Objectives In comparison to other forms of marine renewable energy, tidal energy offers distinct advantages in terms of predictability and high energy density, making it a highly viable energy resource. Tidal current turbines are the most common devices used to generate energy from tides. Based on their fixation method, tidal current turbines can be categorized as fixed or floating. Floating tidal turbines offer significant advantages over fixed turbines, including greater deployment flexibility, easier maintenance, and the capability to operate in deep-water environments. They can provide a reliable power supply for offshore scientific research vessels and operational ships. However, the performance of floating tidal current turbines is dynamically influenced by the complex interplay of multiple factors, including wave action, current flow, and motions of the floating carrier. The hydrodynamic loads on the turbine and carrier, along with the mooring forces, determine the system's motion responses. These responses alter the turbine's position and velocity, disrupting the relative flow velocity and thus affecting its hydrodynamic performance. To accurately capture these interactions, a fully coupled, high-fidelity numerical model using the commercial CFD software STAR-CCM+ is developed. The model integrates the horizontal-axis three-bladed tidal current turbines, the semi-submersible floating carrier, and the mooring system, with the goal of investigating the system's dynamic response characteristics under both regular and irregular wave conditions. The bidirectional coupling mechanism between the turbine and the floating carrier, as well as the wake interference effects among multiple turbines, are analyzed.

Methods The Reynolds-averaged Navier-Stokes (RANS) equations were employed in conjunction with the SSTk - \omega turbulence model to simulate viscous flow effects and calculate the hydrodynamic loads on the turbine and carrier. Furthermore, the overset grid technique, combined with the dynamic fluid-body interaction (DFBI) model, was utilized to capture the six-degree-of-freedom (6-DOF) motions of the floating carrier and the rotational motion of the turbine blades, ensuring precise representation of the relative movements between the solid structures and the fluid. The volume of fluid (VOF) method was adopted to capture the free surface of the fluid. For the mooring system, the lumped-mass method was implemented to simulate the dynamics of mooring lines. A user-defined program developed in C++ was integrated into STAR-CCM+ to achieve bidirectional coupled simulations between the mooring system and the floating carrier. The floating body's motions and the mooring forces are dynamically synchronized at each time step, without relying on external files. Grid convergence analysis was conducted using three sets of grids (coarse, medium, and fine) to ensure the reliability of the numerical results, with the medium-density grid selected for subsequent calculations to balance computational accuracy and cost.

Results A systematic comparison was conducted of the effects of current velocity, wave parameters, and turbine configuration on platform motion and turbine performance. Under regular wave conditions, the integration of a single tidal current turbine was found to reduce the surge and pitch motions of the floating carrier: it suppressed the low-frequency surge response peak and the natural-frequency response peaks of pitch and heave motions. Specifically, compared to the system without a turbine (at a current speed of 2 m/s), the single-turbine system reduced the pitch motion response amplitude by 16.5% under regular waves and decreased the low-frequency surge response under irregular waves. The increasing number of turbines led to a further enhancement of the pitch damping, with the pitch motion response of the three-turbine system being 53.3% of that of the system without a turbine. Moreover, the system's motions were observed to significantly amplify the fluctuation amplitudes of the turbine's hydrodynamic coefficients. The amplitude of the floating turbine's thrust increased by 46.1% and the amplitude of the torque increased by 40.9% compared to the fixed turbine. In the three-turbine configuration, the downstream turbines exhibited slightly improved performance compared to the upstream turbine. Spectral analysis under irregular wave conditions revealed that the peaks of the turbine's thrust coefficient spectrum and power coefficient spectrum were dominated by wave frequencies. Additionally, the presence of the turbine increased the mean value and fluctuation amplitude of the mooring force in the single-turbine system were 1.18 and 1.195 times those of the turbine-free system, respectively, and further increased to 1.58 and 1.40 times in the three-turbine system.

Conclusions These findings indicate that the presence of turbines can effectively restrict the movement of the floating carrier and improve the system's overall stability. Compared to fixed turbines, floating turbines require special consideration of the negative effects caused by amplified load fluctuations, such as increased fatigue damage risks. Furthermore, optimizing the layout of floating turbine arrays can improve the overall energy capture efficiency. In conclusion, the fully coupled numerical model under consideration provides a reliable tool for predicting the dynamic responses of floating tidal current turbine systems. It provides a foundation for further research on extreme sea condition performance, connection structure optimization, and quantitative wake analysis.