Properties of NREL 5MW-OC4-DeepCwind SemiSubmersible wind turbine in FAST and reference

Dear Dr. Jonkman,

Recently I have studied the reference below about the NREL 5MW-OC4-DeepCwind SemiSubmersible wind turbine.
[1] Robertson, A , et al. “Definition of the Semisubmersible Floating System for Phase II of OC4.” (2014).

And I am also looking into the file in the FAST v8 and OpenFAST file about 5MW-OC4-DeepCwind SemiSubmersible wind turbine, which in FAST v8 it refered to the Test 25.
In the file called ‘‘NRELOffshrBsline5MW_OC4DeepCwindSemi_ElastoDyn’’,
-8.6588 PtfmCMzt - Vertical distance from the ground level [onshore] or MSL [offshore] to the platform CM (meters)
2.56193E+09 PtfmRIner - Platform inertia for roll tilt rotation about the platform CM (kg m^2)
2.56193E+09 PtfmPIner - Platform inertia for pitch tilt rotation about the platform CM (kg m^2)
4.24265E+09 PtfmYIner - Platform inertia for yaw rotation about the platform CM (kg m^2)

while in the Table 3-3 (Floating platform structural properties) in the reference [1],
-13.46 PtfmCMzt
6.827E+09 PtfmRIner
6.827E+09 PtfmPIner
1.226E+10 PtfmYIner

Therefore,i am not sure which number is correct and which model to trust. At present, i have built a OC4 Semi model by myself,which needs to be validated with the FAST model output. However, some parameters in the FAST model are difficult to determine which needs to be determined by the Reference [1]. For example, in reference [1], i can know the Center of buoyancy below SWL is 13.15m. while in FAST v8, i cannot find in the file called ‘‘NRELOffshrBsline5MW_OC4DeepCwindSemi_HydroDyn’’. The choice of these parameters has a great influence on the response of the OC4 wind turbine. Therefore, I am looking forward to your reply,
thanks,
Yifan

Dear Yifan,

A similar question was asked and answered in the following forum topic: http://forums.nrel.gov/t/5mw-semisubmersible-oc4-deepwind-model/2031/2.

Best regards,

Dear Jason,

thank you very much.

Best regards,

Dear Jason,

recently I have met a problem to identify the natural frequency of the tower of the NREL 5MW OC4 semisubmersible wind turbine.
From the report and FAST V8, we can know the details as below:

---------------------- DISTRIBUTED TOWER PROPERTIES ----------------------------
HtFract TMassDen TwFAStif TwSSStif
(-) (kg/m) (Nm^2) (Nm^2)
0.0000000E+000 4.6670000E+003 6.0390300E+011 6.0390300E+011
1.0000000E-001 4.3452800E+003 5.1764400E+011 5.1764400E+011
2.0000000E-001 4.0347600E+003 4.4092500E+011 4.4092500E+011
3.0000000E-001 3.7354400E+003 3.7302200E+011 3.7302200E+011
4.0000000E-001 3.4473200E+003 3.1323600E+011 3.1323600E+011
5.0000000E-001 3.1704000E+003 2.6089700E+011 2.6089700E+011
6.0000000E-001 2.9046900E+003 2.1536500E+011 2.1536500E+011
7.0000000E-001 2.6501800E+003 1.7602800E+011 1.7602800E+011
8.0000000E-001 2.4068800E+003 1.4230100E+011 1.4230100E+011
9.0000000E-001 2.1747700E+003 1.1363000E+011 1.1363000E+011
1.0000000E+000 1.9538700E+003 8.9488000E+010 8.9488000E+010

As we known, the third and fourth columns represent the bending stiffness EI of the tower in fore-aft and side-side direction. so later i tried to calculate the frequency by modelling dynamic. it was about 2.4308e+03 Hz of the tower fore-aft vibration frequency as the orders of the magtitude of the mass is small to the stiffness of the tower. while in FAST V8, I identified the natural frequency of the tower fore-aft vibration is 0.428Hz. The method i used is to set the initial condiiton of fore-aft vibration is 1 while other vibration is 0 under still water. And then to get the output of the FAST. So I doubt if the numbers are accurate.

looking forward your suggestion,
thanks,
Yifan

Dear Yifan,

I’m not sure I understand how you obtained a frequency of 2400 Hz, but this is much higher than it should be. The frequency of 0.43 Hz is much closer to what I expect it to be.

