Drill collars supply the weight and stiffness near the bit that keeps the more flexible drill pipe above them in tension, and slick, spiral, and nonmagnetic designs cover most applications. Sizing and connection selection belong to the complete BHA design - bit and hole size, mud weight, hole geometry, fishing clearance, and connection capacity - rather than to any single rule of thumb.
Work from API Spec 7-1, API Spec 7-2, API RP 7G, and API RP 7G-2 alongside current manufacturer data, and let the approved inspection program set inspection scope and frequency.
Drill collars are a critical part of the bottomhole assembly, or BHA. Their weight and stiffness help transfer controlled weight to the bit, support directional objectives, and keep the more flexible drill pipe above them from operating in compression under normal drilling conditions.
Although the basic purpose of a drill collar is straightforward, selecting and managing drill collars requires careful engineering. Outside diameter, inside diameter, connection type, material, weight, well geometry, drilling parameters, fishing clearance, and inspection requirements must all be considered as part of the complete BHA design.
This guide provides a practical overview of drill collars and the standards commonly associated with them. Final equipment selection and operating limits should always be established by the operator, drilling contractor, qualified drilling engineers, applicable manufacturers, and the approved drilling program.
A drill collar is a thick-walled tubular component installed in the lower portion of the drill string, normally close to the drill bit. Its primary purpose is to provide weight on bit and stiffness within the BHA.
Unlike conventional drill pipe, a drill collar is generally machined from a solid bar. A bore is machined through the center to allow drilling fluid to circulate to the bit, and rotary-shouldered pin-and-box connections are cut directly into the ends. The result is a heavy, rigid component with no separate welded tool joints.
Most standard steel drill collars are manufactured from heat-treated alloy steel, commonly AISI 4145H modified or an equivalent material specified by the manufacturer. Nonmagnetic drill collars use specialized low-permeability alloys so they do not interfere with magnetic surveying instruments.
Wall thickness varies significantly with the collar's outside and inside diameters. For example, a 4 3/4-inch OD collar with a 2 1/4-inch bore has a radial wall thickness of approximately 1 1/4 inches, while larger collars may have walls exceeding 3 inches.
Key point: Drill collars supply weight and stiffness near the bit. Drill pipe primarily transfers rotation, fluid, and axial load between the rig and the BHA.
Drill collars perform several related functions within the BHA:
The neutral point is the location where axial loading changes from compression below to tension above. In conventional drilling practice, the BHA is designed so the neutral point remains within components intended to carry compressive loading rather than moving into the conventional drill pipe.
Rules of thumb based on buoyed drill-collar weight are sometimes used during preliminary planning, but they are not substitutes for a complete design. Actual loading is affected by hole inclination, friction, drag, wellbore curvature, stabilizer placement, vibration, hydraulic forces, bit requirements, and the total BHA configuration.
Drill-collar stiffness can also affect directional behavior, but the collars do not control the well path by themselves. Stabilizer placement, mud motors, rotary-steerable systems, bent housings, bit selection, formation response, weight on bit, and rotary speed all influence how the BHA builds, drops, or holds angle. The SLB Energy Glossary outlines how these components are combined in a typical assembly.
Slick drill collars have a smooth, cylindrical outside surface. They provide the greatest weight for a given outside diameter and are commonly used where differential sticking risk and wall-contact concerns are manageable.
Spiral drill collars have helical grooves machined into the outside diameter. The grooves reduce the surface area in contact with the wellbore and can help reduce differential-sticking risk. The exact reduction in contact area and weight depends on the manufacturer's design, so published product data should be reviewed rather than applying a single percentage to every spiral collar.
Nonmagnetic drill collars are manufactured from low-magnetic-permeability alloys. They are commonly placed around magnetic survey instruments and certain measurement-while-drilling tools to reduce magnetic interference. The required nonmagnetic spacing depends on the survey system, BHA design, well direction, and geographic location.
Pony collars are shortened drill-collar sections used for spacing or positioning components within the BHA. "Pony" describes the length rather than the outside profile or material, so a pony collar may also be slick, spiral, or nonmagnetic.
Square collars and other specialized profiles are available for particular hole conditions or directional objectives, but they are less common than standard slick, spiral, and nonmagnetic designs. Their use should be based on the approved BHA program and manufacturer guidance.
Drill collars are normally identified by:
Nominal lengths near 30 to 31 feet are common, although shorter and specialty lengths are available. The exact dimensions, tolerances, connection, and weight must be confirmed using the manufacturer's current data.
The following table shows representative configurations. The weights are approximate and will change with bore diameter, connection geometry, spiral machining, recesses, and other features.
| Collar OD | Example Bore | Approximate Weight | Common Connection Examples | Typical Application Range |
|---|---|---|---|---|
| 4 3/4 in. | 2 1/4 in. | About 47 lb/ft | NC38 | Smaller hole sections |
| 6 1/4 in. | 2 13/16 in. | About 83 lb/ft | NC46 | Intermediate hole sections |
| 6 1/2 in. | 2 13/16 in. | About 91 lb/ft | NC46 or other engineered selection | Intermediate hole sections |
| 8 in. | 3 in. | About 147 lb/ft | NC56 or 6 5/8 REG | Commonly associated with 12 1/4-in. hole programs |
| 9 1/2 in. | 3 in. | About 217 lb/ft | 7 5/8 REG or other engineered selection | Larger hole sections |
These are examples only. The drilling program, bit and hole size, BHA design, connection capacity, availability, and fishing plan determine the correct configuration.
