Tubing and Casing Connections

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This article descirbes the Tubing and Casing Connections Functional and Operational requirements.

Functional requirements:

- strength -sealing properties, -resistance to damage, corrosion or erosion.

Operational requirements:

-easy to make-up and break-out in the field (e.g. handling, stabbing, testing, etc); -reusable;

Connection types

For low pressure wells API 8RD thread connections have been the standard in the tubing strings. Non upset has proved more effective than upset (EUE) 8 RD tubing.

Connections provided with metal-to-metal seals are commonly referred to as Premium connections.

Threaded 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

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 OD and ID close to the pipe.

-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 connection has a higher torque capacity than the threaded and coupled connection. Designed with an external torque shoulder, while most threaded and coupled connections have the torque shoulder is located at the pin nose.

-There is a risk corrosion (ringwork) at the upset region of joints in the presence of CO2.

Threaded and coupled connection

The joint is externally threaded on both ends of the pipe. The single joints are joined by an internally threaded coupling, to form the connection.

Comparison of integral and threaded/coupled connections

In recent years there has been a move away from integral type connections, towards the use of 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.

- Less risk of leakage due to geometric errors in the machined connection parts

Thread forms

·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.

·Modified buttress threads, used for Premium connections.

Connection sealing

Threaded connections utilise three basic mechanisms to establish a leak tight joint.

·tapered interference fit thread seal (API)

·metal-to-metal seal (premium)

·resilient seal (semi-premium)

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.

Metal-to-metal seal

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 elastically, so as to be able to seal under changing loads without having a permanently deformed seal.

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. Generally employed is Teflon. The property of the material will tend to change with the time.

Do not use the same seal ring twice.

Testing and qualification

The tests to be performed simulate the load conditions which can be imposed on connections during service:

  • repeated make-up and break-out tests at various make-up specifications;
  • internal pressure sealing tests under different combinations of loading;
  • internal pressure sealing tests during thermal cycling;
  • external pressure sealing tests under axial loading;
  • tensile or burst tests to failure.

Operational considerations

  • in all offshore wells, in deep or high pressure land wells and in all gas wells, use Premium connections, i.e. metal-to-metal seals;
  • in corrosive conditions, give preference to non-upset internal flush connections;
  • where space is at a premium, consider using integral joint tubing;
  • synthetic seal joint rings (usually Teflon) can be used for extremely high gas pressure, and should be replaced every time the joint is broken;
  • for onshore low pressure wells (flowing, gas lifting, pumping) API EUE is recommended. For gas and gas/condensate wells, all the previously mentioned connections with the exception of API EUE and Hydril A95 are recommended. Non-upset API tubing is not recommended for heavily loaded pumping wells.

Comments  

#1 Jonathan Luna 2014-12-17 15:18
I recently machined a 3.5" 8-RD EUE connection into the middle of an 11" 10M flange. I have searched API for what the pressure rating would be on that connection (i.e. would the flange still be rated to 10M?). Could you provide any feedback on this inquiry?
Thanks,

Jonathan Luna
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