amount of rotating a transducer can overcome the fact that the relative elevations of the transducer, the obstruction/structure, and the bottom profile can create a "shadow" that prevents survey beyond it and thus reduces the total footprint that can be surveyed. The maximum depth of the structure can thus create a "shadow" which can only be overcome by dropping the survey transducer beneath this maximum depth. A pier deck that extends well below the water level creates a "shadow" for the multibeam swath that cannot be removed without dropping the depth of the transducer. The deeper that this maximum depth exists beneath the water surface, the more that the feasibility of using a vessel-mounted survey system dramatically decreases. The operational and safety considerations of having a large over-the-side mount reaching to significant depth and attached to a small inshore survey vessel make these approaches less attractive.
Special Projects Demands Alternative
NISMO, charged with maintaining the U.S. Navy's decommissioned vessel fleet, wanted to learn the bathymetric conditions under and around the carriers prior to any plans to permanently move the carriers from their moorings. Moving the vessels from their berths temporarily to complete this latest survey was considered prohibitively expensive and risky. Both vessels were moored to the piers with heavy weather, long-term moorings that
would be costly to undo. The previous surveys of the berths had been performed in 1998, and the underwater conditions in the berths since then were unknown, risking a possible grounding of the carriers. The pier measures approximately 1575 ft. in length and the survey footprint of each berth was approximately 1900 by 400 ft. Each aircraft carrier measures approximately 1000 ft. in length with a maximum beam at the waterline of 130 ft., and current drafts in the neighborhood of 30 ft. beneath the waterline. The stern of the decommissioned aircraft carrier USS Forrestal at Naval Station Newport, RI. The mooring lines and chains for the long-term heavy-weather mooring configuration, and the hull itself, prevent survey of the bottom underneath the footprint of the carrier by a vessel. The dimensions of the survey areas and the carriers are particularly important when considering the objectives of the survey, which were two-fold. The primary objective was to survey the entirety of each of the berths, including the footprints under each of the carriers. The secondary objective was to determine the distance between the hulls and the mudline based on the existing draft conditions at the time of the survey. Several options were considered for conducting the survey. Each option consisted of a traditional, vessel-based hydrographic survey with a single-beam echosounder in the accessible portions of the berths. To address the footprint under each moored aircraft carrier, diver and remote
The total hydrographic survey at pier 1. The area in red represents the footprints in both berths where data collected with the ROV (armed with an aided INS and profiling sonar) completed the restricted access portion of the survey. 24 MTR
The vessel-based hydrographic survey at Pier 1 was used to develop one-ft. elevation contours in the berths surrounding the pier.
January 2008
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