Casing has couplings, and tubing has couplings - their purpose of connecting tubing is the same. On the other hand, there are still some differences between the two.
Tubing Couplings
◆Typical Dimensions and Types
Tubing couplings typically have a maximum outside diameter of 4 1/2". Since tubing is used for drilling activities (the drill/pump sucker rods are inside the tubing), the coupling types are more varied and stringent than for casing. Externally upset (EUE) couplings, also known as EU couplings, are more commonly used and considered the most advantageous type of connection for tubing. Tubing with EU end (EUE) coupling connections is also called UPTBG or Upset Tubing with Bottleneck Geometry.
◆Advantages of EUE Couplings
Due to the upset (thickened) ends of the tubing, EUE connections have superior stress capability compared to buttress couplings. The tubing ends connect to the coupling's inner wall via a tapered thread. The coupling ends connect to the tubing using the same pitch of straight threads. This coupling design mitigates fatigue cracking issues from stress concentrations seen in existing couplings. EUE couplings effectively prevent tubing string failure accidents in the well.
◆Other Tubing Coupling Types
Buttress couplings, where the coupling wall thickness matches the tubing body, are another type used for tubing connections. These are called NU end (NUE) tubing or TBG (Tubing with Grooved Geometry).
Casing Couplings
◆Dimensions
Casing couplings have dimensions greater than 4 1/2 inches. Unlike tubing couplings, which generally do not require upsetting (thickening), so they are non-upset couplings. There are a few key types:
BTC - Buttress Threaded Couplings
These have the same wall thickness as the casing body.
LTC - Long Threaded Casing Couplings
STC - Short Threaded Casing Couplings
The long and short refer to different coupling lengths, but the wall thickness matches the casing.
They are essential for connecting two sections of casing pipe. The manufacturing method is the same as for seamless pipe - the pipe ends have internal threads to connect to the coupling threaded ends above and below. To ensure a tight coupling connection, there are stringent requirements for the thread precision.
◆Steel Grades and Heat Treatment
For both tubing and casing couplings, the steel grade and heat treatment of the couplings must match the connected pipe sections per the relevant API specifications.
◆API 5CT Requirements for Couplings
The API 5CT specification covers the key requirements for casing and tubing couplings:
Material Selection
Per the pipe grade being coupled, the coupling material grade must be properly selected and verified through certification.
◆Mechanical Testing
Tensile strength, impact toughness, sulfide stress cracking resistance (for higher grades) and other mechanical test requirements must be met and documented.
◆Chemical Composition
The chemical composition ranges for each steel grade are tightly controlled.
◆Dimensions and Tolerances
Strict tolerances are specified for outer diameter, wall thickness, length, straightness, threads per inch, thread taper, etc.
◆Thread Specifications
Different thread forms like buttress, taper, etc. have their own dimensional and geometry specifications.
◆Heat Treatment
Normalizing, quenching, tempering requirements are defined.
◆Marking
Specific marking is required on each coupling for traceability.
◆Product Specification Level
Higher PSLs like PSL-2 or PSL-3 involve additional requirements like third-party inspection.
Manufacturing and Testing
Coupling Manufacturing Process
The couplings are typically machined from seamless pipe body sections or coupling blanks. Hot upsetting may be used for certain grades.
Inspection and Testing
Besides the mill's own inspection and testing, third-party inspection involving monitoring of processes and test witnessing may be required for higher PSL levels.
Additional Requirements
◆Specialty Couplings
API 5CT also covers specialty coupling types like combination couplings (different threads at each end), reducer couplings (for transitioning different diameters), seal ring couplings, and special tapered couplings.
◆Non-Integral Connections
While API 5CT focuses on integral threaded and coupled connections, some tubing strings may use non-integral separate compression or wedge-type connectors. These have their own design standards.
◆Effect of Well Conditions
Factors like H2S/CO2 content, high pressures/temperatures, dynamic loading, etc. affect coupling material selection and design requirements.
Casing and tubing couplings play a vital role in the integrity of the well string under various operating conditions. Selecting and using the appropriate coupling type per API specifications is crucial for safe and reliable well operations.
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