Reducing Operational Costs
In this section, mooring issues are discussed for two specific aquaculture use cases: The types of products targeted at aquaculture farm can vary widely from traditional large fish cage operations to seaweed and kelp farming aquaculture—for both energy and food sourcing. Additionally, because seaweed and kelp farming removes CO2 from the water column during photosynthesis, seaweed and kelp farming is presented as a solution for ocean acidification—a growing global problem arising from increased atmospheric CO2 level.
Seaweed and kelp farming away from shore often relies on lines suspended in the water column, onto which various seaweeds are encouraged to grow. By placing the seaweed in the water column, optimal positioning for photosynthetically active radiation (PAR) and nutrients availability can be controlled and monitored.
Cage aquaculture: the extensive catenary mooring configuration required to control the cage movement. Each mooring line has an anchor, chain, and commonly high-strength marine line. (Image credit: EOM Offshore)
Conventional seaweed and kelp farming conducted away from coastal infrastructure often relies on suspended lines moored by some mix of high strength polyline, chain, and heavy anchors. These farms can be either single lines with such moorings at either end, or parallel lines with a common bridle arrangement. The mooring arrangement can be simplified by using EOM stretch hoses as the mooring elements. These stretch hoses can stretch up to 2.5 times the resting length and can have any of a number of elastic moduli. For the two seaweed/kelp farming projects that EOM has worked on, the stretch hose behavior has been tuned to the desired behavior of the application (one was in extremely high currents, the second in normal nearshore water waves and currents). Advantages of the EOM stretch hose include:
- Streamlined deployment
- Simplified mooring configuration
- Limited benthic interactions—anchor only—decreases adverse impacts to benthic habitats like seagrasses
- Reduced snap loads leading to extended lifetimes of all mooring and farm components
Another common configuration is for fish farming, where a large net is moored to the seafloor, to prevent excessive movement (vertical and horizontal) of the net.
Proteus model of fish farm moored with EOM stretch hoses. (Image credit: EOM Offshore)
A standard mooring arrangement takes up extensive real estate. A common arrangement for fish farming is for the farmer to pay for the lease area, depending on the size of the lease area. Clearly, with the extensive mooring arrangement required here, the lease area per farm will be large. To overcome this form factor issue, EOM Offshore has designed compliant moorings that require far less real estate. These moorings are quite strong (up to about 500 kN working load limit) and have some limited compliance to allow the moorings to breathe as waves and currents change. The advantages of this type of mooring include:
- Hose compliance reduces snap loading—extending lifetime of all mooring and cage components
- Cage and mooring footprint is reduced up to four-fold
- Streamlines deployment (typically the moorings are anchored by helix anchors, which have a high holding power and simple installation) and recovery
- No benthic interactions decrease or altogether removes adverse impacts to benthic habitats like seagrasses
This improvement in mooring design is made possible because of EOM’s stretch hose design capabilities. Based on early work by the Woods Hole Oceanographic Institution (WHOI), EOM has taken these concepts and extended them to a design capability to create hoses having various tension-elongation behaviors, for a variety of mooring applications (e.g., small boats, seasonal docks, maritime protection barriers, oceanographic uses, offshore wind applications, etc.). With this new design tool, EOM can create high or low modulus hoses of most any length (up to about 30 m) and of varying diameters.
Providing Renewable Energy
Most aquaculture operations require on-site power. This power can be required to fulfill a number of requirements, such as feeding, cooling, storage, monitoring, and communications. Especially for farming away from the immediate vicinity of shore, a source of renewable power for fish farming would be useful.
Thanks to funding from the US Department of Energy, Triton Systems Inc. has developed and is currently testing wave energy conversion buoys for use with various monitoring systems. With targeted power outputs in the hundreds of watt range, and combined with solar, the Triton Wave Energy Converter (TSI WEC) uses a surface buoy to house its power conversion system, a depth tube to leverage differential pressure in the water column, and solar cells to attain a solution that takes advantage of wave and solar energy.
The initial deployment of this system occurred during summer 2023, when it was deployed for a period of nearly two months. Triton teamed with EOM Offshore, who provided the mooring buoy and stretch hose linkage from the WEC buoy to the monitoring buoy. The deployment proved successful in validating the differential pressure concept of wave energy generation, even in relatively shallow water and in a more northerly geographic location where solar irradiance is not as great as more southerly areas.
EOM Offshore and Triton Systems Inc. have teamed to develop products of use to the aquaculture sector. With the mooring technology of EOM Offshore, and the wave energy conversion concepts from Triton Systems, marine and freshwater aquaculture have two options for upping their game, while becoming more environmentally friendly than alternative technology solutions.
To find out more about EOM Offshore and Triton Systems, visit: www.eomoffshore.com & tritonsystems.com
This feature appeared in Environment, Coastal & Offshore (ECO) Magazine’s 2023 Winter edition Fisheries & Aquaculture, to read more access the magazine here.
