Drill pipe is the core component of the drill string, made from seamless alloy steel and composed of the pipe body, tool joint, and weld zone. It transmits torque, circulates drilling fluid, and bears drill string load. Its specifications and structure vary with different drilling applications, directly impacting drilling safety and efficiency.
1. Drill Pipe Structure
(1) Pipe Body
Material: High-strength alloy steel
Features: Thin wall, light weight, good elasticity
Function: Transmits torque and drilling fluid; main load-bearing section
(2) Tool Joint
Material: Higher-grade alloy steel, heat-treated
Features: Larger OD and wall thickness, threaded connections
Functions:
Connects single drill pipes
Bears tensile, compressive, and torsional loads
Wear-resistant band protects against wellbore friction
Types: IF, FH, REG, NC, and other special threads.
(3) Weld Zone
Joining Method: Friction welding
Requirement: Weld strength ≥ pipe body strength
Status: Friction-welded drill pipe is the industry mainstream
2. Connection Methods
Fine-thread drill pipe (older design, mostly phased out)
Friction-welded drill pipe (mainstream; high strength and stability)
Pipe body ends are usually thickened (internal, external, or both) to enhance joint strength.
3. Common Drill Pipe Specifications (API 5DP)
Outer Diameters
73.0 mm (2-7/8”) – Shallow wells
114.3 mm (4-1/2”) – Medium/deep wells
127.0 mm (5”) – Most widely used
139.7 mm (5-1/2”) – Deep/ultra-deep wells
Weight (Wall Thickness)
Example (5” drill pipe):
Standard: 19.50 lb/ft
Heavyweight: 25.60 lb/ft
Steel Grades (Yield Strength)
E75, X95, G105, S135
Higher grade = higher tensile & torsional strength
Strength upgrade should prefer higher steel grade rather than simply increasing wall thickness
4. Application Differences
(1) Oil Drilling
Environments: Deep/ultra-deep wells, high torque, high hook load
Common sizes: 3-1/2", 4", 4-1/2", 5", 5-1/2"
Common grades: G105, S135
Features: Thick tool joints, wear-resistant bands, optional internal coating
Selection Guide:
Deep wells: 5" S135 / 5-1/2" S135
Medium wells: 4-1/2" G105 / 5" G105
Directional/horizontal wells: High-strength steel + special threads
(2) Geological Exploration
Designed for coring: lightweight, easy to handle, frequent trips.
Common series:
AQ / BQ / NQ / HQ / PQ
Increasing OD and core size from AQ → PQ
Selection Guide:
Mineral exploration: NQ / HQ (mainstream)
Shallow engineering geology: AQ / BQ
Deep-hole coring: HQ / PQ
5. Drill Pipe Length (API Ranges)
R1: 5.49–6.71 m
R2: 8.23–9.14 m
R3: 11.58–13.72 m
