There are several numerical approaches to evaluate the dynamic performances of floating offshore wind foundations (FOWF). Most common practices are to implement a fully coupled modeling of the foundation hull, mooring, tower and rotors. However, this requires significant efforts in the modeling, especially tower and rotor, control effects, whereas the uncoupled (or semi-coupled) method can minimize those efforts resulting in quick turn-around time for the design spiral which is very beneficial to early design stage of an FOWF. The paper presents the comparisons between the coupled and uncoupled methods for the 5MW Y-wind semi-submersible FOWF moored at a water depth of 200m. In the coupled analysis, the wind turbine and platform interact dynamically exchanging loads and motions at each time step. Aerodynamic loadings on blade element with wind-inflow data, tower/blade elasticity, rotating blades, and blade-pitch control are also taken into account. For the uncoupled analysis, the tower and rotors are set as a rigid body on the foundation, but the rotor thrust is modeled with two different schemes such as applying a steady force at the tower top and a disk drag with the same rotor swept area, where the thrust or mean drag is equivalent mean thrust computed from a land based wind turbine. Numerical simulations for the coupled and uncoupled models are carried out and the results in motions, accelerations, offsets, and mooring tensions are compared. The present results indicate that the uncoupled approach gives acceptable range of design results compared with the fully coupled approach and that the uncoupled approach can be utilized effectively in the design phases of the floating wind platform and mooring system.

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