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, the 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.
Mechanical advantage and difficulties
While the mechanical advantage used by a 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-diameter (710 mm) sheaves, eight 24-inch-diameter (610 mm) sheaves, and one 20-inch-diameter (510 mm) 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 toward Ford Island. The Utah recovery effort was abandoned.
Righting of Oklahoma 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).
Righting of Costa Concordia
Following its capsizing and sinking in January 2012, the hull of Costa Concordia lay starboard side to the seaward face of a small outcropping very near the mouth of the harbor of Giglio, Italy, resting precariously on the incline to deeper water. To right the vessel, four key pieces of apparatus were required:
a "holdback" system of chains attached to the island on one end and the hull on the other to ensure Costa Concordia rolled in place; a man-made ledge inserted into the island face to provide a landing surface for the vessel; a series of sponsons attached to the hull's port side so as, when flooded, to increase the torque on the hull and to unburden the strand jacks; an arrangement of cables rising from the edge of the ledge over the sponsons on the port side of the hull. Tensioning the cables started the roll of the ship. At about the halfway-to-vertical position the sponsons were filled with seawater, and Costa Concordia completed its roll to upright upon the ledge. The hull was rotated 65 degrees to become vertical. Parbuckling was accomplished in three phases:
Freeing the hull Phase of rotation using cables Rotation by ballasting with sponsons At the completion of parbuckling, Costa Concordia rested on the ledge at a depth of 30 meters (98 feet).
Holdback system The holdback system consisted of 56 chains in total, of which 22 chains were attached to the port side to go under the hull to the island. Each chain was 58 meters (190 ft) long and weighed about 26 metric tons (29 short tons). Each link weighed 205 kilograms (452 pounds).
Ledge The ledge was part steel and part grout. There were six steel platforms. The three larger platforms measured 35 by 40 meters (115 by 131 feet) each; the three smaller platforms measured 15 by 5 meters (49 by 16 feet) each. The 6 platforms were supported by 21 pillars of 1.6 meters (5.2 feet) diameter each and plunged for an average of 9 meters (30 feet) in the granite sea face of Giglio. The grout filled the space between the land side of the platforms and the sea bed. It totaled 1,180 individual bags with a volume of over 12,000 cubic meters (16,000 cubic yards) and over 16,000 metric tons (18,000 short tons) in weight. The grout bags contained an "ecofriendly cement," and were built with eyelets to aid post-recovery cleanup.
Sponsons Eleven steel sponsons were installed on the port side of the hull: two long horizontal sponsons; two long vertical sponsons and seven short vertical sponsons.
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