Frequency response of multi-degree-of-freedom hybrid offshore systems integrating wind and hydrokinetic turbines

Recent technological advancements have made marine energy a significant focus within renewable energy. Offshore wind turbines are usually installed on monopile foundations up to a certain depth. When these monopiles also support hydrokinetic turbines, they form combined hybrid systems. Natural frequency analysis is one of the most critical design criteria for these systems. Traditionally, offshore wind turbines have been analyzed using single-degree-of-freedom (SDOF) models. However, incorporating hydrokinetic turbines increases the system's complexity, requiring a multi-degree-of-freedom (MDOF) approach. This study performs a natural frequency analysis based on a two-degree-of-freedom model for such hybrid systems. Compared with a previous study, the analytical solution shows a difference of only 2.35 %, indicating strong consistency with the literature. Moreover, a 24.59 % decrease in the system's natural frequency is observed compared to the wind-only configuration, emphasizing the need for higher-order modeling in hybrid systems.