Introduction
Salmon farming is an important industry in the Faroe Islands with a large production in a limited number of sites. The available sheltered farming sites are already in use and production has expanded into highly exposed sites. Further expansion is difficult and will most probably be in highly exposed sites. There has lately been a high focus on open ocean, but there are also possibilities in high current sites if the current speed inside the cages can be reduced to a suitable level for the farmed fish. In the Faroe islands salmon is farmed with high success at sites experiencing tidal currents up to 104 cm/s (measured currents). This is slightly higher than the reported maximum swimming speed of salmon from research (Hvas etal ., 2021) even when the current reduction from the cagenets is included. Therefore, it is not expected that sites with much higher currents can be used with conventional equipment.
CFD (Computational Fluid Dynamics) is used to model flow through fish farming nets and the effect the nets have on the flow (Patursson, 2008 and Patursson etal ., 2010). The present work presents the first phases of the project “Fish farming in Vestmannasund ”, where the use of current barriers is investigated in order to enable fish farming in high current sites previously not suitable for farming. The presented phases include 1) finding a suitable site, 2) using CFD to investigate a possible farm and current barrier design, and 3) the test of a small current barrier at the site to test the basic concept.
Materials and methods
Initial measurements are used for description of the site and for boundary conditions for the CFD model. The measurements are from a bottom mounted ADCP and a boat mounted ADCP. Bottom mounted ADCP measurements provide timeseries of currents at a single location for statistical analysis, while boat mounted currents provide a transect along the boat path. By repeating the same path for just over 12 hours in a semi diurnal tidal site, an overview of the currents at the site through a tidal cycle can be provided.
CFD is used to investigate a full-size farm with a current barrier and a small current barrier. The CFD investigation is performed using steady state RANS, k-ω SST turbulence model and standard wall functions. The boundary conditions are based on the boat mounted current measurement at the time of the highest current speed at the site. The porous media coefficients to describe the nets are based on Patursson (2008). The coefficients for the cage nets are from Patursson etal., (2010), while the coefficients for the current barriers are chosen to provide a suitable current reduction.
The small current barrier was designed to facilitate field measurements of current reduction, mooring force and the vertical movement of the bottom of the current barrier. It was made from two barriers, each barrier was a flat net panel oriented across the flow, one downcurrent from the other.
The current reduction effect of the small current barrier was documented by measuring current upcurrent and downcurrent of the current barrier using two methods 1) One floating ADCP was moored on each side of the current barrier and 2) a boat mounted ADCP was used to measure current transects upcurrent and downcurrent of the barrier. In addition, mooring force in the major current direction was measured and the vertical position of the bottom of the barrier was measured using pressure sensors. The measurements were used to validate the CFD model and to evaluate the effect of the current barrier.
Results
The site experiences strong currents. The strongest currents (maximum during the measurement period is 220 cm/s) are aligned along the line from 280 to 100 degrees. Currents are fairly strong in the opposite direction but are low in other directions. Waves are expected to be low in the area, less than 2.5 m Hs.
The CFD results show that a current barrier, designed as a flat net across the flow slightly larger than the cross section of the cages, can shelter a row of at least 5 cages at the present site. The CFD results for the small current barrier show that the reduction is similar to the large barrier, and the wake is clear and long enough to be recognized in boat mounted ADCP measurements. The CFD work also shows that it is difficult to use measured current profiles from a boat mounted ADCP as inlet boundary conditions.
The measurement of current reduction with moored ADCPs gave the possibility to reevaluate the drag coefficient and the porous media coefficients of the net and the drag force gave another independent evaluation. The CFD results of the small barrier were validated against measurements with the boat mounted ADCP and showed good agreement.
The present work shows that it should be possible to have salmon farming in such high current sites as described above using current barriers. The present work also shows that the methods used for modeling and dimensioning were adequate to describe the small barrier and the effect of it. After the presented work was finished, the project has continued, and the farm is now in production. The next step is to improve the design of current barriers and to design current barriers that can withstand the forces from large waves as well. There is also ongoing work to investigate the difference between using steady state RANS and more detailed models like DES.
References
Patursson, Ø., 2008. Flow through and around fish farming nets. University of New Hampshire, USA.
Patursson, Ø. , Swift, M. R., Tsukrov , I., Simonsen, K., Baldwin, K., Fredriksson, D. W., Celikkol , B., 2010. Development of a porous media model with application to flow through and around a net panel. Ocean Engineering , Vol. 37, No. 2
Hvas, M., Folkedal, O., Oppedal, F., 2021. Fish welfare in offshore salmon aquaculture. Reviews in Aquaculture 13, 836–852.