Traditional API casing threads (such as long round and partial trapezoidal threads) are not well-suited for the development of high-pressure gas fields, primarily due to limitations in air-tightness and tensile strength. To address these challenges, many manufacturers have developed proprietary premium connections with enhanced sealing and mechanical performance. Among them, several gas-tight special thread types have proven suitable for demanding gas field applications:
1. NS-CC Thread (Developed by Nippon Steel, Japan)
Key Features:
Utilizes an improved API partial trapezoidal thread design, offering high joint tensile strength.
Features metal-to-metal sealing between tapered surfaces (primary seal) and inner shoulders (secondary seal), ensuring excellent gas tightness.
The stepped dual-shoulder design enhances resistance to torsion and bending, while also protecting the primary seal.
Reduced circumferential stress helps prevent stress corrosion cracking (SCC).
2. FOX Thread
Jointly developed by Kawasaki Steel (Japan) and Hunting (UK), this thread type provides superior sealing performance in high-pressure environments.
3. TM Thread
An improved design by Sumitomo Metal (Japan), based on the VAM thread, with enhanced sealing and mechanical integrity.
4. SEC Thread
Developed and manufactured by Siderca (Argentina), this connection is engineered for high-performance casing applications.
Additional Background:
Currently, most oil country tubular goods (OCTG)—including tubing, casing, and drill pipe—are produced in accordance with API (American Petroleum Institute) standards. China has largely harmonized its national standards with the API system. While API threads remain widely used, premium connections—often proprietary and protected by patents—are increasingly adopted for critical well applications, especially in sour gas or high-pressure environments.
The most globally recognized premium connections include:
VAM series: Including New VAM, VAM TOP, etc., developed by Vallourec & Mannesmann.
FOX: Further developed after the merger of Kawasaki Steel and NKK into JFE.
TM: Developed by Sumitomo Metal (Japan).
In China, several domestically developed premium threads have emerged:
Pancheng Yihong (a joint venture with Pancheng Steel)
TP-CQ series by Tiangang
BGT series by Baosteel
WSP series by Wuxi Seamless Pipe
HSM series by Henggang Steel
These have all received official certification, have undergone testing in major Chinese oilfields, and are also exported internationally.
Application of Special Threads:
Premium threaded casing pipes are primarily used in conditions where API threads are inadequate, such as:
Wells containing hydrogen sulfide (H₂S), carbon dioxide (CO₂), and chloride ions
High-pressure, high-temperature (HPHT) environments
Situations requiring high-grade steel (e.g., Q125 or higher)
Compared to traditional API threads, premium connections offer:
Superior gas sealing
Greater tensile strength
Improved resistance to environmental corrosion
These advantages make premium threads the preferred choice for the safe and reliable development of complex wells.
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