Question about the Total Pitch Moment in OpenFAST for the 5 MW Barge-type FOWT

Wind & Water Computer-Aided Engineering Software Tools
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Dear @Jason.Jonkman

I hope this message finds you well. I have a question regarding the total pitch moment as calculated in OpenFAST for the 5 MW barge-type FOWT.

From my understanding, two main contributions are responsible for pitching the platform. The first is the moment resulting from the rotor thrust force and the aerodynamic loads along the tower height. The second is the hydrodynamic moment. Therefore, I interpret the total pitch moment as:

Total_Moment = HydroMyi – TwrbsMyt

where the negative sign arises because TwrbsMyt represents the internal bending moment at the tower base, and the corresponding external moment acts in the opposite direction.

Could you please confirm if this interpretation is correct, or clarify how the total pitch moment is defined in OpenFAST?

I would greatly appreciate any insights or references you could share.

Best regards,

Riad

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Dear @Riad.Elhamoud,

I’m not sure I understand what you mean by “total moment”. I would say there are moments from applied forces like those from aerodynamics, hydrodynamics (e.g., HydroMyi), and gravity/weight. And there are reactions moments within various components (e.g., TwrBsMyt at the tower base) that include contributions from applied forces and inertial effects.

Best regards,

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Dear Dr. Jonkman,

Thank you very much for your answer.

I mean by “total moment” the overall pitch moment on the FOWT. That is, if I want to evaluate the total pitch moment acting on the floating platform (i.e., the external moment that drives its pitch motion), would it be appropriate to sum the moments from the external loads — namely the aerodynamic, hydrodynamic, and gravitational contributions — about the platform reference point, right? and since no “transition piece“ exists in the barge (because the tower base is at mean water level), i assume that the output channel “TwrbsMyt” contains the contributions of aerodynamic loads and the weight at the platform reference point which is coincident with tower base. In addition, i will add the output channel “HydroMyi” which represents the total pitching hydrodynamic moment.

Therefore,

total pitch moment acting on the floating platform = TwrbsMyt + HydroMyi

What do you think ? is my interpretation correct ? or i am mixing things uup?

I appreciate your time and help.

Best regards,
Riad Elhamoud

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Dear @Riad.Elhamoud,

I disagree with your calculation because while TwrBsMyt includes the applied aerodynamic moment and the gravitational moment from the turbine, it also includes inertial effects (unless you time-average to average-out the dynamic inertial effects). Also, you haven’t accounted for the gravitational moment from the floater.

The equivalent of the total hydrodynamic moment HydroMyi in HydroDyn is not available in AeroDyn for the total aerodynamic moment, although you calculate that by modifying the source code. Likewise, the total gravitational moment is not currently output from OpenFAST either.

Best regards,

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