Numerical analysis of local and global responses of an offshore fish farm subjected to ship impacts

Abstract

Due to limited nearshore areas and great impact to local ecosystems, the aquaculture industry is moving the fish farms into more exposed sea regions where the environmental conditions are much more severe. Like other offshore installations operating at sea, fish cages are also exposed to the risk of collisions from attendant or visiting vessels. This paper evaluates the structural strength of the world's first offshore fish farm operating in Norway subjected to supply vessel collisions by the use of nonlinear finite element simulations. The fish cage is a semi-submersible floating structure consisting of ring-stiffened columns connected by braces. A standard 7500-ton supply vessel is used as the striking vessel. Both local shell deformation analysis using LS-DYNA and beam deformation analysis using USFOS are carried out. In the local shell analysis, collision scenarios with both decoupled and coupled methods are simulated for several representative impact locations. The decoupled model adopts prescribed paths for ship motions, while in the coupled model, the ship motions are calculated based on hydrodynamic forces from linear potential theory as well as inertia and collision forces. In the global beam-column analysis in USFOS, the force-deformation curves of the supply vessel from local shell analysis are used as the input. The resulting damage and energy absorption are compared with those from local shell simulations. The results are discussed with respect to the impact resistance, energy absorption and structural damage.