Casing connections should satisfy several functional and operational requirements which are presented in this article.
Functional aspects
- to provide a leak resistance to internal or external fluid pressures
- to have sufficient structural rigidity to transmit externally applied loads
- to have good geometry in order not to increase the outer diameter or reduce the inner diameter of the casing string significantly
Operational aspects
- easy to make-up in the field
- easy to break-out in the field
- reusable
To fulfil these aspects, the connections are provided, in almost all cases, with connection threads. Connections based on welding or gluing techniques and snap-on connectors are available for casing but will not be dealt with here.
For many years the API thread connections, with or without a resilient seal ring, have been the standard in well casing strings. These standardised connections are:
- the API round thread connection for casing application;
- the API buttress thread connection for casing application;
- the API extreme line connection for casing application.
However, during the last decades there has been a shift away from relatively simple and inexpensive shallow wells to complicated completions for deep, often corrosive and high pressure/temperature wells. This trend entailed the need for connections with better seals than the API connections, and led to the development of the so-called Premium connections.
All connections that have one or more special features, such as higher strength, better sealing properties, faster make-up, smaller outer diameter of the coupling, internally streamlined and recess free, etc. as compared with API connections, are collectively called Premium connections.
The following thread forms are commonly manufactured today:
- API round type thread a tapered thread with stabbing and loading flanks of 30° and rounded crests and roots.
- API buttress type thread a tapered thread with stabbing and loading flanks of 10° and 3° respectively, and flat crests and roots, parallel to the thread cone.
- API extremeline thread a tapered thread with stabbing and loading flanks of 6°, and flat crests and roots parallel to the pipe axis.
Modified buttress threads used for Premium connections. Several thread forms have been developed which are provided with one of the following modifications or combinations thereof: the thread profile has thread crests and roots parallel to the pipe axis rather than being parallel to the thread cone; a clearance at the pin thread crest, in order to ensure a better control of the thread friction during make-up; a change in the angle of the stabbing flank, ranging from +10° to +45° in order to improve the connection stabbing performance; a change in the angle of the loading flank, ranging from +3° to -15° in order to increase the tensile capacity of the connection; a change in the pitch of the threads (single or double pitch change) in order to provide a more uniform stress distribution in the connection threads under tensile or compressive loads.
Two step thread has two sections of different diameter, each provided with free running, non interfering, threads either straight or tapered. The figure shows a design with three shoulders which has the advantage of an increased over-torque capacity. In contrast, a non-interfering thread has the risk of inadvertently backing-out of the connection.
Wedge shape thread is based on an interlocking dovetail thread profile. The loading flank is machined with a greater pitch than the stabbing flank to produce a thread that wedges together during make-up, eliminating the need for an additional torque shoulder. The applicable make-up torques of these connections tend to be higher than that of connections with modified buttress thread profiles and a shoulder.
