Confusion about IceDyn

[Tianhui.Liu]

Dear all
I am currently studying the interaction process between ice load and fixed offshore wind turbines. During the process, I have several questions that I need help with.

First of all, when I run Icemodel 3, the ice force curve I get is very abnormal no matter how I set it up. Even when I set the parameters according to the IceDyn Manual example, the results are still very strange.

This is what I get when I set the parameters according to IceDyn Manual Example 3-1.

This is what I get when I set the parameters according to IceDyn Manual Example 3-2.
It may seem unbelievable, but I’m pretty sure I choose the right icemodel.

Another thing that I am confused about is the setting of the parameters, for example, the variance of the ice thickness and the average of the ice thickness, what kind of relationship should be satisfied between these two values to be considered reasonable. Of course I know the formula between the variance and the mean, but the parameters I collected are often only the mean but not the variance, so how to set the variance bothers me. This is true for ice thickness and ice speed.
I have another question about the settings of Icemodel 3. In Icemodel 3, we need to set the mean and variance of ice speed and ice thickness to generate random ice loads, does this mean that the ice speed and ice thickness in the general settings do not work when running icemodel 3 ?

Some questions when dealing with calculation results
In my analysis of the calculation results, the output options I selected were mainly tower top acceleration（YawBrTAxp from ElastoDyn）, tower top displacement（YawBrTDxp form ElastoDyn）, and tower base shear forces（ReactFXss from SubDyn）. I wonder if you have the relevant safety norms value. What should be the limits of tower top acceleration and tower top displacement in order to comply with the specification
According to the offshore wind turbine design requirements published by IES, the design specification for ice load in Annex E should be less than 20Mpa for the support structure. According to my understanding, it corresponds to the stress at the substrure mud line, but there is no stress in the output options of SubDyn, do I need to write a pull request on GitHub?
Of course, calculated according to the shear force is also an option.

I would be grateful for any hints.

Best regards,
Tianhui.Liu

[Jason.Jonkman]

Dear Tianhui,

Unfortunately, I have little experience with IceDyn and don’t know the answers to your IceDyn-related questions. IceDyn was developed at the University of Michigan; I suggest reaching out to the original developers (who I don’t believe follow this forum closely) with questions.

I’m not sure what offshore wind system you are modeling. If it is the NREL 5-MW baseline wind turbine atop the OC3-monopile, the design limiting values of tower-top acceleration and deflection have not been defined, so, you’ll have to make your own assumptions for your own purposes. Tower-top acceleration limits are typically not more than a third of gravity, g/3.

The various structural modules of OpenFAST, including SubDyn, output sectional forces and moments (loads) rather than stresses. These loads can be used to calculate local stresses using cross-sectional analysis. This topic has been discussed several times on this forum e.g. search for “stress”, “load”, and/or “cross-sectional analysis”.

Best regards,

[Tianhui.Liu]

Dear Jonkman

I’m currently using icedyn.dat v1.01. I noticed some bugs in the process of using it, then I tried to find a newer version, and the latest icedyn in openfast is already 1.02, but the format is P90 files, not Icedyn.dat as I want. I don’t know if you have a version of icedyn in dat format

thanks a lot

best regards

[Jason.Jonkman]

Dear Tianhui.Liu,

I’m not actually aware of any changes to IceDyn made within OpenFAST. In fact, the sample IceDyn input file still refers to v1.01–see: github.com/OpenFAST/r-test/blob … _Input.dat.

Best regards,

[Tianhui.Liu]

Dear Jonkman

when I run the simulation this is what I got.And to be honest,there are several points in the calculation that I cannot understand.But I’d like to focus on some of them

Figure 1 shows the ice force time curve.This is almost the only load since wavedyn must be set wo 0 when running iceload and I have set the wind speed to 0.001m/s ,the initial rotor speed to zero.Ice load is applied to the waterline position.
I can’t explain why the tower base shear force (ReactFXss from SubDun) is so large, much larger than the ice load, and only so large at the beginning moment.
What does this have to do with the following error report?

Is there any load that I have not noticed?

Best regards
Tianhui

[Jason.Jonkman]

Dear Tianhui,

The warnings you are getting sound similar to the ones you are reporting in the related forum post-- Question about Turbsim. However, if set the wind speed to practically zero, you should also disable wake and unsteady airfoil aerodynamic calculations in AeroDyn by setting WakeMod = 0 and AFAeroMod = 1. Alternatively, you can disable aerodynamic loads altogether by disabling AeroDyn, by setting CompAero = 0 in the OpenFAST primary (*.fst) input file.

