# NREL OC3 Monopile Questions.

I’m sorry, that was my mistake I meant the input “TwrRBHt”.
I understand that monopile is modeled to -20m below MSL. However, is there a way to model the depth of the pile underneath the seafloor?
I think that would change the response of the monopile under loading.

Dear Kan,

A similar question has been discussed in the following forum topic: http://forums.nrel.gov/t/foundation-stiffness-and-damping-in-fast-v8/1059/1. The “apparent fixity” model possible with SubDyn has been discussed in several topics on this forum.

Best regards,

Hi Jason,

I just wanted to clarify some geometric inputs for this turbine.
If you look at the image attached (common Monopile design), I have placed the inputs to the corresponding dimensions.
If this is accurate, why is there a constraint that PtfmRefzt must be smaller than TowerBsHt?
I feel that TowerBsHt would be too small.

Regards,

Dear Kan,

When FAST is used to model an offshore wind turbine with a fixed-bottom monopile, ElastoDyn is used to model the tower, nacelle, and rotor and SubDyn is used to model the monopile. What ElastoDyn refers to as the “platform” is the rigid interface between the tower base and the top of the monopile (i.e., the platform roughly refers to the transition piece). Thus, I would expect TowerBsHt, PtfmRefzt, and PtfmCMzt to be identical. In fact, you’ll see in the FAST model of the OC3-monopile provided as Test19 in the FAST CertTest, that TowerBsHt = PtfmRefzt = PtfmCMzt = 10 m. In your image, while the TowerBsHt is labeled correctly, PtfmRefzt and PtfmCMzt are not.

Best regards,

1 Like

Hi,

I am trying to find natural frequency of the whole structure of offshore wind turbine (Substructure + tower+RNA) in FAST v8 to check if it is within allowable natural frequency (1p and 3p soft-stiff).

But I was not able to find this natural frequency. I noticed that Subdyn calculates natural frequency of the substructure (platform) from sea bed to the interface level (bottom of tower) and in my case it is about 4.5 Hz. But i did not find the whole natural frequency. Does FAST consider internal water and added mass in natural frequency calculations?

I have read in a post in this forum that you recommended to compute the PSD (or FFT) of FAST time series and identifying the frequency peaks. Is there any other way to find the whole natural frequency?

Thank you very much,

Arash,

Dear Arash,

I believe the forum topic you are referring to (http://forums.nrel.gov/t/natural-frequencies-with-fast-v8/1211/1) answers your question. Until the linearization functionality has been added to FAST v8, computing the PSD (or FFT) of FAST time series and identifying the frequency peaks is the most straightforward way of identifying full-system natural frequencies.

Best regards,

Hi Jason,

I ran FAST with just hydrodynamic loading (White noise spectrum as you recommended in one of your posts) and I did not include wind. Then, I generated time history of for-aft displacement at top of tower. I computed the FFT of this time history and plotted. I have some doubts about it

1- Is my method OK? (fft of displacement at top tower (F-A) when we just generate wave spectrum (white noise))

2- I expected few frequency peaks (1st natural frequency around 0.29 , 2st around 1.3 , 3st around 3.5, and so on). But in the plot I can see the first natural frequency is 0.183 hrtz and it is different from what it should be and I can’t tell other peaks (In case of not having filter).

3-Then, after searching I found out that I need to use low pass filter for my frequencies. But I noticed that it is very sensitive to filtering coefficients (order and …). I changed (b) and (a) in butter command and the best response I got is from these values. But I am not sure it is right.

[b,a]=butter(16,2piTs,‘low’);
Ts is my time step =0.01 sec

I uploaded time history and FFT plots with filtering and without filtering.

Could you please kindly have a look at them and give me some hints?

Thank you again.

Best Regards,

Arash Hemmati,

Dear Arash,

I’m not familiar with the filtering approach you’ve taken; if filtering is needed, I would typically filter the PSD directly rather than time series.

I’m not sure where you are getting the frequencies of 0.29, 1.3, and 3.5 Hz. For the OC3-monopile model, I would expect the 1st fore-aft bending mode frequency to be around 0.25 Hz and the 2nd fore-aft bending mode frequency to be around 2.5 Hz.

Otherwise, your approach sounds OK. From your data, it doesn’t look like your white noise extends beyond about 0.5 Hz, which is probably why you can’t see the higher modes. Have you set the high cut-off frequency (WvHiCOff) in HydroDyn to be around 0.5 Hz (WvHiCOff is actually specified in HydroDyn in rad/s)? If so, I would increase this to a higher frequency to excite the higher-frequency modes. If you haven’t already, I would also disable the 2nd-order waves (both sum and difference frequency) in HydroDyn to ensure that the white noise waves have equal energy across all frequencies of interest.

Best regards,

Dear Jason,

Actually I modified the substructure of OC3-Monopile, that is why I expected different natural frequencies.

According to your reply, I increased high cut-out frequency and disabled second order waves and now I can identify other natural frequencies.

Best Regards,

Arash Hemmati,

Dear Jason,

I just have a quick question regarding Platform mass in Elastodyn.

