An Experimental Study on the Deformation of the Net Cages

Transcription

1 J. Fish. Soc. Taiwan, 26(4): An Experimental Study on the Deformation of the Net Cages Kei-Chien Yu 叭, Syue-Sinn Leu 1 and Yau-Shou Chow 2 (Received, May 7, 1999; Accepted, July 20, 1999) ABSTRACT Experiments were conducted on preventing deformation of net cage from waves and strong current by reducing the deformation rate through adding the stable rope in the net cage. In this study, a 40 cm x 40 cm x 32 cm traditional net cage a new design mooring rope net cage and a stable net cage were used as models in a current - wave water tank. There were 4 kinds of sinker weights fastened at the bottom of the net cage. The shape and volume of cage were determined and measured under different flow velocity (V) and wave heights (H). Theresults are as follows In case of V=15cm/sec and H=O (waveless condition), we can determine that the relative volume ratio of the traditional net cage (35%) raised to 68% of the stable net cage. The volume deformation ratio of the traditional net cage was 90%, reduced to 50% of a stable net cage during against strong waves and current. Due to the elastic space of a stable net cage creates the more inner volume of cage, the fishes reared in the cage had much more survival space then traditional net cage against strong waves and current. AII this results will be used in the development of an anti-wave net cage for increasing the fish aquaculture production. Key words: Relative volume ratio, Volume deformation ratio, Elastic space, Stable net cage. INTRODUCTION The fish culture of the net cage was regarded as an important method to enhance the fishery production in many coastal countries such as Japan, Norway, Hong Kong, and Italy etc (Yu, Terhune, 1992 and Banchini, 1990). In T aiwan, it was also widely extended to fishermen ín recent years. Because the net cage is employed in the coastal water, the waves and the tidal current affects it. Previous studies suggested that the tidal current was a factor for cage culture. For example, the fish cultured in aquaculture cages will die resulting from the shortage of dissolved oxygen when the current speed is less than 0.1m/s in a river (Chen, 1994). On the contrary, the rough current w ill deform the shape of the net cages, resulting in the decrease of its space volume (Wang, 1993; Lee, 1997), and subsequently reduce the rate of subsistence of fish bred in the marine aquaculture cages. However, the subsistence rate of fish is not directly related to the current speed, but a large amount of fish will die from the strike of rough wave (Yu, 1998). Due to the effective space volume of the net cage could be decreased to 50%, by the motion of wave action, in the extern sea condition (Lee, 1997). The reducing effective space volume of the net cage may be affecting the subsistence rate of fish, so how to :Oepa 川 ment of Fisheries, Natio 叫 Kaohsiung Institute of Ma 巾 e Technology, Kaohsiung 811, Taiwan ιoepartment of Fisheries science, National Taiwan Ocean University, Keelung 202,Taiwan. * Corresponding author

2 214 Kei-Chien Yu, Syue-Sinn Leu and Yau-Shou Chow increase the effective space volume of the net cage against wave and current is an important study subject. Hence, to elucidate the mechanism of high dead rate resulted from the deformation of net shape, the change of the shape and effective space volume of the net cage under the interaction of current and wave is examined in this study. Knowledge obtained from this study is expected to improve the design of the net cage and increase the productivity of the marine aquaculture cage. 1. materials MATERIALS AND METHODS The size of the square floating net cage model employed in this study was measured 40 cm x 40 cm x 32 cm. The respective sinkers were (A type), (8 type), (C type), and (0 type), which were fastened at the 4 corner of the bottom side for a traditional net cage (as in Fig. 1), a new mooring rope net cage (Fig. 2) and the stable net cage (Fig. 3). The specifications for these model net cages are listed in Table 1. The Artificial wave-current water tank belonged to National Sun Yat - Sen University was employed in this study. The length was 40 m, the width was 1 m and the depth was 1.2 m. The effective depth is 1 m. The maximum rolling speed of the flow velocity controller was 1700 rounds per second. It can create waves simultaneously and the maximum wave height is 35cm. The experimental apparatus is illustrated in Fig 2. The model net cage was hung on a floating frame by four ropes parallel to the water tank. An AOV type flow meter and a MKS type wave meter were installed on the top of the water tank. A Hi 8 camera was installed at the front of. the water tank. 11. Methods In order to understand the shape change of the net cage against the current and wave, a traditional net cage, a new mooring rope net cage and a stable net cage, attached with 4 kinds of sinker weights were employed in the water tank. A rectangular white board on which plant of grids of 2 cm on each side were plotted, was arranged in the location where the net cage was set. The Hi 8 camera was arranged at the front of the water tank, and during the experiment, the pattern of the net cage could be drawn and measured by computer (K04600, Graphtec). The volumes of the net cage were obtained by multiplying the length (L) 門洞Fig. 1. Diagram of the model of traditional net cage.

