Free-Vibration Response of OC3HYwind Spar turbine

Dear Dr. Jonkman

I have another query regarding OC3HYwind (Test24) and will appreciate your help. I am trying to understand FAST and dynamic response of floating body. I subjected the OC3Hwyind turbine to initial displacement of 2 m in surge direction. I flag the potential flow model “0”, and use only strip theory. Also in strip theory i made all the coefficient to “0”. I requested “HydroFzi”," AddFzi" in the output. There is some force in “HydroFzi”, which i am unable to understand. I have attached the plot of “HydroFzi”, “AddFzi”, “initial buoyancy from hydrodynamic summary”. I know about the source of “AddFzi” and “Buoyancy” but i don’t know about “HydroFzi”. I will be thankful if you can help in understanding “HydroFzi”.

Additionally, If you can send me unofficial “Fast theory manual” and “MoorDyn input file for Test24”. It will help me to understand the FAST more deeply.
i really appreciate your help and cooperation. (my email id: ktk.zahid@gmail.com)

Regards,
Zahid
AddFzi.PNG
Bouyancy.PNG
HydroFzi.PNG

Dear Zahid,

The “odd” square shape you are seeing in the time series is likely due to the small round-off error that results from the fact that FAST is only outputting 4-digits of precision based on the setting of OutFmt = “ES10.3E2” in FAST’s primary input file. My guess is that the shape would look more rounded if you added at least one more digit of precision in the output format. See the following forum topic for more information on OutFmt: http://forums.nrel.gov/t/fast-integrator-step-size-precision/247/1.

I’ve sent you the “Unofficial FAST Theory Manual” by e-mail.

I don’t have copy of the MoorDyn input file for OC3-Hywind model of Test24, but it is probably not too difficult to make one yourself because the MAP++ and MoorDyn input files have many similarities.

Best regards,

Dear Dr. Jason Jonkman
I added one more digit precision as you suggested and now the results are better…more round shape. I also wanted to know about the source of force in “HydroFzi”. I have made all the coefficients zeros but still there is force in “HYdroFzi”. My first assumption was that may be its the the initial buoyancy force in the “HdroFzi”. but the force is smaller than “buoyancy” , so i am confused to track the source of force in “Hydrofzi” vector. Please Help me on the issue. (May be my statement is not very understandable so i attached figures).

About the Moordyn input file i used the input file available with MoorDyn program, but its shows some format error and gives reading error.
Always been thankful for you help.
HydroFzi.PNG
AddFzi.PNG
Initial Buoyancy.PNG

Dear Zahid,

I’m not sure how you are plotting buoyancy, but the value of HyrdoFzi = 8.07E7 N matches the static buoyancy reported in the OC3-Hywind specifications report: nrel.gov/docs/fy10osti/47535.pdf. The small oscillation in HydroFzi is from the small oscillation in AddFzi, which is included in the calculation of HydroFzi.

I can’t comment on the issue with MoorDyn without knowing more about what file you are using an what error you are receiving.

Best regards,

Dear Dr. Jason Jonkman

Upon your suggestion i looked into the buoyancy force in the hydrodynamic summary file and also in the report you referred. There is difference in the buoyancy force in the summary file, however the submerged volume is same in both “the report” and “HD.sum”. I have attached the “HD.sum file” and also a snapshot of it. In the snapshot there is total structure volume and submerged volume, if I multiply the submerged volume with “10259.8066", then i get buoyancy force equal to that given in the report, but if i multiply “total volume” with "10259.8066”, then i obtain buoyancy force which is given in the summary file. Please help me to figure out the problem. (i am using only strip theory and flagged the WAMIT to False)Thank you very for your time.

Regards,
Zahid
HD summary file.txt (205 KB)

Dear Zahid,

Indeed, there is a bug in HydroDyn whereby the external and total buoyancy written to the HydroDyn summary file is incorrect for the version of HydroDyn included in FAST v8.15 and v8.16. This bug has been discussed in the following forum topic and will be fixed in the next release of HydroDyn: http://forums.nrel.gov/t/water-ballasting-buoyancy-loads/1380/4. The time series is not effected.

Best regards,

Dear Dr. jonkman

I am thankful for clarification on buoyancy force. For the MoodrDyn error, i have attached input file i am using, and snapshot of the error. I think the input file is not in the proper format, but i don’t how to fix it.

Regards,
Zahid
NRELOffshrBsline5MW_OC3Hywind_MoorDyn.txt (2.34 KB)
Error.PNG

Dear Zahid,

I don’t see that you have the Echo, NTypes, NConnects, or NLines input parameters in your MoorDyn input file. I suggest that you do a file compare to an existing MoorDyn input file for comparison (e.g. NRELOffshrBsline5MW_OC4DeepCwindSemi_MoorDyn.dat in the CertTest\5MW_Baseline directory of the FAST v8 archive).

Best regards,

Dear Dr. Jonkman
I wanted the response of the system at location near to center of mass (instead of water surface) therefore following your previous suggestions, i specified “PtfmRefzt = -78 (for OC3hywind)” in ElastDyn input file. It is working fine however i am little confused about HdryoDyn outputs which i want to understand.
1). If i specify “PtfmRefzt”, different than water surface level,Then the output e.g. AddMxi, AddMyi, AddMzi, are calculate with reference to mean sea level or with reference to “PtfmRefzt = -78” ?