The first part of your simulation appears to be related to a start-up transient, e.g, the tower deflecting as a result of the gravity load. I would not start post-processing the results until after this start-up transient diminishes. It may help to run a simulation without ice loads (without any load except gravity) just to assess the results and to quantify the length of the start-up transient before applying the ice loads. I start seeing the effect of the ice loads on the reaction load at the seabed (ReactFXss) at about 58 s into your simulation.

I hope that helps.

Best regards,

[Tianhui.Liu]

Dear Dr.Jonkman

First of all, I thank you very much for your help, that does help.After setting AreoDynamic=0 in the .fst file ,no more errors were reported during the calculation .Following your suggestion, I tried turning off all loads, leaving only gravity, and got the following response.

In order to minimize the impact of transient effects, I decided to process the results after 100s in the future.

You can see that by setting the start of post-processing to 40s you are already able to see the effect of ice loads in the tower top acceleration and tower base shear, but there are some problems.

1. The tower top acceleration is apparently too small, at least according to my research.
2. The displacement of the top of the tower basically does not show any effect, which is what I can’t understand.
   I wonder if you have any suggestions for this.

Best regards,
Tianhui

[Jason.Jonkman]

Dear Tianhui,

The effect of the ice loading on the tower-top acceleration and displacement will depend on the rigidity of the monopile. (If the monopile were rigid, there would be no effect of monopile loading at the tower top). It looks like the ice load has a very minor effect on the tower-top acceleration, which when integrated twice shows minimal effect on the displacement. Regardless, my guess is that if you computed a PSD of either signal, you’d see the frequency of ice loading showing up in these signals, at least by a small amount.

BTW: I would normally use the tower term “tower base” to refer to the top of the transition piece above the monopile. My understanding is that you are plotting the shear force in the monopile at the mudline (ReactFXss from SubDyn).

Best regards,

[Tianhui.Liu]

Dear Dr.Jonkman
Indeed,I was talking about the shear force in the monopile at the mudline (ReactFXss from SubDyn).
In IceDyn’s calculations, the velocity of the ice does not affect the magnitude of the ice load, but only the period of ice loading.
The shear force in the monopile at the mudline I calculated is shown below

The value of z-axis is ReactFXss and the horizontal coordinate is the velocity of the ice at intervals of 0.1m/s,the processing time is 40s to 100s.
You must have noticed that the ReactFXss value is too small, that’s because due to a problem with my settings, it should be multiplied by 10^5 here, but it doesn’t show up here.
My main concern is that there is no significant trend in ReactFXss as the ice speed increases.Is this normal?It may not be obvious, but the maximum value actually occurs at 0.4m/s.The effect of ice thickness change is rather obvious

Best regards
Tianhui

[Jason.Jonkman]

Dear Tianhui,

I’m afraid I can’t answer your question regarding the impact of ice velocity on the ice load output from IceDyn. This would be a better question for the developers (Prof. Dale Karr et al) at the University of Michigan, who I don’t believe follow this forum.

Best regards,

[Tianhui.Liu]

Dear Jonkman
Many thanks for your kind help.If it weren’t for you, I would still be stuck in the middle of a mistake that I didn’t even notice.

Best regards
Tianhui

[Tianhui.Liu]

Dear all
The next question I want to ask is not exactly about ice loads, if I had to categorize it I would categorize it as a post-processing question.
I use ice speed as a variable to study the response of offshore wind turbines under different loads. In the time domain I get the following results
iceforce

Tower top acceleration

Obviously, when the ice speed is equal to 0.3 and 0.7, there will be a resonance-like result, and this phenomenon will be more obvious if the calculation time is extended
the shear force in the monopile at the mudline (ReactFXss from SubDyn)

I didn’t find much useful information about this image

[Jason.Jonkman]

Dear Tianhui,

I’m not sure I understand what your question is. Please clarify.

Best regards,

[Tianhui.Liu]

Since my findings in the time domain were not sufficient to support my article, I decided to continue my research in the frequency domain.
For simplicity, I only selected the response corresponding to 0.9m/s ice speed for spectral analysis.
This is the result I got by processing with wafo’s dat2spec function, which is actually a lot of window functions added on top of the Fourier transform.

The first two diagrams in the previous post correspond to the first two here. The third picture in the previous post corresponds to the fourth one here.
The third graph here corresponds to what should be the acceleration at the waterline, as follows

The frequency spectrum of the excitation load and the frequency spectrum of the acceleration at the top of the tower do not correspond exactly.
I wonder if there is someone who is good at spectrum analysis to teach me how to analyze my calculated
Now what puzzles me most is that the ice load is the only excitation load, while the spectrum of the acceleration at the top of the tower is concentrated in high frequencies, which is completely different from the spectrum of the ice load

thanks a lot

best regards
Tianhui.Liu

[Tianhui.Liu]

Sorry I didn’t make my question clear. Since I can only post three pictures in one post so I split it into two posts.

