Dear Dr. Jason.
I am using the FAST v8 for analysis of DeePCWind semi-submersible wind turbine. I knew the platform mass is 1.3473E7 kg in your report (nrel.gov/docs/fy14osti/60601.pdf). However, in the given example file of FAST v8.16, the platform mass (PtfmMass in NRELOffshrBsline5MW_OC4DeepCwindSemi_ElastoDyn.dat) is 3.85218E6kg, which is much smaller than the value in the report. The weird thing is that even though the platform mass is very small, the equilibrium (Buoyancy = Total Weight + vertical tension caused by Mooring wet mass) is well matched in the simulation when the given input files are used.
I summarized various mass and buoyancy like following, which is included in “NRELOffshrBsline5MW_OC4DeepCwindSemi_ElastoDyn.dat”
Rotor Mass : 110,000 kg
Nacelle Mass : 240,000 kg
Tower Mass : 249,718 kg
Platform Mass 3,852,180 kg
Total Mass : 4,451,898 kg
Total Weight(Platform+Wind Turbine): 43,673,119 N
Mooring Weight : Mooring length (835.35m) * MassDensity(113.35kg/m)*Number of Mooring(3 ea)*9.81=2,786,636N
Vertical mooring tension will be smaller than 2,786,636N because a lot mooring parts will be touched to seabed, then the total weight + vertical mooring tension has to be less than 46,459,756 N.
Total Weight + vertical mooring tension <= 46,459,756 N
Displaced Volume of water in given input file: 13,917 m^3 in “NRELOffshrBsline5MW_OC4DeepCwindSemi_HydroDyn.dat”
Buoyancy : 139,938,914 N
Thus, the buoyancy is much larger than total Weight + vertical mooring tension. It means that the equilibrium condition can not be matched. But in the simulation, the equilibrium is matched well when I check the platform heave motion in the still water. Are there any other mass terms I missed in the input files, such as ballast water?