Cite this article as: Jensen Ø, Lader P, Kristiansen D, Mendiola D, Gabiña G, Sanz V, Rico A (2013) Sea-load exposure. In: PREVENT ESCAPE Project Compendium. Chapter 6.5. Commission of the European Communities, 7th Research Framework Program. www.preventescape.eu ISBN: 978-82-14-05565-8
authors: Østen Jensen1, Pål Lader1, David Kristiansen1, Diego Mendiola2, Gorka Gabiña2, Veronica Sanz2 & Antonio Rico2 1 2
SINTEF Fisheries & Aquaculture, Norway AZTI-Tecnalia, Spain
INTRODUCTION Escapees can have negative ecological and genetic effects on populations of wild fish, and the present level of escapees is regarded as too high and a problem for the sustainability of seacage aquaculture (Naylor et al. 2005). The escapes problem is largely caused by technical and operational failures of fish farming equipment. Atlantic salmon (Salmo Salar), rainbow trout (Oncorhynchus Mykiss) and European seabass (Dicentrarchus labrax) primarily escape after structural failures of containment equipment, whereas Atlantic cod (Gadus morhua) and gilthead seabream (Sparus aurata), escape due to holes in the net made by the farmed fish as well as predators (Jensen et al. 2010). Structural failures is by far the dominating cause of escapes and may be generated by severe environmental forcing in strong winds, waves and currents, which may occur in combination with how the components have been installed or operated (Jensen 2006; Jensen et al. 2010).
OBJECTIVE The objective in this task was to study the dynamic behaviour of typical nets and cages used in European aquaculture. The emphasis was to develop knowledge on how extreme weather loads lead to loss of structural integrity. The model tests focused on net deformation and how holes in the net form due to contact between net cage and weight system whereas the numerical simulations was motivated by a need to better understand how multiple cages in a system behave when subjected to waves and current.
METHODS Model tests were performed in a tow tank at the United States Naval Academy to investigate at which combinations of waves and current the net deforms and come in contact with the sinker tube chain. The wave considered in this study is a severe swell with a full scale period of 12.7 s and a wave height of 5.6 m. A model in scale 1:40 was used to represent a fish cage with a circumference of 120 meters and net depth of 40 m. The cage model was composed of a model net, a flexible floating collar of the circular plastic type and a weight system. Two different net design and four different weighting systems were tested (Figure 6.5.1, Figure 6.5.2). The model was moored to a towing carriage and subjected to regular waves in the small wave tank. Multiple tow tests were conducted in a single run by starting the towing carriage at the lowest velocity, and by stepwise increasing the speed until all the conditions were conducted. Tension forces in the mooring lines were measured during the towing at a sampling rate of 10 Hz. The model was filmed by an underwater camera placed at the side of the model pointing perpendicular to the towing direction so that the geometry of both the cage and the weight system was visible. A modified floating collar design with purpose to reduce the probability of contact between support chains and net was also tested. The modified collar had a larger diameter of the outer ring compared to the conventional floating collar. This caused the separation between the support chains and the net to be increased. Detailed numerical simulations of a typical Spanish fish farm were performed to investigate the structural integrity of the floating collar and the mooring grid. Wave and current parameters were chosen to represent condition found at the Canary Island, Mediterranean Sea and Atlantic Ocean. Detailed models were developed to represent typical mooring grid (Figure 6.5.3) and net cage designs. For more details on the numerical models see.
Figure 6.5.1. Sliding and fixed connection between weight system and lower end of net cage.
Figure 6.5.2. Net and weight system test setup.
Figure 6.5.3. Model used for mooring analysis.
RESULTS In Figure 6.5.4 and Figure 6.5.5 the net deformation for the different combinations of net design and weight system when subjected to current and waves are presented. Both figures show clearly how the deformation increases with increasing current. This follows the same pattern of deformation with increasing velocity as reported in previous studies (Lader and Enerhaug 2005), the front of the cage deforms more than the rear and the bottom rises due to the front and rear deformation. For the case where the model was subjected to waves, the two pictures shown were taken when the model had the largest horizontal displacement and deformation, approximately when the wave crest (bottom picture) and wave through (top picture) passed the centre of the cage. In the figures the contact between the net and the weight/sinker tube chain is indicated by a thick black line. This contact was found by visual
Figure 6.5.4. Deformation and contact for a cylindrical net in current and waves.
