Built by Norwegian Contractors at Hinnavågen in Stavanger, the GBS comprises 19 cells arranged in a concentric formation. Most of these are used for oil storage. Three have been extended upwards as shafts to carry the topside. The platform, which measures 254 metres in height from the seabed to the top of the drilling derrick, was towed to the field in 1977 and came on stream in November 1979.
Oil was exported via a loading buoy located a few hundred metres away. The original articulated loading platform (ALP) was later replaced by a specially developed Ugland-Kongsberg offshore loading system (Ukols).
The rig comprises a derrick, modules for such functions as mud mixing, and pipe storage. Wells are drilled through two of the concrete shafts supporting the topside. A total of 42 slots are provided. Both production and gas/water injection wells have been drilled, with an average depth of 2 500-2 800 metres. The derrick stands on skids which allow it to be positioned over the various wells with the aid of hydraulic jacks.
The topside measures 86.2 metres long by 83.6 wide, providing a surface area of 5 000 square metres and total deck space of 21 000 square metres. Total height to the helideck is 43.2 metres, while the base is 28 metres above the sea at its lowest point.
The lowest of the steel decks, the cellar deck is built into the module support frame (MSF) which carries the rest of the superstructure. Divided into individual sections with double bulkheads, it forms an integrated steel structure. The flare boom extends for 116 metres at an angle of 45 degrees from its eastern end to burn off all associated gas for brief periods.
Located above the cellar deck, this has acquired its name because it is partly composed of prefabricated modules. The south-western corner, with the M11, M12, M14,M15, M17 and M18 units, forms part of the cellar deck’s integrated steel structure.
Upper module deck
With the exception of UM7, this deck is built up entirely of modules and lies immediately above the module deck. UM7, which is part of the drilling module, has been constructed as an integrated steel structure together with the cellar deck and the south-western part of the module deck.
The living quarters provide 100 cabins. These were originally two-berth but converted for single occupancy in 2000-2003. The six-storey structure comprises three modules. A partial seventh floor is provided by the helideck, radio shack and helicopter fuel tanks. The marine control centre was positioned there in the early years, but later moved to Bergen. In addition to the cabins come a canteen and recreation rooms. A lift and internal staircase connect the floors, while external staircases serve as emergency exits. The illustration shows the layout of a typical cabin in the living quarters.
The tops of the uppermost modules form the weather deck. This area is used for storage and for loading/discharging cargo by the large platform cranes.
The lower section of the GBS comprises 19 concrete cells. Sixteen of these are used for storage, while three extend upwards to form the shafts supporting the topside.
With an internal diameter of 19 metres and a height of 67 metres, the storage cells are individual concrete cylinders which each have a capacity of 80 000 barrels.
Fifteen of the cells are used to store crude oil, and can collectively hold about 1.2 million barrels. Cell 6 is equipped with an internal cylindrical tank to store up to 21 750 barrels of diesel oil. The annulus between the internal cylinder and the cell wall provides 63 500 barrels of storage space for the oil-water emulsion (sludge) which forms during platform operation. All 16 of the cells are partly filled with sand ballast covered by a concrete lid. They are kept filled with liquid at all times. When oil is exported, the cells take in water to maintain their liquid content.
One of the three shafts supporting the topside, this contains a series of decks carrying pumps for fire, ballast and sea water, ballast water tanks and pumps for crude oil loading.
These are completely filled with conductor tubing for a total of 16 wells in each shaft. The illustration shows the distribution between production and water/gas injection wells.