Considerationsfor Longline Culture Systems Design:Scallops production
German E. Merino Universidad Catolica del Norte Coquimbo, Chile
Introduction Mollusk culture in cagessuspended in the waterbegan in Japanwith theoyster Crassostreagigas! 300 years ago, but only in themiddle of thiscentury did theJapanese develop a specialculture structure where the main characteristic is to give an artificial environmentto grow sessileorganisms in a vertical watercolumn. This is a greatadvance in the optimizationof the productionper unit of surfacearea. These structures belong to thegeneral class of culturesystems known as longline systems, andthey are usedcommonly for scallopsand oyster commercial culture, with some modifications in several countries,but all inspiredin the Japanesesystem Dupouy, 1983;Imai, 1977;Illanes & Akaboshi, 1985;Barnabe, 1991!. To designand dimension the longline system, it is necessaryto knowthe depth of placement,the current speed, the longline flow exposurearea, the respectivedrag coefficients,and an estimateof the fouling growth on the system Merino, 1996!. In this work the basicphysics and operational concepts for design,dimensioning and installation of a commerciallongline for scallopproduction will be discussed. LonglineSystems and Structures Thespatial disposition of thelongline in thewater column couldbe totally immersed or at thesurface. The immersed longlineis usedin unprotectedculture areas and in areaswith a considerablefouling biomass,because wave and fouling effects are minor under the water surface. The surface longline generallyis usedin protectedareas with a lowfouling biomass. This generalrule could reduceinvestment and operations cost in a commercial mollusk culture Merino, 1996!. In the "longline culture system,"three subsystems can be found: the mooring-anchorsystem, the floatation system,and the growing system,each of them are assembledfrom structures like ropes, buoys, pearl-nets, etc. Figure I!. The mooring-anchor system is used to stabilize the system againstthe ef1'ectsof dynamicstresses, both vertical and horizontal, to which the longline i» continually being subjected within the marine environment. The floatation system is used to maintain suspension of the culture system, and i» usually composed of different kinds of plastic buoys,or other manufacturedmaterials, wi1h varying sizes and capacities. The growing system i» used to grow the mollusk in captivity, and one of several growth structures is used, dependingon the stepof the culture and also on the selected species.In scallopand oystercultures, pearl-net, lanterns, bags, ear-hangers,and other basketsystems are generallyused.
Figure l. Typical Chilean aquaculture longline fa~ilitv setup. I! culture units, 2! main line, .3! buoy rope, 4! first buoy rope, 5! floats, 6! buoy,7! mooring line, 8! atuhor.system,9! anchor rope, l0! anchor buoy.
Knowing the Environmental Forces The currents, waves, wind, system weight and managementoperations are the main variablesthat determine the dimensions and the magnitude of the stress that acts upon the longline system's structures.
Marine currents make horizontal tension a maximum in each of the extreme ends of the longline, and they are
146 quantifiedthrough the hydrodynamic resistance of the longline's structures.The magnitude ol' this forcedepends on thecurrent speedand the exposure area ol' the individual structures. Waves makehorizontal and vertical tensions,and both are maximumat thewater surface, and are progressively lessened under the surface.Winds also provide a horizontaltension, and its magnitudedepends on the v,ind speed and exposure area of the surfacestructures. I'inally, the system weight animals,fouling, ropes,etc.! provides gravitational forces, and they can be compensatedthrough the addition of floatsduring the system design. Operationalmanagement requires the longline to belifted to theboat for seeding,material changes, or harvest,and also whenit is necessaryto moveor relocatethe anchor and mooring systems. Design Principles Theprimary design challenge is to identifyand evaluate theforces that are present on andin thelongline, to determine thestrength needed for thelongline's structure to resistthe naturalforces and to reducethe culture labor risk. With sufficienttechnical dat« it is possibleto sizea longlinefor a specificgoal and the specif'ic environmental conditions of the culturearea. This paper will showwhat is neededto specifya commerciallongline system to growmollusk».
Buoyancy force In accordwith the ArchimedesPrinciple, all bodes,either totallyor partiallyimmersed in a fluidof density ! experience anupward» force commonly referred to asbuoyancy force, similarin magnitudeto theweight of theliquid volume displacedby the body. Floatationor thegravity force on an immersedbody Floatationis a conceptdirectly relatedto ihe buoyancy force,for thefloatation of a bodywill dependon thebalance thatexists between the buoyancy force upward»force! and the 147 body weight downwards force!. Thus, it is possible to obtain the other force, a result of the addition of both forces, which is named floatation. Consequently, for any body: Floatation= Dry bodyweight * [I . /6 !] l! Bfld fluid density Kg/m'! = bodydensity Kg/m'!
