Visund, North Sea Northern, Norway

Visund was developed using a semisubmersible steel drilling, production and quarters platform, connected to a subsea completion system. The wellheads for 21 wells are installed under the platform, with a separate template for two other wells, located 8km away, to the north of the platform. Plans call for gas production to begin around the year 2006.

LOCATION

Located roughly 140km west of the Sogne Fjord, Visund comprises four reservoirs. It lies in licence 120, blocks 34/7 and 34/8. The license was awarded in late 1985 in the tenth round. Visund lies 27km away from the Gullfaks field, in a water depth of 335m.

PRODUCTION

For the overall field development, a total of 21 wells are planned in the vicinity of the platform. These can be broken down into 12 oil producers, two water injectors, six gas injectors and one gas producer. There will also be four spare slots. Two further oil producers will be drilled 8km to the north-east. Overall reserves in the field are estimated at 304 million barrels of oil and 51 billion m³ of gas.

RESERVOIR

Visund is developed from two reservoir groups called Statfjord/Amundsen and Brent.

STATFJORD/AMUNDSEN

The depth to the Statfjord/Amundsen reservoir is 2,900m and that to the oil/water contact is 3,095m. The depth to the gas/oil contact is 2,811m. The reservoir has recoverable reserves of 13.5 million m³ of oil and 5.5 G m³ gas. The oil column is 150m high and the gas column 90m high. The reservoir has a 266/512m³ gas/oil ratio. The reservoir pressure is 450bar and the temperature is 115°C.

BRENT

The water depth to the Brent reservoir is 2,900m and the depth to the oil/water contact is 2,820m. The depth to gas/oil contact is 2,904m. The field has recoverable reserves of 35 million m³ oil and 51 G m³ of gas. The oil column is 40-100m and the gas column is 110m. It has a 180-230m³/m³ gas/oil ratio. The reservoir pressure is 425bar and the temperature is 115°C.

DEVELOPMENT STRATEGY

Due to the overall economics and the operational flexibility needed to handle the complex reservoir conditions, as well as to operate in water 335m deep, Norsk Hydro selected a semisubmersible production/drilling/quarters (PDQ) facility for the Visund development. The platform is located directly above the subsea completed wells. It is 121m long and 95m wide. The height to the upper deck is 27m and that to the lower deck is 20m. It has a 21m draught and a displacement of 52,600t. The platform has a topside weight of approximately 25,000t, including process facilities for both oil and gas.

The Visund semisubmersible has enough space for 28 risers on the production side, as well as the capacity for ten non-hazardous risers in the living quarters area. The processing facilities have a design capacity of 16,000m³/d of oil and 28,000m³/d of liquid. It has a water-injection capacity of 18,000m³/day. Gas production is rated at 10 million m³/d and gas injection is 10 million m³/d. The total gas-production capacity is 18 million m³/d.

LIVING QUARTERS

Visund has a maximum capacity for 120 people in its living quarters, although it will normally house just 28 people during normal operations. The living quarters are a modular aluminium structure, located above the utility module.

The construction of the Visund platform was carried out under an Engineering Procurement and Construction (EPC) contract with Umoe. An Umoe/GVA 8000 semisumbersible design was selected in March 1996, based on functional specifications and process data.

The structure was fabricated at three locations. The substructure was fabricated by Kvaerner Rosenberg in Stavanger. The living quarters were built by Leirvik Sveis at the Stord Island yard and the remaining two deck sections were fabricated at the Haugesund yard.