3 前-4EL An Experimental Study on the Deformation of the Net Cages 215 flow yelocity meter E Fig. 2. Diagram of the model of the new design mooring rope net cage flow velocity meter w 9u v e l m e A C P S Anchor Chain Pulley Sinker nunu--uyem uose opa ye -ue e nr go tp he b yiaa RUDaoo R Hi-R Camer a Fig. 3. Diagram of the model of th e stable net cage used in this experiment The scale conversion The water tank depth was 1m, due to the maximum wave height be created was 0.25 m, so that the utility depth of water tank is 0.87 m, and used 85 cm as depth standard. Using the dynamic similarity theory and Froude number (Fn) equality principle (Fn is the ratio of gravity and inertia) showas 2..2 V';,., V~ Fn = -:-'-"-= ~ 一一一 一 ---(1) n - glm glp V 三 Vf Fromeq. (1) we can obtain ~ =+ Lm Lp L~ V~ L 三 /T~ the period is 7 止 =-4= lhl Lp V 吉 L~/Tt the wave height is H~ L~ 一 LL= ll Hp Lp V (cm/ sec) : Flow Velocity 9 (cm/sec 2 ) : Acceleration of Gravity L (cm) : Length of the net cage H (cm) : Wave Height p : the field experiment m the model experiment. The water tank depth, compared with the actual depth of 22 m determined A scale factor of 1/25. The net cage was reduced to 40 cm x 40 cm x 32 cm accord 開 ing to this scale. The sinker weight was reduced by the cubic of the scale factor. IV. Simulating the sea conditions as Regarding bad weather on the sea, set RESULTS

4 216 Kei-Chien Yu, Syue-Sinn Leu and Yau-Shou Chow The field condition model condition The maximum wave height as H=6 m H=0.25 m The wave period T=10.70 sec T=2.15 sec The wave length L= m The water depth d=22m d=0.85 m The maximum flow velocity V=0.75 m/sec V=0.15 m/sec Table 1. Specification of model net cage Type A Type square L(cm) 40 D(cm) 32 d (cm) (mm) SA (g) 7.80 L 1 (cm) 50 L 2 (cm) 200 L C1 (cm) 40 W C1 (g) W Sb (g) 17.8 d B (cm) 3 Ls (cm) L C2 (cm) W C2 (g) L: Length of netting cage d: diameter of twine 1: Length of twine bar SA: Sinker weight L C1 : length of chane d : B diameter of buoyant Ls: Length of stability chain W : C2 sinker weight D: Depth of netting cage L 1 L 2 : length of mooring rope W : Sb sinker weight L : C2 length of chain B Type square ~ CType DType S Type Square square square ~:dá") 7.74*2 1. The factors influencing net cage deformation In order to understand the factor which influence the deformation of a net cage, a traditional net cage, new design mooring rope net cage and a stable net cage attached with 4 kinds of sinker weights against various flow velocities were employed in the water tank. A side view of the net cage is illustrated in Fig. 4 and Fig. 5. A trend was obtained thatwhen the sinker weight was constant; the shape of the net cage would be skewed backward

5 (OO6 B -告訴 自gkrvoEAn Experimental Study on the Deformation of the Net Cages 217 的\詞 )b,也a type 一WmLB d一,向lvpa 衍Ctype -m u ρuvm Dtype 已二斗它 ~L: 斗 ~ 斗 文 ~~~~ 孔斗 心 \ 丘之忌 Q N 且心 竺心 予公茫 Fig. 4. The size of traditional net cage against the flow velocity under waveless condition og0\)b-z 自3 6 (Sinker Type B A茁回回 bd D unit cm 2 可王三六日叮叮冶均可忍 日c S 9 有已可 ~~ ~ 弋茁斗 12 它可及為它可是 b\~~ 旦入 六亞三 b 15 ~ 心之 '::Jc:::::. 苛政三 ~ 立之 ~~ 忍這六百三 b Fig. 5. Size view of new design mooring rope net cage and stable net cage against the flow velocity under waveless condition. once the cage was attacked by the flow. The greater the flow velocity, the greater the net cage would become skewed and the lesser the volume under waveless conditions. The inner volume of the net cages was measured. The relative volume ratio (Rc) was the inner volume of a net cage compressed by the current (UC) compared to the inner volume of an undisturbed inner volume of a net cage (UO). Show as Rc = (UC/UO) Results of the regression analyses of

