API casing threads are not suitable for the development of high-pressure gas fields, so many manufacturers have developed special joints with their own characteristics. Among them, the gas-sealed special joint thread types suitable for the development of gas fields are:
1. NS-CC thread (developed by Nippon Steel), its main features are:
1) The thread adopts improved API partial trapezoidal thread. High tensile strength of the joint;
2) Cone surface and cone surface metal main seal and inner shoulder metal secondary seal, so that the joint has good air tightness;
3) The stepped double straight shoulder structure makes the joint have good anti-torsion and bending ability, and is beneficial to protect the main seal;
4) The circumferential stress on the joint is lower, which helps prevent stress corrosion cracking.
2. Fox thread (co-developed by Kawasaki Steel in Japan and Hunting in the UK).
3. TM thread (Japan Sumitomo Metal's improved design based on VAM thread)
4. SEC thread (designed and produced by Siderca, Argentina)
At present, oil well pipes (tubing, casing, and drilling tool pipes) are all implemented API standards (American Petroleum Institute), and China’s standards are basically converted in accordance with API standards. Major oilfields implement API standards and special threads. Compared with API threads (the main models are long round threads and partial trapezoidal threads), special threads are generally developed by casing manufacturers and have patent protection.
The most widely used internationally are VAM buckles and VAM buckles based on the development of NEW VAM, VAM-top and other buckle types, and the most used domestically are VAM buckles (developed by Varurek and Mannesmann), TM buckles ( Sumitomo Metal Development), FOX buckle (developed by JFE Company after the merger of Kawasaki Steel and NKK). China is mainly produced by Pancheng Yihong Co., Ltd., which is jointly invested with Pancheng Steel. In addition, the TP-CQ series buckle developed by Tiangang, Baosteel The BGT series of buckles, WSP series buckles of Wuxi Simles, and HSM series buckles of Henggang Steel have been developed in recent years and have been certified by relevant authorities. They have been tested in major oil fields in China, and a large part of them are exported.
Special threaded oil casing pipe is mainly used in wells where traditional API threads are not suitable. Traditional API threads have problems such as sealing and tensile strength. Traditional API threads have poor air-tightness. For wells containing hydrogen sulfide, carbon dioxide and chloride ions, Special threads can effectively overcome these shortcomings. Other high-grade steel pipes (such as Q125 and above) generally use special threads.