Conclusions
For a recent hydrographic survey adjacent to two decommissioned aircraft carriers, a standard, vessel-based hydrographic survey would be restricted from accessing significant portions of the berths due to the carriers and their mooring hardware. The carriers could not be moved from their berths to accommodate the survey, but it was necessary to survey under the carriers in order to supplement the bathymetric data collected during a traditional vessel-based hydrographic survey. It was determined that divers alone would not be sufficient to accurately determine the bottom elevations under the carriers. Difficulties in accurately positioning the divers horizontally and vertically, and very sparse data, led us to conclude that divers would not provide enough high-quality data to properly supplement the vessel-based hydrographic survey. Mounting a swath multibeam or profiling sonar system was considered for this project. However, the draft of the carriers at 30 feet, and the beam at the waterline of 130 feet, prohibited the use of a vesselmounted sonar system that could be rotated to more directly survey under the carriers. Though this approach could provide accurate data with high-quality horizontal and vertical positioning for the soundings, large portions of the survey footprint under each carrier would be left without survey coverage unless we mounted the transducer near 30 feet below the water surface. This was deemed to be unnecessarily difficult and potential hazardous to the survey vessel and equipment. To complete the survey under each of the aircraft carriers in this restricted-access environment, SeaVision Marine Services LLC teamed with SeaView Systems, Incorporated to deploy a remotely-operated vehicle armed with an aided inertial navigation system and profiling sonar provided by CD Limited. The technology generated a geographically referenced bathymetric dataset within the footprint of the restricted access areas. Post-processing of the data allowed us to generate a complete bathymetric data model of each berth that was referenced to the horizontal and vertical datums of the project. The use of an ROV to perform this hydrographic survey proved to be an effective solution to collecting accurate bathymetric data and managing risk for planned future operations at the site. Other survey options suffered from a lack of data density, poor positional accuracy, or incomplete coverage of the berth floor. Our solution affords the opportunity to collect hydrographic survey data that is comparable in accuracy and density to a traditional vessel-based hydrographic survey
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dataset so long as standards such as horizontal accuracy control, vertical measurement control, and vertical datum control are maintained throughout the survey operations. When the stakes are high enough and complete data is necessary to support the design, construction, or operations at a site with restricted access conditions, this solution can generate a valuable data set that outperforms diver-based or vessel-based surveys in terms of accuracy, density, and/or coverage.
About the Author
Jeff Snyder is the President of SeaVision (SeaVision) Marine Services LLC, based in Naugatuck, CT. He can be contacted at jsnyder@seavisionmarine.com or 203-605-8959.
Marine Technology Reporter 31
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