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Rotary Drilling

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Rotary drilling is characterized as a method of drilling that employs a sharp, rotational drill bit to bite its way through the earth’s crust. One of the most effective and common methods of drilling, it is used in the construction, mining, and oil industries for its ability to cut through even the most challenging and hardest formations.

The rotary drill rig consists of a prime mover, hoisting equipment, rotating equipment and circulation equipment, all of which perform tasks crucial to drilling a well or hole. [1]

With the help of pneumatics, hydraulics or an electric motor, high torque and rotation are transmitted from the prime mover to the drill, which sits on a mast above the hole.

Contents

[edit] History

[edit] Early Rotary Drilling

Rotary drilling has been in use as early as 3,000 B.C. in China, and later in Egypt. The concept of rotary drilling has evolved and grown over time. Leonardo da Vinci created a design for a rotary drill in the early 1500s; it closely resembled the rotary drilling method employed today. Even though rotary drilling has had an early beginning, its use did not gain popularity until the late 1800s to early 1900s.

Though there were many patents for rotary drills as early as the 1830s, the first one was invented by Englishman Robert Beart in 1845. Early rotary drilling methods involved attaching a drilling device to a mule that would walk in circles to achieve the rotation needed for drilling.[2]

[edit] Drilling for Oil

Rotary drilling was first used for drilling oil wells in 1901 when Captain Anthony Lucas and Patillo Higgins applied it to their Spindletop well in Texas. By 1925, the rotary drilling method was improved with the use of a diesel engine.[3] The first attempt at rotary drilling involved a device that sent a stream of water down a hollow shaft of the drill to remove cuttings. This was in contrast to the cable drill, whereby the operation is interrupted and cuttings are removed singularly before the drilling can resume.[4]

The early rotary drill was originally used for drilling in softer formations in areas such as California and the Gulf Coast; but this changed with the production of roller and cone bits comprising steel alloy. This addition made rotary drilling suitable for harder formations, such as rock, and it was eventually used for boring deep holes and wells.

Rotary drilling became valuable to the oil industry because it was able to produce holes of a smaller diameter than cable drilling. This became possible because a hole produced by a rotary drill was the same diameter at the lower end as it was at the top.

Rotary drilling provided many improvements to drilling applications. The nature of the rotary drill proved important when oil well cements were being produced. The process of unloading continuous sacks of cement to construct a well required the drilling techniques of the rotary drill, which could accommodate these new materials.

Early oil drilling applications that were completed with cable tools or churn drills were laden with problems, such as erupting oil fountains that resulted in fires and the loss of valuable oil. The issues that arose from the high pressure of oil wells were remedied with the use of rotary drilling.[5]

[edit] Basic Rotary Drilling System

The basic rotary drilling system comprised prime movers, hoisting equipment, rotating equipment, and circulating equipment. Although this was a basic system, it was also a complex one. Previously, the system was powered by steam engines. After World War II, they were powered by gas, and eventually, diesel engines.

The ability of the rotary drill to automatically deposit water in order to remove cuttings (via the circulation system) meant that the operator would not have to interrupt the operation, as would be required in a cable drilling application. It also reduced the chances that mud would cause the hole to cave in.

[edit] Process

[edit] Rotary Drilling Components

Varying degrees of technology has contributed to rotary drilling. The basic process of rotary drilling involves a sharp drill bit being rotationally driven into rock. Using a system of prime movers, hoisting equipment, rotating equipment, and circulation equipment, the basic rotary drilling technique can operate without problems.

[edit] Source of Power

The diesel engine is the most common source of power for rotary rigs. Natural gas and gas engines are also used, as well as reciprocating turbines, which are capable of producing energy on-site. The energy is transmitted from these prime movers to the rotary, hoisting and circulating equipment. On larger rigs, power might also be transmitted for lighting, water, and compression requirements.

