Non-stationary GEV models for estimating design sea-states in a changing climate – applications to offshore wind farms along the French coasts

Abstract. The rapid expansion of the French offshore wind sector requires a critical reassessment of structural durability in the face of evolving marine conditions driven by climate change. Traditional design methodologies assuming stationary conditions are inadequate for climate-resilient infrastructure, as they fail to account for evolving wave patterns, storm tracks, and extreme event frequencies (Amlashi, 2024; Barkanov et al., 2024). This study introduces a non-stationary GEV framework to quantify future changes in significant wave height using monthly maxima, reducing uncertainty in design tools for offshore wind farms under climate change. While non-stationary GEV models are well established, their application to offshore wind design using climate projections remains limited. The main objective is to derive an equivalent stationary design level that accounts for time-varying extremes over the lifetime of structures. Based on CMIP6 climate models and reanalysis data, results reveal a projected trend towards a more pronounced seasonal contrast along the French Atlantic and English Channel coasts under future scenarios (SSP1-2.6 and SSP5-8.5), whereas the French Mediterranean Sea exhibits weaker increases in extremes and larger uncertainties (inter-model spread). Projections indicate more intense winters and calmer summers, along with a shift in the seasonal cycle. Overall, the multi-model ensemble suggests an increase in the design levels for extreme sea states. The research concludes by defining a new methodology for calculating an equivalent design level over the structure's operational lifespan. This tool is deemed essential for ensuring the resilience and economic viability of future offshore wind farms in a changing climate.