How deep-sea oil platforms swim

Crude oil from 3000 meters under the sea

The oil reserves below the mainland or near the coast are drying up. That is why companies have been trying for some time to open up deep-sea oil storage facilities. So-called offshore platforms are already producing oil and gas in water depths of over one kilometer. But the engineers are currently developing technologies with which it should be possible to penetrate deeper sea depths.

Ten years ago, all the drilling rigs were firmly anchored in the ocean floor. Ships dragged the steel frames out to sea, where they were set down on the seabed by floating slewing cranes. Greater water depths soon required ever higher headframes, the erection of which became increasingly difficult. When Shell opened up oil fields 300 meters below sea level in 1978, the steel framework was assembled on site for the first time.

At a depth of 570 meters, however, a limit was reached. Those winding towers - the tallest structures human beings - required extremely flexible steel structures in order not to be set into dangerous vibrations by currents and waves. Ropes, link chains and anchoring piles were therefore integrated into the steel framework. There are still plans to build 900 meter high scaffolding - that would be twice as high as the Petronas Twin Towers in Kuala Lumpur, the largest skyscrapers in the world. For depths over 570 meters, however, other constructions, the swimming platforms, have become established.

Before the first swimming platforms could go into operation, difficult questions had to be answered: How do you keep the islands in position? How do you connect the kilometer-long pipelines rising from the ocean floor with the platform? Which material can withstand the extreme environmental conditions? How do you anchor the island? How do you allow oil and gas to rise easily into an environment with a completely different temperature and much lower pressure?

The assembly work in the deep sea was also a major problem. People can dive a maximum of 300 meters deep. Special robots are now doing the work in depth. From ships, the sometimes truck-sized devices are steered from one job to another before they reappear.

So-called tension leg platforms (TLP) have existed since 1994. Its base, the size of a football field, is supported at the corners by four pillars, which in turn are supported by pontoons underwater. Twelve steel cables with a diameter of half a meter fix the island to the ground. The buoyancy of the pontoons tensions the ropes so that the platform hardly moves even when the waves are rough. The oil flows through a pipeline that runs vertically from the sea floor to the platform. Several TLPs are anchored in the Gulf of Mexico over water depths of up to 1140 meters. In 2004, the Conoco company plans to operate a TLP at a water depth of just under one and a half kilometers.

All elements of the offshore platforms are exposed to extreme conditions in these water depths. The temperatures are only just above freezing point, the water presses at 300 times atmospheric pressure and threatens to crush everything. Strong currents permanently stress the material, which can tire and eventually break. "It works like a paper clip that breaks when one of its ends is repeatedly bent back and forth," explains José Roesset, director of the Offshore Technology Research Center (OTRC) in Austin, Texas.

At the OTRC, Roesset and colleagues simulate the behavior of miniature platforms in wave pools in the laboratory. The tests suggest the use of composite materials made of a resin matrix with carbon and glass fibers for construction, which are hardly susceptible to corrosion and material fatigue. Due to the lower weight compared to steel structures, the platforms can be smaller and float despite the lower water displacement. "Every weight reduction of one kilo lowers the total costs by around ten euros," says Roesset. However, it is particularly difficult to predict long-term effects on the plants. For this reason, test procedures have been developed that constantly monitor the material on the drilling rigs in use. It is hoped that this will also enable damage to the interior of the structures to be discovered in good time.

Until recently, the water depth record of all offshore platforms was held by Exxon Mobil's floating drilling rig "Hoover Diana" in the Gulf of Mexico, at 1,481 meters. The system, a so-called Deep Draft Caisson Vessel (DDCV), which costs more than one billion euros, reaches the height of an 80-story high-rise, with most of it under water. In contrast to the TLP, a DDCV does not rest on four, but on one huge float. The large draft stabilizes the platform. The submarine "skyscraper" also functions as a tank and protective jacket for the ascending pipelines. Like a TLP, the "Hoover Diana" is fixed to the ground with steel cables. And just like on a TLP, the DDCV is used to drill, control and deliver oil with remote-controlled pumps. The "Hoover Diana" also exploits oil fields 50 kilometers away from the platform with pipelines running horizontally on the seabed.

The current depth record was set in the Atlantic off the west coast of Brazil. There, converted tankers from the Petrobas company are extracting oil from a water depth of 1853 meters. Ships cannot be stabilized as well or equipped as well as drilling platforms. However, due to their greater flexibility, they are increasingly being used for exploratory drilling and the development of smaller oil fields.

The financial risk for deep sea projects has decreased in recent years. This is also thanks to the improved exploration methods used by petroleum geologists. The first licenses for springs in water depths of up to 3000 meters have now been issued. The planning of the corresponding platforms has already started.