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.
Threaded casing connections can be divided in two groups, namely the integral connections and the threaded and coupled connections. Each group can further be divided into several types, depending on the sealing mechanism and the existence of a torque shoulder.
1 Integral connection
The geometry of the pipe ends are different so that they can be connected without using an intermediate part. Two types of integral connections are common:
Upset type connection:
this type of connection has pipe ends with an increased wall thickness. The pipe may be externally upset, internally upset or both.
Non-upset or flush type connection:
this type of connection has pipe ends with the same OD and ID as the pipe. It has reduced strength efficiency, compared to upset type of connections, in all cases. Sometimes the pin-end is swaged to a slightly smaller diameter, which will then affect the drift diameter. Other connections have an increased box diameter or designs with combinations of these.
2 Threaded and coupled connection
The casing joint is externally threaded on both ends of the pipe. The single joints are joined by an internally threaded coupling, to form the connection.
The coupling can be made with several varying outer diameters, the following having its influences on the dimension:
- Regular, as specified in API Spec 5CT.
- Special clearance, which have a smaller OD than the regular coupling. In most cases this coupling will have a reduced strength efficiency. However, a coupling with a higher yield strength material might be considered to negate this.
- Resilient seal, in order to incorporate a resilient seal and maintain the required cross-sectional area to keep the same capacity, it is sometimes required to increase the OD of the coupling.
- Matched strength, these couplings are designed in order to achieve a 100% efficiency. These optimum couplings may have an external diameter larger or smaller than the API couplings.
3 Comparison of integral and threaded/coupled connections
In recent years there has been a move, in many companies, away from integral type connections, towards the use of threaded and coupled connections. Listed below are the characteristics of the integral connections and those of the threaded and coupled connections:
Integral connections
- integral connections halve the number of threaded connections, and thus the number of potential leakage paths.
- there is no possibility of receiving a coupling made of a different, and thus wrong, material
- in general, the integral type of connections has higher torque capacity than the threaded and coupled connection. This is because integral connections are generally designed with an external torque shoulder, while for most threaded and coupled connections the torque shoulder is located at the pin nose.
- there is a risk of "ringworm" corrosion. This corrosion can occur at the upset region of joints in the presence of CO2. During the upsetting process the pipe ends are heated and heavily deformed, which results in a difference in steel microstructure compared to the pipe. It has been found that this microstructure is highly sensitive to CO2 corrosion so that pits can form quite rapidly. The observed corrosion has a characteristic morphology called ringworm attack. To avoid this problem it is necessary to use tubulars which have been fully heat treated after upsetting.
Threaded and coupled connections
- threaded and coupled connections are generally cheaper to produce and the pipe ends can be re-cut should the threads be damaged.
- the manufacturing process of threaded and coupled connections is a lot simpler than that of integral connections as no upsetting or swaging is required.
- with threaded and coupled connections there is less risk of leakage due to geometric errors in the machined connection parts. Generally, the geometric error in machined couplings is smaller than the error in machined pipe ends. Pins and boxes, machined on long tubulars, may show geometry errors in the shape of a clover leaf. This is usually caused by movements of the long unsupported section of the casing joint.
- there has also been a move towards the use of more highly alloyed steel grades which cannot be satisfactorily hot-worked to produce the upset pipe ends necessary for an integral connection.
4 Thread forms
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.