# Free decay analysis and natural frequency

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

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

and here is the power spectral density functions for it.

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

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,

Dear jason,

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

Kindly go through it and see what is the problem

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

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 (http://forums.nrel.gov/t/community-rules/706/1) 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,

Dear Jason,
I would just like to know your comments on the zoomed in frequency responses i have sent you a pic of.

regards

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,

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

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,

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

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

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,

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

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,

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

Dear Purushotham,

Input WtrDpth in HydroDyn is set to 200 m in the FAST model NREL has provided for the MIT/NREL TLP.

You’re understanding of how to set a free-decay response is correct.

If you are still experiencing an error, even under small initial platform displacements, your problem could be caused by the use of the FEAMooring mooring module, and the known sensitivity to FEAmooring inputs Tension and LUnstrLen. See the following forum topic for more information: http://forums.nrel.gov/t/for-tlp-in-still-conditions-pretension-not-matching-with-output-tension/1418/4. As a result of this sensitivity, it may be difficult to set up FAST models using FEAMooring with initial platform displacements. The other mooring modules in FAST (MAP++ and MoorDyn) do not have similar problems, so, you may wish to switch to one of those.

Best regards,

Dear jason,

i have enclosed my heave free decay response , kindly go through it also jaosn what would be the expected natural frequency in heave period and also what would be the damping ratio, i am getting a response which is not qualitatively consistent with the other free decay results , that is the trend is not same kindly reply on this aspect.

Dear Purushotham,

The natural frequencies of the MIT/NREL TLP in the undisplaced position are documented in Table 6 of Denis Matha’s MS thesis-turned NREL report: nrel.gov/docs/fy10osti/45891.pdf. Here it is reported that the platform-heave natural frequency is 0.4375 Hz, which matches the results you’ve shown. Your results show a very small level of damping, likely because only hydrodynamic radiation damping is included in the heave direction in this FAST model.

Best regards,

Dear jason,

How do i include the other forms of damping in this model in fast so that i get a more damped reponse, which is the case with all other degrees of freedom except with that of the heave, i want this information to calculate the damping ratio, which files need to be changed so that i can get a more damped output. kindly reply in this regard.

regards

Dear Purushotham,

For the heave direction, if radiation damping from potential-flow theory is not giving a satisfactory level of damping, you can either add additional linear damping or axial quadratic drag through the strip-theory formulation in HydroDyn. See the draft HydroDyn User’s Guide and Theory Manual for more information: wind.nrel.gov/nwtc/docs/HydroDyn_Manual.pdf.

Best regards,