Dear @Lei.Xue,
I agree that in the absence of wind or wave excitation, the draft of the NREL 5-MW baseline wind turbine atop the OC4-DeepCwind semisubmersible is 20 m, which is the draft that balances the system mass/weight, buoyancy, and vertical mooring pretension. The WAMIT solution should be derived about the mean position of the body, which includes this 20 m of draft.
Please find the WAMIT input files that we used to generate the 1st-order WAMIT solution for the OC4-DeepCwind semisubmersible here: WAMITInputFiles_OC4_Semi_1st.zip - Google Drive.
Best regards,