Shipwrecks

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SS Central America
SS Gairsoppa
SS Mantola
HMS Victory
SS Republic
Black Swan
HMS Sussex
Tortugas
Blue China
Other Shipwrecks
The bell was the first artifact recovered from the
Spanish Colonial olive jars on the
A mariner's astrolabe is recovered from the 1622
A mariner's astrolabe from the
Hundreds of silver cob coins were recovered from the 1622 “Tortugas” wreck site

"Tortugas" Operational Overview

Soon after the discovery of the "Tortugas" shipwreck site over 1,300 feet (405 meters) deep, a Phantom Remotely Operated Vehicle (ROV) outfitted with video and still camera capabilities and a manipulator arm for recovering artifacts was launched from the Research Vessel Seahawk. The first artifact retrieveda bronze bellwas used to establish an admiralty claim to the "Tortugas" site. The video revealed a site approximately 10 meters (33 feet) wide and 15 meters (49 feet) long with evidence of wooden timbers, piles of ballast stone and dozens of 17th century Colonial-style ceramic amphorae known as “olive jars.”

The depth of the "Tortugas" site required a robotic archaeological excavation which at the time had never been tried before. New systems were developed, technologies were adapted, and innovation was vital to every phase of the project. During the 1990-1991 excavation seasons, the "Tortugas" site yielded nearly 17,000 artifacts, and would make history as the first deep-ocean remotely-operated archaeological excavation of a shipwreck.

To begin excavation of the "Tortugas" wreck site, another vessel, the 210-foot recovery ship Seahawk Retriever was dispatched to the site. The twin-screw supply vessel would serve as the archaeological platform on the ocean surface, and for this purpose was retro-fitted for the excavation with a host of new equipment: winches for rolling cable, cranes for hoisting equipment and artifacts, electronic equipment for communications and security, a four point deep-water mooring system for holding the ship in place over the site, and a control module for the large Remotely Operated Vehicle (ROV) nicknamed Merlin—designed and reconfigured specifically for archaeological excavation in deep water. The Seahawk Retriever was also equipped with living facilities for its 30 person team of archaeologists, ROV pilots, technicians and additional crew.

Fitted with manipulator arms capable of lifting up to 113 kg (250 lbs) each, a suction dredge and positioning systems, the ROV weighed approximately three tons out of water. Buoyancy blocks of syntactic foam made Merlin 600 pounds positively buoyant while six hydraulically-powered positioning thrusters allowed it to work above the seafloor without stirring up sand or silt. Merlin floated and used thrusters to push itself down, enabling it to hover safely over the seabed without damaging the wreck site. Thrusters also held Merlin steady while lifting artifacts and other objects without altering Merlin’s position in the water. Position was determined and recorded by a system of transponders installed on the site and on Merlin, and a transducer located above on the recovery vessel.

Lowered from the recovery vessel Seahawk Retriever, the ROV was operated by a crew in the control room aboard the surface ship. Merlin captured on video a 180-degree view of the underwater excavation which was relayed to the control room via fiber optic cable and was observed on three 30-inch video screens. Three cameras mounted on Merlin provided additional still photography. Using its two titanium manipulator arms, each capable of lifting 250 pounds, the ROV picked up artifacts and placed them into recovery baskets, which were then raised to the surface while the ROV continued to work. ROV Pilots aboard the Seahawk Retriever were able to observe the excavation in real time on monitors and commanded the manipulator arms using joy sticks at the control console.

Historically, shallow-water archaeology had been conducted by physically constructing a pattern over the wreck site. The grid is essential for identifying the location where specific artifacts are found during the excavation. But constructing a physical grid on a deep-water side is not practical. Instead, at the "Tortugas" shipwreck site a unique electronic grid was constructed remotely using sonar transponders and computers capable of quickly providing the location of retrieved artifacts in three dimensions, X, Y, and Z—the latter being depth. The transponders sent sound waves to hydrophone sensors on Merlin. Computers on the ROV calculated the exact distance of each transponder from the ROV. This information, along with Merlin’s depth sensor, would then give the precise 3-dimensional coordinates of the artifact being recovered.

The system used specialized computer software to generate Merlin's position coordinates every five seconds. The data was passed to an On-Line Data Storage and Logging System designed specifically to manage artifact recovery and document artifact provenience. The system was operated by a data logger who assigned artifact numbers to each item as they arrived on the surface vessel. The artifacts were then photographed, provided the appropriate triage conservation and then placed in storage to await transport to the land-based conservation facility. All operational events were recorded on a paper log transcript and in an electronic database for further review by the archaeologist and for future study and publication. In effect, the history of every individual artifact recovered from the "Tortugas" wreck site was tracked and documented beginning with its first observation on the ocean floor, through excavation and transport to the surface vessel, followed by more extensive conservation and recording at the land-based conservation and storage facility.

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