A model of extreme environmental conditions for wind–wave–swell and related structural analysis

Jiang Yunmu; Yu Dinghao; Li Gang; Dong Zhiqian

doi:10.12284/hyxb20250121

Volume 47 Issue 12

Dec. 2025

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Jiang Yunmu，Yu Dinghao，Li Gang, et al. A model of extreme environmental conditions for wind–wave–swell and related structural analysis[J]. Haiyang Xuebao，2025, 47(12)：70–83 doi: 10.12284/hyxb20250121

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A model of extreme environmental conditions for wind–wave–swell and related structural analysis

doi: 10.12284/hyxb20250121

State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology, Dalian 116024, China

Received Date: 2025-06-24
Rev Recd Date: 2025-12-03

Available Online: 2025-12-17

Publish Date: 2025-12-31

Abstract

Abstract

Accurate assessment of a marine structure’s long-term extreme response is fundamental to its survivability, yet an unclear taxonomy of external environmental conditions hampers such assessments. While many studies prioritize wind and wind sea, real sea states are frequently multimodal, with wind sea and swell superposed. Unimodal-spectrum time-series methods cannot represent this multimodality or the statistical dependence among wind, wind sea, and swell, leading to underestimated joint extremes and biased reliability/safety evaluations. Swell—a low-frequency component whose intensity can rival wind sea—readily excites low-frequency resonance in flexible systems such as offshore wind turbines, amplifying dynamic responses and cumulative fatigue. Recent standards (IEC 61400-3-2: 2025 and China’s guideline for integrated analysis of floating offshore wind turbines) explicitly require swell to be treated as a mandatory load case. Accordingly, we treat swell as a co-equal hazard with wind and wind sea. Using reanalysis data from representative stations in the South China Sea, East China Sea, Bohai Sea, and Yellow Sea, we build a joint probabilistic model of the three drivers and, via correlation analysis, Granger causality tests, and conditional probability analysis, reveal region-specific dependence structures. For the South China Sea, the environmental contour method is then used to construct an extreme-environment model that explicitly includes swell. Results show that incorporating swell markedly increases the complexity of environmental-variable combinations; omitting it distorts the environmental model and underestimates extremes. By extending the conventional wind–wave framework to include swell and demonstrating its necessity as a hazard, the study clarifies condition categories and supplies a more complete and accurate environmental input for long-term extreme-response assessment.
- long-term extreme response,
- multi-hazard model,
- extreme environmental combination,
- structural safety and reliability,
- coupling mechanism analysis

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