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Although a spar platform is used for offloading in the Brent field, this will be the first deepwater spar production platform in the Norwegian Sea.
Innovation is key to deepwater development. One of the new technologies being touted for operations in the North Atlantic Ocean is a variation of the spar platform, called the “belly spar.”
Aker Solutions presented a paper at the Offshore Technology Conference 2012 in Houston, April 30-May 3, that describes the company's design for moving spar production platform technology out of the Gulf of Mexico (GoM) and into the North Atlantic. A variation of the design using a concrete spar concept also has been developed for Arctic applications.
Statoil is sold on the idea, having awarded Aker a FEED contract for the development of the Aasta Hansteen (formerly Luva) field in the Norwegian Sea. The contract is for the design of the world's largest deepwater spar platform – a hull of 193 m (633 ft) and a draft of 170 m (558 ft).
"The Aasta Hansteen spar will be the first production platform on the Norwegian continental shelf with steel catenary risers. With a water depth of 1,300 m (4,265 ft), this is probably the only riser technology that can meet the challenges of the field," said Henning Ostvig, head of front-end and technology, Aker Solutions.
But not just any spar platform will do. For the most part, spar platforms were limited to the GoM, which has certain metocean conditions such as mostly mild weather with occasional loop currents and hurricanes.
The Norwegian Sea is quite different. The metocean conditions include long-period swell and create fatigue induced by normal operational seas. Add to that the water depth. The platform will be designed for 1,300 m and will include condensate storage in the spar.
The spar consists of a cylindrical hull with three sections – an upper spar hard tank, midsection, and soft tank. A flooded center well houses the risers.
The hard tank has a wide lower section (belly) and a narrow upper section (neck), which results in low heave excitation forces and a high natural heave period.
For oil or condensate storage, tanks can be arranged in the lower part of the hard tank with a favorable vertical center of gravity. The belly starts below the wave surface and extends down to the hard-tank depth. The upper portion of the hard tank consists of void spaces for the required buoyancy for the spar. Model wave tests have shown that the concept is robust.
The analyses indicated that the belly spar performs well in the long-period extreme storms without any nonlinear or chaotic responses. In the return-period sea state in a 10,000-year event, the most likely maximum heave is 5 m to 6 m (16.5 ft to 19.8 ft), and maximum pitch is less than 10°.
The FEED is due in October this year. It will be interesting to see where this latest deepwater technology will take the industry.
Editor's Note: Information from "The Belly Spar - Design and Verification of an Ultradeepwater Solution" (OTC-23461) by Rolf Loken, Lars Laukeland, and Henrik Hannus, Aker Solutions, was used in this article.