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A tension-leg platform (TLP) is a floating structure that is secured below the water with anchors and cables of varying tension. The platform's buoyancy atop the water helps to keep the cables taut, though the way they are strung can also have an impact. These structures are most commonly used for production of gas and oil, though they have also seen some use with offshore wind turbines. They are also known as extended tension leg platforms (ETLP).
The cables in a tension-leg platform are designed to be flexible, but sturdy. They allow a small amount of horizontal movement, in order to avoid breakage and wear due to constantly fighting the pressure of the sea. The cables are tightened just enough so that vertical movement or bobbing are not possible. This helps to improve productivity and safety on the platform.
A tension-leg platform is kept buoyant by a hull on top and secured by anchors and pilings that are driven deep into the seabed below. This security, coupled with the flexibility in the cables, make it possible to keep the wellheads for gas and oil production on the deck of the platform. Before tension-leg platforms came into use, this equipment was situated on the ocean floor, which was more expensive, difficult, and dangerous to manage.
The platforms are a square shape, with supporting legs at each corner which are filled with air. They typically have living quarters, drilling equipment, and a power source up on deck. Each platform also has a rig for the production of oil and gas. If the platform is being used for a wind turbine, the deck will be less elaborate and most likely consist primarily of a single tower.
All the pieces of a tension-leg platform are constructed on land and then towed to its location in the sea. Some portions of the structure, such as the pilings, are permanent once they have been put in place. As many as 16 pilings can be driven into the sea floor to secure a leg of the platform. The top portion of the structure is more flexible and can still be moved once it has been attached.
While it is being towed out to see, the cables for a tension-leg platform are retracted. The anchors for the piece are kept close to the underside of the platform during transport. Upon arrival at the site, the cables are released, with the attached anchors descending to the sea floor. Then the cables are adjusted while the structure is attached to the support structure below.