Information on couplings needed

Dear OpenFAST team,

I am running simulations of a floating turbine on a spar-type platform.
The wind ramps up to 11 m/s during the initial 300 seconds.
The rotor is rigid and so is the tower.
The only DOFs active are:
True PtfmSgDOF - Platform horizontal surge translation DOF (flag)
True PtfmSwDOF - Platform horizontal sway translation DOF (flag)
True PtfmHvDOF - Platform vertical heave translation DOF (flag)
True PtfmRDOF - Platform roll tilt rotation DOF (flag)
True PtfmPDOF - Platform pitch tilt rotation DOF (flag)

I believe everything is defined correctly and the situation presented is symmetric about the xi axis in terms of wind direction and map++ mooring lines. There are no waves or current either.

Here is the plot, please notice the steady state displacement in platform sway and roll

What are the couplings between the rotor and the platform motions that I should be aware of that will have an effect on the platform position in the steady state?
If I turn on platform yaw DOF:
False PtfmYDOF - Platform yaw rotation DOF (flag)
to “True”
and try to run the previous simulation (omega constant at 10 rpm, wind ramp from 1 to 11 m/s), the motoring turbine is falling over v. quickly, is it a coupling effect as well?



Dear Bartosz,

The aerodynamic loads on the rotor will include a torque, which will tend to induce roll/sway of the floater. There will also be gyroscopic loads from the spinning rotor, which mainly induce yaw, but when the rotor is tilted (due to shaft tilt or floater heal), could also induce roll.

I’m not sure what you mean by “falling over v. quickly”, so, I’m not sure I can comment on your case with platform yaw, but it sounds like your model may not have adequate yaw stiffness in the mooring system or that the model has gone unstable for some reason.

Best regards,

Dear Jason,

Thank you for your answer,
Now all the degrees are working OK. I have all the DOFs on True except these 4:

False TeetDOF - Rotor-teeter DOF (flag) [unused for 3 blades]
False DrTrDOF - Drivetrain rotational-flexibility DOF (flag)
False GenDOF - Generator DOF (flag)
False YawDOF - Yaw DOF (flag)

and the results for the platform displacements are:

The views below:

continuing from previous post,
these are the rotor characteristics:

The rotor is tilted -5 degrees.

In your opinion, from your experience, do you think these displacements seem realistic? Could they be caused by the reasons you are mentioning? I mean above all the sway and pitch of the platform. Can you comment on the blade out of plane deflections seen in one f the images? Does that seem excessive?

Dear Bartosz,

I’m not seeing anything out of the ordinary with these results.

Best regards,