What Is A Rotary Derrick - Types Of Oil Drilling Platforms
What is the Rotary Derrick and How does It Work?
The rotary derrick is a type of drilling tower. The derrick has to be high enough so that sections of pipe can be added or removed. Each pipe section is 10 to 15 m long and, as a group, they are joined together to form the drill string.
At the top of the derrick is the crown block, and at the side is the power plant, usually a large diesel engine. The power plant provides power for the draw works, which are used to lift sections of pipe in and out of the borehole. It also powers the mud pumps. The mud is used for lubricating the bit, as well as the turntable.
The traveling block is suspended by a cable from the crown block and can move up and down. The swivel is attached to the traveling block - a swivel is something which is free to rotate. The swivel allows the pipe to rotate freely.
The traveling block raises or lowers the pipe. The kelly is a heavy pipe approximately 13 m long. lt is square or hexagonal in section and is suspended from the swivel. lt fits through the slot in the turntable and connects to the top joint of the drill string.
The rotary derrick works as follows. First, the power plant turns the turntable. Then, as the turntable rotates it turns the kelly, which rotates the drill string. At the end of the string of drill pipes is the diamond-toothed bit. As the pipe turns, the bit cuts the borehole.
Gravity or Fixed Oil Platforms
Gravity or fixed platforms sit on the seabed. They often have a concrete base with three to four hollow columns acting as legs with a steel deck built on top. A gravity or fixed oil platform is used in water up 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.
Jack-up Oil Platforms
Jack-up oil platforms sit on steel legs. The legs can be lowered to raise the platform above the sea, like a car jack. Jack-up platforms 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 these rigs and their safety record is poorer than other types.
Semi-submersible Oil Platforms
A semi-submersible oil 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 semi-submersible platform 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. Semi-submersible oil platforms need more support vessels because they have limited storage.
Spar Oil Platforms
Spar oil platforms are floating platforms moored to the seabed.
They come in three forms: a cell spar has a body composed of a number of vertical columns, a conventional spar has one column, and a truss spar 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.
Spar oil platforms are cheaper and more stable than tension leg platforms.They are suitable for depths up to 1800 m. They can be moved horizontally.
Tension Leg Oil Platforms
Tension leg oil platforms 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 used in depths of 200 - 1100 m. Tension leg oil platforms are inexpensive to make compared with other platform types, and can be moved to new locations, but are less stable than fixed platforms.
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