56 JUNE 2015 • WORLD AQUACULTURE • WWW.WAS.ORG the floating cage, inclination of the floating collar is undesirable because cultured fish could easily escape. For this reason, the buoyancy of the floating collar can be maximized by increasing the diameter of the pipe. Net shrinkage is stronger in the front and bottom of the cage with ballast weights at the bottom (Model 2) (Fig. 4). Constriction in size will reduce the projected area, resulting in decline of drag force or low mooring load. As current speed increases, there was a slow increase followed by a rapid rise of current load on the mooring system of both cage types. However, the magnitude of the response was different for each cage model. Tensile loads on the mooring system of Model 1 exceeded Model 2 at all current speeds (Fig. 5). The two factors that contribute to the high load on Model 1 are the high resistance to deformation and an increased projected area of the floating collar beneath the water surface. Furthermore, Model 1 has greater mooring tension than Model 2, even at flow rates greater than 0.9 m/s. As the current speed increases, the front wall deflected inward and the rear side deflected outward and these resulted in a loss of inner volume (Fig. 5). However, this situation was more pronounced in the net-cage with ballast weights (Model 2). The reduction volume ratio was used to describe the volume of the cage models under different flow rates and assumed that the cross-sectional area is proportional to cage volume. The volume reduction ratio is calculated as the ratio of cross-sectional area of the side panel before and after the current is applied. With flow rate, there was a decrease in volume reduction ratio. However, the cage with a ring at the bottom (Model 1) had less percentage inner space loss caused by current at each flow rate. Differences in volume reduction between the cage models at 0.4 m/s, 0.5 m/s and 0.6 m/s were 16 percent, 18 percent and 19 percent, respectively (Fig. 6). At high current speeds, there was a considerable loss of volume in each cage model, especially in Model 2. The difference in the reduction ratio of volume between the two cage models can be explained by the different ballast weighting methods. The ballast-weight collar of Model 1 is rigid and consequently flexibility is reduced. As the flexibility of net structure is reduced, it remains more resistant to external forces and hence less deformation and simultaneous increment of drag force was observed. For Model 2, the ballast system is not a single rigid system but rather units that are dispersed and with a high degree of freedom to move independently and thus with less influence on net flexibility. This implies indirectly that deformation of the cage likely to be dependent on cage design and geometry. As tension in mooring lines increased, cage volume constricted in a similar manner in both models. Two-dimensional displacement of selected points illustrate the change in shape of the cross sectional area of cages by current action. The change in cross-sectional area obtained at a specific current to the current at initial states is compared (Fig. 7). The deflection angle is formed between the vector positions of the upper side of the cage panel, before and after current application, and front and rear deflections are considered in the analysis. The various sections of panel indicate distinct impacts on the total volume of the cage. The various sections of panel have distinct impacts on the total volume of the cage. The inward deflection by the front wall reduces the outward deflection by the rear wall. As water passes through the front of the net, velocity slows due to the front wall screening effect. The rear portion of the net is thus confronted FIGURE 4. Shapes of model cages at various current speeds. LEFT, FIGURE 5. Mooring tension in relation to current speed in cage Model 1 (with a sinking collar) and cage Model 2 (with independent ballast weights). RIGHT, FIGURE 5. Relationship between current speed and volume reduction of cage Model 1 (with a sinking collar) and cage Model 2 (with independent ballast weights). FIGURE 7. Figure 7. Two-dimensional deformation of model cages determined by digitizer.
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