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

Nicolas Raillard, Coline Poppeschi, Tessa Chevallier, Youen Kervella, Laurent Dubus

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.

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, 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.
Paper Structure (16 sections, 6 equations, 9 figures, 2 tables)

This paper contains 16 sections, 6 equations, 9 figures, 2 tables.

Table of Contents

  1. Sea-state data
  2. Sea-state data
  3. CMIP6 Climate models
  4. Reanalyses
  5. Bias correction
  6. Modeling of extreme values
  7. Models for annual maxima
  8. Models for monthly maxima
  9. Methodology
  10. Computation of return levels
  11. Long-term non-stationary models
  12. Methodology
  13. Computation of design sea-state in a non-stationary climate
  14. Results
  15. Discussion
  16. ...and 1 more sections

Figures (9)

  • Figure 1: Locations of the Selected Representative Point (SRP) along the French coasts.
  • Figure 2: CDF-t validation over the Eastern Channel point.
  • Figure 3: Comparison of auto-correlation function (ACF), left, and partial auto-correlation functions (PACF), right; on East English Channel, before modeling the non-stationarity of monthly maxima (left columns of each figure) and after (right columns of each figure).
  • Figure 4: Comparison of monthly 99th quantile, at the East English Channel Representative point, obtained from the completely non-stationary model of eq. \ref{['full-non-stat']}.
  • Figure 5: Comparison of monthly 99th quantiles, at the East English Channel Representative point, obtained from the completely non-stationary model of eq. \ref{['full-non-stat']} at different years (2010 for Historical run, 2100 for Scenarios SP1-2.6 and SSP5-8.5).
  • ...and 4 more figures