The flotation ring, a vital component of the gravity-type fish cage in open ocean engineering, oscillates under the action of water waves. The wave forces on the element of the flotation ring can be represented by Morison formula. However, the computing results according to the common Morison formula can not be in good agreement with the experimental data. Therefore, the attempt to substitute the relative acceleration of water particle in the inertial forces is made. The fresh results improve the surge displacements in the negative direction to some extent. It is said that that the modification for Morison formula may provide another angle of view to describe the wave forces acting on the floating structures.
Due to the decreasing fishery products and the lack of the offshore marine resources, fishery aquaculture in the open ocean is playing an increasingly important role, believed to be a very efficient farming technique. The deep-water anti-storm fish cages are frequently employed, which is a typical open ocean aquaculture technique. As a new form of offshore structures, fish cages have received the extensive investigations. Fredriksson (2003) studied the heave response of a central spar fish cage using the physical and finite-element model and the field observations. Huang (2006) proposed a numerical model of analyzing the dynamic characteristics of a net cage, and numerical results obtained was in good agreements with the experimental data. Herein the gravity-type fish cage is considered. It is composed of a flotation ring as the main load-bearing structure component, cage net to ensure that fishes are encircled inside, and mooring cables fastening the fish cage from floating away in waves and flows. And the focus is on its flotation ring. In this paper, numerical model of flotation collar was set up to simulate its hydrodynamic behaviors in waves.