Friday, November 5, 2010

Basics of Offshore Well drilling

Common drilling of wells are two basic types —exploratory, i.e. to find new oil or gas deposits  and development, i.e. to prepare the discovery for production. Water depths range from 50 to 400 feet for jack-up rigs to up to 10,000 feet for semisubmersibles and other types of vessels like drillships.

Before drilling an exploratory well, an operator will conduct geologic surveys of an area to determine the potential for oil or gas deposits. The operator then hires a drilling contractor to drill exploratory (also term “wildcat”) wells offshore. The oil company chooses the location and supervises the operation, which may take as little as 15 days or as long as 12 months to drill a single well depending on the complexity of the project.

Offshore rigs are designed for efficiency in living and working, with emphasis on keeping the rig steady in gulf or ocean waters. Offshore wells are drilled in much the same way as their onshore or typical landrigs  —with several allowances for the offshore environment. A string of tubes made from lengths of steel pipe permits drilling fluids to move between the rig—at the water’s surface—and the sea floor. This tube is called a “riser.” Such riser exists only on deepsea semi drilling, as for jackups, we normally call them drill string. The riser is fitted with ball-and-slip joints that permit the long string of riser pipe to move up and down and bend slightly with the wave-induced movement of the rig.

The well is drilled using a length of slender steel pipes and other tools that, connected, comprise a “drill string.” At the bottom of the string of pipes is a hole-boring device called a “drill bit.” Heavy sections of pipe, called “drill collars,” add weight and stability to the drill bit. Each ordinary pipe in the string is about 30 feet long and weighs few hundred pounds; drill collars can weigh 4,000 Pounds or more per 30-foot length.

As drilling proceeds, and the well gets deeper, the drilling crew adds new sections of drill pipe to the ever-lengthening drill string. Hydraulic devices keep constant tension on the drill string to prevent the motion of the rig and riser from being transmitted to the drill bit.

The drill string is lowered through the riser to the sea floor ( this is the case for semi drilling), passing through a system of safety valves called a “blowout preventer” (BOP, pronounced “B.O.P.”). This stack of multiple safety valves is designed to contain any natural pressures that the drillers might encounter beneath the Earth’s surface. Its purpose is to prevent a possible “blowout”—an uncontrolled eruption of oil, gas or wellbore fluids due to excessive natural pressure.

Basics to Drilling_choong

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