ZUG, Switzerland – Work has started on underwater tie ins of the second Nord Stream gas pipeline in the Baltic Sea.
During the two-week operation, two of the 1,224-km (760-mi) pipeline’s three sections offshore Finland will be joined inside a hyperbaric welding station.
As with the parallel Line 1, the three sections feature reduced pipe-wall thicknesses as the design pressure of the gas drops from 220 to 177.5 bar (3,191 to 2,574 psi) on its journey from Portovaya Bay, northern Russia to Lubmin on the German Baltic Sea coast. This design means there is no need for an interim compressor station, reducing the amount of steel required and allowing faster pipelay.
The hyperbaric tie-ins are being performed at two offshore locations where the design pressure changes from 220 to 200 bar (3,191 to 2,901 psi) and from 200 to 177.5 bar (2,901to 2,574 psi), respectively.
Connection of the central and southwestern sections will take place in June off the Swedish island of Gotland in a water depth of around 110 m (361 ft).
Welding operations will be set up by divers and remotely controlled from Technip’s DSV Skandi Arctic, using equipment from the pipeline repair system administered by Statoil on behalf of a pool of pipeline operators.
Three pipe-handling frames will be lowered from the vessel and positioned over the pipeline ends on the seabed. The frames will move the ends of the overlapping parallel pipeline segments to aline them for welding after they are cut to the correct length. Pipe ends will then be beveled and the pipes lifted and moved into place.
Welding should take a day. The weld will be inspected with ultrasound and, assuming an acceptable outcome, the welding equipment will be retrieved to the vessel while the pipe-handling frames lower the pipeline back on to the seabed.
All water will be removed from the completed pipeline during the summer followed by drying of the evacuated pipeline.
Nord Stream 2’s onshore and offshore sections will be connected early in the fall, and after testing, the line is scheduled to come onstream before end-2012.