Introduction The use of Autonomous Underwater Vehicles (AUVs) in all areas of ocean exploration and mapping is expanding very rapidly. Initially built primarily for military purposes and for scientific exploration, these vehicles are now regularly equipped for seabed mapping using a full suite of sonar systems, including multibeam echo sounders to provide swath bathymetry, multifrequency side-scan sonars, and subbottom profilers. There are over 20 of these full-function sonar mapping AUVs in operation today, and more being built. Over a hundred more specialized AUVs are operating with cameras, laser sensors, magnetometers, and a wide variety of oceanographic and chemical sensors.
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Full function seafloor mapping AUVs provide very high resolution throughout their operating range, typically to 3,000 meters water depth, because they operate at low altitudes -- often 30-50 meters above the seafloor. These AUVs use precise inertial navigation systems that are aided with position control from surface vessels and/or navigation transponders on the seafloor to maintain position accuracy that can be made accurate to a few meters. The sonar data acquired by a state-of-the-art AUV typically includes bathymetry that is accurate to a few tens of centimeters,
resolution of geologic features and other objects that are only a few tens of centimeters in size, and very precise acoustic profiles of the uppermost 10-30 meters of seabed sediments. The resultant data are as precise as would be expected using specialized survey vessels on the surface in water depths of less than 50 meters. AUVs maintain this precision to their full operational depth. To date AUVs have not been used for telecommunication cable surveys because they are considerably more expensive per route-kilometer, and because cable installations do not typically require the very high precision data that is necessary for pipelines and other complex seafloor installations. Cables are routed around areas of unfavorable seafloor, avoiding where possible the need to pick a fine route through specific small hazards on the bottom. The cable, and cable plows, can not be positioned on the seafloor with anywhere near the precision of an AUV map.
The use of AUVs in the oil and gas industry will continue to grow at a very fast rate as this industry pushes into deeper water. Present predictions are that production in water depths greater than 2,000 meters will nearly double in the next three years, and continue this rapid growth in the near term. This growth will buoy the AUV market, particularly offshore West Africa, Brazil, and even Egypt. Most of the full function AUVs in operation today are large vehicles, such as the Hugin 3000 shown in Figure 1, that require a large dedicated surface support vessel. The mobilization of these large AUVs, and transporting them between jobs on the specialized vessel, adds considerably to their cost. Several smaller AUVs that still have the full suite of swath bathymetry, side-scan and subbottom sonars,
Present AUV Applications By far the most use for full function AUVs in today’s market is for oil and gas development and production in deep water. Pipeline routes and the placement of drilling and production platforms require very precise seabed data. Sonar data acquired from surface vessels begin to lose the necessary resolution at around 300 meters water depth. Deep towed sonars do not have the necessary navigational accuracy unless they are positioned with a chase boat (adding considerable expense), and typically do not operate deeper than about 1,000 meters. Remotely Operated Vehicles (ROVs) can conduct sonar surveys, but are slow and require a DP vessel – so are even more expensive than AUVs.
Figure 1: The full-function Fugro Echo Surveyor (a Hugin 3000) normally operated for large projects on a dedicated vessel. There are presently five Hugin 3000’s in commercial operation, with more on order.