Drill pipe

Drill pipe, is hollow, thin-walled, steel or aluminium alloy piping that is used on drilling rigs to facilitate the drilling of a wellbore which comes in a variety of sizes, strengths, and weights but are typically 27 to 32 feet in length (Range 2). Longer lengths, up to 45 feet, exist (Range 3). They are hollow to allow drilling fluid to be pumped through them, down the hole, and back up the annuls.

Because it is designed to transfer drilling torque for combined lengths that often exceed 1 mile down into the Earth's crust, the tempered steel tubes are expensive, and owners spend considerable efforts to reuse them after finishing a well. A used drill stem is inspected on site, or off location. Modified instruments similar to the spherometer are used at inspection sites to identify defects in the metallurgy, in order to prevent fracture of the drill stem during future wellboring. New drill pipe is classed as new (N class), becoming premium (P-class) and finally down to C (C 1 to 3) as the body outside diameter is worn down by usage. Eventually the drill pipe will be graded a scrap and marked with a red band.

Drill pipe is a portion of the overall drill string. The drill string consists of both drill pipe and the bottom hole assembly (BHA) which is the tubular portion closest to the bit. The bottom hole assembly will be made of thicker walled heavy weight drill pipe (HWDP) and drill collars which have a larger diameter and smaller internal diameter and provide weight to the drill bit and stiffness to the drilling assembly. Other BHA components can include mud motor, measurement while drilling collar (MWD), stabilizers, and other specialty downhole tools. The drill stem includes the entire drill string and the kelly which provides rotation and torque to the drill pipe.

See Drilling rig (petroleum) for a diagram of a drilling rig.

Manufacturing Process

Modern drill pipe is made from the welding of at least three separate pieces: box tool joint, pin tool joint, and the tube. The green tubes are received by the drill pipe manufacturer from the steel mill. The ends of the tubes are then upset to increase the cross sectional area of the ends. The tube end may be externally upset (EU), internally upset (IU), or internally and externally upset (IEU). Standard max upset dimensions are specified in API 5DP, but the exact dimensions of the upset are proprietary to the manufacturer. After upsetting, the tube then goes through a heat treating process. Drill pipe steel is commonly quenched and tempered to achieve high yield strengths (135 ksi is a common tube yield strength).

The tool joints (connections) are also received by the manufacturer as green tubes. After a quench and temper heat treat, the tool joints are cut into a box (female) and pin (male) end. Tool joints are commonly 120 ksi SMYS, rather than the 135 ksi of the tube. The connection is generally more stiff than the tube, increasing the likelihood of a fatigue failure. The lower SMYS on the connection increases the fatigue resistance. Higher strength steels are typically harder and more brittle, making them more susceptible to cracking and subsequent stress crack propagation.

Tubes and tool joints are welded using inertia or friction welding. The tube is held stationary while the tool joint is revolved at high RPMs. The tool joint is the firmly pressed onto the upset end of the tube while the tool joint is rotating. The heat and force during this interaction welds the two together. Once the "ram horns" or excess material is removed, the weld line can only be seen under a microscope.