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 jackup. This kind of rig sits on top of a floating barge. The barge is towed to the drilling site, and then the jackup extends its legs down onto the ocean floor (but they don't penetrate the floor). Then the jackup continues to ratchet its legs until the platform rises above the water. This way the rig is safe from waves and tidal motions. Jackups can operate as deep as 525 feet (160 meters) below the water’s surface.
A 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. 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. When a MODU hits oil, the engineers seal the well bore using plugs. Once the well is capped, a production rig can take over from the MODU and capture the oil.