Mechanical design of an anchoring system for autonomous underwater vehicles

Autonomous Underwater Vehicles (AUVs) are a critical technology that enable monitoring and inspection of the complex subsea environment. AUV deployments are often limited by the endurance of the platform. One option to increase endurance, particularly when constant operation is not required, is to anchor to the seabed in a reduced power state. Here we show the mechanical design, development and testing of a novel anchoring system for the 1500m depth rated Autosub Long Range (ALR). The ALR, like most AUVs, is typically ballasted to be slightly positively buoyant so in the event of failure the AUV should rise to the surface. However, to successfully anchor the vehicle to the seabed the AUV plus anchor system will need to be negatively buoyant. The anchor system developed solves the issue of buoyancy change and anchoring by combining them into a single action. This is achieved by deliberately flooding a pressure vessel to make the AUV negatively buoyant, this pressure vessel is then released from the AUV (while still attached via a tether). When the pressure vessel settles on the sea floor, it acts as an anchor while the AUV floats above. When the AUV is ready to depart, the tether is detached using a galvanic burn wire release. To enable repeated anchoring sequences, the system needs to have multiple anchors installed. However, each system is ballasted to be neutrally buoyant, so that once the AUV completes an anchoring sequence, it returns to its original slight positive buoyancy. The anchoring system was tested in both lab tests and two separate field trials, with the first in Ardmucknish Bay, Scotland, April 2023, and the second in Shetland, Scotland, October 2023. During testing it was shown that the vehicle and anchor system could hold position at anchor with water current speeds of up to 0.6m/s. The holding power of the current anchor design is variable depending on the substrate of the sea floor, with hard surfaces likely providing less holding power than silt, mud or sandy substrates. Further testing is required to determine the limits of the anchors holding power, while further design is also required to ensure that none of the components corrode while at anchor, that the anchors do not induce excessive drag to the vehicle, and environmental testing to ensure the mechanisms operate correctly after an extended period in the ocean.