Drill pipe
Drill Pipe Overview
Drill pipe is a crucial component in drilling rigs, characterized by its hollow, thin-walled steel or aluminum alloy structure. This design facilitates the pumping of drilling fluid down through the hole, passing through the bit, and back up the annulus. Drill pipes are available in various sizes, strengths, and wall thicknesses, with typical lengths ranging from 27 to 32 feet (classified as Range 2). Longer lengths, up to 45 feet (Range 3), are also available.
Background of Drill Pipe
Drill stems are engineered to transfer drilling torque over lengths that often extend several miles into the Earth's crust. They must withstand pressure differentials along with bearing the weight of deeper components. Tempered steel tubes are typically used for deep wells due to their strength and durability, making these tubes expensive. After completing a well, considerable efforts go into inspecting and reusing the drill stems. Used drill stems undergo on-site or off-location inspections utilizing ultrasonic testing and instruments akin to spherometers to identify fatigue-related defects and prevent future fractures during drilling operations.
Drill pipe is predominantly premium class, characterized by having 80% remaining body wall (RBW). If inspection reveals that the RBW falls below 80%, the pipe is downgraded to Class 2 or "yellow band" pipe. Over time, the pipe is further downgraded to scrap, marked with a red band.
Drill pipe forms part of the drill string, which also includes the bottom hole assembly (BHA). The BHA is the section closest to the bit, comprising heavier, thicker-walled heavy weight drill pipe (HWDP) and drill collars. These components have larger diameters, contributing weight to the drill bit and lending stiffness to the drilling assembly. The BHA may also feature a mud motor, measurement-while-drilling (MWD) apparatus, stabilizers, and specialized downhole tools. The drill stem encompasses the entire drill string, including the kelly at the top, which imparts rotation and torque to the drill pipe.
For a diagram of a drilling rig, see the "Drilling rig (petroleum)" reference.
Drill Pipe Manufacturing Process
The manufacturing of modern drill pipe involves welding at least three distinct pieces: the box tool joint, pin tool joint, and the tube. The primary tube is obtained from the steel mill, and its ends are upset to enhance the cross-sectional area. This upsetting can be external (EU), internal (IU), or both internal and external (IEU). Maximum upset dimensions are standard per API 5DP, though exact specifications are proprietary to the manufacturer. Subsequent to upsetting, the tubes are heat-treated, typically undergoing quenching and tempering to achieve high yield strengths, commonly around 135 ksi.
The tool joints, received as green tubes, are heat-treated similarly, yet they are typically specified to a minimum yield strength (SMYS) of 120 ksi to bolster fatigue resistance. The tool joints are cut into box (female) and pin (male) threads and are often stiffer than the tube, which may lead to fatigue failure at the junction. Higher-strength steels are generally more brittle, raising susceptibility to cracking and stress propagation.
The welding process for tubes and tool joints employs rotary inertia or direct drive friction welding. This process involves rotating the tool joint at high RPMs while pressing it firmly against the stationary tube, generating heat and force to weld them together. Post-welding, excess material (ram horns) is removed, and the weld line remains visible only under a microscope. Direct drive friction welding offers finer quality welds monitored up to 1,000 times a second, often eliminating the need for complete post-weld heat treatment.
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