Open Ocean Aquaculture Engineering: System Design and Physical Modeling

Abstract

An open ocean aquaculture net pen system was developed and deployed for an exposed demonstration site south of the Isles of Shoals, New Hampshire in 55 meters of water. This component of the project is part of an interdisciplinary effort at the University of New Hampshire involving engineers, biologists, economists and commercial fishermen. Initially, two cages were specified for the growout of summer flounder (Paralichthys dentatus). The design process included physical model testing conducted in a wave/tow tank using 1/22.5 scale models. To select an optimum system, experiments were performed using gravity-type and central spar-type cages. Vertical taut line and catenary moorings were evaluated. Data acquisition included drag and wave forces on the cages, mooring line forces and heave, pitch and surge motion of the cages.After comparison of the results and holding a design review including outside experts, a central spar configuration was selected for both cages. This system exhibited both reduced force loadings and less extreme motion, and its rigid frame would resist volume changes under storm conditions. In the final mooring design, each cage was separately deployed using a four anchor system to ensure redundancy. A mid-depth, square, horizontal grid was employed in order to reduce anchor footprint area, which is very expensive under New Hampshire permit rules. Mooring system components were sized to meet loadings scaled up from the physical model results. The system was deployed in June of 1999 and has performed well in all weather forcing conditions to date.