Resistance force
To maintain an object in equilibrium in a moving fluid, there must be applied to the body a force of equal magnitude and opposite in direction to the resultant of the forces exerted by the fluid particles over it. This force, which will maintain the body in static equilibrium, is called "fluid resistance or drag" R!, and it is a function of the speed of the fluid v!, mass fluid density !, fluid viscosity lp! and by the body geometryand surface roughness. The last two elements can be representedby a drag coefficient Cd! and the projected body area A!, upon a plane normal to the direction of the flow. Thus the fluid resistance is given by: R='/ CdA6v ! These data are determined by observation and experiment empirical data!, not on theory, and upon the evaluation of the body resistance in a fluid. Generally the Cd value depends on the body size and of the Reynold's number Re!. Knowing the Reynold's number and the coefficient of drag Cd!, it is possible to obtain the respective resistance coefficient for a given body geometry and body cross sectional area from the numerous and widely published empirical tables. Longline Rope Sizing To size the structural longline ropes, the following should be considered: Evaluation of tensions in the mooring line Safety factor Characteristics of the materials used Mooring line: The recommendedlength for preliminarydesign is: n=j*h ! n = mooring-anchor rope length m! j = constant 3.0 j < 5.S! h = placement depth m! It shouldbe notedthat for a high length/depthratio, the natural vertical changes acting on the anchor system are minimal, but at the same time the relative cost for this system is high due to the needfor more rope. Consideratingthe forceson the extremeends of the main line and the diagonalconfiguration that will be adoptedby the mooring-anchorrope subject to thoseforces, the tension will be: T = T '+ T "-!"s ~! ma h v T = mooringline rope maximumtension N! T = maximumhorizontal tension in themain line N! T = maximum vertical tension in the main line N! Once the maximum tension at the mooring line rope is determined,it is possibleto determinethe rope diameter d!, using the tensionvalue for an approximatesafety factor: d = T *F!' /C ! where, d = rope diameter mm! T or T = maximum tension in the rope N! F = safety factor C = resistance constant N/mm !
Main line: The main line's lengthis determinedby the desired productionlevel and the site characteristics. The following are the most importantfactors Merino, 1996!: Production level: This is in function of the investment available,supply of seeds,market demand, marine characteristic of the aquaculture site, etc.
149 Productivity:The productivity of the aquatic environment will determine the maximum stocking level; from this one can establish an optimum culture density. Economics:Usually the ropesare usedcompletely from anchor to anchor, so it is important ni>t to introduce into the longline any elementtlrat could be the causeof structural failure during the period of i>per;ition. Designaspects: some design parameters, like the separationbetween each growing culture unit, determinethe main line lengthand factor directly in the stockingdensity of the culture system. Also to quantify the forcesacting over the maiii line, it i» possibleto solve the problemby usingcoordin ites at the one end of the main line: Ti Rh+ Fh+ T ! T = Hi>rizontal force at the main-line end N! h R = Hydrodynamic resist;mcc of the niain line and h culture units N> F = Horizontal force due to dynamic lorces N! waves, il winds! T = Horizontal force due to gravitational fi>rccs N! rope weight. biofouling weight!
Buoy ropes: For the determination of the buoy rope diameter it is necessaryto know the working tensionto which the>will be submi1ted.First it is required to calculate the iinmersed weight that should be lit'ted by the flotati<>nmain-line systen>.
a! Main-line total wei lht calculus W ITI11111 .. I/Ill' !. The immersed tot;il weight of the main-line system should be calculated from the immersed weights sum of each si.ructural component PSi!. Then, froin the equation l ! results: W .. = ZPSi 8! b! Main line weight per unit length calculation w!, The
150 main-lineweight W .. ! perunit lengthis simplythe W dividedby themain-line length s!. Knowing this, it is possible to knowhow many buoy» per unit lengthwill heneeded to maintainthe main line at the surface.The following equation givesthe longline lineal v eight: w =W, . /s [kg/m! 9! c! Buoyrope diameter calculation: Assuming that the adoptedconfiguration by themain line, between two buoy», is like a catenary,it i» possibleto utilizethe catenary expression to determinethe tensionin the line. Oncea maximumis determined,a ropediameter can he chosen. and an adequate safetyfactor incorporated, using a methodologythat is similarto thatused in otherportions of thelongline systems Equation 6!.
