During an offshore military exercise, a foreign Navy had the misfortune of losing a Remus 600 AUV. The Remus AUV is approximately 3.25m long and weighs about 240Kg. Thrust comes from a single propeller while three variable fins provide maneuverability. Since the AUV travels at speeds up to 5 knots, it requires a fair amount of warning from its sensor inputs to make course adjustments, such as when it encounters an obstacle or an extreme variation in bottom terrain.
Once it was determined that the AUV was not going to return to the surface on its own power, the Navy immediately began tracking the unit with its built-in navigation system. Over the course of a week, the Navy was able to pinpoint the location of the AUV several times in a fixed position. This lead to the conclusion that the AUV was somehow stuck and not necessarily lost. At this point, SeaBotix was contacted to help locate and recover the Remus 600 using the LBV MiniROV system.
The Navy provided SeaBotix with the general coordinates of the AUV and proposed a search area. SeaBotix determined that the LBV150SE-5 would be the ideal system. The unit on hand was equipped with 250m of umbilical, an upgraded depth rating of 200m, both scanning and multi-beam sonars, USBL tracking system, multi-function Grabber, and Integrated Navigation/Control Console with a sunlight readable 20" LCD monitor. A 13m vessel with hydraulic boom (to land the AUV) was sourced locally as a suitable operating and recovery platform.
Operations on the first day were performed in less-than-ideal conditions with high winds, a 2m swell, and 1.5 knots of current. While the Skipper held position over the first GPS coordinates, the LBV was deployed off the stern using an umbilical sheave on the boom and a clump weight 30m back from the LBV. At 105m, the LBV reached the seafloor and transects were begun. The visibility was amazing, requiring no light for the sensitive high-resolution cameras.
A new set of GPS coordinates was searched on the second day of operations. About two hours into searching the steep slopes, the LBV turned to make another pass and Remus 600 was spotted off in the distance, stuck under a rocky ledge. As the LBV approached, it became apparent that the lifting handle and guide loop at the bow of the AUV had hung up on the jagged rock ledge. Without the ability to provide downward thrust, the AUV was unable to free itself. The LBV was maneuvered into position and the manipulator was used to grasp the propeller. Using its powerful brushless thrusters, the LBV was able to work the Remus free and drag it backwards. The LBV's powerful dual-vertical thrusters were then used to assist in the 137m ascent to the surface, and a lifting line was attached to hoist the AUV onto the aft deck.
The incident was clearly determined to be a combination of bad luck as the Remus was unable to respond to the incline quickly enough as it was conducting programmed transects combined with inaccurate and incomplete maps. In an attempt to adjust its course, the AUV drove straight up and into the cave/ledge.