I am trying to measure the heave value from the center of mass instead of mean sea level (MSL).
For this, I have to change the value of PtfmRefzt from 0 to =-8.6588 which is similar to PtfmCMzt=-8.6588. However, my heave value remains unchanged and it still seems to be measured from MSL. Do I also need to make changes somewhere else?
I am using 5MW_OC4Semi_WSt_WavesWN, Open FAST v3.0.0.
With such a small moment arm, I would guess the heave displacement would only be minimally effected by pitch and roll motions when comparing the motion relative to the MSL versus relative the CM.
Alternatively, you could keep the platform reference point at PtfmRefzt = 0 and shift the outputs to the CM with a bit of post-processing, as discussed, e.g., in the following forum topic: http://forums.nrel.gov/t/time-domain-analysis-by-aqwa/1946/2.