A physical model approach to nonlinear vertical accelerations and mooring loads of an offshore aquaculture cage induced by wave-structure interactions

Abstract

A grand aim of aquaculturists is to investigate the interactions between the oceanic environment and aquaculture structures, in order to make the structures more durable to loads caused by waves and current. To that end, we investigate the effects of extreme wave conditions by leveraging a physical model approach. We assume that the floating collar of the cage moves with the wave in heave, leading to extreme conditions. Thus, the wavelength is much larger than the size of the floating collar model and the wave frequency is much lower than the natural period of heave. For the floating collar model without netting, under wave-only conditions, the first- and second-harmonic components of the vertical acceleration are proportional to the wave amplitude ζa, and square of the wave amplitude ζa2, respectively. The third- and fourth-harmonic components exhibit oscillations at non-dimensional wave number νa≈0.055 due to the change of free surface. For the floating cage model under combined waves and current conditions, the first-harmonic component is more likely proportional to ζa, while the second-harmonic component shows erratic behavior. The effects of mooring loads in front and aft mooring lines are also assessed.