This work examines the dynamic response of a single semi-submersible wind turbine (SSWT) based on different hydrodynamic theories. Comparisons of platform motions and structural responses in the wind turbine are shown for simulations for a model with linear potential flow solution and quadratic drag and simulations with only Morison-type forces. The SSWT modelled in this study is based on WindFloat and carries the NREL 5MW wind turbine and should be considered a large volume structure. This implies that diffraction effects should be considered by using potential flow theory and viscous effects by Morison's equation.A new coupled simulation code was developed by linking the SIMO and RIFLEX hydrodynamic, structural, and control system computational tools, from MARINTEK, with the aerodynamic forces and wind field generation capabili–ties of AeroDyn and TurbSim, from NREL. In contrast to other available simulation codes, this combination enabled the implementation of these two different hydrodynamic theories and offered the possibility of finite element mooring line models. Wave-only simulations were considered first, in order to tune and compare potential theory versus the inertia term in Morison's equation. Some limited coupled wave-wind simulations give an indication of the extent to which hydrodynamic modelling affects the global response.The SSWT case study showed that the Morison model with forces integrated up to wave elevation gave a good representation of the motions compared to the potential flow model with quadratic drag forces. It also showed that mo–tions are sensitive to choice of added mass coefficients, stretching and dynamic pressure under the columns. Combined wind and wave simulations, using a non-optimized control approach, showed that pitch motions influence the power production and blade bending moments

As more floating farms are being developed, the wake interaction between multiple floating wind turbines (FWTs) is becoming increasingly relevant. FWTs have long natural periods in certain degrees of freedom, and the large‐scale movement of the wake, known as wake meandering, occurs at very low frequencies. In this study, we use FAST.Farm to simulate a two‐turbine case with three different FWT concepts: a semisubmersible (semi), a spar, and a tension leg platform (TLP), separated by eight rotor diameters in the wind direction. Since wake meandering varies depending on the environmental conditions, three different wind speeds (for all three concepts) as well as two different turbulence levels (for the semi) are considered. For the below‐rated wind speed, when wake meandering was most extreme, yaw motion standard deviations for the downstream semi were approximately 40% greater in high turbulence and over 100% greater in low turbulence when compared with the upstream semi. The low yaw natural frequency (0.01 Hz) of the semi was excited by meandering, while quasi‐static responses resulted in approximately 20% increases in yaw motion standard deviations for the spar and TLP. Differences in fatigue loading between the upstream and downstream turbines for the mooring line tension and tower base fore‐aft bending moment mostly depended on the velocity deficit and were not directly affected by meandering. However, wake meandering did affect fatigue loading related to the tower top yaw moment and the blade root out‐of‐plane moment.

The dynamic responses of a spar, tension leg platform (TLP), and two semisubmersible floating wind turbines (FWTs) in selected misaligned wind and wave conditions are investigated using numerical simulation with an aero-hydro-servo-elastic computational tool. For a range of representative operational conditions, the platform motions and short-term fatigue damage in the tower base and tower top are examined. Although some misalignment conditions result in increased motions both parallel and perpendicular to the wave direction, aligned wind and waves cause the largest short-term tower base fatigue damage for the studied platforms and conditions. Several factors which lead to larger fatigue damage for certain platforms in particular conditions are identified, such as tower resonance due to the 3p blade passing frequency in low wind speeds; surge and pitch motions, particularly in the wave frequency range; and the variations in first-order hydrodynamic loads due to wave direction. A semisubmersible platform with large displacement suffers the least damage at the base of the tower.