Seamless casing couplings to API 5CT in grades J55, K55, N80, L80 and P110, in STC, LTC and BTC thread forms, from 4-1/2 inch to 20 inch.
- Standard: API 5CT, API 5B, ISO 11960
- Grades: J55, K55, N80, L80, P110, Q125
Specifications
| Outer diameter | 4-1/2" - 20" |
|---|---|
| Wall thickness | Per API 5CT coupling OD and length tables |
| Length | Per API 5CT, typically 200 - 300 mm |
| Ends | Internally threaded both ends, STC, LTC or BTC |
| Surface treatment | Phosphate coated threads, zinc plated on request |
| Manufacturing process | Seamless tube, machined and threaded |
| Connection type | STC, LTC, BTC |
Why the Coupling Grade Matters
The coupling is often the weakest link in a casing string, so its grade and heat treatment should be specified deliberately rather than defaulted. API 5CT requires that the coupling be of the same grade as the pipe body unless otherwise agreed, and be subjected to the same heat treatment. In practice a higher grade coupling is sometimes specified to increase joint efficiency, or a coupling of the same grade but with controlled hardness is used for sour service. Couplings are seamless because a welded coupling would introduce a seam in the highest stress region of the connection. Each coupling is thread gauged with API 5B plug and ring gauges and marked with its heat number so it can be traced back to the pipe it was made up with.
Thread Forms and Gauging
STC and LTC both use an API round thread but differ in thread length, with LTC being longer and therefore stronger. BTC uses a buttress thread with a 3 degree load flank and a 10 degree stabbing flank, giving a much greater shear area and higher tensile efficiency. All three are inspected with API 5B gauges. Thread compound is applied before make-up, and the correct compound must be selected for the service: standard compounds for normal service, and compounds containing no free metals such as zinc or copper for sour service, since these metals can accelerate sulfide stress cracking. Make-up torque must follow the mill or API recommendation, and torque-turn monitoring is recommended for critical strings.
What is the difference between STC, LTC and BTC couplings?
All three are API 5CT casing connections. STC has the shortest round thread and the lowest joint strength, suited to shallow wells. LTC has a longer round thread giving higher joint efficiency, used for medium depth wells. BTC uses a trapezoidal buttress thread with much greater shear area, giving the highest tensile efficiency and the best resistance to jump-out, and is the standard for deep and high pressure wells. BTC requires more careful make-up because cross-threading is easier with the buttress form.
Are couplings supplied separately from the casing?
Yes. Couplings can be supplied as a complete package already made up on the casing, or separately as loose items for spares and field repair. When ordering separately, state the casing outside diameter, weight per foot or wall thickness, thread form, grade and quantity. Note that couplings are size and weight specific: a coupling for 7 inch 23 lb/ft casing is not interchangeable with one for 7 inch 29 lb/ft casing even though the outside diameter is the same.
Materials
- Seamless carbon steel
- Alloy steel
Typical applications
- Joining casing joints in a casing string
- Replacement and spare couplings
- Casing repair operations
- Well completion assembly
- Tubing and casing accessories
Inspection & testing
- Thread gauging with API 5B plug and ring gauges
- Dimensional inspection of OD, length and drift
- Hardness testing
- Tensile testing
- Magnetic particle inspection on request
- Third-party inspection (SGS, BV, TUV)
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