What’s a drill collar?

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A drill collar is a solid steel bar used in oil well drilling to weight down the drill bit and allow it to drill into rock. It forms the lowest element of a drill string and can vary in diameter and external configuration. Equipment used with drill collars includes clamps, slides, and die collars.

A drill collar is a device used in oil well drilling to weight down the drill bit, allowing it to drill into rock. It is a solid steel bar, plain carbon steel or a non-magnetic steel alloy, drilled lengthwise to allow the passage of drilling fluids. A drill collar forms the lowest element of a drill string, comprising all elements of a drilling process from surface to rock bit.

These devices are typically 31 feet (approximately 9.45 m) long and threaded at both ends, male at one end and female at the other, to allow multiple drill collars to be joined above the drill assembly. The number of drill collars attached to a drill string will depend on the material composition of the strata at the drill site and the probable depth of the well. A relatively shallow well with less dense geological structure through which the bit has to pass will require fewer drill collars than a deep well through dense material.

Typically, drill collars will be of constant length but can vary in diameter and their external configuration can be smooth or spiral. The outside diameter can range from approximately 3 inches (7.62 cm) to 11 inches (27.9 cm) and larger. Reference to the smooth or spiral external configuration refers to the machining of the external surface of the collar.

A smooth surface simply refers to a machined collar with a uniform cylindrical shape. A spiral collar is machined to have a helix pattern etched into its outer surface. The purpose of the helical flutes is to allow debris to pass over the drill string and prevent it from getting stuck in the wellbore.

The pressure applied to the drill bit assembly by the collar and other elements of the drill string must be carefully regulated for effective drilling. The weight of the drill string is monitored at the surface and the operator slowly lowers the drill string into the hole until the recorded weight changes. If the bit is resting on the bottom of the hole and the monitor shows a reduction of 10,000 pounds (4,540 kg), there should be a corresponding increase in pressure on the drill bit assembly.

Equipment used in conjunction with drill collars in oil well drilling includes drill collars, drill collar clamps, and die collars. The drill collar slide is a device used to handle drill collars when attaching new sections and is adjustable to a variety of diameters. Drill collar clamps are also used when handling drill collars to keep them from falling down a well. In the event that a drill string fails and a drill collar and bit are at the bottom of the shaft, the die collar is pulled down and with a self-tapping bit a drive connection is threaded into the drill collar, allowing it to the recovery.

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