Hydrodynamic performance of nested rectangular aquaculture cages under wave action

The global expansion of seafood demand has accelerated the development of offshore aquaculture systems that utilize open-ocean environments. Protecting these systems from both hydrodynamic forces and external threats, such as predation by wild fish, is essential for maintaining stock health, minimizing losses, and enhancing production efficiency. The present work numerically investigates gravity wave interactions with a fish-culture cage protected by a concentric rectangular outer cage within the framework of linearized potential flow theory, with supporting experiments conducted in a wave flume. Numerical solutions are obtained using the dual boundary element method, incorporating a quadratic pressure-drop formulation across the porous interfaces. The results are validated using experimental data obtained in the present study and supported by the literature, with statistical metrics employed to assess the congruence between the solutions. A parametric analysis examines the effects of porosity, wave height, pontoon width and submergence, and cage spacing on scattering (reflection, transmission, and dissipation) and force coefficients across shallow, intermediate, and deep water regimes. Wave scattering is primarily governed by inner cage porosity, enhancing blocking at intermediate depths. The outer cage provides substantial shielding, reducing horizontal forces by 35%-38% and vertical forces by up to 44% in intermediate and deep-water regimes. Increasing pontoon spacing amplifies scattering and energy loss, yielding up to 25% reduction in transmission, while variations in pontoon width and submergence produce non-monotonic changes in vertical forces. These findings provide guidance for designing protected aquaculture cages and wave-shielding strategies, highlighting the critical role of cage configuration and porosity in optimizing hydrodynamic performance and structural safety.