Figure 6.5.5. Deformation and contact for a conical net in current and waves.
inspection of the images and is thus somewhat approximate, but gives nevertheless a good impression of the amount of abrasion in each condition. The sinker tube chain on the front side goes into slack, and this represent a potential for snap loads which can cause dangerous high loads in the chain (Lader and Fredheim 2006). The experiments showed that the sinker tube performs better than individual weights. Both for a cylindrical net in current only and for conical net in current and waves, contact occurred at a lower current velocity when using individual weights compared to using a sinker tube (see Figure 6.5.4 and Figure 6.5.5). In addition it was observed that contact occurred at a higher velocity and over a smaller area when using the conical net compared to the conventional cylindrical net. This was independent of weighting system (sinker tube or individual weights, sliding or fixed) and true for both current only and combination of waves and current. No difference was observed, between the sliding and fixed condition used to attach the net to the sinker tube or individual weights. This final observation may, however, be influenced by how the sliding connection was modelled in the experiments. If the friction between the sliding connection and the modelled sinker tube chain were too large, compared to the full scale cage, the overall behaviour of the system may have been different for the model compared to a full scale cage. Additional results are presented by Lader et al. (in preperation).
The modified floating collar with increased diameter of the outer ring was tested using the conical shaped net. Deformations of the net at the downstream side of the cage lead to contact between the net and the support chain for the two largest towing speeds, as with the conventional floating collar. However, the model tests suggests that the modified cage design will reduce the probability for contact at operating conditions of full scale cages as the velocity where contact first occurred was significantly higher, see "Figure 6.5.6 Net deformation and contact with weight system when using modified cage with larger outer ring". Hence, contact between the net and the support chains should be expected for typical full scale net cages at least for a dimensioning current speed of 1 m s-1. Similar results from the 2009 tests with the normal floating collar design and the same net model, showed that contact between the support chain and the net occured for current velocities equal to 0.5 m s-1 and above. Hence, the modified collar design yields a significant improvement of the contact problem relative to the normal collar design.
Figure 6.5.6. Net deformation and contact with weight system when using modified cage with larger outer ring.
Based on the experiments it can be concluded that independent of net and weight system design, contact will occur between the sinker tube chain and the net even at moderate current levels with the conventional net, floating collar and weight system design. This observation is confirmed by recent incidents in Norway; more fish have escaped due to hole in the net (Figure 6.5.7). Contact between the net and the sinker tube chain has occurred at multiple instances (Figure 6.5.8) and abrasion damages on the side of the net has been detected during net inspections (Figure 6.5.9). From the pictures it can be seen that the biofouling has been removed by the chain rubbing against the net. The strength of the net was tested in the laboratory, and it was confirmed that the contact had introduced a significant reduction of the strength of the material (Chapter 6.2.2, this compendium). In addition to visual observations of net deformation, force acting on the net cage were measured. The dominating hydrodynamic forces acting on the structure due to the waves can be divided into viscous drag forces on the net, and Froude-Kriloff, diffraction, radiation and viscous drag forces on the floating collar. Mean peak to peak force amplitudes for the conical shaped net are obtained from time intervals of about 10 wave periods from the measured time-series. Obtained values are compared with estimated theoretical values in Figure 6.5.10. There are several wave periods where cancellation of the wave-induced horizontal forces occurs. For the model tested, effects of force cancellation are found to be most pronounced for and where, according to theory, the total horizontal force has a local minimum. There is a local maximum of the wave-induced horizontal forces when. Correspondingly for the full scale cage, cancellation of wave-induced forces occur for the wave periods and, while the
Figure 6.6.7 Contact between net cage and weight system at a commercial farm - hole in the net.
Figure 6.5.8. Contact between net cage and weight system at a commercial farm.
