Some oil deposits are buried deep under the ocean's floor, and it can be very dangerous to try to access them. Oil companies first use sonic equipment to determine which drilling sites are most likely to produce oil. Once they know exactly where to drill, they use something called a MODU, or mobile offshore drilling unit, to dig the well. Some MODUs convert to production rigs; once they find the oil, they also capture it. But usually the oil company uses a more permanent rig for the capturing part of the process.
One kind of MODU is called a submersible MODU. Usually this is a barge that rests about 30 to 35 feet (9.1 to 10.7 meters) underwater, on the sea floor. Steel posts on the barge's deck extend up above the water line and hold a drilling platform. Submersible MODU rigs are usually used in areas where the water is calm.
The MODU drills down into the ocean floor until it finds oil deposits. The riser is the part of the drill that goes down below the deck and through the water. The riser allows drilling fluids to pass from the ocean floor to the rig. A drill string (a series of 30-foot/9.1-meter pipes) is lowered through the riser. As it goes down, more pipe is added. At the sea floor, the MODU has a BOP or blowout preventer. This uses hydraulically powered clamps to close off the pipe in case of a blowout. To make the well more stable and to keep it from collapsing, the engineers use metal casings lined with cement walls.
When a MODU hits oil, the engineers seal the well bore using plugs. Seawater or drilling mud holds down the first plug while the engineers put a second one in place. Once the well is capped, a production rig can take over from the MODU and capture the oil.