Versatile Åsgard inspection pig permits downsizing of riser base

• The meshed monitoring unit of the Åsgard B multidimensional pig, seen from the rear. [30,358 bytes]
• The Åsgard B multidimensional pipeline inspection tool, showing the driving unit at the front and the monitoring unit with magnetic bars at the rear. [23,125 bytes]

Pipetronix, which will build the pig at its Canadian subsidiary, has worked closely with Statoil on pigging technology since 1993. Also, a contract has also been awarded to Kværner for the design and construction of the export riser base, including the subsea pig launcher.

Multidimensional pigs already exist for cleaning purposes, but Egil Ekrene, the Åsgard Transport project director, believes this is the first time such a pig has been used for inspection, especially in a major pipeline system.

The Åsgard gas export line will run 719 km from the base of the Åsgard B gas production platform off mid Norway to Statoil's Kaarstoe gas terminal. The line will be installed by McDermott-ETPM's lay-barge LB200, which will lay the southern section this year and the remainder in 1999. The line is due in operation in October 2000.

Plans are already being laid to export gas from other mid Norway fields, including Heidrun, Norne, Draugen and Haltenbank South, through the Åsgard pipeline system.

Inspection will be an important function for the pipeline system, not least as it will be expected to remain in operation over several decades. Gas from the Åsgard fields is slightly sour and has a high carbon dioxide content, but the pipeline is made of modified carbon steel - the use of exotic materials with high corrosion resistance would be uneconomic for a line several hundred kilometres long.

The use of a multidimensional pig allows a much smaller export riser base to be used. Sized for a 42-in. pig launcher and associated equipment, the base would have been an immense structure weighing 1,000 tons or more. Now the first km of the pipeline will have a diameter of only 28 in., allowing the size - and cost - of the base to be substan-tially reduced.

"The dimensions have not been finally decided, but we expect it to be about half the size," says Per Lothe, Project Manager for the development of both the multidimensional pig and the export riser base.

The pig will be launched into the 28-in. section of pipeline. As it enters into the wider section of pipeline, each part of the pig will expand. This process will take place automatically as the various parts of the pig will be spring-loaded. This will make it possible to use the same inspection tools in the parts of the line with different diameters.

The decision to go ahead with the full development of a multidimensional pig was taken after the completion of extensive testing of the mechanical process on a full-scale loop at the Kaarstoe terminal.

This showed that the magnets used as sensory devices would expand and mesh together when moving into a wider diameter pipe section in such a way as to provide uniform coverage of the pipe-wall and the required degree of accuracy. Statoil applies some of the strictest standards for inspection accuracy, including the ability to detect defects equivalent to 5% of the wall thickness.

The electronics for the system were not involved in these tests. The next step is to integrate them with the mechanical process, Lothe says. There is also plenty of scope for optimizing the system, especially on the launching side.

Initially, the pig will be launched subsea. Eventually, it is hoped to launch it through a service riser from the platform to the export riser base. The initial section of the pipeline was sized at 28 in. because this is the largest diameter which could be used for the rigid riser through which future platform launching would take place.

The development, which got under way little more than a year ago, has come too late to enable great savings to be made. On the other hand, it has been taken on board without adding to the NKr 8 billion budget for the Åsgard gas export project, and it will undoubtedly bring benefits to future pipeline developments, according to Ekrene.

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