6 60 (法) 出218 Kei-Chien Yu, Syue-Sinn Leu and Yau-Shou Chow Rc for traditional net cage and flow velocity are shown in Fig. 6. Rc was decrease as the flow velocity increased from 100% of V =0 cm/sec reduce to 31% of V=15 cm/sec for A type sinker weight net cage. Then adding sinker weight for experiment, when V=15 cm/sec, Rc increased as the sinker weight increased from 31% of A type sinker weight net cage raised to 35% of D Type sinker weight net cage. From the above, we know that the current influences the relative volume ratio much more than sinker weight does. In order to understand the space volume change of the different mooring method net cage, A Type sinker weighs (7.8 g) attached to a traditional net cage, a new mooring rope net cage and a stable net cage respectively for the experiment. Rc would be detected and measured, computed the regression analysis the Rc and flow velocities. Results are shown in Fig. 7. The Rc curve of the stable net cage was more steady then other method of the net cage. When V=15cm/sec, Rc of the traditional net cage was 31%, the new mooring rope net cage was 35% and the stable net cage was 68%. This study indicated that the Rc ofthe stable net cage is much greater than the traditional net cage. The stable net cage is better standing against the deformation under the current. 11. The motion of traditional net cage against wave and current action The wave is another important factor of the deformation of the net cage. In order to understand the motion of the traditional net cage against wave and current action, A type sinker weight traditional net cage was employed in the water tank for exper 卜 ment. In the case of V=15 cm/sec, H=25 cm and wave periods was 2.15 seconds. The motion of a traditional net cage corresponding to the wave action was investigated by Hi 8 camera; results are shown in Fig. 8. When the net cage is at position A (wave trough), the inner volume was 310 cmζ. When the net cages move to the front of the crest (position B), the volume of the net cage was increase to reach up to 510 cm 弋 which was the maximum volume in the wave period. Then the net cage moved to position C (wave crest), here the frame of the net cage rose to the top of the wave and the bottom webbing of cage would rise gradually and the volumes of a net cage was decreased to 438 cmζ From position C moving to position D, the back webbing of cage would extrude together at the bottom of the cage, where the smallest amount of volume of 200 cm". Finally, the net cage would return to the wave trough (position E) and revolve around and around in the same motion as A type 圖 D type 3 '6 9 Flow velocity (cm/sec) Fig, 6, Relationship between the Rc of traditional net cage and flow velocity

7 An Experimental Study on the Deformation of the Net Cages 戶 80 法 -- + 怕 吋 4 70 主 50 Fb O 40 -F 吋 ωvc>d 吋 4 -q 30 ~ New design mooring rope -Traditional methods ~ Stable type Flow velocity ( cm/s ) 15 Fig. 7. Relationship between the Ru and flow velocity using A Type sinker wei ght. Ef/ 六 \ 拉克不? 可 A B A:trough B:at the front of crest C:crest D:at the back of crest E:troegh Fig. 8. The shape of traditional netting cage within the period of wave in case of V=15 cm/s, H=25 cm in this experiment E 可 shown in Fig. 8. The volume change of the net cage was very great subjected to the wave for a range of wave periods. This is the greatest problem for the net cage pisciculture industry. The motion of a stable net cage against waves and currents were also investigated by the camera and shown in Fig. 9. Because the stabilizer rope gives the elastic space, the shape of the cage was very stable in the wave period The volume deformation ratio of the net cage (Rd) against the wave and current From above, the volume deformation ratio of the net cage (Rd) is determined for the largest volume of a net cage (Umax) minus the smallest volume of the net cage (Umin) which against wave and current,

