Parbuckle salvage

Parbuckle salvage, or parbuckling, is the righting of a sunken vessel using rotational leverage. A common operation with smaller watercraft, parbuckling is also employed to right large vessels. In 1943 USS Oklahoma was rotated nearly 180 degrees to upright after being sunk in the attack on Pearl Harbor, and the Italian cruise ship Costa Concordia was successfully parbuckled off the west coast of Italy in September 2013, the largest salvage operation of that kind to date.

While the mechanical advantage used by the laborer to parbuckle a cask up an incline is 2:1, parbuckling salvage is not so limited. Each of the 21 winches used to roll the Oklahoma used cables that passed through two 17-part tackle assemblies (17:1 advantage). Eight 28-inch (710 mm) diameter sheaves, eight 24-inch (610 mm) diameter sheaves, and one 20-inch (510 mm) diameter sheave comprised just half the mechanical effort.

A major concern during salvage is preventing rotational torque from becoming a transverse force moving the ship sideways. USS Utah, lost like the Oklahoma in the Pearl Harbor attack, was meant to be recovered by a similar rotation after the Oklahoma. As the Utah was rotated, however, its hull did not catch on the harbor bottom, and the vessel slid towards Ford Island. The Utah recovery effort was abandoned.

Oklahoma weighed about 35,000 short tons (32,000 metric tons). Twenty-one electric winches were installed on Ford Island, anchored in concrete foundations. They operated in unison. Each winch pulled about 20 short tons (18 metric tons) by a wire operated through a block system which gave an advantage of seventeen, for a total pull of 21×20×17, or 7,140 short tons (6,480 metric tons). In order to increase the leverage, the wire passed over a wooden strut arrangement (a bent) which stood on the bottom of the ship about 40 feet (12 meters) high. Oil had been removed from the ship through the bottom. The ship was lightened by air inside the hull. There was a large amount of weight in the ship which may have been removed prior to righting, but not all could be accessed. About one-third of the ammunition was taken off together with some of the machinery. The blades of the two propellers were also taken off, but more to avoid damage to them than to reduce weight. Tests were made to check whether restraining forces should be used to prevent sliding toward Ford Island. It was indicated that the soil under the aft part of the ship prevented sliding, whereas the bow section rested in soupy mud which permitted it. To prevent sliding about 2200 tons of coral soil were deposited near the bow section. During righting, excess soil under the starboard side was washed away by high-pressure jets operated by divers. The ship rolled as it should have and was right-side up by 16 June 1943, the work having started 8 March 1943. The mean draft of the ship after righting was c. 50 feet (15 meters).

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