Dear Tiasa,
The initial rotor speed should be set to equal the expected mean value for a given hub-height wind speed. You can calculate this by running the model to steady state. The mean values are also published in Figure 3-1 of the IEA 15-MW RWT specifications report: nrel.gov/docs/fy20osti/75698.pdf.
DLC 1.6 from the offshore wind turbine design standard calls for the 50-year extreme significant wave height conditioned on mean hub-height wind speed. These may be lower than the unconditioned 50-year extreme significant wave height.
Best regards,