What are semisubmersibles in oil drilling?

Using sonic equipment, oil companies are able to search for oil deep beneath the ocean's floor. Then they use a MODU, or mobile offshore drilling unit, to dig the well in exactly the right location, which their equipment has determined will be the most likely to produce oil.

A “semisubmersible” is a particular kind of MODU that floats above a submerged pontoon boat. Some semisubmersibles need to be towed to drilling sites by another vessel while others use propulsion systems to navigate their own way. Semisubmersibles use up to a dozen anchors to maintain their orientation and stay in one place over the well. The tension on the anchor chains is controlled by computers in order to correct for drift.

A semisubmersible and other types of MODUs use a riser to drill down into the ocean floor. Drilling fluids pass from the ocean floor through the riser and into the rig. Engineers then lower a drill string (a series of 30-foot/9.1-meter pipes) down through the riser. The engineers continue to add pipes to the string as they lower it down. A BOP -- blowout preventer -- seals off the pipe at the sea floor. Metal casings lined with cement walls stabilize the well and keep it from collapsing. When a MODU hits oil, the well needs to be plugged up. Engineers put in a plug, which is held down by seawater or drilling mud. After the well is capped, a permanent rig may come in to capture the oil, or the MODU may convert to a production rig and do the capturing itself.