OIL RIGS

One of the earliest offshore oil rigs was used in 1869. It was set in shallow water, but was an anchored, four-legged tower much like today's modern platforms.

INSTALLING OFFSHORE OIL RIGS

The first stage in setting up an oil rig is in drilling an offshore well and then lowering a large drilling platform into the ocean and attaching it to the seabed. In order to provide a base for these rigs, an underwater guided base is moved into precise position using the long-range navigational system (LORAN) and satellite technology. Then a wide, relatively shallow hole (about 100ft deep) is drilled into the ocean floor. After the surface hole for the oil rig has been drilled and the hole has been filled with a casting, blowout preventers (BOPs) are installed to control underground well pressures and fluids during drilling. Drilling then resumes following BOP installation. Blow outs can occur when pressure builds up underground and, basically, the drilling hole explodes throwing oil, drilling mud, pipes and rocks into the air. This can be harmful to the environment and dangerous if the oil rig catches fire.

PERMANENT DRILLING RIGS

Permanent drilling rigs are the largest, and most complex offshore structures. These massive platforms are placed in areas where multiple wells will be drilled, and production is high. The Offshore oil rigs in the North Sea are some of the largest structures ever built. They can be constructed in over 500ft of water because of their massive size. They must be durable, able to withstand waves over 60ft high, and winds in excess of 90 knots. Often, the larger rigs are constructed in part while they are being towed to the well site. Some of the larger oil platforms are 450ft in diameter at their base, weigh over 500,000 tons, and rise some 750ft from the sea bed to the tip of the derrick. The cost of these drilling platforms can easily exceed 1 billion dollars. Permanent rigs are held in place by concrete, steel, or tension legs. Many oil rigs that are in the water have skimmers attached to the legs. They are connected to both the decks and the drains. This means that anything that flows off the deck can be collected and separated properly.

SPECIALLY DESIGNED OIL RIGS

In the arctic, other specially modified drilling rigs are needed. Some oil rigs have been built into large floating sections of ice in areas where there is no danger of the ice melting. Others have been built on man-made islands of gravel and sand. These oil rigs are commonly known as singles, doubles, or triples, based on how many lengths of pipe can be stacked on the derrick.