Individual joints of casing are connected together by a threaded connection. These connections are variously classified as: API; premium; gastight; and metal-to-metal seal. In the case of API connections, the casing joints are threaded externally at either end and each joint is connected to the next joint by a coupling which is threaded internally (Figure 5). A coupling is already installed on one end of each joint when the casing is delivered to the rig. The connection must be leak proof but can have a higher or lower physical strength than the main body of the casing joint.
A wide variety of threaded connections are available. The standard types of API threaded and coupled connection are:
• Short thread connection (STC)
• Long thread connection (LTC)
• Buttress thread connection (BTC)
In addition to threaded and coupled connections there are also externally and internally upset connections such as that shown in Figure 4. A standard API upset connection is:
• Extreme line (EL)
The STC thread profile is rounded with 8 threads per inch. The LTC is similar but with a longer coupling, which provides better strength and sealing properties than the STC. The buttress thread profile has flat crests, with the front and back cut at different angles. Extreme line connections also have flat crests and have 5 or 6 threads per inch. The EL connection is the only API connection that has a metal to metal seal at the end of the pin and at the external shoulder of the connection, whereas all of the other API connections rely upon the thread compound, used to make up the connection, to seal off the leak path between the threads of the connection.
In addition to API connections, various manufacturers have developed and patented their own connections (e.g. Hydril, Vallourec, Mannesman). These connections are designed to contain high pressure gas and are often called gastight, premium and metal-to-metal seal connections. These connections are termed metal-to-metal seal because they have a specific surface machined into both the pin and box of the connection which are brought together and subjected to stress when the connection is made up.
Surveys have shown that over 80% of leaks in casing can be attributed to poor make-
up of connections. This may be due to a variety of reasons:
• Excessive torque used in making-up the connections
• Dirty threads
• Using the wrong thread compound.
The casing string should be tested for pressure integrity before drilling the subsequent hole section. Most of the causes of connection failure can be eliminated by good handling and running procedures on the rig.
The recommended make-up torque for API connections is given in API RP 5C1. These recommended torques are based on an empirical equation obtained from tests using API modified thread compound on API connections. The recommended make up torque for other connections is available from manufacturers.
Institute of Petroleum Engineering, Heriot-Watt University