Hello! I am making up some examples for a class and have been looking at Definition of the Floating System for Phase IV of OC3, NREL/TP-500-47535 and see that the mass moment of inertia in pitch and roll is approximately 4.33 x 10^9. In Figure 4-4 the added mass moment of inertia (A55) shown appears to be about 3.8 x 10^10, suggesting a rather large value for the added mass moment of inertia coefficient. Could you tell me where that coefficient came from?

Dear @James.Manwell,

The hydrodynamic added mass coefficient for the OC3-Hywind spar shown in Figure 4-4 of the report you reference was derived from WAMIT.

I suspect your confusion is because of different reference points for these moments of inertia. The mass moments of inertia are expressed relative to the platform CM while the added mass moments of inertia are expressed relative to the platform reference point located at still water level.

Best regards,

Thanks! The added mass moment of inertia term looks much more reasonable now. Is it also reasonable to assume that WAMIT will automatically find a new added MMOI term when the full turbine is modelled, i.e. with the RNA and tower included? Is that term accessible? The MMOI of the entire structure would then be considerably greater than that of the platform alone, but it is not obvious how different the added MMOI term would be.

Dear @James.Manwell,

The first-order hydrodynamic added mass matrix computed by WAMIT only depends on the wetted surface of the substructure in the absence of any displacement, so, this term will not depend on structural properties or any features of the structure above the still water level.

Best regards,