Tony George presents a comparison of data collected with the first and second generation autonomous underwater vehicles
Commercial, deepwater survey AUVs used primarily in the oil and gas industry are approaching nearly a decade of use. The speed and efficiency at which these systems operate in deepwater have saved exploration and production companies millions of dollars in development survey costs when compared to conventional survey techniques. Project timelines from the discovery well to first oil production have decreased significantly due to AUV technology. Larger AUV platform designs allow space for more powerful sensors, longer life batteries and stronger electronic housings capable of withstanding increased water depth pressures. Sensor upgrades in the latest generation of AUVs have resulted in improved high-resolution geophysical data.
C & C Technologies Inc (C&C) entered the deepwater AUV survey market in January 2001 with the commissioning of a Simrad HUGIN 3000 AUV, C-Surveyor I. A team of company engineers and technicians integrated a geophysical system payload of side scan sonar, subbottom profiler and multibeam echosounder. Processing routines were developed for merging the positioning and geophysical data. C-Surveyor I is equipped with three primary geophysical sensors: Edgetech Dual-Frequency Side Scan Sonar, Edgetech DW-216 Chirp Subbottom Profiler and Simrad EM3000 Multibeam Echosounder. The 3000 metre depth rated system operated primarily in the Gulf of Mexico during the first two years of service. The system was plagued with significant downtime, but the mission success rate was improved with time as lessons were learned and improvements to subsystems were made. C-Surveyor I has been working on long-term contracts in Brazil for the past several years.
Deepwater survey market demand in 2005 resulted in C&C investing in a second Simrad AUV, C-Surveyor II. This AUV is a Simrad HUGIN 3000 with the same geophysical payload as C-Surveyor I. The subbottom profilers in these systems worked well in the Gulf of Mexico (Fig.1), but very poor records were logged with the system in the deepwater of West Africa (Fig.2).
In 2007 the deepwater survey market looked promising and C&C invested in C-Surveyor III, a Simrad HUGIN 4500 AUV with an increased water depth rating of 4,500 meters. This AUV is larger than its predecessors. The side sonar system in this AUV is an Edgetech DF (Dynamically Focused) 3200 Side Scan Sonar (230kHz). The sonar transducers are segmented providing high-frequency sonar imagery out to 200metres on each channel (Fig.3).
Significant effort was put into building a superior subbottom profiler for C-Surveyor III. Edgetech was contracted to design and build an AUV chirp subbottom profiler with increased source output power, larger receiver array, reduced beamwidth and a lowered frequency content. The Edgetech DW-106 Subbottom Profiler resulted from the effort. A single transducer for the source was replaced with a 2x2 transducer array. The larger transducer array reduced beamwidth and increased the dB output of the source. The receiver array sized was increased from two 19-in hydrophones to eight 32-in hydrophones. A lowered, frequency modulated source pulse in the range of 1 to 6 kHz results in increased depth of penetration.
Figs.1 and 2 show data examples of the improved DW-106 Subbottom Profiler compared to the DW-216 Subbottom Profiler. Subbottom data in the West Africa is drastically improved in resolution. The Gulf of Mexico example shows nearly double the penetration depth in some locations. An upgrade of the subbottom profiler in C-Surveyor I is anticipated for August 2009.
Earlier this year, C&C added a digital camera to the C-Surveyor III payload (Figs.4 and 5). The ultra compact camera is capable of recording two-megapixel, black and white photographs of the seabed. The anticipated AUV altitude for camera operation is 6.5metres and field of view is 4x3 with the longer view in the alongtrack direction. The low-power camera takes a picture every 1.5 seconds with a strobe light supplying illumination. This firing rate allows approximately 25 per cent overlap between pictures. Pipeline or wellhead inspections typically performed with ROV are now possible with this AUV. The primary advantage of using an AUV over an ROV is the speed at which data images can be collected (3.8vs. 1.5knots). The camera will allow pre-route survey inspections to ensure no debris or obstacles were missed with the other acoustic systems. Groundtruthing of sonar targets for deepwater UXO investigations or archaeological assessments is another application for the AUV camera. Usually a second mobilisation of an ROV is needed to investigate sonar targets. Environmental concerns or studies, such as chemosynthetic communities or deepwater corals, are another application of the AUV camera.l
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Robert A 'Tony' George is with C & C Technologies Inc, Lafayette, LA, USA. www.cctechnol.com