Equilibrium condition of DeePCwind Semi-submersible wind turbine in FAST V8 example files.

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?

Dear Daniel,

What you are missing in your equilibrium calculation is the water ballast, which is physically modeled in the FAST v8 model of Test25 (via the HydroDyn module).

FYI: According to the report you refer to, the vertical mooring pretension of the OC4-DeepCwind semisubmersible is -1839000 N.

Best regards,