I noticed that in OC3-Monopile input files, platform mass and platform inertia for roll and pitch are 0. But in OC4-Jacket input files, they are non-zero (666 ton for PtfmMass).
I was thinking maybe 666 ton is weight of transition piece for the Jacket
What is exactly platform mass and when do we need to consider it?

Thank you very much for your support.

Best Regards,

Arash,

Dear Arash,

When modeling fixed-bottom offshore wind turbines in FAST v8, the platform mass and inertia in ElastoDyn can be used to model heavy and rigid transition pieces that one would not want to model as a flexible body in either the ElastoDyn tower or SubDyn substructure models. In the OC4 jacket, for example, the substructure is connected to the tower via a large and heavy concrete block. For our FAST model of the OC4 jacket, we used the platform mass and inertia in ElastoDyn to model this concrete block.

I hope that helps.

Best regards,

Dear Jason,

Given the fact that I dont model transition piece (Actually in the area that I have transition piece, I add thickness of transition piece to thickness of monopile), platform mass is zero. But I have some doubts about PtfmCMzt and PtfmRefzt.

Could you please kindly tell me if it is correct to assume PtfmCMzt=14.8 and PtfmCMzt=14.8 according to my model’s configuration?

Sorry about this question, I read NREL OC3 Monopile Questions. and also FAST manual. But the figure 20 in the manual is a bit confusing as tower base in lower than water level.

Thank you again for your support.

Best Regards,
Arash,

Dear Arash,

If you don’t have any platform mass, the location of PtfmCMzt doesn’t really matter. Also, PtfmRefzt should locate the logical connection point between tower base in ElastoDyn and the substructure top (interface nodes) in SubDyn. Per your graphic, setting PtfmRefzt to 14.8 m looks correct, assuming that the entire monopole (from the seabed to the tower base) is modeled in SubDyn.

Best regards,

Hi Jason,

I have a question regarding finding natural frequencies from time history results.

According to your suggestion, I ran FAST with just hydrodynamic loading (White noise spectrum as you recommended in one of your posts) and I did not include wind. Then, I generated time history of for-aft displacement at top of tower. I computed the FFT of this time history and plotted. It gives me natural frequencies in the range that I expected 0.25 hrtz (Please see the first figure).

But when I do this procedure on the time history results when I just have wind (for example turbulent wind with 12m/s), the first natural frequency lies around 0.06 hrtz that is much lower than the expected first natural frequency (Please see the second figure).

Thank you,
Best Regards,

Arash,

Dear Arash,

It is difficult to answer your question without knowing more about your model, e.g. did you somehow generate white-noise turbulence or is this based on a given wind spectrum, what rotor speed is the turbine is running at, etc? I’m not really seeing any clear peaks in your second plot, except maybe at 0.28 Hz. When a turbine operates under turbulent wind excitation, I would expect to be strong peaks at 0 Hz (the mean thrust), at three-times-per revolution (3P), at 6P, and if there is a rotor imbalance, at 1P. In your second plot, I don’t see a strong 0P. Perhaps 0.28 Hz is 3P (corresponding to a rotor speed of 5.6 rpm)?

Best regards,

Hi Jason,

I generated turbulent wind file in Turbsim with Kaimal Turbulence model with 14.6% turbulence intensity and 12m/s wind speed. As wind speed is larger than rated wind speed 11.4, rotor speed should be 12.1 rmp.

Regarding rotor speed, I don’t change RotSpeed in Elastodyn for different wind speeds. Should I change it? I assumed it (RotSpeed) is initial rotor speed and I do not need to change it as after transition period it will balance with control system. So, I just kept that 12.1 rmp. Is it correct?Do I need to change RotSpeed in Elastodyn for every wind speed?

Thank you very much again.

Best Regards,
Arash,

Dear Arash,

See the following forum topic for my recommendations on setting initial conditions for FAST simulations: http://forums.nrel.gov/t/initial-conditions-for-tower-and-rotor/609/1.

For a simulation at 12.1 rpm, I would expect to see a strong excitation of the tower at 3P = 0.6 Hz, which I don’t really see in your plot.

Best regards,

Dear Jason,

Thank you for your reply. According to the post, I need to find the initial condition from figure 9-1 FAST user manual.

Regarding calculating FFT from time history, I plotted time history of the result I used (TwHt1TPxi for-aft top tower displacement). Do you think my time history looks reasonable given the wind condition (Wind speed is 12m/s with 14.6% turbulence intensity and 12.1 rmp rotor speed)?Or something is wrong?

Thank you very much.

Best Regards,

Arash,

Dear Arash,

It is hard to look at one plot in isolation to judge the correctness of the response. I suggest that you also look at the rotor speed, blade-pitch angle, blade deflections, and loads.

Best regards,

Hi,

I have a question regarding Platform inertia for yaw rotation about the platform CM in Elastodyn input file.

In OC3-Monopile and OC3-Tripod Elastodyn input files, Platform inertia for yaw rotation about the platform CM is 1.534E+06 kg m^2 and 2.48448E+06 kg m^2, respectively.
How are these values calculated?
Both values have the same order of magnitude (10^6). Are they arbitrary values?

Regards,

Gabriel.