Building huge structures on water developed through the need to extract oil and gas from fields that lie under the ocean. Altogether there are now more than 1,500 oil platforms stationed in the oceans of the world. The North Sea and the Gulf of Mexico are the offshore areas that have seen the most development.
There are two main types of permanent offshore platform. In most cases, the oil men live and work on steel structures that contain the drilling rigs and processing equipment as well as sleeping quarters, cinemas and restaurants. It is the supporting structures that vary. One type consists of lattice steel supports, which stands on the sea bed on four or more legs. Other rigs have concrete bases, which are floated into position and then sunk.
Floating structures
As well as these platforms, which are all fixed to the sea bed, the offshore oil and gas industry has spawned various floating structures. The most impressive of these are crane barges, which can lift up to 14,000 tonnes.
Steel supporting structures, called jackets, once had to be floated out to sea on barges and up-ended when they reached their destination. However, the huge cranes now available can lift most jackets into place from the deck of the barge Steel piles – giant pipes – are then driven through the base of the jackets deep into the sea bed to anchor them in place.
The discovery of oil and gas in the North Sea led to existing technology being taken to undreamed-of-dimensions. For instance, the cylindrical steel piles used to nail the four steel platforms in the Forties oil field (in the North Sea) to the sea bed where 1,37 metres in diameter and 270 metres long – nearly twice the height of the British Telecom Tower in London. Each platform needs 36 of them. Once the jacket has been fixed into position, the decks are placed on top, either in one piece or in separate modules.
Concrete towers
The concrete structures used to support some oil platforms are the heaviest floating structures man has ever devised. How do such immensely heavy objects float?
The answer is that they have giant buoyancy tanks build into them. These hold enough air to make the structure float and, by emptying and refilling tanks, the jacket can be made to rise and fall in the water. The deck is floated out on a barge to a point in deep water to be mated with the jacket. The jacket is lowered in the water so that the barge can float over it. Then the jacket is raised to lift the deck off the barge.
Once a survey and test drilling have showed that it should be economic to build a permanent rig on a site, planning and construction can begin. A typical modern rig takes at least two years to build. The development of all this technology has encouraged people to look at building other massive structures on water