8 220 Kei-Chien Yu, Syue-Sinn Leu and Yau-Shou Chow I II V increase Tsinα>FB III Fig. 9. The motion of stable net cage against the current and wave. divided by constant volumes (Uo) caused by the same flow velocity and waveless condition. Show as Rd = (Umax - Umin) / (Uo) In order to understand the change of Rd, A type sinker weight attached to a traditional net cage for experiment. The Umax and Umin of the traditional net cage under the various wave heights and flow veloci ties are drawn; Results is shown in Fig. 10. The Umax and Umin of a new mooring rope net cage attached with 4 kinds of sinker weight, and a stable net cage attached with A type sinker weight against the wave are drown; Results are illustrated in Fig. 11. The Rd also was measured and obtained; In case of V=15 cm/sec and A type sinker weight, results of the Regression analyses of Rd and wave height are shown in Fig. 12. From the Figu 悶, a trend was obtained that the Rd was increased when wave height increased. The Rd curves of the stable net cage were very calmly, and lower then a traditional net cage. When H=25 cm, The Rd of a stable net cage was reduce to 50 %, it was smaller than the Rd of a traditional net cage (90%) and Rd of a new mooring rope net cage (77%). The results indicate that the stable net cage reduces the deformation ratio of the net cage, especially under strong wave height and current conditions, which produces more rearing space, and to raise the survival ratio of the fish reared in the Flow velocity (cm/sec) MM1阿JUu 個 1n UI 取祖 三 b 弋文斗吭毛丸~ 峭耳之 ~ 10 \:: 二 3 丈二 3 之三 弋之三 b b己之 之 ~ 譯 之三" """ 之二 ~ 已吭雨咱這之立 h ~ 弋之三斗 nh 之三 b 六二 ~9阿15 0 弋二三 20 0 弋二 3 """""'" 之三 七吭 UFig. 10. The shape of the largest (Umax.) and smallest (Umin.) volume of A type sinker traditional 可5 丈二 )~ net cage against the wave 日height (H) and flow velocity (V). 12 仙丸之一一(g)HSZS25L 二 3 弋二三 七

9 怠 這An Experimental Study on the Deformation of the Net Cages 8吼)叫H221 Sinkertype (一一 -Â. 一一一 -.ll. 一一一一一 -L 一一一一 ~ 一一一一 --s 一 Umax Umifi Umax Umin Umax Umin Umax Umin Umax UmÌn 5 jn.. 吋荒之三 '" 之三 ::,,""" 弋么 ~"""'" 之二這弋乏這 ~ 弋亡三 弋二 C 亡三 λ 10 ~-.: 之三且可三 :::, =::::;:: 之 六二 ~ 弋己 弋 ~ 弋之三 之二 15 究之 2 ~ 弋 2 咱可三口之三 亡三 h 弋之三追究試弋之三荒之三 20 """"'\: 三 :::,-= 咒之三 ::,= 之三 "'--- 亡三 ~~ 弋之三 ~ 弋之三斗 之二 諱25 究已占中 六二 ~ --=:::::;" 六二 ~~ 六二 λ 司令六二三 :,~ Fig.11. The side view of various type net cage against the wave when velocity is 15 cm/s. ~ 100 r. Tradition 一 Cage 法 90 旬 出 J 80 ~ 團 New-rnooring Cage 出 + 何 ~ 70.. New-Stable Cage O V=15cm/sec O Wave Height ( cm ) Fig. 12. Relationship between the Volume-Deformation Ratio and wave height in case of three type netting cage when A Type sinker weight is used. cage DISCUSSION Net cages employed in the sea to rear fish. The survival ratio of fish that are reared in net cages corresponds to the effective space volume of the net cage (Lee, 1997); The volume of the net cage is affected by the magnitude of the current (Yoichi, 1985). The flow velocity influences the inner volume of the net cage more than sinker weight or O/L. As in the model experiment, when the flow velocity is greater than 0.4 m/s and in case of O/L=0.1, the inner volume of the net cage decreases to less than 60% of the original volume (Wang 1993). Although the inner volume of a net cage is reduces by the current, the effective space volume of the net cage was constant under the flow velocity. 80 that the current is not a principal factor of fish survival. 8uch as the Yeung An 8ea Farm of Kaohsiung County, although it has the largest flow velocity over the year in full, but the fish survival rate will reach up to 90%. When winter, Northeastern winds are prevalent and after the summer typhoon season, many dead fishes were observed in the cages, the more north wind speed the more fish die in the cage (Yu, 1998). The wind speed created the wave height, If the wave date is not available, then the wind date may be adopted to generate wave height by using suitable theories (Lee 1997). 80 that wave height

