Assembling deepest offshore platform

Perdido’s hull and platform weigh around 55,000 tonnes — the decks alone are as big as two football fields — and it is moored in about 2,450 metres (8,000 feet) of water. The scale of the project demanded international expertise, with equipment brought in from around the world.

Moving the spar into place

The hull, or spar, was built in Pori, Finland, using steel from a mill 500 kilometres (310 miles) away.  It is nearly as tall as the Eiffel Tower and took over two million man hours to build. A giant barge shipped the 22,000-tonne spar on a 13,200-kilometre (8,200-mile) journey from the Baltic Sea to Ingelside, Texas, on the Gulf of Mexico. Here the barge was flooded to partially submerge it, allowing the spar to float onto the water. Two tugboats towed it to its final home above the Alaminos canyon 320 kilometres (199 miles) from the shore.

Working through hurricane season, a team of workers raced to secure the giant floating spar to the seabed. To rotate it from a horizontal to a vertical floating position, they pumped water through hoses attached to the top of the spar, gradually filling tanks near the bottom. It took over 20 hours to finally get it upright. They attached nine mooring lines and anchored the spar to piles in the seafloor, then evacuating before hurricane Gustav hit in early September.

Hurricane resistance

Iron ore ballast at the bottom of the spar helps keep it upright and stable. As a precaution ahead of hurricanes the spar can increase ballast by adding water to internal tanks. That means it won’t move up and down much more than a few metres with the waves, reducing the tug on the tubes that bring oil and gas to the surface from the seabed.

Perdido’s 22,000-tonne hull was designed never to capsize, even in the roughest seas. It can withstand the most powerful storms, the type expected to occur only once in 1,000 years. “It bobs up and down like a cork,” says Clarence Holmstrom who oversaw the building of the spar.

To prevent damage to mooring lines from scraping on the seafloor and against the spar, the 25-centimetre-thick (10-inch) polyester lines have a 300-metre (984-foot) section of chain at the top and bottom. Winches on the platform make it possible to move the spar into the most secure position before a hurricane hits.

Assembling a perfect puzzle

The platform’s three decks — or topsides — support oil and gas processing units, living quarters for 150 people and a drilling rig. Yet all that equipment had to be light enough so that special floating cranes could lift the assembled topsides onto the upright spar. “The deck had to go on a diet,” says Kurt Shallenberger, topsides project leader. By weighing all materials and every piece of equipment — including over 21 kilometres (13 miles) of pipes — the team achieved a topsides weight just one tonne over the exact target.

“The Perdido facility is like a gigantic jigsaw puzzle,” says Shallenberger. “There’s lots of pieces and parts that all have to work, that all have to be safe, and everyone’s got to agree how they fit together.” 

Laser-aided measurements

Temperature differences between Finland and Texas posed another challenge to assembly, as the steel components built in the cold of northern Europe expand in the heat of the Gulf of Mexico.

A team of experts calculated dimensions for components on the deck and hull using advanced computer modelling. This allowed for potential expansion, as well as flexing of steel when cranes lifted the topsides. Computer-guided lasers marked out the measurements during construction to ensure everything fit perfectly. In March 2009, cranes lowered the 9,500-tonne topsides onto four posts on the spar, slotting it into position exactly and setting a record for the heaviest topside lift in the USA.