A fully coupled methodology for simulating floating offshore wind turbines is developed within the framework of CONVERGE CFD software. This methodology utilizes the following models which are solved together in a coupled manner: wind-wave models, turbine rotor/blade models, mooring cable models, and a 6-degree-of-freedom rigid-body fluid-structure interaction (FSI) model. The methodology is benchmarked with the OC4 FOWT experimental and simulation data under various wind-wave conditions. This approach is then used to study tandem floating offshore wind turbines under various wind-wave conditions to gain an in-depth understanding of the interactions of floating offshore wind turbines (FOWTs) in real-world conditions. It is found that the power efficiency of an offshore wind farm can be improved by the wave-induced enhancement in the wake recovery.