Figure 6.5.9. Contact between net cage and weight system at a commercial farm - biofouling removed.
wave period corresponds to the local maximum. The wave induced drag on the sinker tube is negligible due to the exponential decay of the particle velocity with depth. The effect of net deformation is neglected when computing the wave induced drag force on the net cage. Current-induced drag forces on the fish cage model were investigated for the two net design with different weight configurations. The conical net experiences significantly lower drag forces compared to the cylindrical net, primarily due to a reduced exposed area, see Figure 6.5.11. The drag coefficient on net panels shows a strong dependency on the Reynolds number (Rn) when Rn is small, typically Rn < 100 (Fridman 1986). This effect of Rn on the induced drag is observed for the lowest test velocities in Figure 6.5.12 as an increase of the non-dimensional drag force with decreasing current velocity. As the deformations of the net increases with increasing current velocities, the projected area of the net in the towing direction is reduced. The result is a reduced drag force compared to that of an undeformed net for the same current condition, as illustrated by the solid and stippled lines in Figure 6.5.13 obtained from theory. In Norway, all main components of a fish farm (e.g. floating collar, net cage, mooring system and feed barge) are required by legislation to be product certified according to the technical standard NS9415. In the standard the significant wave height and corresponding peak period are calculated based on wind velocity and measured effective fetch lengths. It is not required that the designer consider how the calculated dimensioning wave lengths correspond with the geometry of the cage or system of cages. As Figure 6.5.10 shows, combinations of cage geometry and wave conditions can give cancellation of force, which in a design and dimensioning process could be critical as other combinations of wave length and height which at first glance appear less critical can give significantly higher forces on the system. It was found that there are many wave periods where cancellation of wave induced forces on the model occur. These cancellation wave periods are within the range of dimensioning wave periods commonly used for testing of fish farm structures and hence are important to be aware of. The simulations showed that when current and wave direction have the same angle of attack, three locations (Canary Island, Mediterranean Sea and Atlantic Ocean) have common lines which collapse, with the exception of the Mediterranean Sea, which have fewer number of lines that break. In fact, failures occur in the rope section, not in chain lines. In general, the environmental conditions are worse at the Atlantic Ocean location, causing the mooring line
loads to be larger than at the Canary Island and Mediterranean Sea locations. According to the simulations, the cages will deform prior to the bridle lines reaching its breaking load. The integrity of the cage is only guaranteed when the supported stress is below a third of the breaking load from grid to cage lines. It is necessary to remark that the weakest cage components are the brackets due to the manufacturing process, where the material properties are weakened. These brackets are the origin of cage collapse due to the appearance of cracks. This is consistent with findings in the Spain and Norway industries (Figure 6.5.13).
Figure 6.5.10 Drag forces during towing tests on different net and weight system designs.
Figure 6.5.11. Wave induced drag forces on net cage.
Figure 6.5.12. Drag coefficient as a function of Froude number.
Figure 6.5.13. Cracks in bracket.
RECOMMENDATIONS s Equipment used at a site should be designed and dimensioned, using validated methods, to ensure that the equipment is suitable to withstand the environmental conditions at the site. s It should be validated through analysis or by other means that equipment used at a site (such as floating collar, net, weighting system and mooring) fit together without potential of the individual parts damaging other parts. s Effort should be put into designing new solutions for net cages, floating collars and/or weight system since systems used today often experience abrasion between net and weight system even at moderate current velocities. s Mooring grids should be designed to withstand the expected environmental conditions at a site, the environmental conditions should be determined using appropriate methods and the mooring grid should be designed in such a manner that failure of one mooring line should not lead to a total loss of integrity of the entire farm.
Fridman A L (1986) Calculations for Fishing Gear Designs. Fishing News Books Limited. Jensen Ø (2006) Assessment of technical requirements for floating fish farms—based on escape incidents January 2006. Rep no SFH80 A066056. SINTEF, Trondheim (in Norwegian) Jensen Ø, Dempster T, Thorstad E, Uglem I, Fredheim A (2010) Escapes of fish from Norwegian sea-cage aquaculture: causes, consequences and prevention. Aquaculture Environment Interactions 1:71-83 Lader, P. and B. Enerhaug (2005). Experimental Investigation of Forces and Geometry of a Net Cage in Uniform Flow. IEEE Journal of Ocean Engineering 30(1). Lader, P. and A. Fredheim (2006). Dynamic properties of a flexible net sheet in waves and current - A numerical approach. Aquacultural Engineering 35(3): 228-238. Lader P, Kristiansen D, Jensen Ø, Fredriksson D., (In preperation) Experimental study on the interaction between the net and the weight system for a gravity type fish farm.