Question about OC3-Phase4 Free Decay FAST Results

Hello all! I’m hoping someone can shed some light on the origin/orientation of the Ptfm motion outputs (surge, sway, etc.) of the OC3-Phase4 free decay tests. I’m having trouble orienting these results properly such that I might compare to them. Most notably the pitch results.

  • Is the tower rotated about the PtfmRef point (i.e. the MSL) or at the platform CM?
  • Are the outputs measured at the PtfmRef point?

It seems that a positive pitch (tower leaning ‘back’) at the MSL would displace the system CM only about 1.5m. The surge of the platform follows the pitch (at first) and travels to about -17m as predicted by FAST. I’m failing to see how that rotation causes a 17m response in surge, although I assume that I’m simply misoriented. Can someone answer these questions?

Sean Q

Dear Sean,

The origin/orientation of the platform motion outputs (surge, sway, etc.) of the OC3 Phase IV results ( Basically, the origin (platform reference point) is defined at the intersection of the still water level (SWL) and the spar centerline for the undisplaced system.

The system center of mass (CM) of the OC3-Hywind model (including the rotor + drivetrain + nacelle + tower + spar) is 78 m below the SWL, so a 10-degree platform pitch angle will cause a lot more than 1.5 , of surge motion of the system CM.

Best regards,

Dr. Jonkman,

Thanks for the prompt reply! I did, indeed, post some bad math in that first post… probably a degree/radians error. I have determined the issue I was having, which was related to improper coordinate transformation (my frame originates at the CM). I look to be properly reproducing the numbers in question. Thanks again!

Sean Q