58 JUNE 2015 • WORLD AQUACULTURE • WWW.WAS.ORG wave force action on the cages increases at constant wave periods. Increasing the wave period will result a declining wave force. For model 2, as the wave height increased in ascending order of 10, 20 and 25 cm, with a 3-sec period, the wave load increased (2.1, 5.9 and 6.8 N respectively). However, for model 1, the load was 3.9, 9.2 and 11 N, respectively. As the wave period increase from 2 to 3sec at 0.5-sec intervals, the tension force decreased. For model 1 and 2, the deviation is there but it is not as substantial as the trend associated with an increase in wave height (Figure 11). From these experimental results, the cage with the bottom sinking collar (Model 1) had greater tension force than the cage with the bottom ballast weights (Model 2) at each wave height. The tension force increased with increasing wave height or decreasing wave period. Generally the results can be explained with linear wave theory. Water has a velocity that is directly proportional to wave height and inversely proportional to wave period under regular wave conditions. So, an increase in the wave height will result an increase in velocity and wave force. Conclusion and Suggestion A cage with a sinking collar (Model 1) had a smaller volume reduction ratio, higher inclination angle of the float and a lower mean angle of deflection than Model 2 at each current speed. For Model 1, the front net panel had less inward deflection but greater outward deflection of the rear panel, hence creating more inner space in the cage. The rigid ring at the bottom of the net increased the flexural strength or stiffness of the cage to resist external forces that result from water currents. This ballast weighting system also enhanced stretching of the net. This in turn increased the openness and porosity of the cage, resulting in faster movement of water. However, the cage with the sinking collar had a greater load on its mooring systems at each current speed. The cage with the sinking collar has a greater drag force from currents and waves than the cage with independent weights. Safety factors should be applied in the structural design of similar cages. We suggest that the cage with the sinking collar is more tolerant and resistant to volume deformation under the action of strong currents and waves and thereby providing a more conducive environment for cultured species. Notes Gebremeskel Eshetu Kebede, Chun Woo Lee, Jihoon Lee and Subong Park, College of Fisheries, Pukyoung National University, and Division of Marine Technology, Chonnam National University, Korea References Huang , C.C., H.J. Tang and J.Y Liu. Effects of waves and currents on gravity-type cages in the open sea. Aquacultural Engineering 38:105-116 Huang, C.C., H.J. Tang and J.Y. Liu. 2006. Dynamical analysis of net cage structures for marine aquaculture: numerical simulation and model testing. Aquacultural Engineering 35:258-270. Lader, P., T. Dempster and A. Fredheim. 2008. Current induced net deformations in full-scale sea-cages for Atlantic salmon (Salmo salar). Aquacultural Engineering 38:52-65. Lader, P.F. and B. Enerhaug. 2005. Experimental investigation of forces and geometry of net cage in uniform flow. Oceanic Engineering 30:79-84. Lee, C.W., Y.B. Kim, G.H. Lee, M.Y. Choe and K.Y. Koo. 2008. Dynamic simulation of a fish cage system subjected to currents and waves. Ocean Engineering 35:1521-1532. Moea, H., A. Fredheim and O.S. Hopperstad. 2010. Structural analysis of aquaculture netcages in current. Journal of Fluids 26:503-516. Zhao,Y.P., Y.C. Li, G.H., Dong, F.K. Gui and B. Ten. 2007. A numerical study on dynamic properties of gravity cage in combined wave-current flow. Ocean Engineering 34:23502363. Zhao,Y.P., Y.C. Li, G.H., Dong, F.K. Gui and B. Ten. 2007. Numerical simulation of the effects of structure size ratioand mesh type on three-dimensional deformation of the fishing-net gravity cage in current. Aquacultural Engineering 36: 285-301. FIGURE 11. Relationship between maximum front mooring line tension and wave period at constant wave heights for model 1 and model 2; ‘H’ wave height; ‘T’ wave period.
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