Methodology and validation of multi-physics coupled co-simulation framework for multi-rotor multi-tower FOWT: A VX-case study

As offshore wind turbines develop towards deeper waters and larger sizes, the design of new wind turbines is also shifting from conventional centralized structures to distributed ones, such as multi-rotor multi-tower floating offshore wind turbines (FOWTs). Distributed structures could provide a completely new design perspective for enhancing higher wind energy capture efficiency and improving system stability, but there is a lack of corresponding analytical tools and methods. This study proposes a generalized, fully coupled aero-hydro-servo-elastic co-simulation framework capable of modeling a wide range of unconventional FOWTs. The framework integrates aerodynamic, hydrodynamic, structural, mooring, and control subsystems to capture complex system-level dynamics with high fidelity. A representative case study of a VX-type FOWT configuration – featuring two coaxial dual rotors with two blades (X-shaped) installed on symmetrically inclined towers (V-shaped) on a semi-submersible platform – is conducted to demonstrate the capabilities of the framework. Subsystem validations against GH Bladed, CFD simulations, and AQWA confirm the modeling accuracy across structural, aerodynamic, and hydrodynamic domains. Fully coupled dynamic analyses characterize the overall system responses of the VX-type FOWT under different load cases. The framework also enables the assessment of control strategies, with the phase-synchronized approach showing potential benefits in dynamic load reduction and platform stability enhancement. These results highlight the robustness, flexibility, and practical applicability of the proposed framework for the design, dynamic performance evaluation, and optimization of next-generation floating offshore wind turbine systems.