In the oil and gas OCTG (Oil Country Tubular Goods) industry, casing connections are mainly divided into 3 core types, the specific introduction of each type is as follows:
Conventional API standard connections
This is the most widely used traditional connection defined by the API 5B standard, it includes three mainstream types: STC (Short Round Thread Casing), LTC (Long Round Thread Casing) and BTC (Buttress Thread Casing). It relies on thread compound for sealing, the production cost is low and the matching process is mature, and it is mainly suitable for conventional shallow and medium-depth wells with mild working conditions.
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API standard connections specification
| Parameter Category | Parameter Item | STC (Short Round Thread) | LTC (Long Round Thread) | BTC (Buttress Thread) |
|---|---|---|---|---|
| Standard | API Specification | API Spec 5B | API Spec 5B | API Spec 5B |
| Thread Profile | Thread Angle | 60° | 60° | 7° (load flank) / 45° (backup flank) |
| Thread Form | Thread Type | Round, dome-topped | Round, dome-topped | Asymmetric buttress |
| Thread Pitch | Threads per Inch | 8 TPI | 8 TPI | 5 TPI |
| Taper | Cone Ratio | 1:16 (0.0625 in/in) | 1:16 (0.0625 in/in) | 1:16 (0.0625 in/in) |
| Sealing Method | Seal Type | Thread compound (non-metallic) | Thread compound (non-metallic) | Thread compound (non-metallic) |
| Connection Type | Joint Design | Requires coupling | Requires coupling | Requires coupling |
| Torque Requirements | Make-up Torque (ft-lb) | J55: 1,200–1,800 N80: 1,800–2,600 P110: 2,600–3,500 |
J55: 1,300–1,900 N80: 1,900–2,800 P110: 2,800–3,700 |
J55: 1,400–2,000 N80: 2,000–3,000 P110: 3,000–4,000 |
| Connection Strength | Relative to Pipe Body | 60–75% | 65–80% | 75–85% |
| Max Operating Pressure | Pressure Rating | ≤5,000 psi | ≤5,000 psi | ≤6,500 psi |
| Temperature Limit | Operational Range | -20°C to 120°C | -20°C to 120°C | -20°C to 120°C |
| Thread Length | Minimum Full Crest (Lc) | Varies by OD (see below) | Varies by OD (see below) | Varies by OD (see below) |
| Applicable OD Range | Outer Diameter | 4-1/2" to 20" | 4-1/2" to 20" | 4-1/2" to 20" |
| Applicable Steel Grades | Material Compatibility | J55, K55, N80, L80, P110, Q125 | J55, K55, N80, L80, P110, Q125 | J55, K55, N80, L80, P110, Q125 |
| Application Scope | Recommended Use | Shallow wells (<3,000 m), low axial load | Medium-depth wells (3,000–5,000 m), moderate axial load | Medium to deep wells (up to 6,000 m), high axial tension/compression |
| Key Limitation | Failure Mode | Prone to slip, leak under cyclic loading | Higher stress concentration at thread ends | More sensitive to thread misalignment and tolerance |
API Premium connections
It is a non-standard high-performance threaded connection that meets the overall quality specification requirements of API 5CT, it adopts the metal-to-metal sealing structure instead of the traditional thread compound sealing method, with higher connection strength, better air tightness and stronger anti-fatigue performance. It is mainly used for harsh working conditions such as ultra-deep wells and high-pressure high-temperature (HPHT) gas wells.
API Premium connections Specification
| Parameter Category | Parameter Item | Conventional (STC/LTC/BTC) | API Premium (VAM® TOP / Hydril PH6™ / TMK UP™) |
|---|---|---|---|
| Standard | Applicable Standard | API Spec 5B / 5CT | API Spec 5CT |
| Sealing Method | Seal Type | Non-metallic thread compound | Metal-to-Metal (MTM) |
| Thread Profile | Thread Angle | 60° | 60° |
| Thread Type | Round / Buttress Thread | Optimized geometry with load flank angle | |
| Thread Pitch (TPI) | 8 (STC/LTC), 5 (BTC) | 5–8 (varies by OD) | |
| Taper | 1:16 | 1:16 | |
| Connection Design | Joint Type | Requires coupling | Integral (coupling-free) |
| Torque Requirements | Make-up Torque (ft-lb) (J55 / P110) |
1,200–1,800 / 2,600–3,500 | 2,400–3,500 / 6,000–9,500 |
| Performance Limits | Maximum Sealing Pressure | ≤5,000 psi | ≤10,000 psi |
| Temperature Range | -20°C to 120°C | -40°C to 175°C | |
| Strength Ratio (vs. Pipe Body) | 60–85% | 85–95% | |
| Applicable Steel Grades | Compatible Grades | J55, K55, N80, L80, P110, Q125 | Same as above + 13Cr, 22Cr |
| Application Scope | Recommended Use | Wells <5,000 m, conventional conditions | Wells >6,000 m, HPHT, horizontal, sour service |
| Key Advantages | Engineering Benefits | Lower cost, high compatibility | No thread compound required, superior fatigue resistance, high-pressure/temperature capability |
| Primary Limitations | Operational Risks | Prone to leakage and slip-out | High installation precision required, higher cost |
Proprietary premium connections
It is a private patented premium connection independently developed by professional OCTG manufacturers, the common mainstream products include VAM® series, Hydril series, TMK series and so on. Compared with general API premium connections, this type of connection has more targeted performance optimization for extreme operation scenarios, and can meet more demanding special working condition requirements.
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FAQ
Q：Do you offer NACE MR0175 certification for sour (H₂S) service applications?
A：Yes. All products for corrosive H₂S environments (such as L80-13Cr, 22Cr steel grades) are certified to NACE MR0175/ISO 15156, and we can provide complete material composition analysis and corrosion test data.
Q：Do you support custom steel grades? Such as super 13Cr, 22Cr or duplex stainless steel?
A：Yes. We can produce custom high-alloy steel grades including L80-13Cr, 22Cr, 25Cr and 28Cr to meet the requirements of deep water wells, sour gas fields and extremely corrosive environments.
Q：What is the difference between your conventional API connections and premium connections? When should I use which?
A：Conventional API connections (STC/LTC/BTC) rely on thread compound sealing, with low cost and mature interchangeability, suitable for conventional shallow and medium-depth wells with mild working conditions. Premium connections adopt metal-to-metal sealing, with higher strength and better fatigue resistance, suitable for ultra-deep wells, HPHT gas wells and other harsh working conditions.
