OpenFAST vs Orcaflex Free decay test comparison

Wind & Water Computer-Aided Engineering Software Tools
1

Dear NREL Forum users,

Hello. I am currently using OpenFAST and Orcaflex to build the same model and perform Free decay test.

While comparing them, I have some questions and would like to ask for opinions.

The following figure shows the results of the Free Decay test for a model including the upper floating body, turbine, etc., excluding the mooring system.

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As can be seen, the results are very similar, so I believe the construction of the remaining system without the mooring system is complete.

However, as shown in the results below, when the mooring system is applied, significantly different results are observed.

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I believe I have applied all the input data values for OpenFAST - Moordyn identically in Orcaflex, but I’m curious if there’s any issue I might be overlooking that could be causing the differences in the graphs.

Or could there be a possibility of differences in the results due to limitations in the implementation capabilities of Moordyn?

I’m in quite a predicament due to the previous issue… If you have any insights or suggestions, please, I kindly request your input.

Additionally, I am providing my Moordyn input data as follows:

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Best regards,

Jisu Lim

2

Hi Jisu,

I’m not quite sure what is happening with you simulations, but I can point out 2 things that could be causing issues, but they may not be the total solution.

  1. In the MoorDyn file, your vessel fairleads seem to be +13.475 m, above the water. Is this intentional?
  2. When comparing the free decay time series, the OrcaFlex runs with and without the mooring system seem to be very similar whereas the OpenFAST runs with and without the mooring system are obviously different. Is there a chance that the mooring system in OrcaFlex is not hooked up properly and the OpenFAST run is correct?

Stein

3

As a third possibility, I would double check the diameter inputs in MoorDyn, as MoorDyn uses the volume-equivalent diameter in the LineTypes section, and OrcaFlex uses the nominal diameter.

Stein

4

Dear Stein.Housner

In response to your feedback, I would like to reply as follows:

  1. The intentional design places the location of the fairlead at +13.475m above the water surface.

  2. Are you suggesting that the similarity in Orcaflex results depending on the presence of the mooring system is accurate? As shown in the figure below, the difference in results depending on the presence of the mooring system is clearly evident in Orcaflex as well.

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  1. I have confirmed that there are differences in how diameter is applied in Orcaflex and OpenFAST. So, how can we align the two to match?

Do you have any other suggestions? I’m really curious about the reason why the period shifts, even though the magnitude of the peak is similar in the Pitch Free Decay test results.

Best regards
Jisu Lim

6

Hi Jisu,

In response to 3, you have to convert your hydrodynamic coefficients between MoorDyn and OrcaFlex because OrcaFlex applies them to the nominal diameter, while MoorDyn applies them to the volume equivalent diameter. For example, MoorDyn drag coeff = OrcaFlex drag coeff * Nominal Diameter / Volume Equiv. Diameter.

Ericka

7

Dear all,

Due to the progress of other research, responses and inquiries have not been carried out in a long time.

I understood and applied the difference in the method of recognizing the diameter of the chain mentioned by @Ericka.Lozon above, but there is still a discrepancy in the values.

As shown in the figure below, there is currently a slight difference in the Yaw motion, and I would appreciate opinions on the following questions related to this.

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  1. I think the difference in amplitude and period in Yaw is due to a synchronization issue with mooring system. Please let me know your opinions on this matter

  2. When comparing OpenFAST and OrcaFlex, should i adjust not only the Cd value but also the mass, given the difference in how diameter is considered? (Due to the buoyancy difference caused by the different diameters)

  3. Should the mass value applied in MoorDyn account for the reduction in mass due to buoyancy when submerged ?

Aligning parameters between different tools seems to be a challenging task.

I would appreciate your opinion on the questions.

Thank you,

Best regards
Jisu Lim

8

Hi Jisu,

  1. I’m not completely sure what is causing this issue. Maybe MoorDyn and OrcaFlex account for the mooring lines differently when they’re above water? (assuming your fairleads are still +13.75m above water)
  2. Yes, the drag coefficients on the line types should be relative to the volumetric diameter of the line. The mass input is the dry mass of the line per meter.
  3. Same as #2

Stein