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AquaMap ShipHull performance in difficult areas of a hull, and other challenging situations

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This entry discusses the perfomance of AquaMap ShipHull in specific locations on a hull and situations that might be considered 'challenging'. Please see the related question for a general performance review of the system.

Performance on the side of a hull:
AquaMap ShipHull performs well on the side of a hull. The operator selects the Starboard or Port view as the active view (S or P key command), thereby informing the system that this side of the hull is being surveyed. AquaMap then is aware that line of sight to several of the baseline stations is not available and adjusts its operating method to effectively deal with the situation.

A test was done on the USS Peterson, DD969. The mobile station was held immediately next to the hull at the waterline. It was then slowly lowered to below keel depth, while continously being tracked by the system. The test was repeated in several positions from the stern to near the bow. The straight vertical track line at each location shows that the mobile station was tracked throughout the vertical transit. The detail picture shows the horizontal drift or displacement, which represents the total test error including positioning error and any actual displacement due to currents. It is on the order of tens of centimeters to one meter, depending on location.

The mobile station was held next to the hull of the USS Peterson and slowly lowered to below keel depth.

The ship-side tracking test was repeated in several locations along the hull, each indicated by a vertical track line. The plot has a 10m x 10m grid.

Here is a detail view of two of the vertical transits (thrird and fourth location from the bow, see above). The grid is 1m x 1m.

Performance around the running gear:
Bottom navigation (B command key) is used when inspecting the area around the running gear. Our tests and our customer's experience has shown that AquaMap in general will perform in this region and is suitable to guide and distinguish among locations such as port or straboard gear, location forward or aft of the prop etc. Yet, we have also seen instances where navgation becomes unreliable or there is loss of position fixes in specific locations. In these cases, positioning can be re-acquired by moving the vehicle or diver somewhat. We found that by combining visual cues with the navigation data, experienced ROV operators can reliably operate around the running gear.

ROV operation around the running gear of the USS Cannon, side view, 2m x 2m grid

Top view plot of the USS Cannon operation. The port gear was inspected.

Performance with little or no clearance under the hull:
We have done two tests on hulls where clearance to the mud below was one foot or less. In those ceses, the baseline stations should be deployed just a few inches above the mud line, with the transducer facing down. Lower each station until you feel it touching bottom, then pull up a few inches and secure. In general, we have found that operation will work fine as long as there is enough clearance for the inspection vehicle or diver to pass under the vehicle. However, we also did one test on the USS Barry in the Washington Naval Shipyard. This is a destroyer now serving as a museum ship. At the time of our visit, the forward half of the ship was in the mud and it was not possible to pass underneath the hull. The plot shows that tracking still worked all along the hull. When the small VideoRay ROV made an excursion into some mud cavity under the hull, the tracking system maintained track and showed it under the hull. While AquaMap is a line-of-sight technology, the signals can sometimes penetrate through materials such as water saturated mud. Yet, the operator should consider this a case-by-case scanerio. If a vessel has run aground or is in the mud, tracking may not work along parts of the hull and the operator should be aware of potential position errors.

ROV operations around the USS Barry. The forward portion of the ship was resting in the mud.

Inspection of very small hulls:
On very small hulls, there is probably a limited need for positioning technology. Still, we have done some tests on small vessels such as this 13-meter Coast Guard boat. The system's accuracy is generally independent of vessel size. Including some allowance for errors such as in baseline station placement or baseline station drift, our and customer experience has shown that a 1m error is a conservative (safe) assumption. This number holds true on small vessels as well, but of course a 1m error will 'look' larger on a smaller vessel than it does on a large vessel.

Inspection of a 13-m U.S. Coast Guard boat.

Inspection of very large hulls:
Our own data base only includes vessels up to destroyer size, although our customers report having done work on aircraft carriers and large cargo ships. We will try to obtain some test data for this knowledge base entry. Repeated testing has shown that our 'regular' transmit power systems (182 dB source level) are sufficient for inspecting ships up to 200m / 600ft length. Our estimation is that this version is in fact sufficient for vessels up to 350m length. Yet, for reasons of performance confidence we recommend that our high-power option is used when inspecting vessels larger than 200m with a single setup. The high power option yields 192dB transmit source level, i.e. 10 times the transmit power of the regular version. An alternative to the use of the high-powered hardware is to inspect large vessels in sections by moving the baseline stations forward or aft as needed.