The Oysterbot: Developing a Ropeless Bottom Cage Retrieval System for Nearshore Oyster Farms

Objective 1 - Identify the mechanical attributes of an ROV required to achieve the goals of maneuverability underwater, identification of unique bottom cage markings, and ability to attach a lifting line to a cage harness for retrieval.

Drawing on the expertise of our Technical Adviser, the BSS farm team developed a list of specifications that were deemed important in the design of the OysterBot. These specifications cover details such as swimming speed/power and maneuverability of the ROV, resolution of the camera under ambient and enhanced light conditions, gripping strength of the robotic arm, ease of operation and other parameters that were suggested as important during the design discussions. These criteria are included in Table 1.
Table-1_OysterBot-specs

Following evaluation of the OysterBot, which was constructed to meet the specifications outlined in Table 1, it was determined that the criteria established under Objective 1 were sufficient to produce a field ROV that was capable of providing the services required to complete this project.

Objective 2 Design and construct an ROV that meets the qualifications outlined in Objective 1 (i.e. the OysterBot);

In collaboration with the Technical Adviser, BSS assembled a collection of off-the-shelf components for the OysterBot. Using the design criteria compiled under Objective 1 (Table 1), the BSS farm team worked with our Technical Adviser to identify and purchase the components needed to assemble the OysterBot (Table 2). The bulk of the components were available through Blue Robotics (https://bluerobotics.com/) (Figure 1).

Table 2_OysterBot Components

Figure 1 Blue Robotics ROV2 description.

The ROV components arrived in May/June 2022 and the vehicle was assembled, primarily by our Technical Adviser, through the summer/fall of 2022. Since late September 2022, the OysterBot has undergone a series of test dives in the ROV test tank affiliated with DSL/WHOI. Numerous operational bugs were corrected as a part of the testing series. In December 2022, the OysterBot was tested at a BSS farm site for further debugging (Figure 2).

A critical factor in designing and assembling the OysterBot is the installation of the robotic arm that will carry the lifting line and attach the line to the bridle on the submerged cage. To allow the cage to be hoisted to the surface without tangling the ROV in the process, the arm will need to securely hold the lifting line in position as it descends to the cage, attach the lifting line to the cage, and subsequently release it from the ROV. Blue Robotics offers a gripper arm for their ROV2 and we purchased and installed it on the OysterBot.

During the process of identifying and ordering the components required to build a functional ROV and assembling the ROV, it became clear that this endeavor required extensive technical expertise and engineering skills. Considering that one is assembling a piece of equipment that is entirely dependent on electronic components then submerging that unit in highly damaging seawater at pressure, it is critical that every step in the assembly is not only meticulously completed but then tested to eliminate the risk of flooding by seawater. Often, the testing requires specialized equipment and/or test tanks for submerging the ROV at specific build intervals. Without the expertise, skill set and experience of our Technical Advisor, BSS would not have been able to design and construct the OysterBot from individual components as proposed. This does not preclude an oyster farm from utilizing an ROV for work on the farm as there are numerous preassembled models that are commercially available but it is not practical for an oyster farmer to design and assemble an ROV from scratch, as was successfully undertaken with this study.

Objective 3 - Evaluate the OysterBot on a commercial farm to assess its capacity to replace surface lines for cage retrieval.

Field evaluation of the OysterBot was completed at the BSS West Island Farm in Fairhaven, MA. This site is a nearshore farm in ~12 feet of water. Initially, a general cage location was identified using a down/side looking sonar on the vessel. The sonar-based fish finder has been successfully used to locate bottom cages on the BSS farms as part of a SARE funded Farmer’s Grant in 2021 (SARE Project Number FNE21-989). With the sonar, we were able to locate individual cages placed on the bottom to identify the area for deploying the OysterBot retrieval system. The cages consist of large wire mesh structures populated with up to 18 stocked oyster bags initially weighing in the vicinity of 200 lbs. and have been deployed in a set array on the bottom. We could clearly demarcate the location of large oyster cages on the bottom and, with integrated GPS, the precise location of bottom cages were observed and recorded.

During a routine field test, a cage location was identified by sonar and the surface vessel deployed the OysterBot to locate the individual cage. Because the OysterBot provides real-time video imaging, the on-deck operator was able to "fly" the OysterBot to the cage location and survey the cage positioning on the bottom (Figure 3) as well as trace the lifting bridle suspended at the top of the cage (Figure 4), to allow for attachment of a lifting line.

During the initial field test, it was extremely difficult to observe the video output of the OysterBot on the topside computer screen in an open boat under sunny conditions. During the initial trial, it was necessary to shroud the operator and laptop with a curtain to allow for adequate visibility of the screen. Subsequently, a set of VISIONHMD BIGEYES H3 head-mounted video glasses were evaluated for observing the video output of the OysterBot under daylight conditions. The video glasses interface with the computer control via a conventional USB port thus were simple to use and proved to be effective in allowing the ROV pilot to observe the scene projected by the OysterBot in open sunlit conditions.

