A drilling rig is a machine which creates holes (usually called boreholes) and/or shafts in the ground. Drilling rigs can be massive structures housing equipment used to drill water wells, oil wells, or natural gas extraction wells, or they can be small enough to be moved manually by one person. They sample sub-surface mineral deposits, test rock, soil and groundwater physical properties, and also can be used to install sub-surface fabrications, such as underground utilities, instrumentation, tunnels or wells. Drilling rigs can be mobile equipment mounted on trucks, tracks or trailers, or more permanent land or marine-based structures (such as oil platforms, commonly called 'offshore oil rigs' even if they don't contain a drilling rig). The term "rig" therefore generally refers to the complex of equipment that is used to penetrate the surface of the earth's crust.
Drilling rigs can be:
- Small and portable, such as those used in mineral exploration drilling, water wells and environmental investigations.
- Huge, capable of drilling through thousands of meters of the Earth's crust. Large "mud pumps" circulate drilling mud (slurry) through the drill bit and up the casing annulus, for cooling and removing the "cuttings" while a well is drilled. Hoists in the rig can lift hundreds of tons of pipe. Other equipment can force acid or sand into reservoirs to facilitate extraction of the oil or natural gas; and in remote locations there can be permanent living accommodation and catering for crews (which may be more than a hundred). Marine rigs may operate many hundreds of miles or kilometres distant from the supply base with infrequent crew rotation.
Petroleum drilling industry
Oil and Natural Gas drilling rigs can be used not only to identify geologic reservoirs but also to create holes that allow the extraction of oil or natural gas from those reservoirs. Primarily in onshore oil and gas fields once a well has been drilled, the drilling rig will be moved off of the well and a service rig (a smaller rig) that is purpose-built for completions will be moved on to the well to get the well on line. This frees up the drilling rig to drill another hole and streamlines the operation as well as allowing for specialization of certain services, i.e., completions vs. drilling.
History
Until internal combustion engines came in the late 19th century, the main method for drilling rock was muscle power of man or animal. Rods were turned by hand, using clamps attached to the rod. The rope and drop method invented in Zigong, China used a steel rod or piston raised and dropped vertically via a rope. Mechanised versions of this persisted until about 1970, using a cam to rapidly raise and drop what, by then, was a steel cable.
In the 1970s, outside of the oil and gas industry, roller bits using mud circulation were replaced by the first efficient pneumatic reciprocating piston Reverse Circulation RC drills, and became essentially obsolete for most shallow drilling, and are now only used in certain situations where rocks preclude other methods. RC drilling proved much faster and more efficient, and continues to improve with better metallurgy, deriving harder, more durable bits, and compressors delivering higher air pressures at higher volumes, enabling deeper and faster penetration. Diamond drilling has remained essentially unchanged since its inception.
Mobile drilling rigs
In early oil exploration, drilling rigs were semi-permanent in nature and the derricks were often built on site and left in place after the completion of the well. In more recent times drilling rigs are expensive custom-built machines that can be moved from well to well. Some light duty drilling rigs are like a mobile crane and are more usually used to drill water wells. Larger land rigs must be broken apart into sections and loads to move to a new place, a process which can often take weeks.
Small mobile drilling rigs are also used to drill or bore piles. Rigs can range from 100 ton continuous flight auger (CFA) rigs to small air powered rigs used to drill holes in quarries, etc. These rigs use the same technology and equipment as the oil drilling rigs, just on a smaller scale.
The drilling mechanisms outlined below differ mechanically in terms of the machinery used, but also in terms of the method by which drill cuttings are removed from the cutting face of the drill and returned to surface.
Drilling rig classification
There are many types and designs of drilling rigs, with many drilling rigs capable of switching or combining different drilling technologies as needed. Drilling rigs can be described using any of the following attributes:
by power used
- mechanical - the rig uses torque converters, clutches, and transmissions powered by its own engines, often diesel
- electric - the major items of machinery are driven by electric motors, usually with power generated on-site using internal combustion engines
- hydraulic - the rig primarily uses hydraulic power
- pneumatic - the rig is primarily powered by pressurized air
- steam - the rig uses steam-powered engines and pumps (obsolescent after middle of 20th Century)
by pipe used
- cable - a cable is used to raise and drop the drill bit
- conventional - uses metal or plastic drill pipe of varying types
- coil tubing - uses a giant coil of tube and a downhole drilling motor
vertical drilling machine $ horizontal drilling machine.
by height
- single - can drill only single drill pipes. The presence or absence of vertical pipe racking "fingers" varies from rig to rig.
- double - can hold a stand of pipe in the derrick consisting of two connected drill pipes, called a "double stand".
- triple - can hold a stand of pipe in the derrick consisting of three connected drill pipes, called a "triple stand".
by method of rotation or drilling method
- no rotation includes direct push rigs and most service rigs
- rotary table - rotation is achieved by turning a square or hexagonal pipe (the kelly ) at drill floor level.
- top drive - rotation and circulation is done at the top of the drillstring, on a motor that moves in a track along the derrick.
- sonic - uses primarily vibratory energy to advance the drill string
- hammer - uses rotation and percussive force (see Down-The-Hole Drill)
by position of derrick
- conventional - derrick is vertical
- slant - derrick is slanted at a 45 degree angle to facilitate horizontal drilling
Drill types
There are a variety of drill mechanisms which can be used to sink a borehole into the ground. Each has its advantages and disadvantages, in terms of the depth to which it can drill, the type of sample returned, the costs involved and penetration rates achieved. There are two basic types of drills: drills which produce rock chips, and drills which produce core samples.
Auger drilling
Auger drilling is done with a helical screw which is driven into the ground with rotation; the earth is lifted up the borehole by the blade of the screw. Hollow stem Auger drilling is used for environmental drilling, geotechnical drilling, soil engineering and geochemistry reconnaissance work in exploration for mineral deposits. Solid flight augers/bucket augers are used in construction drilling. In some cases, mine shafts are dug with auger drills. Small augers can be mounted on the back of a utility truck, with large augers used for sinking piles for bridge foundations.
Auger drilling is restricted to generally soft unconsolidated material or weak weathered rock. It is cheap and fast.
Percussion rotary air blast drilling (RAB)
RAB drilling is used most frequently in the mineral exploration industry. (This tool is also known as a Down-The-Hole Drill.) The drill uses a pneumatic reciprocating piston-driven 'hammer' to energetically drive a heavy drill bit into the rock. The drill bit is hollow, solid steel and has ~20 mm thick tungsten rods protruding from the steel matrix as 'buttons'. The tungsten buttons are the cutting face of the bit.
The cuttings are blown up the outside of the rods and collected at surface. Air or a combination of air and foam lift the cuttings.
RAB drilling is used primarily for mineral exploration, water bore drilling and blast-hole drilli
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