Giant riser base is prepared for Åsgard gas startup

A key link in the gas export system for Statoil's Åsgard Field off mid-Norway is the export riser base (ERB) which was engineered and built by Kværner Oilfield Products.

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A key link in the gas export system for Statoil's Åsgard Field off mid-Norway is the export riser base (ERB) which was engineered and built by Kværner Oilfield Products. The ERB has been installed in a water depth of 350 meters and is located within the 500 meter exclusion zone of the Åsgard B platform, although not directly below it. Like the platform, it has been designed for a 50-year life.

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Docked on a porch, the Big Brother RTS tool is being prepared for the pull-in of a 14-in riser.
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The Åsgard project involves the production and export of large reserves of oil, gas, and condensate. Gas production will take place on a semisubmersible platform, Åsgard B, the first time a floater has been used primarily for this purpose. The platform, which is due to be installed in spring, is also the biggest such production platform yet built, and has the capacity to export 38 MMcm/d of gas.

Gas will be sent from the platform to the ERB through three 14-in flexible risers. On the ERB, the three streams are manifolded into one. The size set for the export pipeline, which is known as the Åsgard Transport System and runs 720 km to Statoil's Kårstø terminal, is 42 in. This gives it a much greater capacity than Åsgard itself requires, as it will also carry gas from other fields in the area. For inspection and cleaning purposes, pipeline pigs will be launched from the riser base itself.

However, an ERB large enough to accommodate a 42-in. pipeline and pig launcher would be a truly massive structure, and instead Statoil opted for the novel solution of making the initial section of the pipeline 28 in. in diameter, and using a pig which can adjust its diameter when the initial section broadens out into 42 in. - the so-called multi-diameter pigging tool (MDPT).

ERB structure

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Crane-barge Thialf prepares to lower the Åsgard export riser base to the seabed.
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The ERB consists of a 28-in header, two riser manifold modules, each with two 14-in hubs for connecting the export risers, a valve module, and the 12-meter-long pig launcher. Each of these modules can be installed or removed independently. The only planned retrieval is for the pig launcher, which will be pulled to the platform to be loaded with pigs. Pigging will normally be carried out once a year.

In March, the risers and connectors were due to be installed and hooked up. A new connection tool has been developed for this job. Known as Big Brother Remote Tie-in System (RTS), it measures 5 meters by 3 meters by 4.5 meters and weighs nine tons. Big Brother is lowered to the seabed on guidelines, and docks onto the porch where the pull-in is to be performed. It is operated by ROV, through which it also receives its power.

The tool has high capacity in terms of pull-in (40 tons) and stroking (60 tons). It already has performed the pull-in of the 28-in spool which forms the first section of the ATS. It was also used for the pull-in of 15-in and 20-in risers on Norsk Hydro's Troll C project, where it performed very well, Feginn says.

Another feature which KOP has developed specifically for the Åsgard ERB is the flapper valve connectors which sit between the two riser manifolds and the 14-in risers export spool and act as the main emergency shutdown valve. In the event of riser rupture, or other occurrence which might cause the pressure to drop or the flow to reverse, it automatically drops down and closes the pipeline, thus preventing the danger of kickback.

Gas from several other fields also will be routed through the ATS, entering it through pre-installed tee branches also based on the use of KOP's clamp connector and tee technology. Tee modules with welded-in ball valves are due to be installed to the branches this summer. Gas from Heidrun and Norne will link into the ATS a short way downstream of the ERB, where a tee for a future link from the Kristin Field has also been installed.

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