Modeling the dynamic responses of a 2 × 2 gravity-type fish cage group under combined wave-current conditions

The hydrodynamic response of gravity-type cage groups under harsh open-ocean sea states is closely linked to long-term production safety. However, severe wave-current forcing may produce response transitions and internal load-path changes that remain insufficiently resolved for interconnected cage groups. This study develops a numerical model of a 2 × 2 cage group using the lumped-mass method and examines how wave height (H), wave period (T), and angle of attack (α) govern its motion responses, volume loss rate, and mooring tensions. The model is validated against published experiments for net deformation, drag force, cultivation volume, and mooring tension. The cage motions and volume loss grow monotonically with H but non-monotonically with T. Beyond H = 4 m, periodic submergence of the windward floating collar suppresses heave and volume loss growth while raising the risk of fish escape. The maximum tension concentrates on the central upstream anchor line, rising by 18.65% per meter of H and by 45.66% across the tested periods without saturation. Compared with the grid-based configuration, the star-based configuration distributes the anchor loads more evenly at α = 45° while using fewer anchor lines, but increases the maximum tension in anchor line #1 by an average of 5.44% over α = 0° - 30°. These findings support severe-condition assessment and targeted mooring designs or optimizations of the comparable gravity-type cage groups.