Chapter 6

 Moveable rigs include drilling barges, drill ships, jack-up rigs, and semisubmersible rigs.  Offshore rigs use a device called a top drive, instead of the drawworks and associated cables used on a land rig.  A Posted drilling barge is designed to float to the site of interest and then be sunk, resting on the bottom for stability in water depths as deep as about 25 feet.  The jack-up is the most common, generally used in water as deep as 400 feet.

 In Jack-up Rigs the lower Hull is called the mat.  A semisubmersible rig, or semisub, is partially submerged to increase stability. The unit is typically towed to the drilling site. The drilling platform and derrick are connected by columns to pontoons that extend 30–50 feet below the water’s surface.  As with the semisub, the drill ship uses a dynamic positioning system to keep it on station.

 Dhirubhai Deepwater KG2 drill ship is equipped to work in water as deep as 12,000 feet and is outfitted to construct wells as deep as 35,000 feet.  For a well drilled by a jack-up rig (in relatively shallow water), several hundred feet of large- diameter (26- or 30-inch) conductor casing is set into the seafloor.

 In deep water drilling, the drill string itself is used first to lower a temporary guide base—a hexagonal steel framework—and then a guide frame to the seabed. Those two units are used to position the drill bit, and a hole 30 or 36 inches in diameter is drilled to about 100 feet.  In deep water drilling, foundation pile is the first string of casing that will be placed in the hole.

 Marine Riser is a flexible, hollow metal tube and is attached to the BOP stack and to the drilling rig.  In a Drill Ship, Foundation pile is the first string of casing that will be placed in the hole.  The wellhead equipment on a completed subsea well is dry (enclosed in chamber maintained at atmospheric pressure).

 The gravity-based platform design is typically used in water as deep as about 500 feet deep. One or more reinforced concrete columns, or handles, support the work platform that is mounted on top of them.

 Hibernia platform of Mobil Oil (about 200 miles off the coast of Newfoundland) in 260 feet of water includes a concrete gravity-based structure weighing about 45,000 tons.  In the tension-leg design platforms, long, hollow steel tubes, typically two feet in diameter, are connected to the bottom of the platform deck. These tubes, called tendons, extend down to the seabed, where large weights hold them in place, or they are bolted to pilings sunk into the seabed.

 With its smaller “footprint,” the compliant tower is suitable for installation in water 1,000 to 3,000 feet deep without any need for guy wires or heavy anchors.  Thunder Horse PDQ—the largest semisubmersible vessel in the world, displacing 129,000 tons—is moored in 6,050 feet of water in the Gulf of Mexico, 150 miles southeast of New Orleans.

 The world’s first FDPSO—floating drilling, production, storage, and off-loading vessel— was developed in 2009 for Murphy Oil, for operation off the coast of the Republic of Congo.  Strakes (resembling fins) installed on the outer surface of a cylindrical spar can help lessen the effects of wave action and water movement on the platform.