Every API 5DP drill pipe has forged upsets at both ends where the tool joints are friction-welded. The upset design — EU (External Upset) or IEU (Internal-External Upset) — determines the tool joint OD, the pipe’s internal flow area, and the connection’s torsional and tensile capacity. Understanding the difference prevents specifying a drill string that either cannot pass through the casing or cannot transmit the required torque.
EU: External Upset Only
External upset drill pipe has the pipe body forged outward at each end, increasing the OD in the upset zone to provide sufficient wall thickness for the tool joint weld. The internal diameter remains unchanged through the upset, maintaining a consistent bore from pipe body through the tool joint.
EU drill pipe is used predominantly in smaller-diameter strings (2-3/8″ and 2-7/8″) where the pipe body ID is too small to accommodate internal upsetting without excessively restricting the flow area. The consistent bore is important for hydraulic efficiency: the pressure drop through the drill string is a function of the minimum internal diameter, and any restriction caused by internal upsetting reduces the flow rate available for bit cleaning and cuttings transport.
IEU: Internal-External Upset
Internal-external upset drill pipe has the pipe wall forged both outward and inward at each end. The combined upset provides a much thicker wall in the upset zone, which in turn supports a larger-OD tool joint with greater torsional capacity and longer thread engagement. The trade-off is a reduced ID in the upset zone, creating a flow restriction.
IEU is the standard for larger-diameter drill pipe (4″, 4-1/2″, 5″, 5-1/2″, 6-5/8″) where the pipe body ID is large enough that the IEU restriction does not significantly affect hydraulic performance. The larger tool joint OD enabled by IEU provides substantially higher torsional strength — typically 30–50% more than EU for the same pipe size — and the longer thread engagement provides greater joint tensile efficiency.
Tool Joint Sizing
The tool joint OD is constrained by the casing through which the drill pipe must pass. The maximum tool joint OD must provide adequate clearance inside the smallest casing in the well — typically the production casing or intermediate casing through which the drill string rotates. Industry practice requires a minimum clearance of 0.75–1.0 inch between the tool joint OD and the casing drift diameter to allow for drill string vibration, wellbore deviation, and cuttings passage in the annulus.
IEU’s larger tool joint OD means it may not be suitable for drilling through small-bore casing strings where EU’s smaller tool joint provides the necessary clearance. This is a common consideration in slim-hole drilling and in re-entry wells where the drill string must pass through existing production tubing.
Connection Types
Tool joint connections use API numbered connections (NC26, NC31, NC38, NC46, NC50) or API regular and full-hole designations. The connection number corresponds to the pitch diameter of the connection thread. Larger connections provide higher torsional capacity but require larger tool joint ODs. The connection selection is driven by the required torsional capacity (matched to the rig’s make-up torque capability) and the casing clearance constraint discussed above.
Practical Selection
For small-hole drilling (2-3/8″ and 2-7/8″ pipe): EU with NC26 or NC31 connections. For standard mid-range drilling (3-1/2″ and 4″ pipe): IEU with NC38 or NC46 connections. For large-hole and deepwater drilling (5″ and 5-1/2″ pipe): IEU with NC50 or 6-5/8 FH connections. Always verify that the selected tool joint OD clears the smallest casing in the well program.
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