Design and analysis of an automated feed-buoy for submerged cages

Abstract

A research prototype feed-buoy was developed to supply a submerged net pen at an exposed site south of the Isles of Shoals, New Hampshire, in the Gulf of Maine. The system, designed for a quarter-ton feed capacity, consists of a surface feed-buoy, rubber tether moorings attached to a submerged grid, a feed transfer hose, feed dispensing machinery, and telemetry/control components. The buoy is taut-moored above the cage by compliant members in order to allow for the tidal range and large storm waves. The feeding mechanism uses a small, electric powered pump to actively force feed slurry down to the cage. A wind generator and solar panels provide power to this pump, bilge pumps, and the telemetry/control system. The control system switches the power to the various pumps on a user-set schedule and also monitors the operation of the electric power system. A spread spectrum radio is used to send diagnostic and status information to shore and via Internet to the project manager. The high-stretch feed hose has integrated, spiral-wound conductive wires to transmit power down to, and receive data back from, instrumentation in the fish cage.The mooring system design was analyzed using the University of New Hampshire (UNH) developed, finite element program, Aqua-FE. Modified to include nonlinear material behavior, Aqua-FE was applied to evaluate various buoy-mooring line-submerged cage/grid configurations. To complement the computer modeling, a 1:15.2 scale model of the buoy was built for wave tank testing. Free release tests were conducted to determine heave and pitch natural frequencies and damping ratios. Heave and pitch responses to single frequency waves were measured in order to characterize seakeeping behavior.The three-point mooring system has operated successfully for over 10 months at the site. After correcting start-up problems, due in part to a winter ice storm, the internal systems have functioned as designed enabling the buoy to provide regular, metered fish feeding.