In the present work an extensive verification and validation study is performed to evaluate the accuracy and credibility for CFD simulations of the hydrodynamic responses of a semi-submersible Floating Offshore Wind Turbine (FOWT) platform under bichromatic waves and random waves. A dynamic mooring model is coupled with the CFD code to accurately simulate the mooring system. For the bichromatic wave case, the surge, heave and pitch RAOs at wave frequencies, mean surge offset and mean surge force of the semi-submersible platform are investigated. The numerical uncertainties of the above metrics are quantified, which are primarily sourced from the discretization uncertainty. For the random wave case, the surge, heave and pitch PSD sums in wave frequency range and low frequency range are validated against the experimental results. The numerical uncertainty derived from the bichromatic wave case is applied in the validation of the random wave case. The PSD sums in wave frequency range have achieved the validation within the validation uncertainty. Though the PSD sums in low frequency range are under-predicted, the results with the utilization of the CFD code agree more with the experimental value than the mid-fidelity tools.