A design methodology for a monolithic composite spar-buoy platform supporting a 15~MW floating offshore wind turbine is developed and demonstrated. The concept originates from the need to explore composite alternatives to steel structures, with geometry and material choices initially informed by literature on comparable designs and the WindCrete reference model. The structural layout was parametrized in Grasshopper and coupled to a Python-based workflow integrating aero-hydro-servo-elastic simulations (OpenFAST, BModes, HAMS), post-processing (MExtremes, MLife), and optimization (OpenMDAO), enabling automated updates of simulation inputs from parametric geometry changes.

Extreme and fatigue design load cases (DLC6.1, DLC6.3, DLC1.2) following IEC~61400-3 were simulated to assess yielding, buckling, and fatigue performance. Due to the high computational cost of flexible spar simulations, a rigid spar approach was employed, focusing on the tower base as the critical section. Initial assessments revealed that the filament-wound layer was insufficient under extreme loads, which was addressed by increasing its thickness.

An optimization using 10{,}000 sampled designs identified a configuration with reduced composite thicknesses and an increased tower base diameter, lowering capital expenditure (CAPEX) to 22.67~million~\texteuro{} while meeting structural and stability constraints. The optimized CAPEX lies within the reported range for steel spar and tower designs, indicating economic competitiveness. Further improvements, such as a tapered thickness distribution, are expected to enhance cost-effectiveness. The results demonstrate the technical feasibility and economic potential of composite spar-buoy platforms for large-scale FOWTs.