In addition to locating individual large bottom cages, the OysterBot was also successful in locating a series of smaller oyster bags that had been accidentally lost overboard due to cage damage following a storm (Figure 5). In a third effort (Figure 6), the OysterBot was able to provide images of a series of derelict cages discovered when surveying the farm site with sidescan sonar, thereby allowing us to retrieve those cages as we were able to discern their positioning and accessibility.

Initial field testing uncovered a few more bugs (e.g. the auto-depth setting on the ROV) that were corrected by our Technical Adviser at the DSL-WHOI test facility during the winter/spring of 2023. Subsequent tank and field testing through the summer of 2023 led to the final debugging of the mechanics and control of the OysterBot. As part of final testing, the OysterBot was loaned to the Sea Education Association to be used as a training platform for high school students interested in marine robotics (https://sea.edu/news-post/sea-teams-with-whoi-for-high-school-robotics-cruise/). The end result of that week-long training session at sea was an informational video outlining the session and featuring the OysterBot (https://youtu.be/ucOhXB63NdY).

Final training for operating the OysterBot occurred in the fall of 2023 where both proper handling and piloting of the ROV as well as required maintenance was covered by our Technical Adviser. As a part of the training, the final aspect of field evaluation included assessing the capacity of the OysterBot to carry an attachment line from the surface to the submerged cage and to attach it to the lifting bridle on the cage. To that end, a simple J-hook (Figure 7) was attached to a 3/8” Dyneema® line for lifting the cage. Dyneema® was selected because of its high tensile strength (3/8” Dyneema® has a tensile strength of 13,900 lbs. and is slightly buoyant). The J-hook was gripped in the arm of the OysterBot and carried to the cage where the J-hook was carefully engaged with the bridle of the cage. With a slight tension placed on the Dyneema®/J-hook line, the OysterBot gripper was released from the J-hook and the ROV was carefully reversed away from the bridle/lifting line combination. Subsequently, the cage could be lifted to the work barge for maintenance or harvest.

Due to unforeseen circumstances, the final testing of the OysterBot with retrieving cages at a commercial scale and comparing the performance of the OysterBot retrieval system to the conventional buoyed line could not be completed. The oyster survival and growth performance at the Seal Rock site proved to be subpar over the previous two years of using a bottom cage growing strategy. Because of that observation, the immediate use of the Seal Rock site was discontinued in the summer of 2023 to allow the owners to focus on maximizing the output of the floating cage system at the West Island Farm. BSS will revisit the Seal Rock site during the spring/summer of 2024 to continue research on making that farm site a productive oyster site. Currently, BSS is using the Seal Rock Farm to evaluate kelp culture.

Overall, the utilization of the OysterBot to carry forth and attach a lifting line to a submerged cage proved to be a successful effort. With that come a few caveats in terms of practicality for application in a farm setting. While we were able to detect cages from the surface with the sidescan sonar and subsequently place the ROV at the cage site, reading the tags attached to the submerged cage worked well until biofouling proliferated. To ensure long-term readability of the cage tags will require either routine cleaning of the cage tag, which may be a problem depending on the frequency of handling of an individual cage, or devising a tag label that has antifouling properties built into the tag. For example, fabricating tags of plastic and printed using an antifouling paint may keep the tag clear of fouling and readable by the OysterBot for an extended period of time.

Another consideration is the arrangement of cages on the bottom. As originally proposed, we anticipated dropping the cages along a trawl line in a somewhat straight line configuration. This proved to be more difficult than anticipated as the initial strategy was to use the trawl line as an anchor line with the barge holding tension on the trawl line as the cages were dropped heading down wind. However, two problems arose. First, if you have an array of multiple trawl lines on the bottom then you can only deploy lines when the wind is in the specified direction thus limiting your opportunity to place trawl lines in high density to only that time when the wind direction is favorable. Secondly, by using the trawl line as an anchor for deploying subsequent cages on the line, we discovered that we were flipping previously deployed cages onto their side and upside down with the drag from the surface vessel. While we could detect this event after happening with the sidescan sonar, it required us to remove and redeploy the trawl line to correct the situation. Subsequent to this realization, we dropped cages in groupings rather than straight lines where the trawl line was not used to anchor the surface vessel during cage deployment. However, that made it more difficult to identify specific cages from the sidescan image and subsequent ROV detection as numerous cages were grouped randomly in a relatively compact foot print.

Objective 4 - Outreach this technology to other farms that are operating in nearshore water.

As this project has progressed, the Principal Investigator has made presentations on the development and application of ROV technology for deeper water oyster farms. The presentations have included at the National Shellfisheries Association annual meeting in Baltimore, MD on 27 March 2023 (Title: Developing the OysterBot for oyster cage retrieval) and at the 2023 Smart Shellfish Aquaculture Summit in Grasonville, MD on 24 August 2023 (Virtual presentation title: Fun on the Farm). Additionally, the PI will be showcasing the OysterBot at a farm tour associated with the Northeast Aquaculture Conference and Exposition in Providence, RI on 10 January 2024. An article outlining the application of the sidescan sonar and the OysterBot is in preparation for publication in the newsletter of the East Coast Shellfish Growers Association and is anticipated to be published in the spring/summer of 2024.

At this point, the OysterBot is available for demonstration at individual farms upon request. That information will be included in upcoming presentations and discussions on the application of technology on developing shellfish farms.