the survey lines. Care had to be taken to avoid fouling in the cathodic protection system in place on each aircraft carrier Care also had to be taken to maintain an elevation of 0.3 meter or better above the bottom so that the DVL could properly track the bottom and feed the velocity drift corrections to the INS. Clearly, survey operations at high tide were preferred because they afforded the greatest separation between the hulls and the bottom. The profiling sonar was configured to collect data in a full 360-degree vertical profile circle around the ROV and profile both the berth floor and the hull so that bathymetric soundings of the berth could be compared with actual elevations of the hulls to determine under-keel clearance. Periodic follow-on dockings throughout the progress of the survey, of approximately 5 to 10 minutes per docking event, allowed us to observe the performance of the INS and re-set the position to the known dock location, thereby minimizing long-term positioning errors and maintaining the horizontal positioning error to 1 meter or less, which is consistent with the accuracy generated by the Trimble DSM 132 Differential GPS used during the vessel-based survey.
Survey Processing and Delivery
CDL had previously prepared this instrument package for a flooded tunnel inspection and modeling effort in the Middle East. We worked with CDL to modify the data collection routines in order to better accommodate a hydrographic survey project. We used CDL's Tunnel Mapper acquisition software for data collection and integration. All time-stamps, offsets, position, attitude, and position (both horizontal and vertical) were recorded topside for later post-processing to generate a geographically referenced bathymetric data set. The data, when integrated in the CDL Tunnel Mapper software and brought into the CDL Tunnel Viewer software, created a three-dimensional, geographically referenced data set of bottom and hull elevations. The raw data from the survey merged both the berth floor data and the hull data into a 3-D image model within CDL's Tunnel Viewer software (developed for flooded tunnel inspections). Notice the sloping wall of the hull of the aircraft carrier, and the slope approaching the vertical feature on the right - where the berth meets the slope under the pier. For our purposes, we needed to separate the hull elevations from the bottom elevations so that the bottom elevations could be corrected for tide and referenced to the
30 MTR
project datum of Mean Lower Low Water. To do this, we developed custom software that would read the standard CDL file format and first separate the hull data from the bottom data. This was rather easy, simply by looking at the profiling sonar bearing and creating separate data files according to when the sonar pointed up (for the hull) or down (for the bottom). After separating the data sets, we used Hypack to generate a tide correction file that tracked the tidal elevations relative to MLLW. We then subjected the bottom data set to a look-up algorithm where the time stamp of the profiler data was compared to the time of tide, and the depth of the ROV was corrected according to the height of tide at each time. Note that the hull data was not subject to tide corrections, as the elevation of a floating hull is independent of the vertical datum; the elevation is a function of water level and vessel loading. After correcting the bottom data sets for tide, we imported the hull data and the bottom data back into CDL Tunnel Viewer in order to export geographically referenced xyz bathymetric data of the bottom and the hull. The CDL Tunnel Viewer exports data in the Universal Transverse Mercator (UTM) grid, so we used Hypack to convert into the project horizontal datum of NAD 1983 Rhode Island State Plane feet. There, the bottom elevation data collected with the ROV from underneath each of the aircraft carriers could be merged with bottom elevation data during the vessel-based hydrographic survey. The combined data was then presented to in a series of deliverable drawings consisting of plotted soundings, plotted contours, and differential plots illustrating the under keel clearance between the hulls and the bottom at Mean Lower Low Water. The total hydrographic survey at Pier 1, Naval Station Newport, was used to develop 1-ft elevation contours in the berths surrounding the pier. 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. Some statistics, comparing the results of the vessel based hydrographic survey have been performed to determine the correlation of the vessel-based hydrographic survey with the ROV-based hydrographic survey. The standard deviation of the overlapping soundings was 0.5 feet or less. Sixty percent of the overlapping soundings fell within one standard deviation, and ninety-five percent fell within two standard deviations. It should be cautioned, however, that we only had twenty-one soundings where overlap occurred; while the statistics are promising, they are hardly conclusive.
January 2008
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