMxt in FAST), platform translational surge displacements(PtfmSurge in FAST) except for the mooring line tension. The PSDs results(showed in the attachments) still puzzled me a lot. I choose the sea state(u=10m/s,hs=1m,tp=6s) for a example.
Thank you for your attention!
Best regards,
Yanjian Wu
attachment.docx (324 KB)

4

Dear Yanjian,

I see 4 dominant excitation/response frequencies in your results:

  • 0.008 Hz – corresponds with the platform surge/sway natural frequency
  • 0.034 Hz - corresponds with the platform pitch/roll natural frequency
  • 0.17 Hz - corresponds with the peak of the first-order wave spectrum
  • 0.5 Hz - corresponds with the first tower-bending mode fore-aft/side-to-side natural frequency

These are excitation/response frequencies that are expected for this OC3-Hywind model. Which frequency has the most energy (i.e. contributes to the most standard deviation in the time series) likely depends on the exact wind and wave conditions.

Best regards,

5

Dear Jason,
Thanks for your answers. I am so sorry to disturb you again. I met some problems on the results of filtering the mooring line tension and I hope to get some useful help and suggestions from you.
Because of the limited attachments of pictures,I add a word file to express my calculation and problems.The attachment is followed.
attachment.docx (303 KB)
Please forgive me for causing you a lot of trouble.
Thank you for your attention again!
Best regards,
Yanjian Wu

6

Dear Yanjian,

I would not expect a strong once-per rev (1P) frequency transmitted to the platform/mooring loads (in the fixed frame), unless there is a significant rotor imbalance (mass or aerodynamic). Instead, excitations in the rotating frame of a three-bladed rotor end up as 0P, 3P, 6P etc. excitations in the fixed frame, as has been discussed several times on this forum.

Even though the wave energy is large in the first-order wave-frequency range, the natural frequencies of the OC3-Hywind floating wind turbine are not designed to fall in the first-order wave-energy range. The response is large at the platform natural frequency because of resonance excitation and low damping levels.

Best regards,

7

Dear Jason

Thanks for your answers.
I still have a question about the magnitude of the WF components force and WTF components force.
From the result of filtering the mooring line tension, we can see that

  1. The max value of mooring line tension is about 135 kN
  2. The max value of WF components is about 20 kN
  3. The max value of WTF components is about 1.5 kN
    From the order of magnitude, the max value of mooring line tension is about seven times greater than WF components and it is also ninety times greater than WTF components. Can you help me explain whether the magnitude of WF and WTF components is reasonable or not comparing with the mooring line tension in terms of magnitude.

Best regards,
Yanjian Wu

8

Dear Yanjian,

I’m not really surprised by these results. As I said before, the low frequency is where there is a platform natural frequency and low damping. There are no platform natural frequencies at the wave frequency and there should be little excitation in the fixed frame at the 1P frequency (unless there are large imbalances).

Best regards,