Offshore Drilling Basics
Diamond Offshore provides offshore contract drilling services to the energy industry around the globe and is a leader in deepwater drilling.
Our customers are the world's oil companies ("operators"), and our sole job is to drill and complete* wells at the direction of our customers (*completion in industry terms means preparing the well for production. This entails running production casing, stimulation work, and zonal isolation to allow the well to flow hydrocarbons). Because we work offshore, often in remote locations, each of our drilling rigs is largely self-sufficient. When contracted, they come with a full crew and the equipment and supplies needed to carry out the assigned task, be it one short well or multiple years of work. For assignments in the US Gulf of Mexico, crews typically work a rotational schedule consisting of 21 consecutive days aboard the rig followed by 21 days off. In international locations, our crews typically work a rotational schedule of 28 days on and 28 days off.
In a broad sense, operators drill two basic types of wells-exploratory (to find new oil or gas deposits) and development (to prepare the discovery for production). Water depths range from 20 to 400 feet for jack-up rigs to up to 12,000 feet for semisubmersibles and drillships.
Before drilling an exploratory well, an operator will conduct geologic surveys of an area to determine the potential for oil or gas deposits and to identify specific targets. The operator then hires a drilling contractor like Diamond Offshore to drill exploratory ("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, of round-the-clock, seven-days-per-week operation 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. See rig basics.
Offshore wells are drilled in much the same way as their onshore counterparts-with several allowances for the offshore environment. A conduit made from lengths of steel pipe permits drilling fluids to move between the rig-at the water's surface-and the sea floor. This conduit is called a "riser." 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 about 600 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, 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.