Global Sensitivity Analysis of Platform-Mooring Responses for a 15 MW Semi-Submersible Floating Wind Turbine Based on PCE-Sobol and Spearman Methods

For large-scale floating offshore wind turbines, existing sensitivity studies have not fully addressed the combined effects of multiple uncertain input parameters on multiple output responses. Meanwhile, conventional Sobol indices quantify contribution magnitude but do not indicate effect direction. Based on the IEA 15 MW semi-submersible benchmark model, this study investigates the sensitivity of mooring tension and platform motion dynamic responses at a normal operating condition under power production. Integrated dynamic simulations were performed to generate response data. Eight uncertain parameters were considered, including the key mechanical and hydrodynamic coefficients of mooring lines as well as mass distribution and hydrodynamics-related key parameters for the platform. A polynomial chaos expansion surrogate model was used for the global sensitivity analysis, based on the Sobol index, Spearman coefficient, and a newly proposed modified Sobol index. The results indicate weak parameter interactions, with first-order Sobol indices dominating. The platform mass makes the largest contribution, with first-order Sobol indices approaching 1.0 for the mean tensions of all three mooring lines and 0.995 and 0.999 for the mean surge and heave displacements, respectively. The mooring line normal drag coefficient reaches a first-order Sobol index of 0.805 for the standard deviation of the upwind mooring line tension. The pitch response is influenced by multiple parameters. The Spearman coefficients confirmed the dominant parameters and identified their effect directions. By integrating variance contribution with effect direction, the modified Sobol index provides a more interpretable assessment of parameter effects. These findings can support parameter prioritization, mooring system design, and digital-twin model updating for floating offshore wind turbines.