Some drill collars include handling or fatigue-management features such as:
These features are not automatically appropriate for every collar size or application. They must comply with the applicable specification and the manufacturer's dimensional limits.
Several American Petroleum Institute documents are commonly referenced for drill collars and rotary-shouldered connections:
| Standard | General Scope |
|---|---|
| API Specification 7-1 | Manufacturing requirements for rotary drill-stem elements, including drill collars and heavy weight drill pipe. |
| API Specification 7-2 | Threading and gauging requirements for rotary-shouldered connections. |
| API Recommended Practice 7G | Drill-stem design and operating-limit guidance. |
| API Recommended Practice 7G-2 / ISO 10407-2 | Inspection and classification of used drill-stem elements. |
API Specification 7-1 is currently maintained in its second edition with subsequent addenda and errata. API Specification 7-2 has also received recent updates. Users should verify the current edition and any operator-specific supplements before manufacturing, purchasing, inspecting, or repairing equipment. Current editions and revisions are tracked through API's addenda and errata listings.
Manufacturer data published by NOV lists the following minimum mechanical properties for standard steel drill collars:
| Drill Collar Diameter | Minimum Yield Strength | Minimum Tensile Strength | Minimum Hardness |
|---|---|---|---|
| 3 1/8 through 6 7/8 in. | 110,000 psi | 140,000 psi | 285 BHN |
| 7 in. and larger | 100,000 psi | 135,000 psi | 285 BHN |
These values should not be treated as a complete purchase specification. Chemistry, heat treatment, impact requirements, hardness limits, dimensional tolerances, inspection, marking, and documentation must be confirmed against the applicable standard and manufacturer requirements.
Some operators also specify DS-1, NS-1, or their own inspection and manufacturing criteria. Those requirements supplement or modify the basic program only when they are specifically invoked by the owner, operator, or drilling contractor.
Drill-collar selection is part of the complete BHA design. Important factors include:
A traditional fishing-clearance check sometimes compares drill-collar OD, bit OD, and the casing-coupling OD. This can help identify whether washover equipment would have suitable clearance, but it should not be presented as a universal drill-collar sizing formula. The fishing program and actual well geometry must be reviewed.
Similarly, preliminary calculations may estimate the number of collars from buoyed weight and desired WOB. In deviated or horizontal wells, simple vertical-weight or cosine calculations do not capture the full loading condition. Torque-and-drag analysis and qualified BHA engineering are normally required.
Drill collars operate under repeated rotation, bending, axial loading, vibration, pressure, and abrasive contact. Common damage and failure mechanisms include:
Connection fatigue is a major concern because each rotation can create a stress cycle when the BHA is bent. Bending strength ratio, or BSR, is one consideration when matching pin-and-box geometry. A value near 2.5 has traditionally been used as a design reference for many standard drill-collar connections, but acceptable limits must be verified using the current API, manufacturer, and operator criteria for the exact connection and collar dimensions.
Correct makeup, controlled drilling parameters, suitable stress-relief features, proper handling, condition monitoring, and an appropriate inspection program all help manage drill-collar reliability.
Drill collars and heavy weight drill pipe, or HWDP, are both used in the BHA, but they are built differently and serve different purposes.
| Feature | Drill Collars | Heavy Weight Drill Pipe |
|---|---|---|
| Construction | Normally machined from a solid bar with integral connections. | Commonly manufactured with a heavy-wall tube and extended tool joints; integral designs are also available. |
| Relative stiffness | Very stiff for its outside diameter. | More flexible than drill collars. |
| Weight | High weight per foot. | Heavier than conventional drill pipe but generally lighter than a comparable drill collar. |
| Primary role | Provides concentrated weight and stiffness near the bit and BHA tools. | Provides a gradual stiffness transition and can contribute weight within the BHA. |
| Handling | May require lift subs, safety clamps, or dedicated recesses and handling equipment. | Typically handled more like drill pipe, subject to the manufacturer's requirements. |
Many BHAs use both drill collars and HWDP. The quantity and placement depend on the well path, loading, vibration, directional plan, and drilling program. Extended-reach and high-angle wells may use different proportions than conventional vertical wells, but there is no single arrangement that applies to every well. NOV's heavy weight drill pipe product data outlines the construction variations available.
Used drill collars are inspected under an equipment-owner or operator program based on service history, well criticality, operating environment, and the inspection category required by the applicable standard.
Depending on the specified inspection scope, the program may include:
API RP 7G-2 defines inspection methods, qualification requirements, equipment calibration, and classification practices, but it should not be summarized as requiring the same inspection method before every run. The interval and scope must come from the operator's or equipment owner's approved program.
Connection repair or refacing is also limited by dimensional and acceptance criteria. A damaged shoulder is not automatically repairable. Any repair should be performed and verified by a qualified facility using the applicable specification and remaining dimensional limits.
Superior Performance does not supply, design, or inspect drill pipe or drill collars. Our involvement with drill pipe is limited to supplying rental crossover subs and well-control safety valves with connections selected to support the operator's drill-pipe program.
API Standards for Drill Pipe Inspections - Let's take a closer look at some of the most important API standards for drill pipe inspections, including 5A, RP 7G-2 and 5DP