Floating Units

Floating offshore drilling rigs include semisubmersibles and drill ships. Semisubmersibles, because of their design, are more stable than drill ships. On the other hand, drill ships can carry more drilling equipment and supplies, which often make them the choice in remote waters.

Semisubmersibles

Most semisubmersible rigs have two or more pontoons on which the rig floats. A pontoon is a long, relatively narrow, and hollow steel float with a rectangular or round cross section (fig. 35). When a semisubmersible is moved, the pontoons contain mostly air so that the rig floats on the water's surface. In most cases, towboats then tie onto the rig and move it to the drill site. However, some semisubmersible rigs are self-propelled-they have built-in power units that drive the rig from one site to another.

Semisubmersibles get their name from the fact that in the drilling mode the rig is not submerged to the point where its pontoons contact the sea bottom. Instead, rig personnel carefully flood the pontoons to make them submerge only a few feet (metres) below the water's surface (fig. 36). Thus, the rig is "semisubmerged" (If the pontoons contacted the sea bottom, the rig would be "submerged.") With its pontoons submerged below the waterline, waves do not affect the rig as much as they do when it floats on the surface. A semisubmersible rig therefore offers a more stable drilling platform than a drill ship that drills while floating on the water's surface.

Large cylindrical or square columns extend upward from the pontoons. The main deck rests on top of the columns. The main deck of a semi is big (fig. 37). Semis (short for semisubmersibles) often use anchors to keep them on the drilling station. Workers release several large anchors from the deck of the rig. An anchor-handling boat crew sets the anchors on the seafloor.

Besides being good rough-water rigs, semis are also capable of drilling in water thousands of feet (metres) deep. While many semis work in water depths ranging from 1,000 to 3,500 feet (300 to 1,000 metres), the latest are capable of drilling in water depths of 8,000 feet (2,500 metres). Semis can drill holes up to 30,000 feet (10,000 metres) deep. Indeed, semisubmersibles are among the largest floating structures ever made. The biggest ones soar to over 100 feet (30 metres) tall and their main decks can be almost as big as a football field-that's 3,000 square yards (2,500 square metres).

Drill Ships

A drill ship is also a floater (fig. 38). Drill ships are very mobile because they are self-propelled and have a streamlined hull, much like a regular ocean-going ship. A company may therefore choose a drill ship to make hole in remote waters, far from land. A drill ship is a good choice for drilling remote locations. For one thing, it can move at reasonable speeds under its own power. Secondly, its ship-shaped hull can carry a large amount of the equipment and material required for drilling. Frequent resupplying from a shore base is therefore not necessary.

While many drill ships operate in water depths ranging from 1,000 to 3,000 feet (300 to 1,000 metres), the latest can drill in water depths approaching 10,000 feet (3,000 metres), or nearly 2 miles (3,2 kilometres). They can drill holes over 30,000 feet (10,000 metres) deep. These big drill ships are more than 800 feet (250 metres) long, which is almost as long as three football fields laid end to end. They measure some 100 feet (30 metres) wide, or a little wider than a football field. Their hulls tower more than 60 feet (18 metres) high, about that of a six-story building.

Anchors keep some drill ships on station while drilling, but those drilling in deep water require dynamic positioning. Dynamically positioned drill ships use computer-controlled thrusters and sophisticated electronic sensors. Thrusters are power units with propellers that the builder mounts fore and aft o, the drill ship's hull below the waterline. Once the dynamic positioning operator tells a computer exactly where it should keep the rig positioned, the computer, using information transmitted by the sensors, automatically controls the thrusters. The thrusters offset wind, wave, and current forces that would move the rig away from the desired position.

Whether on land or offshore, and whether large, medium, or small, all rigs require personnel to operate them. The people who drill wells usually work for a company whose business involves drilling, either directly or indirectly. So, let's look next at companies involved in drilling and the personnel who do the work.

Дата добавления: 2015-10-26