Anchor Rope: Althoughone might think that the anchor rope could be smallerthan the main line, the working characteristicsof the systemrequire this rop» diameter to besimilar to thatol' the main-linerope. The reason is thatduring installation and relocationoperations ol'the system, the anchor rope i» used for tensioningthe main line. Using this diameter, it i» possible to get its lengthusing the catenary principles.
Buoy Sizing Thebuoy volume can be easily determined by the following equation: V = ! W/ [ 6 ! ' g! ! " F 0! V = Buoyvolume m'! W = Weight to lift per buoy N! = Seawaterdensity l, 5 Kg/m'! 6 = Buoymaterial density Kg/m'! g = Gravityacceleration 9. 8 m/s-! F = Safety factor Anchor System Sizing During operation, the main factors that generate hydrodynamic resistance are the growing units. In effect pearl- nets and lantern-nets produce a wall-like effect against water flow, and that creates a tension in the system that will be resisted by the mooring-line rope to the anchor system. Design and sizing of the anchor system must also consider the bottom conditions of the place where the longline system will be located. The slope, substrate type and its sheer strength, are very important in determining the anchor system design characteristic. The anchor volume could be determined by the following equation:
V anchor=W dry /6 anchor *g 1!
V anchor= anchorvolume m'! W = anchordry weight N!
6anchor = densityof anchormaterial Kg/m'! g = gravity acceleration 9,8 m/s'! The dry weight anchor should be determined using equation 1. The following equation describes the immersed weight in relation with the respective forces: W = T *cosp/lt!+ T *sing 2! W = immersed anchor weight N! T = mooring line rope tension N! p = drag coefficient = mooring line rope angle To decide the kind of anchor that is necessaryfor a specific place, it is possible, from a technical viewpoint to make a pre-selection based upon the concept of a "fixing coefficient" K!. K is the relation between "fixing force" H! and the anchor dry weight W!, given by: K= H/W 3! 152 Table 1 shows several anchor and substrate types with their respectiveK value.
Table 1. Fixingcoefficient K! from differentanchor and substrate types. Anchor type Sand substrate Mud substrate
Stockless 4.4 < K < 16.0 2.0 < K < 7.5 Mushroom 2.0< K < 2.5 5.0 Danforth 14.6 < K < 21.0 7.1< K< 8.5 Stato 20.0 < K < 35.0 15.0 < K < 22.0 Boss 30 0 < K < 55.0 22.0 < K < 35.0 Rodriguez,1996. Beveridge 987! alsostates that it canbe shownthat K dependsupon the angle between the anchor and the seawater surface,and thus the ratio betweenwater depth and line length, and also with the nature of the substrate Table 2!.
Table2. The fixing coefficient K! of sandbaganchors on differentsubstrates and varyingmooring cable length:water depthratios :d!.
Substrate 1:d 1 2 3 4 5
Sand 0.19 0.53 0.63 0.70 0.74 Sandy mud 0.10 0.32 0.36 0.36 0.62 Mud 0.05 0.23 0.27 0.35 0.41 Beveridge, 1987.
Conclusion Thegeometry and the spatial configuration of a longline culturesystem depend on theoceanographic and meteorologic site conditions,the depthof the main line below the surface, andthe type of longlinethat depends on theoperating approachesand procedures. Of these,the oceanographic and meteorologicsite conditions are probably most important. If theyare incompletely understood, the probability of failureis high,or theinitial costmay be unacceptably high.
153 In this paper,means have been formulated to calculateor estimate the forces are acting upon the long-line system. Once they areevaluated, it is thenpossible to determinethe strength that must be built into the long-line's structures by to resist the natural forces and to reduce the risk to laborers during culture. During the designstage it is also necessaryto considerthe projectedlevels of biofouling on systemscomponents and the effectsof ageand sun damage.Biofouling on netting is particularly importantfor it increasesenvironmental loading over the entire long-line system by added weight and drag but it also reduces water circulation in the lantern-nets and pearl-nets and thi» could kill the mollusks. Finally, a wordof caution.Always consideradequate safety factors, for this gives you an additional tolerance in your system designand to somedegree compensates for unanticipated loading events or even unexpectedly high production.
References Barnabe, G. 1991. Acuicultura. 1 diciones Omega S.A., Barcelona. p. 369-394. Beveridge,M. 1987.Cage Aquaculture. Fi »lting IVe»»Borak» Ltd., England, 3s 1 pp. Dupuoy, lk 1983. Lc Pectiniculture a Saint Pierre et Miquelon. etienne et Pi che Bull. In». Peches Marit. France. 13 pp. Illanes, J. & S. Akaboshi. 198S. Tecnologias de Cultivo. En l" 'lhller de At uacultura: Conotimiento Actual v Per»pe