In this paper a detailed verification and validation study is performed to evaluate the accuracy of CFD simulations for a semi-submersible floating offshore wind turbine in pitch free-decay motion. The simulations are carried out with both linear and dynamic mooring models. The linear and quadratic damping coefficients of the semi-submersible are derived from the pitch motion data. In the verification study, a least-squares method using the observed order of convergence in combination with a data quality measure for different spatial and temporal refinement is applied to estimate the discretization errors. The numerical uncertainty is quantified by applying a safety factor to the summation of numerical errors. The numerical solutions are validated against the experimental measurements obtained from wave tank test of OC5-DeepCWind semi-submersible. Validation is achieved in the pitch natural period and the linear damping coefficient within the level of validation uncertainty. While the numerical simulation under-predicts the quadratic damping coefficient. A guideline is provided for verification and validation study of a CFD code application for floating marine structures under decay tests.