OpenFAST vs Orcaflex Free decay test comparison

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.

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.

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:

Best regards,

Jisu Lim

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

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

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.

  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

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