Environmental contours using copulas for extreme load estimate of offshore wind turbines

Accurate estimation of extreme loads is critical for ensuring the structural stability of offshore wind turbines under severe marine conditions. This study evaluates the applicability of copula functions in estimating extreme loads in the southwestern Sea of Korea. Environmental contours were derived using copula-based joint distribution models, where marginal distributions were modeled with either a single or mixed Weibull distribution, and results were compared with those from the Bitner-Gregersen and Haver model. Among the copula models, the Joe copula effectively captured upper-tail dependence, making it the most suitable choice. The analysis revealed that the copula model with a single Weibull distribution provided the most conservative extreme load estimates across all sea states, ensuring structural stability. In contrast, the Bitner-Gregersen and Haver model produced contours closely aligned with wave hindcasting data, but its lack of conservatism may compromise safety under extreme conditions. Contours from copula models with mixed Weibull distributions exhibited trends closer to the data but showed reduced conservatism at wind speeds exceeding 20 m/s, leading to lower extreme loads under extreme sea state conditions. These findings confirm the effectiveness of copula functions in describing dependencies among metoceanic variables and their importance in offshore wind turbine design and safety.