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

TL;DR

This paper develops non-stationary GEV models to better estimate future extreme sea states for offshore wind farm design, accounting for climate change-driven variability and seasonal shifts, thus improving resilience and economic viability.

Contribution

It introduces a novel non-stationary GEV framework for assessing future sea states, integrating climate model data to improve offshore wind farm design under changing conditions.

Findings

Projected increase in extreme sea state levels under climate scenarios

More intense winters and calmer summers predicted along French coasts

Uncertainty is higher for the Mediterranean Sea due to weaker signals

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, which rely on the assumption of stationary environmental conditions, are no longer adequate. This study introduces a novel statistical framework to assess future changes in significant wave height by employing non-stationary Generalized Extreme Value (GEV) models applied to monthly maxima. This approach aims to reduce uncertainty and provide robust design tools adapted to the non-stationary conditions of the future. 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 results that are more difficult to interpret, due to a weaker increase of extremes and large 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.