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Free decay analysis and natural frequency

Posted: Thu Mar 02, 2017 8:18 am
by Purushotham.Chinu
Dear Jason,

I would just like to know how to carry out free decay test in all six degrees of freedom in fast for a floating platform such as a tlp,and also how can i find out the natural frequency of the system in all 6 degrees of freedom through fast, how is it possible to do so,also jason i went through your paper in isope for rao estimation, is there any direct tool available from wherein we can get the raos from the time history of the responses, like the power spectral densities of the various motions divided by the spectral density of the wave elevation gives the rao,and also jason when doing the fft in my case of a tlp , i am getting a peak close to zero can u please tell me whether this is rite or not and if not what could have possibly gone wrong,kindly reply at the earliest.


regards

purushotham

Re: Free decay analysis and natural frequency

Posted: Thu Mar 02, 2017 8:36 am
by Purushotham.Chinu
Dear jason,

In follow up to my above question, i would just like to share my frequency responses, i have simulated it for 600 seconds and carried out a fft in matlab

[img]blob:https://www.flickr.com/d76c95b7-cf21-4439-b612-952cb4dfff58[/img]
Image
Image

and here is the power spectral density functions for it.
Image
http://farm3.staticflickr.com/2867/3282 ... 9956_b.jpg

kindly go through the above pictures and let me know if they are fine or i need to improve on something.

regards

Re: Free decay analysis and natural frequency

Posted: Thu Mar 02, 2017 11:06 am
by Jason.Jonkman
Dear Purushotham,

Setting up a free-decay test in FAST is trivial; simply set still water (WaveMod=0) and whatever platform initial conditions you desire. I posted a link in my post dated Feb 24, 2017 above to a forum topic about how to calculate eigenfrequencies of offshore systems using white-noise waves. NREL has not publicly provided a post-processing script to derive the RAOs from white-noise wave excitation, but it should not be difficult to develop your own post-processors for your own purposes e.g. using MATLAB.

It is very hard for me to comment on your FFTs because all of the data is confined to low frequencies; I suggest you zoom in to the frequencies of interest.

Best regards,

Re: Free decay analysis and natural frequency

Posted: Thu Mar 02, 2017 5:31 pm
by Purushotham.Chinu
Dear jason,

I am sending the zoomed pictures of the responses in the following figures
Image

Image

Image

Kindly go through it and see what is the problem

Re: Free decay analysis and natural frequency

Posted: Fri Mar 03, 2017 6:33 am
by Purushotham.Chinu
Dear Jason,

In regards to the topic i coudlnt find your post where you have given ways to calculate eigen natural frequencies, also jason, i would like to know whether fast directly gives the natural frequencies subject to various conditions or is there a way to do this(in parked and unparked conditions),i would like to investigate the responses in the parked(no wind condition),kindly tell me what are the parameters i need to change to get response in no wind condition ,also for different angles of wave attack i would like to investigate the responses in all six degrees of freedom,also regards to the rao computation can you share the source code so that i can get the raos,also jason since i want to study the frequency responses how long would you advise me to run the simulation would 900 seconds suffice, since i tried for 3600 seconds but the post processing seems difficult,kindly reply in this regard.

regards

Re: Free decay analysis and natural frequency

Posted: Fri Mar 03, 2017 9:16 am
by Jason.Jonkman
Dear Puroshotham,

I feel like I have already answered most of your questions. I can't solve your problem for you. Please be respectful of the forum rules (viewtopic.php?f=17&t=1162) and only ask questions after you've reviewed the documentation and struggled through the problem yourself. Ask your colleagues for help. We try to help where we can, but please realize that we can't offer unlimited support to free data and open-source software. Narrowly defined questions are more likely to get a response, particularly if question targets a potential problem with the software NREL has provided.

