Tomorrow, if all goes well, a small army of engineers, technicians and mariners will attempt to roll the stricken cruise ship, Costa Concordia, upright from where she sank on the island of Giglio on January 13, 2012. Once upright the ship will be refloated and towed to a scrap yard.  If successful, it will be the largest intact salvage of any ship in history.

The process is called “parbuckling.”  Parbuckling is commonly defined as the use of “a kind of purchase for hoisting or lowering a cylindrical burden, as a cask. The middle of a long rope is made fast aloft, and both parts are looped around the object, which rests in the loops, and rolls in them as the ends are hauled up or payed out.”  The salvage companies involved in the project have set up a website to explain what will be done – The Parbuckling Project.  In this case the Costa Concordia is a very large cask, indeed. See also Parbuckle salvage.   Parbuckling has been used successfully to right the battleship USS Oklahoma (BB-37) which was sunk at Pearl Harbor in 1941 and the passenger/ro-ro ferry Herald of Free Enterprise which capsized in 1987.  Both ships were significantly smaller than the Costa Concordia.

Fortunately, the ship itself  is not quite as heavy as is being reported in the media.  There have been repeated references to the Costa Concordia as a “100,000 ton” ship or as a ship that weighs over 100,000 tonnes. Slate is pretty typical in asking, “How Do You Move a 100,000-Ton Capsized Cruise Ship?”   (The obvious answer is – very carefully.)

The good news for the salvors is that ship itself does not weigh 100,000 tons.  The ship has a Gross Tonnage of 100,000 tonnes, which oddly enough is a measure of the volume of the ship rather than the weight.  The confusion dates back to around the 1200s when a “tun,” a standard sized cask used for shipping wine was first used to assess duties.  Unfortunately, a tun was used as both a measure of weight and volume, which has caused problems ever since.

While the Gross Tonnage of the Costa Concordia is 114,137 tonnes, the actual weight of the ship itself is closer to 50,000 metric tonnes. Nevertheless, entrapped water in the hull may raise the figure somewhat closer to the often quoted 100,000 tonnes.

Engineers gear up to right the wreck of Costa Concordia