10 222 Kei-Chien Yu, Syue-Sinn Leu and Yau-Shou Chow was the most important factor for the fish survival ratio reared in the net cage is modify. The volume deformation is effected by the wave height; such as a round free suspended net cage has the largest volume and safety when subjected to wave action. But the volume reduction will reach up to 50% in extreme cases (Lee 1997). But the motion of a net cage against the waves was not investigated. In this experimer 泣, the motion of a traditional net cage under the influence of the wave was studied. In the case of V=15 cm/sec and waveless condition, the relative volume ratio (Rc) f a traditional net cages were reduce to 31 % of original volume, If the sinker weight from increased to 39.1 g, the Rc of traditional net cages will increase from 31% raise to 35%. Adding sinker weight can increase the inner volume, but the cage may be easy to break should be considered. In case of V=15 cm/sec and H=25 cm, the Rd of a traditional net cage would reach to 90% in the wave period. There are almost no survival spaces, that.is the most important factors for the survival ratio of rearing fish. In this experiment, we installed a stable net cage by adding a stabilization rope attached to a new mooring rope net cage. This net cage is more stable than the traditional net cage. The relative volume ratio of a stable net cage is increased to twice the volume of a traditional net cage under waveless conditions. Especially, the Rd of a stable net cage reduces to 50% against strong wave and current, so that there are more inner volume of the net cage than the traditional net cage. The fishes have much more effeated space volume than the traditional net cage under the same conditions. The survival rate of reared fish should be raised and increase the productivity of marine cage aquaculture. ACKNOWLEDGEMENTS This work was partially supported by the National Science Council, R.O.C (NSC 四 E ). REFFERENCES Banchini, -M. L., C. Costa and F. Lombardi (1990). Project for the development on the Silmountains. RIV.-IDROBIOL, 29(1): Fu, E. B., O. Sato., K. Nashimoto and K. Yamamoto (1989). Fluid force on simplified models of aquaculture net cage Examination of mooring method in Aquaculture Net cage. Nippon Suisan Gakkaishi, 55(7): Fujita, S., K. Fukahori and H. I\Iishinokobi (1991). Tension of the mooring ropes on the net cage model of raft type in regular waves. Nippon Suisan Gakkaishi, 57(12): Fu, E. 日, K. Nashimoto., k. Yamamoto., T. Hiraishi and T. Takagi (1994). Model Experiment Examination of mooring method in Aquaculture Net cage. Bull. Fac. Fish. Hokkaido University, 45(1); 9-18 Lee, H. H. and P. W. Wang (19 97). A Preliminary Study on the Free-Suspended Type Fish Farming Cage Subjected to Waves. Proc. 凹的 Conf. on Ocean Engineering 的 Republic of China, 19: Nath, J. H. (1977). Laboratory Validation of Numer 固 ical model drifting buoy-tether-drbught system, NDBO Report, Nagamatsu, K., H. Nishinokubiand and K. Nakasai (1986). The mechanical characteristics of a netting store pot. Bull. Fac. Fish. Nakasaki Univ, 59: Terhune. J. S. (1992). Increasing yields of channel catfish using a combination cage and open pond production systems. J. World Acpacult. - Soc, 23(1): Wang, M. C. and K. Y. Hsieh (1993). The Inner velocity and Volume of Netting Cage. J. Fish. Soc. Taiwan, 20(2): Yoichi, 0., Y. Tawara and H. Taketome (1985). Studies on Behavior of Fish Cage against Flow of Water - On Relationship between Volume of Cage and Current Velocity. Fisheries Engineering, 6: Yu, K. C., S. S. Leu and Y. S. Chow (1998). A study of Seasonal Fluctuation on the Intermediate Breeding in the Marine Aquaculture cages at the Kaohsiung Jouan Sea Area. China Fisheries Monthly, 550: 3-20.

11 An Experimental Study on the Deformation of the Net Cages 223 箱網變形的實驗研究 俞克儉 1* 呂學信周耀然 2 本實驗是為了以防止箱網在大浪強流之衝擊下的網具變形, 利用在箱網加上穩定索來減低箱網變形率 本實驗中, 個 40 cm x 40 cm x 32 cm 之模型傳統箱網 新設計繫留索箱網及穩定 式箱網在模型水槽實驗, 以四種況子量分別繫於箱網之底部, 在不同的流速及波高運作下觀查網形之變化並計測箱網網內容積之變化 其結果如下 ; 當流速為 15 cm/sec 而無波時, 箱網之網內容積率從傳統箱網的 31% 提升至穩定式箱網之 68% 當流強; 良大時, 箱網之網內容積變形率從傳統箱網的 90% 降低至穗定式箱網之 50% 由於彈性空間之發揮, 穩定式箱網有更大之箱網網內容積, 當流強 ; 良大時, 箱網內飼養之魚類比傳統箱網有更大的生存空間 這實驗結果可促進發展抗浪形箱網以提高魚類養殖生產量 關鍵詞 : 相對容積率, 容積變形卒, 彈性空間, 穩定式箱網 1 國立高雄海洋技術學院漁業系 2 國立台灣海洋大學漁業科學系 * 通訊作者