What Is a Drill Pipe?
Drill pipe is the steel tube with threaded connections at both ends that connects the surface drilling rig to the drilling tools and bottom-hole assembly. It performs three jobs at once: it carries drilling mud down to the bit, it transmits rotation and torque from the surface to the bit, and it supports the tensile load of the string. During drilling the pipe must survive high internal and external pressure, torsion, bending and continuous vibration, and in oil and gas operations a string is pulled and re-run many times, so the pipe must also be resistant to fatigue.
Drill String Arrangement
A typical drill string is assembled from top to bottom as: one kelly, the drill pipe section, the weighted drill pipe section, and then drill collars and the bit. The kelly is a square or hexagonal tube that slides through the rotary table and transmits rotation to the string. The number of drill pipe joints depends on the well depth, while the number of weighted drill pipe joints depends on the design of the bottom-hole assembly. Weighted drill pipe has a thicker wall than standard drill pipe and is placed above the drill collars to provide a smooth transition in stiffness.
Design Features of Seamless Drill Pipe
Seamless drill pipe is made by hot piercing and rolling of a solid billet, which gives a continuous fiber structure and better fatigue resistance than welded construction. The pipe body has a thin wall and a low weight per unit length so that the hoisting load stays manageable. To strengthen the connection between the pipe body and the tool joint, both ends of the pipe body are upset, or thickened, before the tool joints are friction welded on.
| Upset form | Abbreviation | Where the thickening occurs |
|---|---|---|
| Internal upset | IU | Inside diameter reduced at both ends |
| External upset | EU | Outside diameter increased at both ends |
| Internal and external upset | IEU | Both inside and outside thickened at both ends |
The upset provides extra wall thickness where the tool joint is attached, so that the threaded connection is not the weakest point of the string.
Material Grades and Standards
Seamless drill pipe is manufactured to API Specification 5DP, which defines four common grades by minimum yield strength: E75 at 75 ksi, X95 at 95 ksi, G105 at 105 ksi and S135 at 135 ksi. Higher grades carry more load for the same weight but are more sensitive to handling damage and require more careful inspection. The pipe body and tool joint are made of alloy steel, and the tool joint is heat treated to a higher hardness than the body for wear resistance at the connection.
Service Conditions and Inspection
Drill pipe fails most often by fatigue, wear and corrosion, or by mechanical damage from slips, tongs and handling. A robust inspection program is therefore part of normal operation: dimensional checks, electromagnetic inspection of the pipe body, ultrasonic testing of the upset and tool joint area, and thread inspection after each trip or on a defined schedule. The pipe is a reusable asset, and its condition history is tracked to decide when joints should be retired or moved to lower-stress positions in the string.
Applications
Seamless drill pipe is used in oil and gas wells, geothermal drilling, water wells and mining boreholes. In oil and gas drilling it is one of the most important pieces of equipment on the rig, because the entire load path and the mud circuit pass through it. Because the pipe transmits the rotational power of the kelly or top drive to the bit and to the drill collars, its reliability directly controls the speed and safety of the drilling operation.
FAQ
Q1: Why is drill pipe seamless rather than welded?
The seamless process produces a pipe with no longitudinal weld seam, giving uniform strength and better fatigue resistance under the alternating bending and torsion loads of drilling. This is important because weld seams are natural initiation sites for fatigue cracks.
Q2: What is the difference between drill pipe and weighted drill pipe?
Weighted drill pipe has a thicker wall, giving more weight per meter and higher stiffness while still having the same tool joint size. It is placed above the drill collars to reduce the transition in stiffness and to provide weight on bit with less risk of differential sticking.
Q3: What do the grades E75, G105 and S135 mean?
The numbers are the minimum yield strength in ksi. E75 resists about 75 ksi yield stress, G105 about 105 ksi and S135 about 135 ksi. Higher grades allow a lighter string for a given load but are less forgiving of damage.
Q4: Why are the ends of drill pipe upset?
Upsetting adds wall thickness at both ends so the friction-welded tool joint sits on a stronger section. This keeps the connection strength close to or above the pipe body strength, so the thread does not become the weak link.
Q5: What causes most drill pipe failures?
Fatigue from rotation under bending, corrosion and erosion inside and outside the pipe, and mechanical damage such as slip cuts, tong marks and thread damage are the main causes. Regular inspection and correct handling prevent most failures.
Q6: Can drill pipe be reused on the next well?
Yes. Drill pipe is a reusable tubular and is normally inspected, graded and returned to service multiple times. Its remaining life is managed by tracking the inspection history and by moving worn joints to lower-stress positions.