Read the text. In small groups, match these descriptions of oil platforms to the correct illustration. Be prepared to justify you answers

1. Gravity or fixed platforms

Gravity or fixed platforms sit on the seabed. They often have a concrete base with three or four hollow columns acting as legs with a steel deck built on top. Used in water to 520 m. gravity rigs are often built in sheltered waters, then floated out to sea and sunk in position. The hollow legs and base can be used for storing oil. They cannot easily be moved when their useful life is over, so disposal can be a problem.

2. Jack-up

These platforms sit on steel legs. The legs can be lowered to raise the platform above the sea (like a car jack). They are used in shallow waters up to about 100 m. They are cheap, and are reusable, as they can be moved to other locations. However, there are towing problems with the rigs and their safety record is poorer than that of other types.

3. Semi-submersible

Semi-submersible platform stands on columns which sit on pontoons below the level of the sea. These provide enough lift to float the platform and enough weight to keep it upright. By altering the amount of water in the pontoons, the platforms can be moved up and down. They are used in water of 600 – 1800 m in depth. They are very stable, even in rough seas. They can be easily moved to new locations. They need more support vessels because they have limited storage.

4. Spar

Spars are floating platforms moored to the seabed. They come in three forms: a cell spar that has a body composed of a number of vertical columns, a conventional spar that has one column, and a truss spar which has a floating hard tank at the top linked to a soft tank at the bottom, which is weighted to provide stability to the structure. Spars are cheaper and more stable than tension leg platforms. They are suitable to depth up to 1800 m. They can be moved horizontally.

5. Tension leg

These are floating platforms where the mooring system prevents vertical movement. This means that the well can be tapped directly from the platform. They can be useful in depth of 200 – 1100 m. They are inexpensive to make compared with other platform types, and can be moved to new locations, but are less stable than fixed platforms.