Motion response of a truss-type cage during submergence: a numerical method combining BEM and power-based averaging method

Large truss-type aquaculture cages are core facilities for deep-sea aquaculture, and submergence operations are critical for stabilizing the internal culture environment. Accurate simulation of the full submergence process is essential for cage structural safety, yet targeted numerical methods remain limited. This study takes the Deep Blue No. 2 aquaculture cage as the object, computing frequency-domain added mass and damping coefficients via the boundary element method. A power-based averaging method generates time-domain equivalent parameters and ballast net weight, which are imported into a Cummins-based time-domain solver for 6-DOF submergence simulation. Numerical results agree well with flume model tests and accurately reproduce three characteristic submergence stages: initial, acceleration and deceleration. Furthermore, motion responses under different environmental conditions are analyzed. At 5 m wave height, the sum of cage maximum sway and drift approaches 1/4 of its length. Pitch peaks at 10–12 s periods are 15%–25% higher than at 14 s in the acceleration stage, while long-period waves dominating other submergence stages. A current velocity of 0.62 m/s is identified as a welfare-based safety threshold. Under wave-current coupling simulation, hydrodynamic load dominance shifts through three distinct phases. Emergency submergence may induce 1.6–4.6 times larger motion responses than normal operations. These findings provide a reliable numerical framework and quantitative safety reference for submergence design of large truss-type deep-sea aquaculture cages.