fter a decade in service in theoffshore oil and gas industry,large Autonomous UnderwaterVehicles (AUVs) such as the

Hugin have demonstrated their worth andare now routinely specified for deep-water surveys. Smaller AUVs havestruggled for acceptance but as thetechnology improves, costs come downand experience is gained, the comingdecade may finally see compact AUVsready for commercial service.

Large versus small AUVsFugro own and operate three HuginAUVs, manufactured by KongsbergMaritime AS. The Hugins are 4.5 m long,weigh up to a ton and require a dedicatedlaunch and recovery system. With thatsize and complexity comes an underwaterendurance of up to 60 hours, a full suite ofgeophysical sensors and a depthcapability of up to 4500 m. The HuginAUVs have successfully filled the role forwhich they were designed, that is, thesurvey of deep-water sites and pipelineroutes.

Although popular within the research andmilitary fields, small or compact AUVshave not had such a compelling reason foruse within the offshore survey industry.Such AUVs generally have a limitedendurance and do not provide any seismicor subseabed information. Within theshallow water arena in which they operate,c o m p a c t A U V s h a v e n o t b e e ncommercially competitive with the well-tested and proven ship-borne surveyspreads which they hope to replace.

Nevertheless, development of the compactA U V form-factor has continued andadvanced to the stage now where acompact AUV can viably perform some ofthe survey tasks traditionally carried outfrom a vessel or by ROV.

The Gavia AUVAdvances in the capabilities of thecompact AUV are exemplified by theGavia AUV manufactured by HafmyndEhf. This vehicle is approximately 3 mlong and weighs 90 kg, hence itsdescription as a low-logistics AUV.

A n u m b e r o f f e a t u r e s have beenincorporated into the Gavia AUV to ensurethat it meets the requirements of theoffshore survey industry:

INS - a Kearfott T-24 InertialNavigation System together with aDoppler Velocity Log (DVL) reducesposition drift to as low as one metreper hour of survey.

Swath Echosounder - a 500kHzGeoSwath Plus echosounder providesbathymetric and backscatter data.

Camera - a downward-looking stillcamera can record colour images of theseabed at a rate of 3 frames per second.

Pipeline Tracking - a SeebyteAutoTracker module can 'lock' theAUV onto a pipeline for high-resolution pipeline inspection surveys.

The above features are in addition to thestandard 900 / 1800 kHz side scan sonar,col l i s ion avoidance sonar, acousticmodem, Wi-Fi connectivity and missionplanning software.

Customer Acceptance TrialsIn late 2009, with the support of WoodsideEnergy Ltd, Fugro took delivery of thefirst Gavia AUV sold into the commercialsector. A series of Customer AcceptanceTrials, designed to confirm the operationalstatus of the various onboard systems,were held in Perth, Western Australia.

The trials began in shallow water in theSwan River, moving over the course ofseveral days into progressively deeperwater. On the final day of the trials theAUV successfully carried out a mission ata depth of 1000 m (its maximum designdepth).

A

ROVS & AUVS

The Gavia AUV can be launched from a vessel or a beach

Image of a lobster pot in 110 m of water and a fish in 650 m of water

60m

Side scan sonar data example of sea grass beds in 7 m of water

Several minor issues were identifiedduring the various tests and have sincebeen rectified, but overall the trialsproceeded smoothly and the Gavia AUVproved itself to be a capable and reliablesurvey platform.

A number of key parameters relevant tothe offshore survey industry weredefined or confirmed over the course ofthe trials and during subsequent testdives.

EnduranceWith two battery modules (the maximumpossible) and all systems operational, theunderwater endurance of the Gavia AUVis between 7 and 8 hours. The mostefficient survey speed is 4 knots, at whichrate a 1 km by 1 km site (total 26 linekilometres) can be surveyed in well under5 hours (including line turns).

Position AccuracyThe Gavia AUV model discussed in thisarticle does not utilise differential GPScorrections. Underwater, the AUVsposition is unaided by acoustic updatesand it relies solely on its INS and DVL.The degree to which the AUVs positionerror will grow over time without anyexternal input depends on a number offactors, including water depth, watercurrents and survey layout.

During the recent trials, the positiondiscrepancy between the same seabedtarget scanned two hours apart (i.e. at thestart and end of a mission) in 100 m ofwater was measured as 2 m. During

another test in 20 m of water, the sametarget, 500 m from the point where theAUV submerged, was scanned by threeseparate missions. The maximumdifference in the measured position of thetarget from each of these three missionswas 3 m.

Data QualityThe side scan sonar at 900 kHz has aneffective maximum range to either side ofthe AUV of 40 m. The range at 1800 kHzthe range is only 10 m, but images have avery high resolution and objects as smallas a car tyre can be clearly resolved.

The GeoSwath Plus echosounder has aneffective range to either side of the AUVof approximately 8 times the height of theAUV above the seabed. That is, with theAUV flying 5m above the seabed, a swathwidth of 80 m (40 m to either side of theAUV) will be recorded. Backscatterdata, equivalent to that from a 500 kHzside scan sonar, is also recorded.

The downward-looking camera recordscolour images at a resolution of 800 x 600pixels. With the AUV 4 m above theseabed, each photograph images an areaof seabed 4 m by 3 m in size. Illuminationis provided by an LED strobe but due tothe rapid absorption light in water, imagesrecorded below 50 m water depth haveonly a limited colour range. Black andwhite photos can be taken at a higherresolution of 1600 x 1200 pixels.

Pipeline InspectionUse of the Gavia AUV is expected to leadto reduced costs and improved dataquality for a number of mapping tasksranging from environmental surveys, to

debris surveys inconfined areasa n d h i g h -resolution seabedsurveys in deep-water.

The primary taskfor which theA U V w a sspecifiedhowever, was the

acous t ic inspec t ion of p ipe l ines ,particularly those which continue intowater depths greater than several hundredsof metres. At those depths a towed sensoris difficult to deploy and position, whileuse of an ROV can be costly. AutoTrackersoftware incorporated into the AUV usesthe side scan sonar data, in real time, tomaintain the AUV at a constant distancefrom the pipeline.

Ongoing Development In order to improve the position accuracyof the AUV further, the ability toincorporate industry-standard differentialGPS corrections such as those supplied bythe S ta r f ix ne twork i s r equ i red .Underwater, the ability to aid the AUVsposition with acoustic updates is desirable.Development of both of these options arewell advanced by the manufacturer andshould be available by the end of this year(2010).

The Gavia AUV utilises a modular designin which various modules (for example,the GeoSwath module or the extra batterymodule) can be added or removed asrequired. The final module needed tocomplete the Gavia AUV as a fully-featured survey tool is a sub-bottomprofiler, the development of which,utilising a low-power, parametrictransducer, is also well advanced.