Draupner-Dunkerque gas pipeline accelerates growth of Norwegian grid

Anchor-handling vessels reset the anchors as LB 200 pulls its way across the North Sea. [18,101 bytes] Another 25-meter section of the NorFra pipeline on its way to the bottom of the North Sea via LB 200's stern ramp. [23,577 bytes] Two vesssels have been clawing their way across the North Sea this summer installing the world's longest subsea pipeline. They are McDermott-ETPM's pipelay vessels LB200 and DLB 1601, which were due to finish installing the 826-km NorFra pipeline in

Two vesssels have been clawing their way across the North Sea this summer installing the world's longest subsea pipeline. They are McDermott-ETPM's pipelay vessels LB200 and DLB 1601, which were due to finish installing the 826-km NorFra pipeline in August.

Laying NorFra is only part of McDermott-ETPM's contribution to Norway's expanding subsea gas grid. As part of the Statoil 2000 program, early in the season LB200 finished laying the 294-km, 40-in Zeepipe IIB line, which runs from the Kollnes shore terminal to the Draupner riser complex in block 16/11. Next year, the barge is to lay the 697-km, 42-in export line from the Aasgard field in the Norwegian Sea to the Kaarstoe terminal.

NorFra, Norway's fourth gas pipeline to the Continent, runs from the Draupner riser complex to Dunkirk, giving it a record length for a subsea pipeline of 826 km. With a 42-in diameter, it will have an initial transport capacity of 11.4 bcm a year. The decision to build it was taken in 1995 after Gaz de France had contracted to buy 40 bcm of Norwegian gas in the period to 2027.

On its way from Norway to France the line runs through Danish, German, Dutch and Belgian waters, crossing no fewer than five other pipelines and eight cables.

For most of its length NorFra runs alongside the Zeepipe line, at a distance of just 50 meters. Rigorous safety measures are implemented to ensure that the pipelaying operation presents no risk to Zeepipe, which is filled with gas at 180 bar.

The bulk of the work - some 600 km - is being done by LB200, a semisubmersible vessel, which is also capable of laying the line faster. In early June, the barge laid 5,348 meters in a 24-hour period, the third time it had broken its own record on this assignment.

DLB 1601, which doubles as a lift vessel as well as a pipelayer, will lay some 280 km. In June, the vessel was averaging around 2,800 meters a day. Her more leisurely pace is mainly due to her monohull shape and floating stinger - LB200 has a fixed stern ramp - which makes her more sensitive to weather interruptions.

Both vessels are essentially mobile welding lines. The pipeline itself is made up of some 67,350 sections or joints, each measuring 12.25 meters. They are shipped to the two barges in a constant stream of supply vessels, each carrying between 99 and 144 joints, from pipecoater Bredero Price's base at Farsund in southern Norway.

Each joint is given a concrete coat 55-110 mm thick, depending on the water depth in which the joint is to be installed - the shallower the water depth, and hence the stronger the seabed currents, the heavier the coating required to give the pipeline stability.

With a wall thickness of 24.5 mm, each joint weighs 18-25 tons, depending on the amount of concrete coating. For the Aasgard job, pipe with a wall thickness of 30 mm will be used.

The same welding process is followed on both pipelay barges. First, at one side of the vessel, two sections are welded together to make a double-joint. The double-joints are then transferred to the main welding line which runs down the centre of the vessel. Here they are welded together into a continuous line at a series of welding stations. Both barges carry welding equipment - Saturne 8 Torches and Saturne AX Bugs - made by Serimer DASA, an ETPM subsidiary.

Even in this high-tech age, the crucial job of aligning two sections or double-joints prior to welding them together is performed visually. All joints are checked by x-ray, both on the double-jointing line and the main line. If a defect is found at the double-jointing stage, the two sections can be separated and re-welded. But once a double-joint becomes part of the main line, any defect has to be repaired in situ.

The joints are protected by bitumen mastic, which is applied with the aid of a mould. On every sixth double-joint there is a sacrificial anode, installed by the pipe-coater, to provide corrosion protection.

Four tensioners placed towards the end of the main production line support the weight of the pipe between the vessel and the seabed. As new sections are added to the pipeline, the barge pulls itself forward with the assistance of its anchors. Every few joints, the anchors must be reset, a job which is carried out by anchor-handling vessels.

All operations - the loading of joints from the pipe-carriers, activites on the welding line and moving the vessel itself and the anchors - are controlled from the bridge or the control room behind it. Computerized systems provide real-time monitoring of key parameters such as the tension in the pipe hanging from the stern.

For the Aasgard job, which involves strong currents and water depths at places in excess of 350 meters, a fifth tensioner will be installed on LB200. As the wall thickness of the pipe will be in excess of 30 mm, use will be made of an additional welding station equipped with Serimer's B'Cat, an automatic welding machine which works from inside the pipe.

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