Threaded casing connections utilise three basic mechanisms to establish a leak tight joint. These mechanisms are:
- tapered interference fit thread seal;
- metal-to-metal seal;
- resilient seal.
1 Tapered interference-fit thread seal
Tapered interference fit thread seals, such as the API round and API buttress threads, are not inherently leak tight, but have helical leak paths included in the design. Leak tightness of these connections is thus obtained by establishing a high contact pressure on the thread flanks and sealing the remaining leak path(s) with a thread compound.
API round thread: Both thread flanks act as sealing surfaces. They are to be loaded by the make-up torque to such an extent that the contact pressure is greater than the fluid pressure to be retained. The two small leak paths of the API round thread are at the crest and the root of the thread. These paths are very long, approximately p times the thread diameter times the number of threads engaged. The smaller the tolerance, the smaller the cross-section of the leak path. Sometimes a soft metal thread surface finish may help in reducing the size of the leak path. Under high axial and bending loads, the sealing capabilities of the API round thread will be strongly reduced because box and pin thread will deform, causing the leak paths to increase in size.
API buttress thread In made-up condition, contacts between the loading flanks and the crest and root of the threads form the seal. The leak path in the buttress thread form is along the stabbing flank and the crest and root radii, and is bigger than that for the API round thread (see Fig. 390). So, more than the API round thread, this type of thread relies on the compound to seal the leak paths. Under certain conditions, like for instance compressive loads the thread contact can change from the loading flank to the stabbing flank. This shift can result in leakage of the connection.
It should be noted that no amount of torque applied to the connection can close the leak paths in round or buttress threads. Added torque sometimes stops leaks in connections with round thread, but doesn't close the leak path. However, once the elastic limit of the material is reached, the additional torque cannot help and may damage the connection. Hence, in many cases, particularly at high temperatures, with alternating load conditions and/or under gas pressure, these types of connections are not capable of providing a reliable seal. Research performed has led to the conclusion that the amount of pressure a connection can hold, depends on the gap width between the threads. The smaller the size of this gap, the higher the pressure it can hold. Therefore small thread tolerances should be requested, so as to increase connection sealing performance. It has not yet been shown that it is possible to design a thread profile that is capable of providing a reliable gas-tight seal on its own, although some manufacturers have made this claim.
2 Metal-to-metal seal
Connections provided with metal-to-metal seals are commonly referred to as Premium connections. Sealing relies on metal-to-metal contact between the two mating sealing surfaces from both pin and box. Therefore, the thread itself does not have a primary sealing function but serves to transmit externally applied loads. At the sealing contact area the surfaces will deform elasticly, so as to be able to seal under changing loads without having a permanently deformed seal. No plastic deformation of the sealing area should occur due to these changes in loads.
Although many Premium connections are based on a similar design principle, the details of the design in many cases result in very different characteristics. Often this is a result of the compromise necessary to ensure good sealing integrity and acceptable running characteristics in the field. For example, radial seals may be more prone to damage due to galling as a result of the long sliding contact of the seal surfaces during make-up or break-out. Increasing the seal interference, in an attempt to improve the sealing performance, will tend to increase the galling tendency. At the other hand, high angle tapered seals tend to be less prone to galling although there may be a greater risk of losing sealing integrity under high tensile loads or following compression-tension cycles, which may cause the tapered faces to separate.
3 Resilient seal
The API round and API buttress thread connections as well as the Premium connections can all be applied with an additional seal made from polymeric material. Their sealing function is either primary or secondary. In almost all cases the polymeric seal ring is incorporated in the threaded part of the box.
Polymeric seal materials can be divided into two groups:
- elastomeric materials;
- plastomeric materials.
Both these groups react differently to the downhole conditions. The properties of these materials will tend to change with the time of exposure to these conditions, although at a decreasing rate. Both groups of materials will tend to absorb hydrocarbons over long periods of time, thus affecting the properties. However, this occurs in the plastomeric materials to a lesser extent.
The polymers which are used most as sealing material within casing connections are the plastomeric materials, for instance, virgin Teflon or reinforced Teflon.
There is a general recommendation not to use the same seal ring twice. This represents an operational problem since from testing it has been found out that the use of a new ring, installed prior to re-using a connection, will increase the risk of galling [301], since a new ring may cause misalignment of the pin as a result of grease trapped behind the ring. Therefore, the seal ring should preferably be installed at the factory, because of the need for an absolutely clean environment. The fact that the boxes may not be cleaned any more, once the rings have been installed will lead to additional operational problems.
From the viewpoint of design and operational aspects, the use of resilient seals is not encourage. However couplings with resilient seals may be used as remedial action to solve operational problems with API type connections.