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What is a Drill String?

By Keith Koons
Updated May 17, 2024
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A drill string is a tool that is used for boring deep holes into the ground in order to locate and extract oil or other resources. The construction of such a device allows for rapid drilling while simultaneously extracting large amounts of rock and ore from an excavation site. Mud also is injected down through the drill string to help cool the drill bit while it is in motion and to soften the surface that it is boring through, lessening the chances of an improper cut and increasing the overall lifespan of the bit. An average drill string extends 15,000 feet (4,572 m) into the ground when assembled on land and up to 30,000 feet (9,144 m) or more when constructed offshore, so the quality and precision of this instrument are paramount in order to avoid system failure and potentially becoming stuck a great distance below the Earth's surface.

Within the drill string assembly, there are four major components: the bottom hole assembly (BHA), the transition pipe, the drill pipes and the drill stem subs. The BHA is the stabilization system that is made up of the drill bit itself and the massive drill collars that apply tremendous amounts of downward force to assist in the boring operation. A transition pipe connects the drill collars to the actual drill pipe, and together these two components provide the much-needed stability in order to ensure that the drill bit remains sturdy at such drastic depths. Drill pipes also make up the majority of the length within a drill string, so they must be constructed using specific chemical compositions and forged under extreme temperatures. There also are giant braces called drill stem subs that hold the other components together, so they also have to be created with a focus on superior durability and craftsmanship.

Most of the components within a drill string are constructed in intervals of 31 or 46 feet (9.4 or 14 m), and two to four of them are combined to make what is referred to as a stand. Each stand is then lowered into the ground before drilling commences, in order to ensure that the drill always remains within perfect alignment. Likewise, they are removed from the ground before the drill is extracted.

At times, the stands can become stuck and become difficult to remove, and specialized retrieval tools called drill string jars and resonant vibrators are used to rectify this otherwise difficult situation. These methods normally are implemented by experienced laborers from the oil companies. Technological advancements discovered during the middle of the 20th century have made drill strings much easier to manage.

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