OREANDA-NEWS. June 1, 2012. Nord Stream started today the first of two all-important underwater tie-ins of its second gas pipeline through the Baltic Sea. In this complex two-week-long offshore operation, two of the pipeline's three sections will be joined together 80 metres underwater inside a hyperbaric welding habitat on the seabed off the coast of Finland.

Nord Stream has designed and constructed its twin 1,224-kilometre pipelines in three sections with reducing pipe-wall thicknesses as the design pressure of the gas drops from 220 to 177.5 bar on its journey from Portovaya Bay in Russia to Lubmin on Germany's Baltic Sea coast. This design enabled Nord Stream to dispense with the need for any interim compressor station with environmental benefits as onshore lines require the gas to be recompressed every 100 to 200 kilometres. It also reduced the amount of steel required, saving costs significantly and allowing faster pipe-lay.

The underwater connection of the three sections by hyperbaric tie-in takes place at the two offshore locations where the design pressure changes from 220 to 200 bar and from 200 to 177.5 bar respectively. The connection of the Central and South Western sections will take place in June off the Swedish island of Gotland at a depth of approximately 110 metres

"The underwater welding operations will be set up by technical divers and remotely controlled from the world's largest dive support vessel, Technip's 160 metre long Skandi Arctic, using equipment from PRS, the Pipeline Repair System pool of pipeline operators administered by Norway's Statoil", says Nicolas Rivet, Project Coordinator for the hyperbaric tie-in operations at Nord Stream.

The Skandi Arctic has a 24-man dive system on board. It has pressurized living spaces for the teams of highly-specialised divers working on the welding operation. The process uses three massive pipe handling frames, which are lowered from the Skandi Arctic and positioned over the pipeline ends on the seabed. The frames move the ends of the overlapping parallel pipeline segments to line them up for welding after cutting them to the right length. The pipe ends are then bevelled and the pipes lifted and moved into place.

The actual welding takes about 24 hours. It is remotely controlled from the Skandi Arctic, and closely monitored by the technical divers. The weld is then inspected with ultrasound and after the weld is accepted, the welding habitat is lifted back on to the vessel and the pipe handling frames lower the pipeline back on to the seabed.

After the two hyperbaric tie-ins, all the water will be removed from the completed pipeline over the summer and the empty pipeline will then be dried. The onshore and offshore sections of the pipeline will be connected in early autumn, and after thorough testing this second line is expected to come on stream on schedule in the last quarter of 2012.

When fully operational, the integrated twin pipeline system will have the capacity to transport 55 billion cubic metres (bcm) of gas a year from Russia's massive gas fields to the European Union.