Explanation of Casing Connections in Petroleum Industry

Explanation of Casing Connections in Petroleum Industry

Connections are one of the most common parts in tools and also one of the hardest to distinguish. Understanding connections is crucial for tool technicians or supervisors. Without knowledge of connections, procuring materials, preparing for operations, and directing work would be impossible and could lead to significant issues. This week’s focus is primarily on recognizing various common connection types, including tubing connections, casing connections, screen and liner connections, seal unit connections, drill pipe connections, etc.

1. Common Tubing Connection Types (Tubing Joint)
There are three common types of tubing connections: EU (External Upset), NU (Non-Upset), and NewVam. These connection types can all be found in the tool shop. It is challenging to distinguish EU from NU solely by their external appearance since both have triangular threads. However, they can be easily identified when viewed as part of the entire pipe string. Specifically, EU stands for External Upset, indicating an externally thickened end, whereas NU stands for Non-Upset, indicating no external thickening. NewVam is a trapezoidal thread (with a rectangular cross-section) that also lacks external thickening, making it easy to identify. Below is a detailed explanation of these three connection types with diagrams.
1) EU (External Upset): Externally Thickened
The EU connection is an externally thickened tubing connection. During the identification of adapters in the shop, you will also encounter three types of biano markings related to EU: EUE (External Upset End) indicates an externally thickened end; EUP (External Upset Pin) indicates an externally thickened pin; and EUB (External Upset Box) indicates an externally thickened box. Besides ‘pin’ and ‘box’, other terms for male threads include ‘external thread’, ‘male’, and ‘male thread’; female threads include ‘female thread’, ‘internal thread’, ‘box’, and ‘box thread’.
Figure 1-1: EU Connection Type
2) NU (Non-Upset): Not Externally Thickened
NU indicates a tubing connection without external thickening. Besides lacking external thickening, NU usually has ten threads per inch, while EU usually has eight threads per inch. NUE indicates a non-thickened end. Here, ‘E’ stands for ‘End’. [This information comes from “Encyclopedia of Petroleum”.]
Figure 1-2: NU Connection Type
3) New VAM
This connection type features a rectangular cross-section, equal pitch spacing, minimal taper, and no external thickening. It is seen at the ends of production sliding sleeves in the shop.
Figure 1-3: New VAM Connection Type

2. Summary of Common Drill Pipe Connections
Common drill pipe connections are typically REG and IF, while others like FH are not commonly found in the tool shop. According to experienced instructors, REG connections usually have five threads per inch, while IF connections have four threads per inch. However, drill pipes larger than 4-1/2 inches with four threads per inch are still classified as REG connections, meaning that REG connections are predominantly used for sizes greater than 4-1/2 inches, while IF connections are more common for smaller sizes.
1) REG (API Regular Thread): Standard Connection According to API
Standard drill pipe connections use this type of thread. Originally designed for connecting internally thickened drill pipes, creating a passage where the drill pipe connection ID is smaller than the thickened section ID, which in turn is smaller than the drill pipe body ID. [From “Well Intervention and Workover Engineering” by Nie Haiguang, Wang Xinhua et al., published by Petroleum Industry Press, Beijing, February 2002, First Edition].
When comparing REG and IF connections by touch, REG feels smoother due to the higher thread density per unit length, while IF feels rougher. Figure 1-4 is a sectional diagram of the REG connection, and Figure 1-5 shows the physical image.
Figure 1-4: REG Connection Type
Figure 1-5: Physical Image of REG Connection
2) IF (API Internal Flush): Internal Flush Connection According to API
Internal flush drill pipe connections use this type of thread. Used for connecting externally thickened or internally and externally thickened drill pipes, creating a passage where the drill pipe connection ID, thickened section ID, and drill pipe body ID are approximately equal. [From “Well Intervention and Workover Engineering” by Nie Haiguang, Wang Xinhua et al., published by Petroleum Industry Press, Beijing, February 2002, First Edition].
Figure 1-6 is a sectional diagram of the IF connection.
Figure 1-6: IF Connection Type
3) FH (API Full Hole): Full Hole Connection According to API
The full hole connection, as the name suggests, means that the inner diameters of the drill pipe connection and the thickened section are generally the same size, though both are usually smaller than the drill pipe body ID. Figure 1-7 illustrates a comparison of the three drill pipe connection types.
Figure 1-7: Comparison of Three Drill Pipe Connection Types

3. Summary of Common Liner and Screen Connection Types
1) LTC (Long Thread Connector): Long Thread Connection
LTC threads look identical to STC threads and can connect, but such connections are generally not allowed because they have different maximum torque ratings.
Figure 1-8: Physical Image of LTC Connection
2) STC (Stub(Short) Thread Connector): Short Thread Connection
Both STC and LTC threads belong to API standard threads, differing only in thread length. Longer threads are LTC, shorter ones are STC. For a 4-1/2 inch outer diameter pipe, after my measurements, threads longer than 3 inches are generally LTC, and those shorter than 3 inches are STC. This experience needs verification.
3) BTC (Buttress Thread Coupling): Buttress Thread Coupling
BTC is easy to distinguish from LTC and STC, but it is very similar to NewVam. The distinction can only be made based on the outer diameter of the pipe and its application.
Figure 1-9: BTC Buttress Thread

4. Common Connection Types for Washpipe
Washpipe is a commonly used string in sand control completion operations, mainly serving as a conduit for completion fluid. Therefore, the string generally does not have couplings or thickening except for 1.9-inch washpipe, which has external thickening. The reason is likely that the wall of the 1.9-inch washpipe is too thin, and direct threading without external thickening could affect the mechanical properties of the entire string. Common washpipe connections in the shop include Hydrill 511, Hydrill CS, and TSWP. 2-7/8 inch washpipes are generally Hydrill 511, 1.9 inch are generally Hydrill CS, and others like 4 inches are TSWP. Both Hydrill CS and TSWP are buttress threads, which are hard to distinguish visually, so they are identified by their outer diameter.
1) TSWP (BAKER OIL TOOLS WASHPIPE)
Figure 1-10a: Cross-sectional Diagram of TSWP Connection
Figure 1-10b: Physical Image of TSWP Connection
2) TSS (BAKER OIL TOOLS SPECIAL)
Figure 1-11: Cross-sectional Diagram of TSS (BAKER Special Washpipe Connection)
3) Hydrill 511
Figure 1-12: Physical Image of Hydrill 511

5. Common Connection Types for Seal Collars
Acme Threads
Figure 1-13: ACME Connection
Seal collars often use ACME threads, characterized by a trapezoidal cross-section (approximating a rectangle), short threads, and a smooth extension before the male threads begin.

6. Other Connection Types
FL-4S Connection
FL-4S connections are found on pressure test plugs and feature a near right-angle trapezoidal cross-section.
LP Connection
LP connections are seen on adapter couplings, usually having short threads, small thread angles, and the root diameter of the male threads being the same as the body diameter.