Hydrodynamic analysis of a concentric monopile-cage integrated structure for offshore wind-aquaculture applications

Integrating marine aquaculture with offshore wind energy presents a promising pathway towards a sustainable blue economy, but introduces complex hydrodynamic challenges. To address the lack of efficient predictive tools for such integrated systems, this paper develops a novel semi-analytical model based on the Matched Eigenfunction Expansion Method to investigate the hydrodynamics of a Concentric Monopile-Cage Integrated Structure (CMCIS). A systematic parametric study reveals critical physical phenomena with significant design implications. It is found that severe biofouling can transform the cage into the dominant source of load, causing the total horizontal force to exceed that on an isolated monopile. More critically, a severe design pitfall is identified: a wide annular gap can excite powerful fluid resonance under long-wave conditions, leading to a catastrophic load amplification that can be nearly six times the force on an isolated monopile and more than double the monopile’s own maximum force under any wave condition. Conversely, the analysis demonstrates that increasing the cage’s submergence depth is a highly effective strategy for mitigating these loads. The developed model and the physical insights gained provide crucial guidance for the safe and optimal design of these integrated systems.