Can anyone confirm my understanding that the origin used in WAMIT to obtain the hydrodynamic coefficients should be the same location specified as the PtfmRef value in HydroDyn. And hence this is also the location at/about which the output parameters PtfmFxt etc. are defined.
Yes, the hydrodynamic coefficients from WAMIT should have the same reference origin as the platform reference point in FAST/HydroDyn (specified by PtfmRef). Moreover, FAST requires that PtfmRef be set to 0.0 (still water level) for floating platforms. The FAST platform loads (PtfmFxi, etc.) are output relative to platform reference point.
I hope that clarifies things.