2). In the HydroDyn input file we specify additional linear stiffness coefficient for hydrostatic stiffness. The K44, and k55 (for roll and pitch) stiffness coefficient depends on the ZCOB, i want to know after “PtfmRefzt” to be different than mean sea level, do i have to input the k44 and k55 with reference to “PtfmRefzt”, or these coefficient must be calculated with reference to mean water level.

I will highly appreciate your help.
Regards,
Zahid

Dear Zahid,

The reference points and coordinate systems in FAST’s ElastoDyn and HydroDyn modules are not tied together. Changing PtfmRefzt in ElastoDyn does not effect the inputs or outputs of HydroDyn. So, when PtfmRefzt = -78 m, AddMxi, AddMyi, AddMzi etc. are still calculated relative to mean sea level and K44 and K55 are still specified relative to mean sea level.

I hope that helps.

Best regards,

It was definitely helpful. thank you Dr. Jonkman for all your help.

Dear Dr. Jason Jonkman

I have another small query about the Free vibration response and your response will be highly appreciated. I want to understand the free vibration response of OC3Hywind, as i have posted many questions. I am using FAST V8.16. All the hydrodynamic forces due to waves, and aerodynamic forces are made zero. My question is regarding the variation of Hydro-static restoring force (AddFzi) with rotational degree of platform (pitch or roll). I applied initial pitch rotation of 0 deg, 2 deg and 4 deg and checked the free vibration response (all other initial displacement initially set to zero). I plot the heave displacement and AddFzi as shown in Figure below. I am a little confused about the AddFzi, because it varies with the pitch rotation which i am unable to understand. I thought it will not vary because the linear hydro-static stiffness is diagonal. I will appreciate if you can look at the plats and help me in understanding the source of variation. I highly appreciate your help and cooperation. (I notice that as i apply initial pitch the magnitude of AddFzi varies at t = 0 as well).

Regards,
Zahid

Dear Zahid,

HydroDyn output AddFzi is the heave forces due to the additional preload, stiffness, and damping. Assuming you haven’t modified the additional preload, stiffness, and damping properties of the OC3-Hywind model that NREL has distributed, then the only nonzero term in the heave direction is AddBLin(3,3) i.e. a an additional linear damping in the heave direction due to heave velocity. Thus, HydroDyn output AddFzi for this case represents the additional linear damping force due to heave velocity. You haven’t plotted the heave velocity, but you do show that the heave displacement varies as a result of different initial pitch offsets.

I hope that helps.

Best regards,

Dear Dr. Jason Jonkman

I wanted to check the effect of hydro-static stiffness, therefore i made the other terms zero (preload and damp). I have attached the heave velcoity and also a snapshot of preload, stiffness and damping terms. I will appreciate if you can look at the figures.


Dear Zahid,

The response looks reasonable to me. Is there something specific you wanted me to comment on?

Best regards,

Dear Dr. Jason Jonkman

First i sincerely apologize for late reply. I wanted your comment on the Addfzi at t = 0 sec. From figures, the heave displacement and velocity both are zeros at t =0, for pitch =2 and 4 deg, but the AddFzi is not zeros at t = 0 sec. As the AddFzi depends on heave velocity and heave displacement (also preload = 0), if these are zeros at t = 0, i thought AddFzi will be zero as well. I want to know the value of AddFzi.

Dear Zahid,

OK, looking more closely at your results I don’t see that the platform heave velocity you are plotting is consistent with the platform heave displacement you are plotting. E.g. the velocity should be zero when the displacement hits a local minimum or maximum, but this is not the case in your plots. Are you sure you are outputting/plotting the correct channels?

By the way, AddFzi is based on the platform motion expressed in the inertial frame e.g. you should be plotting ElastoDyn outputs PtfmTDzi (or equivalently, PtfmHeave) and PtfmTVzi (instead of PtfmTVzt).

Best regards,

Dear Dr. Jason Jonkman

Following your suggestions, i plot the PtfmHeave, PtfmTVzi (instead PtfmTVzt ) and AddFzi. Additionally i wanted the response at PtfmRefz = -78m. So i plotted the the three mentioned quantities at PtfmRefz = 0m, PtfmRefz = -78m for three initial pitch rotation of 0 deg, 2 deg and 4 deg. All the plots are attached below. Basically i am unable to understand that why there is change in heave displacement with change in initial pitch rotation. I was thinking may be it is due to AddFzi where AddFzi is function of AddClin and AddBLin. I have also attached the snap of input file of AddClin and AddBLin. I would really appreciate your help in understanding the variation in Heave displacement and AddFzi with Pitch rotation. If any other input information i will post.
(All the picturte could not be uploaded here so i uploaded few in the next post).
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The same quantifies at PtfmRefzi under same initial pitch rotation of 0, 2 and 4 deg, are shown in figures attached below.
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The Input of addClin and AddBLin for this case are shown in the snapshot.