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 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 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 28adj=midNaNadj=mid sheaves, eight 24adj=midNaNadj=mid sheaves, and one 20adj=midNaNadj=mid sheave comprised just half the mechanical effort.[1]
A major concern during salvage is preventing rotational torque from becoming a transverse force moving the ship sideways., 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.[2]
Oklahoma weighed about 35000ST. Twenty-one electric winches were installed on Ford Island, anchored in concrete foundations. They operated in unison. Each winch pulled about 20ST by a wire operated through a block system which gave an advantage of seventeen, for a total pull of 21×20×17, or 7140ST. 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 40abbr=offNaNabbr=off 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. 50abbr=offNaNabbr=off.[3]
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:
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.[4] The hull was rotated 65 degrees to become vertical.[5]
Parbuckling was accomplished in three phases:
At the completion of parbuckling, Costa Concordia rested on the ledge at a depth of 30abbr=offNaNabbr=off.[5]
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 58adj=offNaNadj=off long and weighed about 26MT.[5] Each link weighed 205kg (452lb).
The ledge was part steel and part grout. There were six steel platforms. The three larger platforms measured 35x each; the three smaller platforms measured 15x each. The 6 platforms were supported by 21 pillars of 1.6abbr=offNaNabbr=off diameter each and plunged for an average of 9abbr=offNaNabbr=off 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 12000abbr=offNaNabbr=off and over 16000MT in weight.[5] The grout bags contained an "ecofriendly cement," and were built with eyelets to aid post-recovery cleanup.[6]
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.
Two steel "blister" tanks were connected together at the hull's bow. They measured 23abbr=offNaNabbr=off in length, 20abbr=offNaNabbr=off in height each, and had a total breadth of about 36abbr=offNaNabbr=off. The whole blister structure (the two blister tanks, the tubular frame and the three anchor pipes) weighed about 1700abbr=offNaNabbr=off. They provided a net buoyancy of 4500MT to the bow section.[5]
The cable system provided a force of about 23800MT to start the Costa Concordia's rotation.[5]
The hull of Costa Concordia rested on two spurs of rock, and was severely deformed from the weight of the ship pressing down on the spurs. This phase began when the strand jacks exerted force and the ship started to return to an upright position. This was "without doubt one of the most delicate phases of the entire recovery plan."[5]
This phase began when the hull lifted from the seabed. Rotation continued by tensioning the cables operated by the strand jacks, and continued until the sponson water intakes reached sea level.[5]
The hull continued to rotate, pulled down by the weight of seawater added to the sponsons. The strand jacks and cables went slack. Redundant systems were designed as a guard against failure. For example, two seawater inlet valves were provided to each sponson.[5]