Development and Optimization of a Novel Semi-Submersible Floater for Floating Wind Turbines in the South China Sea

To mitigate the issue of high-pitch natural frequency in V-shaped floating offshore wind turbines (FOWTs), a novel semi-submersible floater design, termed NewSemi, is proposed in this study. The structural performance of the NewSemi floater is compared with that of two existing 5 MW FOWTs, namely, the V-shaped and Braceless. Frequency domain analysis demonstrates that the NewSemi floater exhibits the most favorable response amplitude operator (RAO) in the pitch direction, along with superior damping characteristics. The result reveals a 16.44% reduction in pitch natural frequency compared to the V-shaped floater. Time-domain analysis under extreme conditions reveals 14.6% and 65.2% reductions in mean surge and pitch motions compared to Braceless FOWT, demonstrating enhanced stability. In addition, compared with the V-shaped FOWT, it exhibits smaller standards and deviations in surge and pitch motion, with reductions of 11.3% and 31.9%, respectively. To accommodate the trend toward larger FOWTs, an optimization procedure for scaling up floater designs is developed in this study. Using a differential evolution algorithm, the optimization process adjusts column diameter and spacing while considering motion response and steel usage constraints. The NewSemi floater is successfully scaled from 5 MW to 10 MW, and the effects of this scaling on motion and structural dynamics are examined. Numerical analysis indicates that as turbine size increases, the motion response under extreme sea conditions decreases, while structural dynamic responses, including blade root torque, rotor thrust, tower-base-bending moment and axial force, significantly increase. The maximum values of blade root torque and tower-base-bending moment increase by 10.4 times and 3.95 times in different load cases, respectively, while the mooring forces remain stable. This study offers practical engineering guidance for the design and optimization of next-generation floating wind turbines, enhancing their performance and scalability in offshore wind energy applications.