In the oil and gas industry, tubing connections determine the overall strength, integrity, and sealing performance of the completion string. According to the API 5CT standard, tubing is primarily categorized into Upset Tubing (EUE) and Non-Upset Tubing (NUE).
Upset Tubing (EUE) – External Upset End
EUE stands for External Upset End. Its defining characteristic is a forging process (upsetting) that increases the wall thickness at the pipe ends before the threads are cut.
Technical Details:
Structure: The wall thickness at the ends is greater than the pipe body. This ensures that after threads are machined, the remaining metal at the thread root is still equal to or thicker than the original pipe wall.
Joint Strength: Due to the reinforced ends, the joint efficiency is typically 100%. This means the connection is as strong as, or stronger than, the pipe body itself, allowing it to support massive hanging weights in deep wells.
Thread Profile: The standard is 8 threads per inch (8rd) using a Round Thread design. The rounded crests and roots reduce stress concentration and minimize "galling" during assembly.
Key Data (Example: 2-7/8" 6.4 lb/ft):
Pipe OD: 2.875" (73.03 mm)
Upset OD: 3.094" (78.58 mm)
Coupling OD: 3.668" (93.17 mm)
Tensile Yield: For L80 grade, the joint strength is approximately 145,000 lbs.
Non-Upset Tubing (NUE) – Non-Upset End
NUE stands for Non-Upset End. These threads are machined directly onto the flat ends of the pipe without any prior thickening or forging.
Technical Details:
Structure: The wall thickness is uniform from the center of the pipe to the very end. Since threading removes metal from the wall, the connection becomes the thinnest and most vulnerable part of the tube.
Joint Strength: The joint efficiency is significantly lower, typically 60% to 80% of the pipe body strength. Therefore, it is not suitable for high-tension loads or deep-well environments.
Thread Profile: The standard is usually 10 threads per inch (10rd). These finer threads are designed for lower stress and lower pressure applications.
Advantage: The primary benefit is a smaller coupling diameter. In wells where the clearance between the tubing and casing is extremely tight, EUE couplings may not fit, making NUE the only viable option.
Key Data (Example: 2-7/8" 6.4 lb/ft):
Pipe OD: 2.875" (73.03 mm)
End OD: 2.875" (73.03 mm) - No change.
Coupling OD: 3.500" (88.90 mm) - Approximately 4.3 mm narrower than EUE.
Joint Efficiency: Significantly lower; prone to "parting" (snapping at the threads) under high tensile stress.
The main difference between Upset and Non-Upset tubing lies in the thickness of the pipe ends and the resulting strength of the connection.
Physical Structure
Upset Tubing (EUE): Before the threads are cut, the ends of the pipe undergo a forging process (upsetting) that makes the wall thicker than the rest of the pipe body. The extra thickness is on the outside.
Non-Upset Tubing (NUE): The pipe is a straight tube. the wall thickness at the ends is exactly the same as the rest of the pipe.
Connection Strength (The Most Important Factor)
Upset (EUE): When threads are cut into a pipe, metal is removed, which usually creates a weak point. Because EUE tubing is thickened at the ends first, the metal remaining after the threads are cut is still thick enough to be as strong as (or stronger than) the pipe body.
Joint Efficiency: ~100%
Non-Upset (NUE): Cutting threads into a straight pipe significantly reduces the wall thickness at the joint. Therefore, the connection is the "weak link" in the string.
Joint Efficiency: ~60% to 80%
Thread Specifications
Upset (EUE): Typically uses 8rd (8 round threads per inch). The threads are larger and more rugged.
Non-Upset (NUE): Typically uses 10rd (10 round threads per inch). The threads are smaller and finer.
Comparison Table
| Feature | Upset Tubing (EUE) | Non-Upset Tubing (NUE) |
| End Shape | Thickened/Bulged | Straight/Uniform |
| API Code | EU | NU |
| Joint Strength | Stronger than pipe body | Weaker than pipe body |
| Threads/Inch | 8 TPI | 10 TPI |
| Cost | Higher (due to forging) | Lower |
| Typical Use | Standard production tubing | Shallow, low-pressure wells |
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FAQ
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