Drill collar specifications are strictly governed by the API Spec 7-1 (Specification for Rotary Drilling Equipment). Because their primary function is to provide Weight on Bit (WOB) and maintain string tension, their specifications focus on extreme weight, wall thickness, and material toughness.
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Drill collar Specifications
To withstand high torque and prevent fatigue failure, API drill collars are manufactured from high-strength alloy steel.
Standard Material: AISI 4145H Modified Chromium-Molybdenum (Cr-Mo) alloy steel.
Heat Treatment: Full-length quenched and tempered to reach a uniform hardness and strength.
Yield Strength: 110,000 psi (758 MPa) minimum.
Tensile Strength: 140,000 psi (965 MPa) minimum.
Hardness Range: 285 to 341 HB (Brinell Hardness).
Charpy V-notch Impact Strength: Minimum 40 ft-lbs at 70°F (21°C).
Drill collar Dimension (Standard Sizes)
The "size" of a drill collar is defined by its Outside Diameter (OD). Unlike drill pipe, the OD is uniform across the entire joint.
| Outside Diameter (OD) | Inside Diameter (ID) | Approx. Weight (lb/ft) | Standard API Connection |
| 3-1/8" | 1-1/4" | 21 | NC23 (2-3/8" REG) |
| 4-1/8" | 2" | 35 | NC31 (2-7/8" IF) |
| 4-3/4" | 2-1/4" | 47 | NC35 / NC38 (3-1/2" IF) |
| 6" | 2-1/4" | 82 | NC44 (4" IF) |
| 6-1/2" | 2-13/16" | 92 | NC46 / NC50 (4-1/2" IF) |
| 7" | 2-13/16" | 110 | NC50 (4-1/2" IF) |
| 8" | 2-13/16" | 150 | NC56 |
| 9" | 3" | 192 | NC61 / 7-5/8" REG |
| 9-1/2" | 3" | 216 | NC70 / 8-5/8" REG |
Drill collar Length Specifications
API drill collars come in three standard length ranges. Range 2 is used in 95% of all drilling operations worldwide.
Range 1: 18 – 22 ft (5.5 – 6.7 m)
Range 2: 30 – 32 ft (9.15 – 9.75 m) - The Industry Standard.
Range 3: 38 – 45 ft (11.5 – 13.7 m)
Drill collar Connection & Manufacturing Features
Thread Types: Rotary Shouldered Connections (NC, REG, and sometimes IF).
Cold Rolling: Thread roots are cold-rolled to compress the metal and increase fatigue resistance.
Phosphate Coating: Threads are phosphated or copper-plated to prevent galling (thread damage) during makeup.
Bore Alignment: Precision deep-hole boring ensures the internal diameter (ID) is centered to maintain balance during rotation.
Stress Relief Features: API relief grooves (Pin) and bore backs (Box) are often machined into the connections to distribute stress.
Classification by Surface Design
Slick Drill Collar: A smooth, uniform cylindrical surface. Used for vertical wells and standard formations.
Spiral Drill Collar: Features three helical grooves machined into the OD.
Spec: Reduces contact surface area by 40%.
Purpose: Prevents Differential Sticking by allowing mud to circulate around the pipe even if it is resting against the wellbore wall.
Non-Magnetic Drill Collar: Manufactured from specialized high-Mn, low-C stainless steel.
Spec: Magnetic Permeability μ≤1.010.
Purpose: Houses MWD/LWD tools to ensure accurate directional survey readings.
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FAQ
What size is a drill collar?
The outer diameter of drill collars commonly ranges from 3-1/8″ to 11″ (79.4–279.4 mm), with specific dimensions selected according to hole design and drilling requirements. The standard length is typically 9.15 meters or 9.45 meters.
What is the length of a drill collar?
Drill collars are typically 9.15 meters or 9.45 meters in length, which is a standard specification designed to match with drill pipes (approximately 9.14 meters per joint), enhancing drilling assembly connection efficiency and downhole stability.
