A mixed stochastic waves model for analyzing offshore structures considering engineering characteristics correlation of wind-generated-wave and swell

In the marine environment, offshore structures face mixed stochastic waves, including wind-generated waves and swells. Current design codes often overlook swells, leading to underestimations of environmental loads and structural responses, especially for flexible structures, potentially causing resonance and instability. Wind-generated waves and swells have distinct generation mechanisms, but their characteristics, such as significant wave height and average period, show a statistical correlation due to their interaction. Thus, a mixed stochastic wave model is needed to reflect both wave types and their correlation accurately. This study investigates the correlation of wave characteristics and the power density spectrum (PSD) between wind-generated waves and swells using measured data, providing a comprehensive understanding of mixed wave interactions. The joint probability density model of wave characteristics is discussed via Vine Copula theory. A framework for obtaining mixed wave loads is developed using the Proper Orthogonal Decomposition (POD) method, treating waves as a 1 dimension - 2 variables (1D-2V) stochastic vector process. The model is applied to assess the dynamic response of a 5 MW mono-pile offshore wind turbine (OWT). Results show that swells significantly increase the internal force and bending moment demands, highlighting the necessity of including swells in offshore structure design and analysis.