The present work highlights some of the dynamic couplings observed in a series of tests performed in a wave basin with a scaled-model of a Floating Offshore Wind Turbine (FOWT) with semi-submersible substructure. The model was moored by means of a conventional chain catenary system and an actively controlled fan was used for emulating the thrust loads during the tests. A set of wave tests was performed for concomitant effects of not aligned wave and wind. The experimental measurements illustrate the main coupling effects involved and how they affect the FOWT motions in waves, especially when the floater presents a non-negligible tilt angle. In addition, a frequency domain numerical analysis was performed in order to evaluate its ability to capture these effects properly. The influence of different modes of fan response, floater trim angles (changeable with ballast compensation) and variations in the mooring stiffness with the offsets were investigated in the analysis. Results attest that significant changes in the FOWT responses may indeed arise from coupling effects, thus indicating that caution must be taken when simplifying the hydrodynamic frequency-domain models often used as a basis for the simulation of FOWTs in waves and in optimization procedures for the design of the floater and mooring lines.

This paper introduces a closed-form analytical formulation for the stiffness matrix of floating systems with shared mooring. The model accounts for surge, sway, and yaw motions (thus, considering only motions on the horizontal plane) and can be applied both at neutral and displaced equilibrium positions. It provides rapid and consistent estimates of restoring forces and stiffness properties, offering a computationally efficient alternative to finite-element or iterative approaches. A two-platform Floating Offshore Wind Farm (FOWF) case study illustrates the formulation. Results show non-intuitive couplings and additional yaw stiffness, confirmed by comparison with finite-element simulations. Analysis of natural modes and periods reveals how independent platform motions transform into collective dynamics when coupling is considered. Modal veering phenomena are also identified when natural periods approach themselves under external mean force loads. It is important to emphasize that this formulation complements high-fidelity numerical tools, providing a fast and insightful framework for early stage assessments of FOWFs with shared mooring.