This Master thesis is focussed in the verification of the capabilities of SeaFEM - FAST tool to carry out the fully coupled analysis of offshore wind turbines. First, a state of art of fully coupled analysis is presented. Then, rules to clasify and to certificate offshore wind turbines is studied. Design load conditions for OC3- Hywind NREL 5 MW Baseline are computed. Next, a procedure to carry out the coupled analysis using tool SeaFEM and FAST is presented. Some Design Load Cases are analyzed. OC3 - Hywind NREL 5 Mw tower is verified against DNV rules. Both analysis, FAST/Hydrodyn and SeaFEM-FAST are compared. Finally some conclusions and further research are showed.