Underwater Welding of Line 1 Completed
21. Juni 2011
The final tie-ins for Line 1 of the Nord Stream Pipeline system have been completed. The first two sections were welded together on May 30. The pipeline is built in three sections, which must be welded together to form the 1,224 kilometre pipeline. This “tie-in” process took place on the Baltic seabed.
The welding took place in an underwater welding habitat. After their completion, the sections were pressure tested, then welded together subsea, which is what is referred to as "hyperbaric tie-in". The first tie-in for Line 1 was carried in May in Finnish waters at a depth of about 80 metres. The final tie-in was completed on June 21.
The Skandi Arctic Controls All Welding Activities
"All underwater welding activities are controlled from the Skandi Arctic, a diving support and offshore subsea construction vessel," says Nicolas Rivet, Nord Stream's Project Coordinator for Hyperbaric Tie-ins. The vessel transports and operates all of the equipment necessary to move, lift, cut and weld the pipeline sections together. The sections are connected in an underwater welding habitat in several automated steps assisted by divers.
"The Skandi Arctic has a 24-man dive system on board. The pressurised system is a living space for the divers who will work on the welding. They live here for the duration of the tie-in process," explains Rivet. A three-man diving bell brings the divers from the diving system to their subsea work area. Divers work an eight-hour shift.
How It's Done
The pipeline segments lay parallel to each other and overlap. Therefore, the ends of each segment are cut before they can be aligned. A welding plug is then inserted into each end of the pipeline segments. The plugs are inflated for a perfect seal, and they separate the water in the pipeline segments from the dry area of the habitat. With the use of a bevelling machine, the pipeline ends are given a smooth finish to prepare them for welding.
Three pipe handling frames (PHFs) lift the pipeline ends and align them before the welding can start. The pipeline segments are welded together inside the welding habitat, which is a dry zone where divers work without diving equipment, setting up the automatic welding machine. The welding is completely controlled from the Skandi Arctic. The weld is inspected and once it is approved, the subsea equipment is retrieved onboard the dive support vessel. The PHFs lower the pipeline back onto the seabed, and a remotely operated vehicle surveys the area.
The delivery of gas through Line 1 will begin at the end of the year. By the last quarter of 2012, both lines will be operational and will deliver 55 billion cubic metres of gas a year.
