Definition
Drill Collar — A drill collar is a thick-walled, heavy steel tube run directly above the bit in the bottomhole assembly to provide weight-on-bit in compression and to keep the lower drill string stiff and under control.
Also known as: drill collars, DC
What a drill collar does
A drill collar is the heaviest tubular in the drill string. Machined from solid bar with a small bore and a very thick wall, it provides the concentrated mass that pushes down on the bit — the weight-on-bit (WOB) needed to drill efficiently. Because collars run in compression, their stiffness also stabilizes the bit and helps control hole deviation.
Drill collars are typically 30–31 ft long and available in slick (plain) or spiral (grooved) bodies; spiral collars reduce contact area against the wall to lower the risk of differential sticking. Non-magnetic collars machined from monel-type alloy are used to house directional survey and MWD sensors.
Design points
- Neutral point — enough collar weight must sit below the neutral point so drill pipe stays in tension and does not buckle.
- Connections — collars use rotary-shouldered connections (NC, Regular, Full Hole) sized to the collar OD, often with stress-relief features to fight fatigue.
- Fishing profile — a smooth OD and recut connections keep collars retrievable if a fish occurs.
Renting drill collars
Collar size and weight are chosen from the WOB and hole size for each well. We rent slick, spiral and non-magnetic drill collars in common ODs — contact us to build the weight into your BHA.