OC4 Phase II results

Hi all,

At the end of the OC4 Phase II results paper (http://www.nrel.gov/docs/fy14osti/61154.pdf) the following reference is made:

[5] Website for OC4 project: http://oc4.collaborationhost.net.
An anonymous login may be used to view results from the
project on this site:
login: anonymous@oc4.collaborationhost.net
password: changeit2

The website appears to be offline.
Is there an alternative mirror to this site available?

Thanks in advance.
Aonghus O’Connor

Dear Aonghus,

Since that paper was published, the OC4 results have been moved to Google Drive: drive.google.com/drive/folders/ … EZLdDRxX2s.

Best regards,

Thanks Jason.

One more thing: Are the FAST input files available for the load cases performed by NREL?

Dear Aongus,

We have not made all of the FAST input files NREL developed for OC4 Phase II available, but you can find the input files for a shortened version of load case 3.7 available as Test #25 in the FAST v8 CertTest. The other load cases use variations of that one.

Best regards,

Thanks Jason,

Do you know what version of FAST that the NREL Load Case results were generated in? If it is an older version than the latest version of FAST (8.16) do you expect that the results would differ much to the latest version do you think?

Best wishes

Dear Aonghus,

The NREL results for IEA Wind Task 30 OC4 Phase II would have been generated with a version of FAST v8 older than newest version (8.16)–likely different load cases would have been run with different versions as FAST v8 was being developed in parallel to the project–but I would have to reach out to my colleagues who ran those cases to find out exactly which version was run for which case.

Regardless, I would expect the results to be generally consistent if the load cases are run with the newest version of FAST (v8.16) using the same features enabled as the original run.

Best regards,

Dear Jason,

The full NREL results for load case 3.7 are unfortunately not within the simulation results given in the drive specified drive.google.com/drive/folders/ … EZLdDRxX2s.

Are these available as a verification that Test 25 is running correctly?



Within the OC4 test results given here drive.google.com/drive/folders/ … EZLdDRxX2s only three load cases, with all DOF enabled, contain results from NREL. These three, load cases 3.1, 3.2 and 3.6 unfortunately contain unreadable data.

Could you point me towards readable results for these load cases please.

Are results for the other load cases with all DOF enabled 3.3, 3.4, 3.5, 3.7, 3.8 and 3.9 available elsewhere?


Dear Dawn,

I asked Amy Robertson of NREL, and she informed me that there were some problems when the data was uploaded. However, she has now uploaded the NREL results again. Please check the Google drive site again.

Best regards,

Hi Jason,

Thanks, I can see that Amy has uploaded the results for L3.7, however they are also unreadable when downloaded, as are the other results for L3 You can look at them as a preview on Google Drive but this just shows 7 columns of data with no explanation to what they are. Can you please ask her to try again.


Dear Dawn,

I asked Amy, and here is her response:

Best regards,

Hi Jason,

I have no problem downloading the files, all are quite large from the other companies too. However all the rest open as text where as NRELs files look like this…

In the preview on Google Drive it shows numbers, but only the first 7 columns. The data is therefore unavailable.

Is there any otherway that this data can be retrieved as it is hard to do any form of comparisson without it. Also all other companies data is labeled but the order that it was generated in differs between them. I feel that having completely unlabeled data is making it unnecessarily confusing for users.


My apologies as I have now managed to open the files as readable data.
Hopefully it will not be to difficult to decide what each column is predicting.
Thank you for your time.

Hi, I am trying to validate the results that I am getting using Test 25 against the NREL results for Load Case 3.7.

When looking specifically at the platform motion I get a good correlation for surge roll and heave.

For sway and pitch the motion is very similar but offset below the NREL results

For yaw it is also offset below the NREL results but also has a larger motion.

I noticed that the rotational speed is higher for the NREL results and that the wave profile is slightly different. Can you please let me know if different wind and wave inputs were used for the OC4 3.7 load case than are used in Test 25 and if so what the OC4 ones were?

If I am on the completely wrong line of thought then do you know what else would be causing the lower offset and the increased yaw motion?

Thanks in advance,

Sorry should have put that Test 25 is calling on

Could you please let me know what the different random WaveSeeds used for OC4 phase II Load Case 3.7 were (semisubmersible)?

Also was the simulation run for 8000s with the first 2000s discarded as for HyWind, or for 4600s with the first 1000s discarded?

Also, how many simulations were averaged for the semisubmersible?


As described by Dawn.Ward, comparing the response of the latest OC4 CertTest Model (Test 25) with the original OC4 simulation results yields some differences. The CertTest model uses AeroDyn15 with BEMT, while the original OC4 simulation used AeroDyn14 with DYNIN.
For the original OC4 simulation the mooring system was modeled through a quasi-static approach and the current Test25 model uses a dynamic mooring line model.

Due to these different modelling approaches it is expected to observe some differences in predicted system response between the two models.

Best regards, Fabian

Fabian Wendt, Dipl.-Ing.
Engineer | National Wind Technology Center

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401

I am verifying the next part of my analysis against the OC4 Phase II results for Load Case 2.6 - RAOs and PSDs with no wind.

I have good results except my platform surge range is approx. 2m, nearly double NREL’s range. I realise this is still a small motion but would like to know if I am missing something important?

I am using MAP for the mooring to match the specifications used by NREL as much as possible, but FAST version 8. The following settings have been implicated

In the.fst
0 CompInflow - Compute inflow wind velocities (switch) {0=still air; 1=InflowWind; 2=external from OpenFOAM}

In ElastoDyn file
0 RotSpeed
False DrTrDOF - Drivetrain rotational-flexibility DOF (flag)
False YawDOF - Yaw DOF (flag)
False GenDOF - Generator DOF (flag)
False FlapDOF1 - First flapwise blade mode DOF (flag)
False FlapDOF2 - Second flapwise blade mode DOF (flag)
False EdgeDOF - First edgewise blade mode DOF (flag)

In ServoDyn file
1 HSSBrMode - HSS brake model {0: none, 1: simple, 3: user-defined from routine UserHSSBr, 4: user-defined from Simulink/Labview, 5: user-defined from Bladed-style DLL} (switch)
0 THSSBrDp - Time to initiate deployment of the HSS brake (s)

In AeroDyn15 file
0 WakeMod - Type of wake/induction model (switch) {0=none, 1=BEMT}

Thanks in advance for any help and advice that can be offered.

Dear Dawn,

Fabian can probably confirm the exact FAST v8 settings he used to run OC4 Phase II load case 2.6, but just a few comments on your settings:

  • This case is meant to involve no aerodynamics, so I would set CompInflow = CompAero = 0.
  • Because you’ve disabled all structural DOFs in the rotor and nacelle, you can safely disable ServoDyn by setting CompServo = 0.
  • I agree with your changes to ElastoDyn.

Best regards,

Going back to a problem I had at the start of my research which I never addressed properly, where my results showed the platform pitch offset when operating in stall compared to feather, although the feather is also offset from zero.

Below shows a comparison of my results with the results given in NREL/TP-500-41958 page 134, for the barge.
The top image shows the NREL results, which have no offset and the bottom is my results but for the semi-sub used in OC4 Phase II.
Both are 18m/s stochastic winds and similar irregular wave conditions.

Any advice on why this is happening and whether it is down to user error or the semi-sub design or if the results in the NREl doc have simply been detrended such that the mean is zero?

Many thanks in advance.