An empirical framework for predicting the design towing resistance for floating wind turbines with application on the design of towing system

Wet-towing is a critical stage in the transportation and installation of floating offshore wind turbines (FOWTs), yet rapid and reliable methods for predicting towing resistance remain limited. This study develops an empirical framework for resistance prediction of large-scale semi-submersible FOWTs in calm-water and waves. The configuration-dependent drag coefficients are assigned separately to the columns and pontoon. The column coefficient is expressed as a linear function for the ratio of submerged height to diameter, and distinct pontoon coefficients are introduced for single-column-forward and double-column-forward towing. For wave conditions, the calm-water formulation is extended by defining an effective velocity that combines the towing speed with the averaged area root mean square (RMS) horizontal wave-particle velocity, together with a wave correction coefficient. The framework is further assessed against experimental measurements for the INO WINDMOOR 12 MW FOWT, OC4-DeepCwind FOWT and Computational Fluid Dynamics (CFD) results for different speeds, drafts, configurations, and wave conditions. The predicted resistance reproduces the experimental and CFD trends with substantially lower computational effort. Subsequently, this method is applied to predict the towing resistance of the International Energy Agency (IEA) 15 MW FOWT. In calm-water, most relative errors remain within 5%, with an approximately 8% deviation for the double-column-forward case at 20 m draft. Under wave conditions, all deviations remain within ±5%, and the maximum absolute error is 4.7%. Resistance increases nonlinearly with speed and rises with draft, while the double-column-forward configuration generally produces greater resistance. Component decomposition shows that the pontoon is the dominant resistance contributor, with average drag coefficients of 1.17 and 1.01 for double- and single-column-forward, respectively. A database of 50 towing cases is additionally compiled to establish a practical drag coefficient envelope. According to the available effective towing force of the tugboat, the towing speed under head-wave is recommended not to exceed 4 kn, and a moderate towing draft of 8-12 m is recommended. The proposed framework supports rapid towing resistance assessment, draft and speed selection, and preliminary design.