[edit] Hoisting Equipment

The hoisting equipment of the rotary system is used to raise and lower the tool in and out of the well or hole. The hoisting equipment comprises a derrick, which sits atop a rectangular structure made of steel 'I' beams; it also has a steel floor to walk on—this is called the "substructure." The hoisting structure, in addition to housing the derrick, acts as a support for cables and pulleys that raise and lower the tool. Hoisting equipment is necessary as the drill can sometimes weigh thousands of pounds. The height of the hoisting equipment helps stabilize the system. It also supports the engines and the 'draw works,' which is the mechanism that holds and controls the cable drums that supply the lifting of the 'elevators' and 'swivel'.

[edit] Rotating Equipment

Rotating equipment is essentially a piece of equipment that interprets the power transmitted from the prime mover and puts it into action, rotating the drill bit. In turn, a swivel, which is attached to the hoisting equipment, provides support for the weight of the drill string in a way that enables it to rotate uninterrupted. [6]

Inside the well, a string extends down as far as the drill pipe, reaching towards the drill bit, the sharp, chiseled bit responsible for chipping away earth and rock. Wells are dug with long strings of drill pipe that extend from the surface towards the drill bit. As rotating equipment rotates the drill bit, the hole becomes deeper and deeper. As this is occurs, the drill string is taken out in 20-foot (6.1-m) sections (as they are joined together to help the pipe extend down the hole).

[edit] Circulation System

The circulation system is responsible for cooling and lubricating the drill bit to keep it at its optimal performance. In addition, the circulating system removes cuttings and debris, and coats the walls of the well with a mud-type cake to facilitate circulation.

[edit] Rotary Drill Bit

The rotary drill bit is located at the bottom of the drill string. It is the hard, chiseled, sharp tip that makes contact with the rock formation and drills through it to produce a hole. It does this by breaking and dislodging rock. There are many different types of drill bits designed to facilitate the breakage of different types of formations. Below are the three main types of drill bits:

  • A blade or drag bit comprises steel and tungsten carbide. The combination produces a shearing action suitable for unconsolidated formations.
  • The steel tooth rotary bit was invented in the 1900s and was one of the most common drill bit types for all drilling projects. The long-tooth roller cone bit was used for softer formations while the short-toothed bit was considered best for hard ones. The result is a cutting action produced through crushing and chipping. Lately, tungsten carbide has been used for a more abrasive result, instead of the usual granite or quartzite.
  • A polycrystalline diamond bit consists of diamond inserts attached to carbide inserts. They are suitable for the hardest formations; they are more effective and 40 to 50 times harder than traditional steel bits.

[edit] The Operation

Unlike percussion drilling or cable drilling, which make use of drills that rotate only at the bottom of the hole, rotary drilling exerts downward pressure and drills rotationally all the way down. In order for this operation to be completed successfully, the rotary equipment works closely with the hoisting and circulatory equipment; as the drill is descended into the hole and driven rotationally, the circulation equipment is clearing the cuttings and debris. [7]

[edit] Truck-mounted Drills vs. Rotary Drilling Rigs

Rotary drilling sometimes employ drills that have been mounted on trucks or crawlers. This type of drilling can bore holes up to eight inches (200 mm) in diameter, and 328 to 656 feet (100 to 200 m) deep. Anything deeper requires a rotary drilling rig. With this type of drilling, holes as deep as 19,685 (6,000 m) can be made. Rotary drilling rigs may differ from truck-mounted drills in that they rely more on hydraulics and winches to raise the stem of the drill. [8]

[edit] Types

[edit] References

  1. Harris, Frank. Ground Engineering Equipment Methods. McGraw-Hill: New York: 1983.
  2. Drilling Methods. Well Drilling School. 2008-10-14.
  3. Drilling History. NTL World. 2008-10-12.
  4. Drilling Methods. Well Drilling School. 2008-10-14.
  5. Rotary Drilling. National Bureau of Economic Research Publications in Reprint. Ayer Company Publishers. 203-204.
  6. Drilling Methods. Well Drilling School. 2008-10-14.
  7. Drilling Methods. Well Drilling School. 2008-10-14.
  8. Harris, Frank. Ground Engineering Equipment Methods. McGraw-Hill: New York: 1983.