32 SEPTEMBER 2012 Aquaculture will play an increasing role in the global supply of seafood in coming years. In the United States, marine finfish farming is predominantly situated in sheltered nearshore waters of Maine and Washington. As farmers seek to expand to new areas, they often are met with stiff resistance from recreational users, local fishermen, the shipping industry and environmentalists. Competition for nearshore space, coupled with the strong demand for aquaculture products, has farmers looking for alternative sites. One solution is to relocate farms into more exposed or open ocean waters. Growing fish in more exposed conditions, however, brings new challenges. Cages, mooring systems and auxiliary equipment must withstand a high-energy marine environment with extreme loading conditions associated with strong winds, currents and waves. In addition, fish monitoring and feeding may be conducted remotely or autonomously, resulting in greater capital and operating costs compared to gear typically used at protected farm sites. In an effort to better understand the operational, engineering, biological and environmental aspects of open ocean aquaculture, the University of New Hampshire (UNH) operated an aquaculture site approximately 10 km from the New Hampshire coast in the Gulf of Maine at a water depth of 52 m (Fig. 1). The overall goal of the project, conducted between 1999 and 2011, was to stimulate the further development of commercial marine finfish aquaculture in highenergy, low-temperature environments. As part of this research effort, a submerged four-cage grid mooring was deployed in 2003 to establish a mooring platform for a variety of biological and engineering research projects (Fredriksson et al. 2004). The mooring system also accommodated the installation of auxiliary equipment, such as feeding platforms, at the site (Rice et al. 2003, Fullerton et al. 2004, Turmelle et al. 2009). In 2010, after seven years of continuous deployment, the mooring was recovered and inspected. This article presents and discusses the design, analysis, deployment history and recovered status of the grid mooring. Design A four-bay, submerged grid in the permitted 12-ha site provided a cost-effective platform for commercialscale aquaculture research projects (Fig. 2). The grid was designed to safely secure four 3,000-m3 fish cages under loading from a 9-m, 8.8-second wave field and a 1 m/s co-linear current (Fredriksson 2001). Compared to employing flotation at the high-energy surface environment, the system was placed at a depth of 18 m to reduce wear and keep a constant pre-tension on the gear, ideal to minimize mammal entanglements. In addition, the depth was accessible to divers, yet sufficiently deep to reduce the rate of biofouling. The grid was supported by nine subsurface buoys and secured to the seafloor with 12 anchor legs, each incorporating 51-mm diameter copolymer rope, 42-mm diameter chain and a1-t drag embedment anchor (Figs. 2 and 3). It is important to note that anchor size was specified in the design to be the weak link in the system, such that anchors would drag if the grid was loaded beyond its capacity, reducing system loads and protect the mooring integrity. Analytical and numerical models were used to design, analyze and finalize the system configuration. Aqua-FE, a UNH-developed software package, was the primary tool used to assess mooring integrity under a variety of environmental loading conditions (Tsukrov et al. 2003). The program has been used to study a variety of cage aquaculture systems and has compared well with physical model testing and in-situ experiments for different cage types and mooring configurations (DeCew et al. 2010 a, DeCew et al. 2005, Fredriksson et al. 2003). A full description of the design approach can be found in Fredriksson et al. (2004). Physical models were also employed to assist with deployment strategies. Assessment of a Mooring System for Offshore Aquaculture Judson DeCew 1 *, Barbaros Celikkol 2, Kenneth Baldwin 1, Michael Chambers 3, Jim Irish 4, M. Robinson Swift 2 and Igor Tsukrov 2 FIGURE 1. The UNH offshore site is located 10 km south of Portsmouth, NH in 52 m of water. The site is fully exposed, with only a small group of islands approximately 2 km to the north. The success of the seven-year deployment, with no structural integrity issues and no loss of fish cages or surface feeding buoys, can be attributed to a sound engineering design approach.
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