What OCTG Drill Pipe Is and Where It Sits in the String
Drill pipe is the basic component of the drill string. It is used to deepen the wellbore, transmit torque from the rotary table or top drive to the bit, carry the weight of the string below the neutral point, and provide a circulation path for drilling fluid. The string is assembled as a square or kelly section at the top, the drill pipe bodies in the middle with a number of joints set by the well depth, and the heavy weight drill pipe and bottom hole assembly at the bottom, with the count of heavy joints set by the bottom hole assembly design.
Because the pipe is run and pulled many times during the life of a well, drill pipe is treated as a reusable engineered component rather than as a consumable tubular. Its dimensions, material and connections are standardised so that joints from different manufacturers can be assembled into one string.
Standards, Sizes and Upset Types
Drill pipe is manufactured in accordance with API 5D for the pipe body and API Spec 7-1 for the tool joints, with API RP 7G providing the design data used for string calculations.
| Item | Range supplied |
|---|---|
| Nominal size | 2 3/8 in, 2 7/8 in, 3 1/2 in, 4 in, 4 1/2 in, 5 in, 5 1/2 in and 6 5/8 in |
| Outside diameter, metric | 60.3 mm, 73.0 mm, 88.9 mm, 101.6 mm, 114.3 mm, 127.0 mm, 139.7 mm and 168.3 mm |
| Wall thickness | 6.5 mm to 12.7 mm and heavier as required by the design |
| Upset | Internal upset, external upset and internal and external upset |
| Length range | Range 1 about 18 ft to 22 ft, Range 2 about 27 ft to 30 ft, Range 3 about 38 ft to 40 ft |
| Grades | E75, X95, G105 and S135 to API 5D, with R780 and similar designations used in some drilling specifications |
Upset design is a practical choice. Internal and external upset pipe has a thicker wall over a controlled length at each end, so that the section removed during friction welding and the bending stress concentration behind the tool joint are both compensated. Range 2 pipe is the standard for most drilling operations, Range 3 reduces the number of connections in deep wells, and Range 1 is still used for shallow and workover work.
Grades and Strength Levels
| Grade | Yield strength, psi | Yield strength, MPa | Minimum tensile strength, psi |
|---|---|---|---|
| E75 | 75,000 to 105,000 | 517 to 724 | 100,000 |
| X95 | 95,000 to 125,000 | 655 to 862 | 105,000 |
| G105 | 105,000 to 135,000 | 724 to 931 | 115,000 |
| S135 | 135,000 to 165,000 | 931 to 1,138 | 145,000 |
Grade selection follows the load case that governs the well: tensile load at the top of the string, torque transmitted to the bit, and the fatigue environment created by rotation in a curved wellbore. E75 is used for light duty and shallow work, while G105 and S135 are used where tensile load and torque are high. Hydrogen sulphide in the reservoir fluid restricts the choice, because resistance to sulphide stress cracking depends on chemistry and hardness control rather than on strength alone.
Tool Joints and Connections
Tool joints are forged and heat treated separately from the pipe body, then friction welded to the pipe ends and machined with the connection thread. Common API connections in the sizes supplied are NC26, NC31, NC38, NC40, NC46 and NC50, together with 5 1/2 FH and 6 5/8 FH full hole connections. The tool joint is furnished in a minimum yield strength grade of 120,000 psi, that is 827 MPa, with hardness controlled so that threads resist galling and the shoulder resists deformation during repeated make up.
Tool joint outside diameter is normally larger than the pipe body, so the pipe rests on the tool joint rather than on the body when it lies in the wellbore, which protects the body from wear. Hardbanding can be applied to the tool joint for casing friendly or tool joint friendly service, and the material is selected so that it does not damage the inner wall of the casing.
Inspection, Service Classes and Shipment
New pipe is verified by tensile, hardness and, where specified, impact testing, with wall thickness measurement and full length electromagnetic inspection.
Used pipe is graded into service classes by remaining wall, remaining tool joint outside diameter and thread condition, following the inspection requirements of the applicable standard.
Tool joints are examined by magnetic particle inspection for cracks in the thread and shoulder area, and threads are gauged and refurbished where necessary.
Each joint is stencilled or tagged with size, grade, wall, upset, connection and heat number, and is supplied with a material certificate and inspection report.
For shipment, threads are coated with a specified compound and fitted with steel or plastic protectors, and joints are bundled or racked so that the body is not damaged in transit. The enquiry should state the size, grade, wall thickness, upset, length range, connection type, hardbanding requirement, inspection scope and the drilling environment so that the correct combination is supplied.
FAQ
Q: What is the difference between drill pipe and drill collar?
Drill pipe is the long, relatively thin walled tube that makes up most of the string and transmits torque and fluid. Drill collars and heavy weight drill pipe are thick walled components run near the bottom to supply weight on bit and keep the pipe body in tension.
Q: Why is drill pipe supplied in length ranges rather than fixed lengths?
Ranges R1, R2 and R3 give the mill an ordered length window while still allowing efficient production. The driller adjusts the last joints to make up the required total string length.
Q: What does the upset designation on drill pipe mean?
It describes where the wall is thickened near the ends. Internal upset thickens the bore side, external upset thickens the outside, and internal and external upset thickens both, giving a smoother bore and a larger outside profile for welding to the tool joint.
Q: Can drill pipe be supplied for sour service?
Yes, but the grade, chemistry and hardness must be selected with sulphide stress cracking in mind, and the inspection programme is normally tightened for sour wells.
Q: How is tool joint wear assessed in service?
The outside diameter of the tool joint is measured and compared with the minimum values for the connection size, and joints that have worn beyond the limit are reworked or removed from the string.
Q: What information is needed to order a drill string?
The hole size and well profile, required size and grade in each section, wall thickness, upset and connection type, length range, quantity, inspection and hardbanding requirements, and the drilling fluid chemistry.
