Nord Stream has successfully completed the second of the two underwater tie-ins of its second gas pipeline through the Baltic Sea on 28 June. The 1,224-kilometre offshore pipeline is now complete and will be de-watered and dried before being joined to the onshore landfall sections in Russia and Germany in August. This second line is on schedule to become operational as part of the twin pipeline system in the last quarter of this year.
Nord Stream’s twin pipelines were constructed in three sections with reducing pipe-wall thicknesses as the design pressure of the gas drops from 220 bar to 200 and from 200 bar to 177.5, arriving at the European mainland in Germany at 100 bar. ?The three sections were joined together underwater inside a hyperbaric welding habitat on the seabed at two locations, off the coast of Finland and off the Swedish island of Gotland in June.
“This complex operation was successfully completed a few days ahead of schedule thanks to the excellent international collaboration that has characterized the whole project,” says Nicolas Rivet, Project Coordinator for the hyperbaric tie-in operations at Nord Stream. “As with Line 1, our diving contractor Technip completed this challenging task outstandingly well, using equipment from the Pipeline Repair System (PRS) Pool operated by Norway’s oil and gas company Statoil.” ?
The underwater welding operations were remotely controlled from the world’s largest dive support vessel, the 160 metre long Skandi Arctic. Highly specialized divers oversaw the complex operations. ?
“New technology has made diving safe and the surveillance of the divers is automated. Divers are today used instead of ROVs (Remote Operated Vehicles) when precision is required,” Jahn Erling Nakkestad, Project Manager from Technip explained. “Overseeing the hyperbaric tie-in operations requires experienced and skillful divers and our previous experience from other PRS projects on the Norwegian Continental Shelf has been crucial for the success of the project,” he added.
The diameter of the Nord Stream pipelines required the PRS equipment to be specially adapted. “The overall tie-in equipment had to be upgraded: the pipe handling frames and the welding habitat had to be enlarged, because such 48-inch diameter pipelines had not been handled before,” said Jan Olav Berge, Statoil’s Pipeline Repair Pool Manager. “The same system as used for the tie-ins will be ready to carry out any repairs to these or other large-diameter pipelines in the future if needed,” he added.
Following the underwater tie-ins, all the water will be removed from of the pipeline in July and the pipeline will then be dried. The onshore and offshore sections of the pipeline will be connected in August, and after thorough testing, this second line is expected to come on stream as part of an integrated twin pipeline system on schedule in the last quarter of 2012.
When fully operational, the pipelines will have the capacity to transport 55 billion cubic metres (bcm) of gas a year from some of the world’s largest gas reserves in Russia to the European Union.