Sea Technology

AUG 2017

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Page 24 of 76

24 st / August 2017 Therefore, to ensure a longer service life, an optimized cop- per-alloy net was produced by adding chromium and cobalt to the existing chemical com- position (Cu-35Zn-Sn), thus changing the mixing ratio and producing a copper-alloy net with a tensile strength of 266 MPa. Even though the use of a copper-alloy net with a smaller diameter would be more cost effective, a 4-mm diameter was chosen considering the specifications of the loom and the operating environment. Furthermore, the copper-alloy net was woven as chain-link netting to disperse the impact of external forces on the system. The copper- alloy net was attached to the buoyant pipe by connecting polypropylene ropes, 40 mm and 22 mm in diameter, to the upper parts of the copper-alloy net and then using them to tie the net to the buoyant pipe. The copper-alloy net cage was bound with mooring lines made of 50-mm polyester at eight locations; specifically, the four corners and four sides of the buoyan- cy pipe. Furthermore, steel anchors each weighing 500 kg were used to provide sufficient support considering the ma- rine environment. Due to the large size of the cage, the copper-alloy net and the buoyant pipe were assembled on site at the port near the installation area and were then placed in the water using a crane. The cage was then towed to the installation area. Each fish farm was stocked with 2,500 Pagrus major fish (mean weight: 800 ± 100 g). Test Methods The underwater behavior of the developed copper-alloy net was observed with a rotary sonar system (881L-GS, Ima- genex Technology Corp.). The operating frequency is related to the spatial resolution, which depends on the detection distance, as well as the beam width. Thus, it is necessary to set the operating frequency so that the overall shape of the cage net can be detected easily. Although 1 MHz is advanta- geous for making detailed observations of the shape in that it offers the highest spatial resolution, field tests revealed that, to detect the fish net, the optimum operating frequency was 675 kHz, given the relationship between the size of the cage net and the detection distance. Meanwhile, because the image sonar is of a rotational type, the spatial resolution changes with the rotational speed. However, the rotational speed of the transducer was set to the standard value because the behavior of the fishing gear under the influence of tidal currents does not change suddenly but rather changes gradually according to the tide. The time required for one rotation was about 30 s. In addition, an electromagnetic current meter (Infinity- EM, JFE Advantech Co. Ltd.) was installed 5 m below the surface next to the cage net to record the changes in the Copper-Alloy vs. Fiber Netting A copper-alloy-net fish cage of a commercial size was designed, and before the offshore installation, the cage was first installed in the Tongyeong coastal area in southern South Korea. The Tongyeong coastal waters feature many ar- eas where the depth is less than 10 m. The depth increases further from shore, and the area is characterized by the pres- ence of many islands. The aquaculture industry has made great progress here because there are fewer waves, smooth- er seas and a rich supply of nutrient salts. Aquaculture cages are installed at a depth of about 20 m where the maximum current is 53.5 cm/s. The copper-alloy-net fish cage was attached to a conven- tional cage frame (12 by 13 m) located in Jeodo, Tongyeong City, Gyeongsangnam-do, in December 2015. The buoyan- cy pipe was rectangular in shape, measuring 13 m long by 12 m wide and was made of high-density polyethylene pipe with a diameter of 420 mm and a buoyancy of 5,392 kgf. The copper-alloy net was formed into a square cage 12.3 m long by 11.3 m wide by 9 m deep, while the netting diam- eter is 4 mm, the mesh size is 40 mm, and the overall weight is about 3,150 kg. If the netting were to be produced using a simple copper alloy, corrosion could occur after a long period in seawater. (Top) The shape of the fish cage according to the currents. At top is a horizontal shape. At bottom left is a vertical shape in calm water, and in high current at bottom right. (Bottom) The fouling state of the nettings after three months at sea. Left: anti-foulant netting; middle: non-antifoulant netting; right: copper-alloy netting.

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