ROVs for Surveys when there is
Restricted Access
By Jeffrey Z. Snyder, SeaVision Marine Services LLC SeaVision encountered a situation where a client desired knowledge of the bathymetry beneath some moored vessels -- a "restricted-access hydrographic surveys." It devised a unique solution using Remotely Operated Vehicles.
In late 2005, the Naval Inactive Ships Maintenance Office (NISMO) approached SeaVision Marine Services LLC (SeaVision) to conduct a hydrographic survey of the berths at Pier 1, Naval Station Newport, RI: the berths are home to the decommissioned aircraft carriers USS Saratoga and USS Forrestal. In the inshore, particularly in developed ports and harbors, there exist a variety of structures that prevent access to certain portions of the water by vessels, and obscure a clear view of the sky for GPS systems. Piers, relieving platforms, piling-supported platforms, bulkheads; all may cover a portion of a body of water, and that portion of the water may only be accessed by divers or remotely-operated vehicles. In the design and/or construction phase of any project, it is often necessary to determine the mudline elevations of the bottom adjacent to structures and especially where the bottom may be obstructed by these structures. Bottom elevations can figure into models to calculate appropriate design loads for structures, be used to calculate necessary dredge or fill volumes, or determine whether a vessel moored for an extended period of time may be moved from its mooring with or without dredging. As many of these structures are inspected by divers, two diver-based techniques may be used during the inspections: hand measurements or pneumo-fathometer read22 MTR
ings. Where project depths preclude easy hand-measurements, tide measurements may be recorded independently and pneumo-fathometer readings at the mudline may be recorded as spot elevations. These observations may later be corrected for tide. Unfortunately, this approach is only precise to within 0.5 feet vertically. Diver-based approaches suffer from deficiencies in data density, as there are only so many measurements that a free-swimming diver may make in a given period of time. And of course, divers are susceptible to factors such as poor visibility, strong currents, and human error that could adversely impact the accuracy of any measurements taken. Truly accurate horizontal positioning remains difficult, as it is highly dependent on the accuracy of the asbuilt structural plans of the structure. Depending on the project, a sparse bathymetric data set with known positioning inaccuracies under a pier may be sufficient to develop intelligent assumptions that meet the demands of the project. However, some projects demand more accurate, dense data in order to properly manage design protocols and cost estimates. In order to overcome the lack of data density and the accuracy issues that may accompany diver-based survey techniques, vessel-based hydrographic surveys may be considered. Global positioning system (GPS) technology provides highly accurate, real-time positioning that can be
January 2008
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