The most common type of casing connection in use is the API 8-rd connection, where 8-rd means 8-round or eight threads per inch and a slightly rounded profile. The profile is a V or wedge-shape but slightly rounded at the crest and valleys of the threads. There is also an API 11.-V thread, which has 11. threads per inch and a sharp V profile. This typically is called a line pipe thread and is seldom used in down-hole applications today. The API 8-rd connection is made in either ST&C (short thread and coupling) or LT&C (long thread and coupling). These two threads are the workhorses of the industry and sufficient for most normal applications. Like most connections, these are not as strong in tension as the pipe body itself because of the reduced net cross-sectional area of the tube, resulting from the threads being cut into the pipe body wall in the absence of an upset. These are interference-seal-type connections. The threads are wedge-shaped, cut on a tapered profile, and made up until a prescribed torque is attained. At full makeup torque, the threads do not achieve a pressure seal, because the threads do not meet in the base of the groove, leaving two small channels at the base of the thread in both pipe and the coupling. How then do they seal and prevent pressure leaks? They form a pressure seal with the use of thread lubricant that fills the voids between the thread roots. The gap is very small and its length is quite long due to the number of turns at a pitch of eight per inch, so the lubricant forms a good seal in most cases. However, one must always use an approved thread lubricant, one that ages and shrinks in time and temperature eventually will leak. Although these connections often are used in gas well applications, they generally are not recommended, because they rely on the thread lubricant for a seal. Another precaution is that because the threads are wedge shaped, they tend to override each other when subjected to high tension or compression. This override mechanism is often referred to as jump out. Because of this jump out tendency, ST&C and LT&C connections generally are not recommended for wells that have high bending stress due to well-bore curvature or applications where temperature fluctuations cause high axial tensile and compressive loads.
Other Threaded and Coupled Connections
A number of types of threaded and coupled connections have different profiles from the API 8-rd. Instead of wedge-shaped threads, many have a square profile or something similar to give them greater tensile and bending strengths. Examples of this type of thread is the Buttress (now an API thread), 8-Acme, and the like. These threads typically are used where higher tensile strengths are needed in the joints. In general, they also rely on thread lubricant to form a seal and are prone to leak in high-pressure gas applications. Most of these connections require less makeup torque than API 8-rd connections. This is an advantage but also can be a disadvantage, because the maximum makeup torque usually is less than that required to rotate the casing in the hole. Where rotation is planned for cementing or orienting precut windows for multilateral wells, these types of connections are to be avoided. Also, because the makeup torque is relatively low, most of these joints have a “makeup mark” on the pipe. When the pipe is made up properly, the coupling is aligned with the makeup mark. If the maximum torque is attained before the coupling reaches the makeup mark, it is an indication that the thread lubricant is the wrong type, the connections have not been properly cleaned, the pipe is not round, or the connection has been damaged. If the makeup mark is reached before the optimum torque is achieved, that is an indication the connections are either worn or the threads were not properly cut. Although not as common, some threaded and coupled pipe also has metal-to-metal seals.
Another type of connection used for casing is one in which a metal-tometal seal is achieved that is independent of the threaded area. These usually are integral-type connections cut into both ends of the pipe with no separate coupling. Some have a smooth tapered seal that seats very tightly when the proper makeup torque is achieved, others have a shoulder type seal, and as mentioned previously, still others have a combination of both. These types of connections give both high tensile strengths (some greater than the pipe body itself), greater bending strengths for curved well bores, and greater pressure sealing for high-pressure gas wells. Some of these threads may be cut in non-upset pipe for use as liners, typically called flush-joint connections because both the inner and outer diameters are the same in both the tube and connection. Most integral and metal-to-metal sealing connections often are referred to as premium connections, but this often is a misnomer. With the exception of API X-line, these should be referred to as proprietary threads. They are patented, and their dimensions and properties are strictly those specified by the manufacturer, even though they usually are on API specific tubes.
Many of the proprietary connections are designed for special applications, where the loading exceeds typical casing design loads. High-tensile loads and high pressures come to mind, but there are other types of loading we often do not consider. One of these is high torsion. If a casing string is to be rotated (for cementing or drilling), the frictional torque often is much higher than the recommended maximum makeup torque of most connections. Additionally, in some wells, where temperatures cycle significantly between flowing and shut-in times, severe compressive loading can take place. That a particular connection may be strong in tension does not necessarily mean that it is as strong in compression. For these applications, special connections have been designed. One proprietary thread is of an interlocking design, so that it may used in hightorque situations, curved well bores where bending due to bore-hole curvature is a possible cause of connection failure, and situations where axial compressive loading is significant. The interlocking-type thread is somewhat unique in that it is wider at the crest than at the base, and its width also is tapered along its length.
One should always consult the individual manufacturer for properties such as strengths and makeup torque. Another important point is that one should follow the manufacturer’s recommendation as to thread lubricant, as some lubricants used with API 8-rd connections can result in loss of pressure seal in some of the proprietary connections. And, on the subject of thread lubricants, it should be mentioned that some connections are coated with special clear lubricants at the mill to avoid the need for field lubrication. This is not a labor-saving process but one of avoiding possible environmental and formation damage from conventional lubricants.