Two-part Kvitebjørn jacket will break North Sea water depth record

The Kvitebjørn jacket, to be built in two parts, will stand in 190 meters water depth, the greatest depth for a conventional steel substructure in the North Sea.

One notable feature of the platform with which Statoil is planning to develop Kvitebjørn in the Norwegian North Sea is that it will have the tallest jacket to be found in North-West European waters. The field lies in a water depth of 190 meters, four meters more than the Magnus field in the UK sector, which has the tallest conventional jacket currently installed in the region.

Alternative solutions for the platform substructure were studied but found wanting for a variety of reasons, according to Kvitebjørn Project Manager Bjarne Bakken. A concrete base would have proved too expensive, especially as the infrastructure for building such structures has largely been demobilized. High pressure in the gas/condensate reservoir - 775 bar - would create problems for a floating installation. Dry wellheads were also preferred.

In the end, it was decided that a two-piece lift-installed jacket offered the best solution, despite the record water depth involved for such a structure in the North Sea. "It's fully feasible, and it will provide us with good behavior over the field's production lifetime," says Bakken. "As long as we design and fabricate it properly, it's a good solution and we believe we can get a reasonably priced structure."

One of the key challenges is how to install such a tall structure, and the installation contractor was involved at early stage of planning for this reason. After a competitive bid, the contract was awarded to Saipem last March. At the start, there were two installation options for a jacket in such a water depth: launching or lifting. A launch jacket would have to be the broad, heavy structure typical of the developments in the UK northern North Sea in the 1970s and 1980s. For Kvitebjørn, such a structure would have weighed in the region of 17,000 tons and the high volume of steel would have sent the cost soaring.

Preferable was a slim-line structure, with only four legs, that could be lifted into place, and this could be achieved by building it in two parts, says Kvitebjørn Jacket Manager Morten Bærheim. The base part, which will be 45 meters high, will be a square measuring 50 meters by 50 meters, withvertical legs. It will weigh 4,200 tons and will be transported upright.

The upper part will be 170 meters tall and weigh 7,200 tons. From its square 50 meters by 50 meters base, the structure will become narrower as it rises, until at its narrowest point 16 meters from the top it will measure 25 meters by 25 meters. The legs will then kink out, to give dimensions at the top of 30 meters by 22 meters. Grouted sleeve connections will be used for fixing the two parts together.

The deck will be 95 meters long and 50 meters wide, and in the longitudinal plane will project considerably beyond the legs. On installation, the topsides will weigh 13,000 tons. As it will be too heavy to be installed in a single lift, the 3,000-ton drilling module will be installed separately. The total operational weight of the topsides will be 23,000 tons.

Riser challenge

A number of challenges are posed by the choice of a two-part jacket, Bærheim says. One of these is riser installation. In a normal single-piece jacket, the risers are pre-installed. For the two-piece Kvitebjørn jacket, two options are under consideration. Once the structure is installed, the upper part of the riser could be lowered from the top and connected to a lower part pre-installed in the lower section of the jacket. There are a number of mechanical connectors on the market which are considered suitable for connecting the two parts, according to Bærheim. Alternatively, the entire riser could be lowered from the top.

The slenderness of the structure also gives rise to a number of dynamic considerations which will be carefully analyzed. The structure has a period of just five seconds of natural oscillation, and may be subject to second and third order harmonics. Its dynamic behavior in North Sea conditions could involve the sort of ringing effect which has been identified with some concrete substructures. If the analysis indicates that the jacket could be subjected to an unacceptably high fatigue loading, measures will have to be taken to strengthen it.

As it is, some of the nodes on the jacket may be cast rather than welded, which is somewhat more costly but gives better resistance to fatigue. As part of the verification process, a model test may be carried out of the joint between the two parts of the jacket.

Contract awards

Two contractors have been involved in the design process - Aker Maritime as design consultant for the early phases, and DNV for verification of specific aspects of the structure. In July an EPC contract for the jacket was awarded to Aker Verdal, which will build the structure. Engineering will be performed by Aker Engineering. Three yards, the other two foreign, took part in the tendering process, which included the performance of paid front-end engineering and design (FEED) studies, which confirmed the feasibility of the concept. In ballpark figures, the provision of the jacket is expected to cost around NKr 600 million.

With installation planned for autumn 2002, the fabricator will be relatively squeezed to perform the detailed design and fabrication of a structure of this complexity in little more than two years, Bakken says.

The jacket will have a 25-year design life. In the development plan, which the Storting was expected to approve by around mid-year, production of Kvitebjørn is forecast to continue until 2014. But there is upside potential in both the field itself and nearby discoveries which could be developed, so the platform is likely to remain in operation for several years after that. When operations finally come to an end, the entire platform will be removed