Numerical Modelling and Analysis on Optimized Ballast Condition of a 15MW Semi-Submersible Floating Wind Turbine

Abstract Large semi-submersible floating wind turbines (FWTs) are promising to harvest substantial offshore wind energy but face significant challenges due to complex interactions with marine environmental conditions. Large-scale rotors are subjected to great wind loads at high wind speeds and require advanced control strategies to maintain safe and efficient operation. Since ballast water contributes over 60% of the displacement in semi-submersible platforms, actively adjusting ballast distribution presents a promising approach to enhance stability, reduce structural loads, and improve overall safety. This study develops an advanced static property calculation method, i.e., the Hydrostatic Model with Ballast Transfer Plus (HMBT+), which can account for ballast transfer effects on the properties and hydrostatic performance of FWTs. Additionally, an equilibrium position assessment module was developed to evaluate FWT pitch motion under specified wind, wave, and ballast distributions, ensuring accurate modeling of operational conditions. Using a 15 MW semi-submersible FWT as a case study, we integrated HMBT+ with OpenFAST to perform time-domain simulations across different ballast states in the designed condition, allowing for a comprehensive analysis of dynamic responses. Results demonstrate that ballast transfer significantly impacts FWT properties by adjusting mass distribution and hydrostatic stiffness, which can ultimately achieve an optimized ballast configuration that balances pitch motion near zero. It reduces tower base fore-aft bending moment (My) by approximately 40%, and slightly enhances generator power output. These findings highlight the critical role of ballast transfer in optimizing FWT performance.