Dear @Jason.Jonkman,
Hope you are doing well. I have a question regarding the platform degrees of freedom in FOWT.
The degrees of freedom of the platform (surge, sway, heave, roll, pitch, yaw) are measured in OpenFAST and all old versions (FAST v7, FAST v8) at the SWL and not the center of gravity of the platform. Right ?
Best Regards,
Riad
Dear @Riad.Elhamoud,
Presumably you are referring to the ElastoDyn platform motion outputs, e.g. PtfmSurge; is that correct? The ElastoDyn platform motion outputs are measured relative to the platform reference point, which is specified via ElastoDyn input PtfmRefzt relative to the mean sea level (MSL) when the platform is undisplaced.
Best regards,
Dear @Jason.Jonkman,
Hope you are doing well. I have a few theoretical questions.
1- I was wondering about the direction of the viscous forces computed using Morison formula. Are these forces normal to the member ? In other word, let’s assume a spar that is undergoing pitch motion in 2D (no yawing and no rolling). So, viscous forces are always normal to the spar (except for the heave component) ?
2- A lot of papers do assume the RNA as a lumped mass at the tower top. Moreover, they do not consider the overhang moment due to the eccentricity of the CM of the hub and nacelle. In my opinion, this eccentricity will make a pitching moment and will affect pitch motion. Am i right ? What do you think ?
Best regards,
Riad
Dear @Riad.Elhamoud,
I agree that the transverse viscous loads from the strip-theory (Morison) formulation of HydroDyn are computed normal to the member; the axial loads (from end effects at joints or from tapered members) are computed along the member.
I agree that the offset of the RNA from the tower-top is important to the pitching moment and motion. So is the inertia of the RNA about this center of mass (not just the point mass).
Best regards,