Thunderhorse oil platform

 Rigzone.comThunderhorse oil platform. Credit: Rigzone.com

Thunder Horse is the world's largest semi-submersible floating production, drilling and quarters (PDQ) to extract crude oil and natural gas. The PDQ unit has a total operating displacement of more than 140,000 tons. The rig will be the center for the development of the Thunder Horse field located in the Gulf of Mexico about 125 miles south-east of New Orleans. Discovered in 1999, it is the largest field in the Gulf of Mexico and lies at a water depth of 6,000 feet. The discovery well was drilled to a depth of 25,770 feet. BP operates the development (75% interest), with co-venturer ExxonMobil holding the remaining 25%.

Semi-submersible platforms have superstructures that are supported by columns sitting on hulls that are ballasted below the water surface. They are designed to provide stability in rough, deep seas. The legs of semi-submersible platforms have enough buoyancy to cause the structure to float, but enough weight to keep the structure upright and immobile. They generally are anchored by cable anchors during drilling operations, but can also be kept in place by dynamic positioning.

 BP AmericaThunderhorse being trasported. Credit: BP America

Daewoo's Shipbuilding and Marine Engineering in South Korea built the 60,000-ton hull of the Thunder Horse. The transport from its fabrication location in South Korea to Texas, where the hull and topsides were joined and fabrication was completed, required a vessel larger than any in existence. The marine transportation firm Dockwise enlarged the Blue Marlin transport vessel to become the only vessel in the world capable of transporting ultra-large structures up to 73,000 tons. The Blue Marlin set sail on July 23, 2004, for the 15,813 nautical mile journey to the Gulf of Mexico, via the Cape of Good Hope. After a voyage of about 8 weeks at an average speed of 11 knots the Blue Marlin arrived in Corpus Christi, TX where construction was completed.

Once on location, the giant rig's pontoons are deliberately flooded, partially sinking it to provide a stable drilling platform and production platform while 16 anchors embedded in the seabed 6,000 feet below hold it in place.

The platform is comprised of two decks; a lower deck measuring 350 by 350 feet, and the upper deck measuring 350 by 450 feet. The platform will also include quarters for 230 persons. Peak production rates are targeted at 250,000 barrels per day of liquids and 200 million cubic feet per day of natural gas. This unit could recover in excess of a billion oil-equivalent barrels, making Thunder Horse the largest discovery to date in the Gulf of Mexico.

At a development cost of approximately $5 billion, the platform features more than 100 industry firsts. For example, the platform has a 100-megawatt power station on board, enough electricity to supply a town of 80,000 homes, making it the biggest offshore power generation plant in the world.

 Offshore Technology.comThunder Horse after Hurrican Dennis. Credit: Offshore Technology.com

In July 2005, the Thunder Horse was found listing precariously at a 20-degree angle in the wake of Hurricane Dennis. The rig had been evacuated as a precaution ahead of the hurricane. To right the flooding platform and flush out its hull, salvage engineers attached hydraulic pumps to the 118-ft. columns holding up the Horse. A week later, it was stable and back to normal position.

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