Pipeline & Gas Journal
January issue 2000:

Project Profile

Welding Proves Challenging
On Äsgard-Europipe 11 Pipeline

by Rodney Byles

Per Aarsleff, one of Denmark’s leading civil engineering contractors, has successfully used its extensive pipeline welding and placing expertise on its most challenging contract so far to weld and place exceptionally heavy sections of large diameter, extremely thick-walled cross-country gas pipeline in southwest Norway. The pipe, which is between two to six times thicker than a conventional gas pipeline, is so heavy and comparatively rigid that Per Aarsleff, using a combination of special beveling, induction heating and internal and external welding equipment, can only weld a few sections together into a short string at a time for the lifting and placing equipment to cope with the weight.

Several separate and adjacent short strings are prepared alongside the top of the trench ready for laying. A second Per Aarsleff team, following on behind with closely spaced pipelaying side booms and crawler cranes, lift and lower the short sections of pipeline in turn into the excavation. Each is butt-welded to the mating string and the welds covered with a heat-shrinking protective sleeve. The cross-country sequence is repeated to the edge of the fjords for the sea crossings. Here, the pipes are welded to the concrete-coated ballasted pipeline which has been pulled across and placed on the bed of the fjord. Per Aarsleff has played the key role in welding, pulling and laying the pipeline across four fjords up to 2.3-km wide and depths of 84 meters.

The approximate 30-km cross country pipeline forms part of the Norwegian state-owned oil and gas producer Statoil’s massive NKr7,200M Äsgard Transport and NKr6,700M Europipe 11 combined gas trunkline project. The vast scheme, one of the offshore world’s largest and most technically challenging projects, connects the Äsgard gas field in the northern North Sea, about 300 km northwest of the Norwegian westcoast port of Trondheim, with the German North Sea coast at Dornum.

Sour gas will be transported from Äsgard through a 730-km long subsea pipeline to the Norwegian landfall at Karsto on the island of Karmy, just south of Haugesund. From Karsto the pipeline goes overland, including crossing three fjords, to Statoil’s existing Karsto refinery, which is being expanded to cope with the additional supply. At Karsto, the refined gas is transferred into the 650-km Europipe 11 trunkline. This initially crosses two fjords and two islands en route to the Norwegian southwest coast at Trosnavgneset, for the remaining subsea trunklink to a receiving station at Dornum and onward distribution throughout Germany, Austria and The Netherlands.
Per Aarsleff is part of the joint venture Kaarstoe Pipeline Contractors, which includes Selmer of Norway and the German firms Bolen & Deyen and Ludvik Freytag, working for client Statoil on the approximate NKr900M cross-country pipeline contract. Before Per Aarsleff could start on the cross-country welding, the site team spent several weeks testing and evaluating different welding materials to ensure that the pipes, which were made by different manufacturers, were compatible and could be welded together and comply with Statoil’s specification to cope with the corrosive untreated gas.

Each individual pipe up to 14.25 meters long and weighing approximately one ton per meter, is delivered from the site’s stockyard to its exact position above the previously excavated trench. The one-meter internal diameter pipe sections, which have a wall thickness ranging from 39-mm to 55-mm depending on their location, are prepared for welding by initially chamfering each end using a special mobile lathe. The self-contained, hydraulically operated machine tool clamps itself onto the ends of the pipes to cut the desired bevel to accept the butt weld.

After machining, induction heating coils are placed on the ends of two mating pipe sections and heated to 120 degrees C prior to butting together to accept an initial internal tacking and sealing weld. Per Aarsleff uses a special compressed air controlled, remotely operated robot to perform this semi-automatic operation. The internal Mig welding machine is first placed in one end of a pipe section and driven along inside to the other end and rigidly clamped at the joint position, using a set of integral expanding clamping pads forced out against the pipe wall. The adjacent section of pipe is lifted by one of Per Aarsleff’s pipelayers and fed over the welding machine’s nose protruding from the end of the pipe. A second set of pads in the welder’s nose clamp hold the two pipes together in preparation for welding. The welding machine, complete with six welding heads, quickly completes the internal circumferential sealing weld.

Both sets of clamps are released and the internal welding machine driven along the pipe to the next joint to repeat the sequence. At the same time, a protective cabin is placed over the outside of the first joint. Working inside the cabin, a pair of welders clamp a special semi-automatic twin-headed pulse Mig welding machine around the circumference of the pipe and then fill the bottom of the V-shaped void with an initial three-pass weld. This provides a sturdy joint for the pipes to be supported on blocks and pipelayers released for supporting the next adjacent pipe.
This first cabin and team of two welders is moved along the pipe to the next joint which has been previously sealed with the internal welding machine. A second cabin is placed over the first joint and a second team of welders, using a similar semi-automatic pulse Mig welding clamped to the pipe, complete a further three-pass weld. This sequence is repeated at the next joint and a third cabin placed over the first joint, which is then completed with about seven to eight passes of flux core weld. The completed butt weld is x-rayed and approved prior to placing a heat shrinking protective sleeve over the finished joint.

The entire cycle of front-end welding operations is repeated to weld six pipe sections together into a single string. Per Aarsleff assembles an average three strings per day ready for placing into the adjacent pre-dug trench. Following closely behind are Caterpillar, Komatsu and Terex pipelayers, together with various crawler cranes, lifting and lowering each string in turn into the trench. Each string is temporarily supported by the pipelayers, while the two adjacent lengths of pipeline are welded together prior to final lowering and backfilling. The entire sequence is repeated along the pipeline with Per Aarsleff completing an average 200 meters per day. P&GJ

Rodney Byles is a veteran technical writer based in the United Kingdom. A second story concerning the unique underwater pipe pulling aspects of this project will be published in an upcoming issue of P&GJ.