ROVS & AUVS
A
fter a decade in service in the offshore oil and gas industry, large Autonomous Underwater Vehicles (AUVs) such as the Hugin have demonstrated their worth and are now routinely specified for deepwater surveys. Smaller AUVs have struggled for acceptance but as the technology improves, costs come down and experience is gained, the coming decade may finally see compact AUVs ready for commercial service.
Although popular within the research and military fields, small or compact AUVs have not had such a compelling reason for use within the offshore survey industry. Such AUVs generally have a limited endurance and do not provide any seismic or subseabed information. Within the shallow water arena in which they operate, compact AUVs have not been commercially competitive with the welltested and proven ship-borne survey spreads which they hope to replace.
Large versus small AUVs
Nevertheless, development of the compact AUV form-factor has continued and advanced to the stage now where a compact AUV can viably perform some of the survey tasks traditionally carried out from a vessel or by ROV.
Fugro own and operate three Hugin AUVs, manufactured by Kongsberg Maritime AS. The Hugins are 4.5 m long, weigh up to a ton and require a dedicated launch and recovery system. With that size and complexity comes an underwater endurance of up to 60 hours, a full suite of geophysical sensors and a depth capability of up to 4500 m. The Hugin AUVs have successfully filled the role for which they were designed, that is, the survey of deep-water sites and pipeline routes.
The Gavia AUV Advances in the capabilities of the compact AUV are exemplified by the Gavia AUV manufactured by Hafmynd Ehf. This vehicle is approximately 3 m long and weighs 90 kg, hence its description as a low-logistics AUV.
A number of features have been incorporated into the Gavia AUV to ensure that it meets the requirements of the offshore survey industry: • INS - a Kearfott T-24 Inertial Navigation System together with a Doppler Velocity Log (DVL) reduces position drift to as low as one metre per hour of survey. • Swath Echosounder - a 500kHz GeoSwath Plus echosounder provides bathymetric and backscatter data. • Camera - a downward-looking still camera can record colour images of the seabed at a rate of 3 frames per second. • Pipeline Tracking - a Seebyte AutoTracker module can 'lock' the AUV onto a pipeline for highresolution pipeline inspection surveys. The above features are in addition to the standard 900 / 1800 kHz side scan sonar, collision avoidance sonar, acoustic modem, Wi-Fi connectivity and mission planning software.
Customer Acceptance Trials In late 2009, with the support of Woodside Energy Ltd, Fugro took delivery of the first Gavia AUV sold into the commercial sector. A series of Customer Acceptance Trials, designed to confirm the operational status of the various onboard systems, were held in Perth, Western Australia. The trials began in shallow water in the Swan River, moving over the course of several days into progressively deeper water. On the final day of the trials the AUV successfully carried out a mission at a depth of 1000 m (its maximum design depth). The Gavia AUV can be launched from a vessel or a beach
Several minor issues were identified during the various tests and have since been rectified, but overall the trials proceeded smoothly and the Gavia AUV proved itself to be a capable and reliable survey platform. A number of key parameters relevant to the offshore survey industry were defined or confirmed over the course of the trials and during subsequent test dives.
Endurance With two battery modules (the maximum possible) and all systems operational, the underwater endurance of the Gavia AUV is between 7 and 8 hours. The most efficient survey speed is 4 knots, at which rate a 1 km by 1 km site (total 26 line kilometres) can be surveyed in well under 5 hours (including line turns).
Position Accuracy The Gavia AUV model discussed in this article does not utilise differential GPS corrections. Underwater, the AUVs position is unaided by acoustic updates and it relies solely on its INS and DVL. The degree to which the AUVs position error will grow over time without any external input depends on a number of factors, including water depth, water currents and survey layout. During the recent trials, the position discrepancy between the same seabed target scanned two hours apart (i.e. at the start and end of a mission) in 100 m of water was measured as 2 m. During
Image of a lobster pot in 110 m of water and a fish in 650 m of water
another test in 20 m of water, the same target, 500 m from the point where the AUV submerged, was scanned by three separate missions. The maximum difference in the measured position of the target from each of these three missions was 3 m.
Data Quality The side scan sonar at 900 kHz has an effective maximum range to either side of the AUV of 40 m. The range at 1800 kHz the range is only 10 m, but images have a very high resolution and objects as small as a car tyre can be clearly resolved. The GeoSwath Plus echosounder has an effective range to either side of the AUV of approximately 8 times the height of the AUV above the seabed. That is, with the AUV flying 5m above the seabed, a swath width of 80 m (40 m to either side of the AUV) will be recorded. Backscatter data, equivalent to that from a 500 kHz side scan sonar, is also recorded. The downward-looking camera records colour images at a resolution of 800 x 600 pixels. With the AUV 4 m above the seabed, each photograph images an area of seabed 4 m by 3 m in size. Illumination is provided by an LED strobe but due to the rapid absorption light in water, images recorded below 50 m water depth have only a limited colour range. Black and white photos can be taken at a higher resolution of 1600 x 1200 pixels.
Pipeline Inspection Use of the Gavia AUV is expected to lead to reduced costs and improved data quality for a number of mapping tasks ranging from environmental surveys, to debris surveys in confined areas and highresolution seabed surveys in deepwater. The primary task for which the AUV was specified however, was the
60m Side scan sonar data example of sea grass beds in 7 m of water
acoustic inspection of pipelines, particularly those which continue into water depths greater than several hundreds of metres. At those depths a towed sensor is difficult to deploy and position, while use of an ROV can be costly. AutoTracker software incorporated into the AUV uses the side scan sonar data, in real time, to maintain the AUV at a constant distance from the pipeline.
Ongoing Development In order to improve the position accuracy of the AUV further, the ability to incorporate industry-standard differential GPS corrections such as those supplied by the Starfix network is required. Underwater, the ability to aid the AUVs position with acoustic updates is desirable. Development of both of these options are well advanced by the manufacturer and should be available by the end of this year (2010).
The Gavia AUV utilises a modular design in which various modules (for example, the GeoSwath module or the extra battery module) can be added or removed as required. The final module needed to complete the Gavia AUV as a fullyfeatured survey tool is a sub-bottom profiler, the development of which, utilising a low-power, parametric transducer, is also well advanced.