Comparison NREL 5 MW Land vs FOWT platforms

Wind & Water
1

Dear all,

Currently I’m working on a research to compare the NREL 5 MW wind turbine model with land based foundation and the 4 FOWT platforms under different conditions. I conducted some preliminary simulations with very simple environmental conditions to verify that everything was OK. This are the conditions :

  • SteadyWind of 8 m/s; conditions set on NRELOffshrBsline5MW_InflowWind_Steady8mps.dat
    1 WindType
    8 HWindSpeed
    90 RefHt
    0.2 PLExp

  • No waves; conditions set on every SeaState file of the 4 FOWTs
    0 WaveMod

  • RotSpeed 8.5 m/s; initial condition set on evey ElastoDyn file to minimize the transient behaviour at the beging of the simulations.
    8.5 RotSpeed
    Value calculated from the expression:
    TSR=V_tip/V_wind=80 (m/s)/11,4 (m/s) ≈7
    RotSpeed (rpm)=(TSR·60·V_wind)/(2pi·R)=(7·60·8)/(2pi·63)≈8.5 rpm

  • I prolonged the simulations to 600 s at the .fst files
    600 TMax

As you can see those are the only files and variables that I have modified. The rest of the files are the original ones that come with every model when you download it from GitHub.

I have attached the results. I was expecting something similar to the GenPwr curve from the land foundation in every FOWT, but only the semi submersible model satisfy my expectations. I can’t understand why the other 3 models (Barge, Spar and TLP) project that output.

Land_SteadyWind_8mps
Land_SteadyWind_8mps560×420 4.45 KB

Barge_SteadyWind_8mps_NoWaves
Barge_SteadyWind_8mps_NoWaves560×420 6.54 KB

Semi_SteadyWind_8mps_NoWaves
Semi_SteadyWind_8mps_NoWaves560×420 5.33 KB

Spar_SteadyWind_8mps_NoWaves
Spar_SteadyWind_8mps_NoWaves560×420 5.78 KB

TLP_SteadyWind_8mps_NoWaves
TLP_SteadyWind_8mps_NoWaves560×420 5.95 KB

I would be grateful if you could enlighten me about the behaviour of those models under such simple conditions.

Kind regards,
Ignacio López

2

Dear @Ignacio.Lopez,

Presumably the first and third plots you are showing are for the land-based and OC4-DeepCwind semisubmersible, respectively; is that correct?

The other models are showing clear signs of a strong start-up transient, resulting from not setting good initial conditions for platform surge and pitch. To avoid this transient, we generally recommend for floating wind systems to set the initial platform surge and pitch angles in ElastoDyn (PtfmSurge, PtfmPitch) to their expected (mean) values conditioned on the mean hub-height wind speed you are using. (The r-test for the OC4-DeepCwind semisubmersible sets good initial for surge and pitch in ElastoDyn based on a mean wind speed of 8 m/s).

Best regards,

3

Dear @Jason.Jonkman ,

Yes, the first and third plots are for the land-based and OC4-DeepCwind semisubmersible, respectively.

As you suggested, I changed the initial conditions of the variables PtfmSurge and PtfmPitch to their expected mean values at a mean wind speed of 8 m/s. Now the GenPwr is behaving normally.

I have some new doubts. The method I used to calculate those values is to check the results of the PtfmSurge and PtfmPitch in the firts simulations (without the correct initial conditions). Then I estimated the mean value and remade the simulations with the good initial conditions.

  1. Is there another more “academic” way to calculate the expected values of PtfmSurge and PtfmPitch under different wind conditions?

  2. If I understand your answer correctly, to calculate those initial conditions, in the case I use a turbulent wind, I should only consider the mean hub-height wind speed, right? But what if I also have irregular waves? Are they not taken into account?

  3. Lastly, what’s the typical transient time in the FOWTs with turbulent wind and irregular waves?

Thanks a lot.

Best regards,

4

Dear @Ignacio.Lopez,

Here are my responses:

  1. Running simulations with a steady wind input and looking at the mean response as the start-up transients die away is a good way to estimate the expected values. We have been working on a direct steady-state solve that will be available in a future release of OpenFAST that will avoid the need to specify such initial conditions, but this work is not yet complete.
  2. I would expect the expected values to be the same whether the wind is steady or turbulent. I would not expect the waves to result in a mean offset of the floater, unless the second-order mean drift or current loads are sizeable relative to the mean wind-induced thrust loads.
  3. This will depend on the lowest natural frequencies of the FOWT, which are typically the rigid-body modes. Setting good initial surge and pitch values (in addition to good initial rotor speed and blade-pitch angles often needed by the wind turbine controller) should minimize the transient time, but most FOWT models will likely still need 60 s or so of transient time even with good initial conditions.

Best regards,

5

Dear @Jason.Jonkman,

I have simulated a simple scenario of uniform wind and no waves with the four types of floating platforms, and I have evaluated the GenPwr. The uniform wind condition I have simulated is a step wind of 8-14 m/s with a transition time of 5 s. Here are the results:

UniWind_NoWaves_MULTI
UniWind_NoWaves_MULTI1920×955 39.8 KB

I think the responses from the Barge, Land, and TLP are something expected, but I am not sure about the Spar and Semi. I don’t think a peak of almost 6 MW is normal. Is it due to my wind conditions being too aggressive or unrealistic?

Best regards,

6

Dear @Ignacio.Lopez,

The NREL 5-MW baseline wind turbine atop the OC3-Hywind spar and OC4-DeepCwind semisubmersible share the same baseline controller, which I presume you are using. This controller responds more slowly (less gain) than the baseline controller of the other systems you are simulating due to the long natural periods of the platform-pitch modes in the spar and semi to avoid controller-induced negative damping. I suspect this controller change is the reason for the overshoot in power you are seeing.

Best regards,

7

Dear @Jason.Jonkman,

Thank you for your quick reply.

So, if the controller in the Spar and Semi responds more slowly and because of that I am obtaining a peak of almost 6 MW, then what is the capacity of the wind turbine to bear an overshoot in power?

I remember reading somewhere that the maximum overshoot that the turbine could withstand was around 10 % of its capacity. That’s why the results from the Spar and Semi didn’t add up for me.

Is this assumption correct or the limit for an overshoot in power is higher?

Best regards,

8

Dear @Ignacio.Lopez,

Well the NREL 5-MW baseline wind turbine is not real, and the amount of power “overshoot” that is permitted is not specified for this “paper” turbine. I agree that 10% sounds more reasonable in reality, but like you said, this specific step change in wind speed is likely unrealistic as well.

Best regards,