Best regards,

Dear Jason,

thanks for your reply, i have solved the problem above. But I have another problem for calculating the added linear hydrostatic stiffness matrix.

i have read “HydroDyn User’s Guide and Theory Manual” and OC4 report and understand the calculation of elements (4,4) and (5,5) for roll and pitch stiffness and known its value: Kh(4,4)= Kh(5,5)=-3.776e8. However, i suspect why the value is negative. In my opinion, the stiffness of the platform under still water is contributed by the stiffness of the hydrostatic restoring matrices and linear mooring system restoring matrices. In the linear mooring system restoring matrices of the OC4 report, Km(4,4)= Km(5,5)=8.73e7. if we add Kh(4,4) and Km(4,4), we will get a negative value for the stiffness. Could you give some explanation for it? Because i want to calculate the natural frequency of the roll and pitch motion by K/M. Thus, i want know the stiffness of the roll and pitch motion of the platform.

BTW, I dont know how to understant the data in the FAST file for the OC4 Semi HydroDyn as below. Because i cannot see the value of the AddF0, AddCLin, AddBlin, AddBQuad, and cannot understand what the number in the matrix in the left side represents for.

---------------------- PLATFORM ADDITIONAL STIFFNESS AND DAMPING --------------
0 0 0 0 0 0 AddF0 - Additional preload (N, N-m)
0 0 0 0 0 0 AddCLin - Additional linear stiffness (N/m, N/rad, N-m/m, N-m/rad)
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 1451298897 0 0
0 0 0 0 1451298897 0
0 0 0 0 0 0
0 0 0 0 0 0 AddBLin - Additional linear damping(N/(m/s), N/(rad/s), N-m/(m/s), N-m/(rad/s))
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0 AddBQuad - Additional quadratic drag(N/(m/s)^2, N/(rad/s)^2, N-m(m/s)^2, N-m/(rad/s)^2)
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0

thanks,
Yifan

Dear Yifan,

Just a couple comments:

  • The values of Kh(4,4) and Kh(5,5) are negative because they reflect the effects from hydrostatic pressure only. In addition to Kh (hydrostatic), and Km (mooring), you should also add the effect from body weight, i.e., Kw(4,4) = Kw(5,5) = -mgz, where m is the full-system mass, g is gravity, and z is the full-system center of mass location (negative valued), so Kw(4,4) and Kw(5,5) are positive. This topic has been discussed for the OC4-DeepCwind semisubmersible more in the following forum topic: http://forums.nrel.gov/t/the-frequency-component-of-flapwise/1730/1.
  • The nonzero (4,4) and (5,5) elements of AddCLin in the HydroDyn model of OC4-DeepCwind semisubmersible model are associated with the water ballast. But please be aware that in OpenFAST v2.6 and newer, AddCLin no longer needs to be specified in HydroDyn when adding ballast; this is because hydrostatics in the strip-theory solution are now computed at the displaced position/orientation of the floater.

Best regards,

Dear Jason,

thank you very much.

Dear Jason,

From what you said ‘Kw(4,4) = Kw(5,5) = -mgz, where m is the full-system mass, g is gravity, and z is the full-system center of mass location (negative valued)’, when i calculate the z above, should i consider the influence of the weight of the mooring system ? if so, how can i identify the center of mass location of the mooring system?

thanks,

Dear Yifan,

In my equations, m and z consider the FOWT without the mooring system. The mooring system is only accounted for in Km and the pretension force.

Best regards,

Dear Jason,

thanks for your reply. In FAST_v8, when i want to run the semi wind turbine under turbulent wind, i cannot find the wind source file. Such as Uniform wind file, binary TurbSim FF, binary Bladed-style FF, HAWC format. looking forward your help.

best,

Dear Yifan,

I’m not sure I understand what you are asking. Are you asking where the wind data is stored for the FAST v8 CertTest of the OC4-DeepCwind semi model, how to generate your own wind data for your own simulation, or something else?

Best regards,

Dear Jason,

i am jusk asking where the turbulent wind data is stored for the FAST v8 CertTest of the OC4-DeepCwind semi model (Test 25.fst)? For the the OC4-DeepCwind semi model (Test 25.fst), i only found NRELOffshrBsline5MW_InflowWind_Steady8mps file for the steady wind, shown as below,

where i cannot find the “Wind/08ms.wnd” and “wasp\Output\basic_5u.bin” .

BTW, i can find a turbulent winds file for the like NRELOffshrBsline5MW_InflowWind_12mps for the turbulent winds on 31x31 FF grid and tower for FAST CertTests #18, #19, #21, #22, #23, and #24.

thanks,

Dear Yifan,

Test25 was set up to use steady wind (WindType = 1). The settings of “Wind/08ms.wnd” and “wasp\Output\basic_5u.bin” etc. for the turbulent wind input Filenames are just placeholders; these files are not part of the FAST v8 CertTest. I recommend using TurbSim to generate your own turbulent wind data for your own purposes.

Best regards,

thanks.