Oil down under

The structure looks like something out of Jules Verne - several cylindrical concrete chambers that would rest on the seabed. Inside would be working and living quarters, complete with a gymnasium, for up to 50 people. Thumping away outside would be an Erector Set of machinery, drawing up crude oil from beneath the ocean floor.

The structure: an underwater oil platform. Although now just a sketch on the pad of a consortium of British companies, it is one of many ideas being looked at by the world's oil industry to draw up more energy from beneath deep offshore waters.

As the hunt for energy supplies forces companies to explore new frontiers, deep offshore waters represent one of the more promising sources. The US government estimates that 10.4 billion barrels of oil, about 13 percent of total US onshore and offshore undiscovered recoverable crude, lies in deep waters. Some 53.6 trillion cubic feet of natural gas is also believed trapped there.

But new technologies and engineering feats will be required to extract them. Slack demand for oil over the past two years has slowed the pace of offshore exploration worldwide. But development of a new generation of deep-water equipment continues, although not as briskly as during the era of spurting oil prices.

Most production platforms today are ''fixed'': rigid, one-piece systems that stand on steel or concrete pillars fastened by piles to the ocean bed. Engineers believe these Tinkertoy-like structures will work fine up to depths of about 1, 600 feet. (Exploratory wells have been sunk below 4,000 feet, but the deepest production platform is Shell Oil Company's 1,025-foot pumper in the Gulf of Mexico.)

But beyond this, entirely new design concepts will be required. The reason is the cost and complexity of building, transporting, setting up, and maintaining structures much taller than the World Trade Center in such an unruly environment. An answer: Platforms without legs.

One way to do this is to anchor a floating platform to the seabed with steel tethers. This is the rationale behind a new type of structure being closely (if somewhat skeptically) watched by the oil industry. Under construction by Conoco Inc. for use in the storm-lashed North Sea, the $1.5 billion system, called a tension leg platform (TLP), will be towed out to sea sometime this fall. It will be tested in 485 feet of water, but Conoco engineers believe it could be adapted to work at depths greater than 6,000 feet.

The platform will be moored by 16 tubular lines to piles driven into the ocean floor. The natural buoyancy of the structure will create an upward force that keeps the lines taut. This, in theory, results in a platform that is vertically stable but can rock horizontally, making it more secure in heavy seas.

''It's like trying to pull a buoy under water by pulling on the mooring lines ,'' says N.D. Birrell, Conoco's chief marine engineer. ''It can only move in one direction. It won't roll or pitch.''

A variation on the ''compliant'' approach - platforms that work with rather than fight the sea - is Exxon's ''guyed tower.'' It was nudged into 1,000 feet of water in the Gulf of Mexico last year. It, too, rolls slightly with the waves during severe storms. Instead of being anchored by vertical cables, though, it is steadied by a web of 20 guy wires. These extend outward, like the cables that support the mast of a sailboat, from the platform to weights on the sea floor.

Both of these are big platforms (58 wells can be bored from the guyed tower), which makes them economical only for large reservoirs of crude. But what about those pint-size pools of 10 million barrels or so? Enter British Petroleum (BP) with its ''single-well oil production system,'' which goes by the acronym SWOPS.

The centerpiece of the setup is an unusual ship that acts as part tanker, part pipeline. First a subsea well is drilled. Then the ship, computer guided and fitted with oil processing and storage equipment, hovers over the wellhead. A ''riser,'' or pipeline, is lowered and snapped onto the well, through which the crude is drawn up.

BP, now soliciting bids for the tanker, expects to put the system to work on a few fields in the North Sea within three years. Similar apparatus could be built for other small offshore sites. But they would have to prove competitive with techniques used on more conventional fields, onshore and offshore, first.

''Technically, we should be capable of going down to 10,000 feet within the next few years,'' says Wayne Ingram, manager of Seafloor Engineers, a Houston-based geotechni-cal consulting firm.

The most revolutionary, if far-off, option open to oil companies is to place production platforms and crews on the seabed itself. Among those who have carried the concept the furthest is McAlpine Offshore, a subsidiary of the big British construction firm McAlpine. It has linked up with three other British companies to devise a submarine oil production plant hous-ed in concrete chambers.

Its most ambitious plan calls for an underwater complex housing 30 to 50 workers at depths down to 3,500 feet. The system would pump some 100,000 barrels of oil a day. Crude would be drawn up and funneled to tankers on the surface through a riser. Crews would be shuttled to the complex in submersibles. Technical snags remain to be ironed out, and McAlpine is now just trying to interest oil companies - appropriately reticent in these days of slack energy demand - in testing a small, unmanned system in shallow waters.

There is probably enough cheaper energy around to keep this idea on the drafting table for some years. But it is a reminder of the extent man may have to go someday to help fuel the world.

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