Modern aquaculture - from Mediterranean sea bream hatcheries to huge Atlantic salmon farms - depends on a continuous flow of quality seawater and the clean return of effluent to the ocean. Traditionally these captures and returns have been solved with surface pipelines or dredged ditches, but both methods are vulnerable to waves, occupy coastal strips and generate conflicts with coastal tourism. As a result, more and more developers are turning to the microtunnelingTrenchless technique: a trenchless technique that installs pipes several meters below the seabed, protected from storms and with almost no visual impact.
Eurohinca has applied this solution -described in the category Seawater harvesting systems for desalination- for numerous outfalls and fish farm intakes. Even so, the project presents its own challenges: hydrostatic pressure, biofouling and sanitary requirements that demand a high-precision design.
Benefits of microtunneling vs. traditional methods
Less exposure to waves and currents
By burying the pipeline one meter or more below the active sedimentation layer, cyclic stresses that break surface pipelines are eliminated. This reduces maintenance costs and avoids interruptions in production, which is critical in farms that rely on a constant flow rate to maintain fish density.
Almost zero visual and environmental impact
The microtunnel avoids dredging and backfilling that muddies the water for weeks. During our Tunnels for fish farms only an attack shaft was opened 300 m inland off the coast of Huelva; the rest of the work was left under the seabed, preserving seagrass meadows, and the Posidonia catalogued as SCI.
Longer and more precise tracings
Thanks to curved feed tunnel boring machines -similar to the ones described in Closed shield - Hydroshield- we can start behind the slope and emerge right at the catchment elevation, overcoming reefs or port accesses without interfering with other infrastructure.
Challenges specific to catchments and returns
Hydrostatic pressure and face equilibrium
On open beaches the pressure at 20 m depth exceeds 2 bar. Maintaining this equilibrium requires control of the bentonite sludge inside the shield chamber - as explained in the entry on pressure management with hydroshield- and coordinate it with the jack's thrust so as not to overpressure the front or allow water ingress.
Biofouling and fouling
Nutrient-rich water favors pipe colonization by algae, mussels and bacteria. A microtunnel allows the installation of GRP pipes with antimicrobial coatings and reduces the sunlight that feeds biofouling, but requires planning of manholes and injection points for periodic treatments.
Environmental legislation and animal welfare
European standards require the return to meet temperature, solids and pathogen limits. Placing the outlet at a sufficient distance from the intake, with adequate diffusion, is easier when subway routing freedom is available. The design should be coordinated with the category of Drainage works to ensure that purge flows do not cause bottom hypoxia.
Hydraulic design strategies
Diameter sizing and flow rate
A common mistake is to oversize the pipe "just in case". This decreases the velocity and favors the deposit of organic matter that clogs the line. Eurohinca's calculations are based on velocities of 0.9-1.5 m/s to prevent settlement without penalizing pumping consumption. This criterion comes from the study "Design Considerations for Tunnelled Seawater Intakes." published in Coastal Engineering 2024, which shows a reduction of 22 % in scheduled cleanings when the velocity exceeds 0.8 m/s ResearchGate.
Protection against sediment intrusion
In sandy bottoms, a metal grating is sufficient; in turbid environments we install low velocity hoods with narrow slots, inspired by desalination plant catchments. This approach is detailed in the category of River and watercourse crossingswhere the sediment load is similar.
High efficiency return diffusers
To avoid concentrated effluent plumes, multiport diffusers are placed to induce mixing with a dilution flow rate of 1:100 in less than 50m. Numerical studies - such as the brine discharge study in Water 2023 - confirm that flush placement and 45° orientation to flow reduce additional salinity to less than 0.2 ‰ in the water column. MDPI.
Eurohinca Construction Solutions
Modular attack well and laser-inertial guidance
To minimize the surface footprint, a circular precast concrete shaft is constructed. The hydro-closed shield starts from the bottom, guided by a laser-inertial system identical to that of our Horizontal directional drilling. This keeps the offset within ±0.05 % and prevents the output from being higher than the pickup level, which would cause air pockets.
GRP pipes with elastic joints
Once the tunnel has been bored, we push GRP shafts DN800-DN1600. The elasto-thermoplastic joints absorb expansion due to temperature and pressure changes, prolonging the service life. This system is based on the experience of Pipe jacking and meets NSF/ANSI 61 for contact with water intended for fish consumption.
Exterior sealing with microcement grout
Tail injection fills the annular space, preventing water seepage and blocking routes for mollusk boring larvae. During a project in the Canary Islands, the permeability of the ground dropped by an order of magnitude after sealing, reducing the input of fine sediment into the pipe.
Success story: salmon farm in the Hardanger Fjord
In 2022 Eurohinca completed a 40 m deep intake and return system under Hardanger Fjord, Norway. The work included:
650 m of DN1200 microtunnel for intake and 700 m DN1000 for return.
Hydroshield working at 4.5 bar, with pressure monitoring.
Multiport duplex steel diffusers resistant to biofouling.
At twelve months, the farm reported a reduction of 35 % in treatments against Lepeophtheirus salmonis (sea lice) due to the increased depth of intake and reduced summer heating. Oxygen sensors installed as part of the IoT system, described in Technical assistance and engineeringconfirmed levels above 7 mg/L throughout the summer.
Good operating and maintenance practices
Periodic rinsing with fresh water
Every six months, fresh water is pumped for two hours to dewater calcifying organisms. This procedure, adopted from Drainage worksprolongs the life of antifouling coatings.
ROV inspection and mechanical cleaning
An ROV equipped with a rotating brush runs through the pipeline once a year. Inspections show that organic growth does not exceed 1 mm, well below the 3 mm threshold required by Norwegian regulations.
Calibration of flow rate vs. biomass
As the farm biomass increases, the capture flow rate must increase to maintain the oxygen/fish ratio; however, increasing the velocity above 1.8 m/s increases the specific pumping consumption. Our algorithm, which is hosted on the Excavation systemsautomatically adjusts the variable frequency pump to balance both needs.
In conclusion, the catchments and returns for aquaculture by microtunneling offer a unique combination of protection from harsh marine conditions, environmental compliance and operational savings. Adopting proper hydraulic design, protecting the pipeline against biofouling and controlling pressure intelligently are essential steps to a reliable system for decades.
If your project involves the expansion or construction of a fish farm, Eurohinca's team can integrate these solutions into a turnkey plan that optimizes both initial investment and OPEX costs.
To learn more: the review Subsurface intakes for seawater facilities (Elsevier 2023) compares different subway collection methods and highlights the efficiency of microtunnels versus infiltrating galleries. ScienceDirect.
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