FAST-OrcaFlex Coupling

Dear Dr. Jonkman,

I am in a phase of getting myself familiar with modelling Fast-OrcaFlex module for OC4 Phase II.
As I was learning how to execute the sample files from the FASTv8 package, I found the discrepancy between the results I got and the majority of the publicly available results (I checked both LC 1.3b and LC 2.2).

For Load Case 1.3b, I got fairlead tensions for line 1, line 2, and line 3 as 129,353 kN, 129,654 kN, and 129,353 kN respectively while the public results seemed to converge to a value around 1,100 kN for all three mooring lines.

For Load Case 2.2, the OrcaFlex simulation file gave me results of 755 kN, 2,691 kN, 737 kN for line 1, line 2 and line 3 respectively while again, the public results for fairlead tensions were around 1,100 kN.

All the results were compared at 59s of the simulation.

I have two assumptions of why I am getting too much tensions in the mooring lines:

  1. The FAST input file for both load cases do not seem to include the effects of the turbine (tower, nacelle and rotor) motion by switching off all the features except for CompElast and CompMooring.

  2. The OrcaFlex simulation model is missing the rest of structural bodies above the sea surface. Also, the platform is consisted of rectilinear volumes instead of cylindrical shapes.

Thus, I am seeing the pure wave loading that leads to large static pretension in the mooring lines.

If I am approaching this issue wrong, your corrections would be greatly appreciated.

Thank you,

Leo Shin

Dear Leo,

I’m not sure I know why you are getting such odd results relative to others from OC4 Phase II, but here are a couple general responses regarding your comments:

  • ElastoDyn is enabled in both simulations and the ElastoDyn model considers the rotor, nacelle, and tower, and platform mass/inertia.
  • The geometry visible in the OrcaFlex GUI is optional and not necessarily consistent with the physics being modeled.

I hope that helps.

Best regards,