[Jason.Jonkman]

Dear Tianhui,

I’m not familiar with the PSD script you are using, but there appears to be a lot of windowing, which I do not believe results in a very realistic PSD in your case. Can you try a simpler PSD script, such as my script that calculates the PSD directly via FFT without windowing (but with optional binning to smooth the PSD). This script, Jason_PSD.m can be found in the following forum post: User defined spectrum from routine UserWaveSpctrm in HydroDYn - #7 by Jason.Jonkman.

Best regards,

[Tianhui.Liu]

Dear Jonkman
First of all, I apologize for taking so long to reply. I was waiting for your reply before and then I realized it was on the second page.
In order to better illustrate my process of processing the results, I have put all the graphs obtained from the process here. And with some basic descriptions:
In the process of processing the data I selected all the results after 100, the total calculation time is 400 is, the time step is 0.005, each data has 6000 values
The picture on the left half is the result in the time domain, while the picture on the right half is the result of my own calculation using fft
The four images correspond to
1.iceforce
2.Tower-top / yaw bearing fore-aft (translational) acceleration (absolute)(From ElastoDyn)
3.acceleration at node 1 of member 3 (located at 0 m, i.e. MSL)(From SubDyn)
4.fore-aft shear force at mudline(From SubDyn)

The graphics output by using your script look like this(On the right is the generated discrete point plot, and on the left is the generated curve)

[f1,Sf1]=Jason_PSD(iceforce,0.005,8192,1); [f1,Sf1]=Jason_PSD(TTA(:,1),0.005,8192,1); [f1,Sf1]=Jason_PSD(TBA(:,1),0.005,8192,1); [f1,Sf1]=Jason_PSD(TowerF,0.005,8192,1);
I’m not sure if I’m using it correctly, so please point out if there are errors.
The first parameter is of course the data, the second parameter is the time step I chose for my calculation, and the third I filled in is 8192 (when in normal fft a larger value than the amount of data is taken here, and it is often the nth power of 2. Each set of data has 6000 values so I filled in 8192 here)

Regarding the previously mentioned WAFO function for calculating the power spectrum, I tried to adjust the smoothness setting and got a relatively normal PSD.

Whether I calculate the fft myself or use WAFO’s function to calculate the power spectral density, one thing I don’t understand is why the excitation load is concentrated at low frequencies while the response (e.g. YawBrTAxp) is concentrated at high frequencies.
Whether I calculate the fft myself or use WAFO’s function to calculate the power spectral density, one thing I really don’t understand is why the excitation load is concentrated at low frequencies while the response (e.g. YawBrTAxp) is concentrated at high frequencies. The script you gave also shows this (although the scale in the frequency domain is slightly different)

Best regards
Tianhui

[Jason.Jonkman]

Dear Tianhui.Liu,

It doesn’t appear that you are using the Jason_PSD.m script correctly. If I understand correctly, your time series goes from 100-400 s (300-s long), with a time step of 0.005 s (for a total of 60000 points). I also think it helps to bin the PSD for smoothing (without binning, df = 1/300 = 0.00333333 Hz; with BinLen = 10, this would coarsen the result to df = 0.0333333 Hz). So, I would expect you to use Jason_PSD.m as follows:

[ f1, Sf1, f1_Bin, Sf1_Bin ] = Jason_PSD( iceforce, 1/300, 60000, 1, 10)

How do these plots look?

Best regards,

[Tianhui.Liu]

Dear Jonkman
Following your advice, these are what I got:

I have to say, this looks a bit confusing due to the large number of data points.

Best regards,
Tianhui

[Jason.Jonkman]

Dear Tianhui,

In your original PSD plots (from WAFO), I see dominant peaks at:
0 rad/s = 0 Hz
15 rad/s = 2.4 Hz
34 rad/s = 5.4 Hz

In the plots from Jason_PSD.m, I see dominant peaks at:
0 Hz
2 Hz
5 Hz
15 Hz

Overall, the agreement seems quite close (the 15 Hz peak is beyond the range plotted in the WAFO results). It may help to show the results from Jason_PSD.m on a linear y-axis to better compare to the results from WAFO. Again, I’m not sure why you see oscillations near the peaks in the WAFO results. The Jason_PSD.m results don’t seem to show these oscillations, at least not with the log scale.

Best regards,