Åsgard re-design cost additions offset by gas and oil production rewards

Separation, injection, export, condensate handling

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Th 6008osasg
Åsgard B is the largest semisubmersible production platform in the world, and the first dedicated primarily to gas handling.
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In October, gas exports are due to start from Statoil's Åsgard Field when Åsgard B, the largest semisubmersible production platform in the world and the first dedicated primarily to gas handling, comes into operation. On average, 10 bcm/year of gas will flow from Åsgard in the Norwegian Sea to the continent.

The start of gas exports means that Åsgard will finally be in full production. Oil production from two of the fields which constitute Åsgard - Smørbukk and Smørbukk South - began in May 1999 through the Åsgard A production ship. The third field, Midgard, will come on stream with Åsgard B.

The Åsgard development has had more than its share of attention, due mainly to the apparently inexorable rise in costs. It was one of the projects executed in accordance with Norsok thinking, a joint industry/government initiative which sought to drastically reduce costs and schedule in one stroke. Like many other projects in the mid-to-late 1990s, and not just in Norway, Åsgard B tried to move into fabrication before engineering was mature, and even before the operative parameters had been properly pinned down.

The platform has therefore cost more than originally intended. The latest estimate of the cost, last February, was NKr 12.2 billion, up from NKr 8.4 billion at the time the development was approved in June 1996. Time will show, however, whether this represents value for money. According to sources close to the project, Åsgard B will be a "money machine" for Statoil and Norway. In this respect, the Åsgard A production ship, which cost 50% more than originally planned, earned gross revenues in its first year of operation, which more than exceeded its cost.

An engineering-procurement-commissioning (EPC) contract for the gas platform was placed with Kværner in February 1997. The company proposed building the Kværner GVA 70 design, which it had developed jointly with GVA Consultants.

Immature studies

There was a lot of reconsidering of the solution required as studies were not mature enough when the project was launched. As a consequence, there were many changes, which pushed up the cost. One example was that a permanent pigging facility was needed, which entailed a new module weighing nearly 1,000 tons.

A second was the decision to double the capacity for MEG regeneration after the contract for the structure had been awarded. This had a knock-on effect on the number of pipe racks, which led to a 5,500-ton increase in the weight of steel structures.

Altogether, the weight of the topsides has gone up by 6,500 tons since contracts were originally placed. There was enough spare buoyancy to absorb an extra 2,000 tons, but capacity had to be increased to take the additional weight, and this meant adding an extra 1,000 tons to the hull weight.

The effect of all these changes could have been very disruptive, but were taken by the work force in stride. The changes have not only improved the platform, but have not occasioned any dispute between Statoil and Kværner. The cost of Kværner's contract has risen from an original NKr 6.6 billion to NKr 10.7 billion, but Statoil has accepted all the increases as justified.

Gas production on Åsgard B is expected to continue for about 30 years, though the platform's fatigue design life is 50 years. Its hull displacement of 84,400 tons makes it almost 50% bigger than other large semisubmersible platforms in the North Sea. The dry weight of the hull is 19,151 tons, while the topsides weigh 33,617 tons. The topsides, which are 115 meters long and 96 meters wide, consist of a 22,060-ton deck box and 14 systems modules, weighing a combined 11,557 tons. Gas, oil and condensate are received and dispatched through 24 flexible risers.

The platform's process system is the most complicated in the North Sea, with gas/liquids separation, gas injection, gas export, condensate/oil separation, condensate stabilization and export, and partial oil stabilization and export. The reservoirs of the three Åsgard fields are very different, and the receiving conditions on the platform vary from sweet gas from Midgard with an operating temperature in the inlet separator of -7 to -25° C, to sour gas from Smørbukk with high hydrogen sulfide and carbon dioxide contents and an arriving temperature of 100° C.

Gas processing on a floater is difficult, and with current technology it is not possible to produce dry gas. The platform will therefore export wet gas at rates of up to 37.5 MMcm/d, and inject gas at up to 10.5 MMcm/d to assist with oil recovery from Smørbukk and Smørbukk South. There are two large compressor trains for export and one for injection. Oil will be transported by pipeline to the Åsgard A ship for final processing, storage and export, and condensate will be piped to the Åsgard C floating storage unit.

The platform was moored in 300 meters water depth to 16 suction anchors, each weighing 63 tons in early summer. The first well, on the Midgard Field, was due to be brought on stream in July, and handover was scheduled for August. The platform is conspicuous for the 111 meters high flare tower which towers over it. Normally, however, only a small pilot flare will be burning, as the platform is equipped with a closed flare system.

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