What Is a Drill Collar?

Drill collars are among the heaviest and stiffest tubular components in a conventional rotary drillstring. Unlike standard drill pipe, which is designed primarily to transmit torque, tensile load and drilling fluid over the main length of the drillstring, a drill collar has a much thicker wall and a relatively small internal bore.

The added metal cross-section gives the collar substantial weight and bending stiffness. A group of drill collars is normally placed in the bottom-hole assembly (BHA), above the bit and around other lower-BHA components such as stabilizers, directional tools or mud motors.

In practical drilling terms, the drill collar section helps transfer compressive weight toward the bit while keeping as much of the drill pipe as possible in tension. This improves mechanical stability and reduces the risk of drill-pipe buckling caused by operating the lighter pipe body in compression.

What Does a Drill Collar Do?

Where Is the Drill Collar Located in the BHA?

A simplified lower drillstring commonly follows this sequence:

This is a simplified arrangement rather than a universal BHA design. Stabilizers, MWD/LWD tools, jars, reamers, subs and motors may be positioned differently according to the drilling program. The number and position of drill collars should therefore be calculated from the required WOB, well trajectory, neutral-point position, BHA stiffness and operational limits.

Common Types of Drill Collars

Slick Drill Collar

A slick drill collar has a smooth cylindrical outside surface. It provides high weight and stiffness for a given base OD and bore and is widely used in conventional vertical and general drilling assemblies.

Spiral Drill Collar

A spiral drill collar has helical grooves machined along the outside surface. These grooves reduce the amount of continuous contact between the collar and the wellbore and provide additional external flow paths. Spiral designs are commonly considered in deviated, directional and sticking-sensitive drilling applications.

Goldenman supplies API 7-1 Spiral Drill Collars with published outside diameters from 3-1/8 to 11 inches, NC23 through NC70 connection options and selected REG connections.

Non-Magnetic Drill Collar

Non-magnetic drill collars are produced from specially selected non-magnetic material for BHA sections where magnetic interference can affect directional-survey, MWD or LWD instruments. Material specification and magnetic-property requirements must be confirmed separately from standard alloy-steel drill collars.

Drill Collar Size and Connection Reference

Drill collar size cannot be selected by OD alone. The outside diameter, internal bore, connection, length, BHA interfaces and required bending relationship must be reviewed together.

The following condensed table uses common configurations from Goldenman’s current Spiral Drill Collar product range as an engineering reference:

Reference values are drawn from Goldenman’s current published Spiral Drill Collar range. Final tolerances, mass, connection dimensions and spiral geometry must be confirmed against the approved product drawing and purchase specification.

Drill Collar vs Heavy Weight Drill Pipe vs Drill Pipe

These three drillstring components work together, but they are designed for different mechanical roles. The difference is especially important when selecting the transition between the main drillstring and the BHA.

Goldenman’s API 5DP Drill Pipe is used for the main drillstring, while the 3-1/2 in NC38 Heavy Weight Drill Pipe provides a controlled transition between standard drill pipe and the heavier drill collar section.

How to Select the Correct Drill Collar

1. Start With Hole Size and Minimum Clearance

A larger drill collar OD increases weight and bending stiffness, but it also reduces annular clearance. Check the open-hole diameter, casing ID, liner ID, stabilizer gauge and the minimum restriction through which the BHA must pass.

2. Determine the Required Weight on Bit

The required number of drill collars should be calculated from the planned WOB together with buoyancy, hole inclination, collar weight per unit length and the desired neutral-point location. A fixed collar count should not be applied to every well.

3. Confirm OD and Internal Bore Together

A smaller bore generally leaves more metal cross-section and increases collar weight, but reduces internal hydraulic area. A larger bore can reduce hydraulic restriction but changes joint weight and mechanical balance.

4. Match the Rotary Shouldered Connection

The connection must match adjacent BHA components such as HWDP, stabilizers, subs, motors or other drill collars. Connection designation alone is not enough: bore, shoulder geometry, stress-relief features and dimensional gauging should also be confirmed.

5. Evaluate Well Trajectory and Sticking Risk

Directional, deviated and extended-reach wells can increase wall contact and differential-sticking exposure. Spiral drill collars may reduce continuous contact area, but they do not eliminate differential sticking. Mud properties, overbalance, filter cake and hole cleaning remain important.

6. Review Handling and Inspection Requirements

Slip recesses, elevator recesses, stress-relief grooves, box bore-back, thread treatment, NDE scope and third-party inspection should be defined before manufacturing rather than added as an afterthought.

Why Drill Collar Weight Matters

Drill collar weight is not simply a purchasing specification. It directly affects the mechanical behavior of the lower drillstring.

• Available weight on bit
• Neutral-point location
• BHA stiffness
• Axial and bending response
• Drill-pipe compression above the BHA
• Rig handling and hook-load requirements

Two drill collars with the same nominal OD can have different weights because bore size, spiral-groove geometry, recesses and overall length change the final metal volume. For replacement projects, the existing product weight and drawing should therefore be reviewed together with OD and connection.

Spiral Drill Collar vs Slick Drill Collar

Drill Collar Connections: Why NC Size Matters

Common drill collar configurations use NC rotary shouldered connections, including NC23, NC26, NC31, NC35, NC38, NC44, NC46, NC50, NC56, NC61 and NC70. Larger configurations may also use selected REG connections.

The connection has to be evaluated together with collar OD and bore. This is because the available pin and box section, bending-strength relationship, shoulder geometry and internal flow path all change with the selected configuration.

When ordering a replacement collar, provide the complete connection designation and, where possible, the existing dimensional drawing or gauge information rather than relying only on a verbal description such as “6-3/4 inch drill collar.”

Drill Collar Manufacturing and Inspection

For oilfield drilling applications, drill collar quality control should cover both the heavy-wall body and the rotary shouldered connections. Goldenman’s Spiral Drill Collar manufacturing and inspection scope can include material traceability, heat-treatment review, ultrasonic examination, magnetic-particle inspection, hardness testing, dimensional inspection, bore drift, straightness inspection and connection gauging.

For spiral designs, groove depth, width, lead and transition geometry also require dimensional control because the external profile affects weight, contact area, annular clearance and local stress behavior.

Related Goldenman Drillstring Resources