Best regards,

Re: Free decay analysis and natural frequency

Posted: Fri Mar 03, 2017 5:03 pm
by Purushotham.Chinu
Dear Jason,
I would just like to know your comments on the zoomed in frequency responses i have sent you a pic of.


regards

Re: Free decay analysis and natural frequency

Posted: Fri Mar 03, 2017 5:15 pm
by Jason.Jonkman
Dear Purushotham,

A few of peaks are clearly visible, which together with the time series, should be able to be used to identify the key platform natural frequencies. I'm not sure what the time series look like, but the pitch and roll results are a bit noisy (and some of the peaks extend beyond the plot), which could perhaps be fixed by filtering.

Best regards,

Re: Free decay analysis and natural frequency

Posted: Sat Mar 04, 2017 7:21 am
by Purushotham.Chinu
Dear Jason,

Thank you for quick reply, what could be the reason for noisy responses in pitch and roll, and is there a means of filtering the responses in fast and also jason the peaks correspond to natural frequency rite , so is it accurate to assume the one corresponding to the highest peak would be its natural frequency, also jason what you mea when you say use the plots in combination with the time series(kindly elaborate on this one), as i suppose the fft plots would suffice, kindly reply on this regard.

regards

Re: Free decay analysis and natural frequency

Posted: Sat Mar 04, 2017 9:01 pm
by Jason.Jonkman
Dear Purushotham,

The more the time series are periodic the more pronounced the peaks will be in the FFT. You didn't generate the FFT using FAST (I'm assuming you used MATLAB as the post-processor); you can provide filtering in the post-processor.

The natural frequencies should show up as peaks in the FFT, but there may be several peaks in each FFT, and analysis of the time series should help you determine which frequency corresponds to which mode.

Best regards,

Re: Free decay analysis and natural frequency

Posted: Thu Mar 09, 2017 1:02 am
by Purushotham.Chinu
Dear jason,

I am trying to conduct free decay test using fast by your said procedure i am setting the wavemod to 0 and also ensuring that there is steady wind or uniform wind supply and i also gave displacements for platforms in the elastodyn file, but i am getting the following error

Image


I also tried changing the hydrodyn file water depth from 600 meters to 200 meters , but that also seem to not help, kindly please go through this and tell me what changes i need to make in which file so that i can get the free decay results, kindly reply as soon as possible.


regards


purushotham.s

Re: Free decay analysis and natural frequency

Posted: Thu Mar 09, 2017 6:22 am
by Jason.Jonkman
Dear Purushotham,

It looks like the initialization of the FEAMooring module is failing for some reason. Is this a model NREL has provided e.g. Test 23 from the FAST CertTest or something else? If it is Test 23, what did you change?

Best regards,

Re: Free decay analysis and natural frequency

Posted: Thu Mar 09, 2017 5:46 pm
by Purushotham.Chinu
Dear jason,

it is the test23 file which nrel has provided it says that it takes 200 m water depth in fea mooring analysis, so accordingly i changed my water depth to 200 , but still it dosent seem to work, kindly guide me on this aspect.



regards

Re: Free decay analysis and natural frequency

Posted: Thu Mar 09, 2017 9:06 pm
by Jason.Jonkman
Dear Purushotham,

Test 23 as supplied by NREL has a water depth of 200 m, so, it doesn't sound like that was a change you've made.

Test 23 is a model of the NREL/MIT TLP. I don't know what magnitude of platform displacement you initialized the model to for a free-decay analysis, but--as you're likely aware--a TLP has limited range of motion. Perhaps you've initialized the model such that tendons become very taut or very slack?--both of which could cause problems with the mooring model.

Best regards,

Re: Free decay analysis and natural frequency

Posted: Fri Mar 10, 2017 6:13 am
by Purushotham.Chinu
Dear jason,

When i mean i changed to 200 m i mean the hydrodyn input for water depth not the default value taken by the fea mooring file, also jason regarding the initial platform displacements i gave about 5m surge with all the rest degrees of freeddom initial displacement zero , also i have changed that to 1m and 2 m and tried with that as well,now is my approach correct because i believe the only two things you need to do is set wavemod to zero and have a constant or steady wind speed, but still seems to be a error, kindly tell me how should i change my input so that i can get the free decay responses.


regards

purushotham