WHile Hibernia is not the biggest offshore oil platform ever built, it is the heaviest.

Once the concrete-and-steel base for Hibernia (the ''gravity base structure,'' or GBS) is finished in mid-1997, and oil drilling equipment and crew quarters are aboard, the structure will weigh about 600,000 tons. It will float in the ocean during transit because the volume of water displaced is greater than the weight of the GBS.

Ballast will be added to ensure stability of the structure as it is towed to the drill site almost 200 miles away on the eastern edge of the Grand Banks. At that point, another 500,000 tons of iron-ore pellets will be added to bring its weight to 1.2 million tons. The added weight will settle the structure onto the ocean bottom, where it will sit in 260 feet of water.

The jagged saw-toothed double wall of the base, called the ice wall, protects the inner oil storage cells. The platform is designed to withstand a hit from icebergs weighing up to 6 million tons. But icebergs in the Hibernia area are generally smaller, about 300,000 tons. The area's relatively shallow water generally keeps big icebergs from getting too close, although officials acknowledge that they they expect to have to tow large bergs away from the rig occasionally.

About 125,000 barrels of oil a day are expected from 83 wells when the rig reaches full production. The concrete base will store up to 1.3 million barrels of oil. Once a week, an ice-hardened double-hulled tanker will tie up at the offshore loading system and load up to 900,000 barrels of crude oil that will be shipped to United States refineries. About 615 million barrels are expected to be pumped over a 20-year period.

Although Canada is marginally a net exporter of oil, the country imports 567,000 barrels per day. Hibernia will represent 6 percent of Canada's total oil production and supply about 800 permanent jobs to Newfoundland, the company says.

Source: Hibernia Management and Development Company Ltd.

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