Assessment of a submerged grid mooring in the Gulf of Maine

The University of New Hampshire (UNH) developed and maintained an offshore aquaculture test site in the Western Gulf of Maine, south of the Isles of Shoals in approximately 50 m of water. This site was designed to have a permanent moored grid to which prototype fish cages or surface buoys could be attached for testing new designs and the viability of the structure in the exposed Gulf of Maine. In 1999, the first moorings deployed consisted of twin single bay grids each capable of each securing one fish cage. These systems were maintained until 2003. To expand the biomass capacity of the site, the single bay moorings were recovered and a new four bay submerged grid mooring was deployed within the same foot print of the previous twin systems. This unique system operated as a working platform to test various structures, including surface and submersible fish cages, feeding buoys and other supporting equipment. In addition, the expanded capability allowed aquaculture fish studies to be conducted along with engineering and new cage/feeder testing. The 4 bays of the mooring system were located 15 meters below the surface. These bays were supported by nine flotation elements. The system was secured to the seafloor on the sides with twelve catenary mooring legs, consisting of Polysteel
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line, 27.5 m of 52 mm chain and a 1 ton embedment anchor, and in the center, with a single vertical line to a 2 ton weight. To size the mooring gear, the UNH software package Aqua-FE was employed. This program can apply waves and currents to oceanic structures, predicting system motions and mooring component tensions. The submerged grid was designed to withstand 9 meter, 8.8 second waves with a 1 m/s collinear current, when securing four fish cages. During its seven year deployment, the site regularly experienced extreme weather events, most notably a storm with a 9 m significant wave height, 10 second dominate period in April 2007. The maximum currents at the site were observed during internal solitary wave events when 0.75 m/s currents with 25 minute periods and 8 m duration were observed. The mooring was recovered in 2010 after 7 years of continuous deployment without problems. The dominate maintenance requirement of the mooring was the cleaning once a year of excessive mussel growth on the flotation elements and grid lines. No problems of anchor dragging or failure of mooring components were documented during the deployment. Upon recovery, critical mooring components were inspected and documented, focusing on items with wear or other areas of interest. The mooring proved to be a reliable, stable working platform for a variety of prototype ocean projects, highlighting the importance of a sound engineering approach taken in the design process.