Reliability and structural integrity of connections

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This article addresses the possible loads on a connection and its sealing and structural capacity. It should be highlighted that these capacities do not always produce the same value.

1 Imposed loads

In general a connection is subjected to two types of load:

  1. Make-up load: Making up the parts will impose stresses in the connection. In general, the stress levels are related to the make-up torque applied. Stresses should be sufficiently high to generate good contact between the sealing surfaces, but below yield to avoid plastic deformation.
  2. Service load: The connection is subjected to the same loads as the total casing string.

2 Structural integrity

There is only one way of expressing the connection structural capacity. The stresses in a loaded connection should be compared to the actual yield strength of the material in use.

However, to present a simple means of assessing a connection capacity the term "efficiency" has been introduced. The efficiency is always expressed as the ratio between an uniaxial capacity of the connection and the pipe body to which it is attached.

The efficiency of a connection can be expressed as a tensile -, pressure -, compression -, bending -, or torque - efficiency.

Since it is not recommended to operate a casing string outside the minimum yield envelope, care should be taken when applying the tabulations of API 5C2 for API connections or when accepting performance values of Premium connection manufacturers. With respect to the API it is imperative to know that this institute defines casing and tubing by providing a cut off based on outer diameter (OD). The API 5C2 considers 41/2 in (0.1143 m) OD and larger as casing and 41/2 in (0.1143 m) OD and smaller as tubing. API 5C2 is based on equations formulated in API 5C3 where two different equations appear: one for casing connections and one for tubing connections.

The tensile joint strength of a casing connection is based on the ultimate tensile strength while the tensile joint strength of a tubing connection is based on the minimum yield strength. As the pipe body yield strength is based on the minimum yield strength this will result in two values for the tensile - efficiency.

It is recommended not to operate a casing string outside the minimum yield envelope, care should be taken when applying the tabulations in API Bull. 5C2. The industry in general is aware of this discrepancy and has introduced the following terms to describe the tensile capacity:

  • Parting load: Load under which the connection will suffer from tensile failure.
  • Joint Elastic Limit: Tensile load under which the connection will yield.

Thus, for 41/2 in (0.1143 m) OD and larger API casing connections the "parting load" is quoted in API Bull. 5C2. Most premium connection manufacturers are quoting both limits.

It follows that when designing a casing string for the production phase it is important to check with the manufacturer what definition has been used to quote the tensile capacity. Here the "joint elastic limit" should be used. For the casing string to be used in the drilling phase the same rule is strongly recommended. However, the designer should be aware that this will downrate the values quoted for 41/2 in (0.1143 m) or larger OD API connections in API Bull. 5C2.

The best approach is to request a full qualification test. This would reveal the connection capacity under triaxial load conditions.

3 Sealing capacity

The sealing capacity of a connection is the ability to prevent leakage while subjected to any of the imposed loads. Leakage is defined as the inability of a connection to withstand a pressure differential over it. Normally a qualification test generates the values for the sealing capacity. A maximum leakage rate of 1ยด10-3 std cm3 gas/sec at the manufacturer's specified pressure rating is acceptable. For API connections it should be highlighted that the tables in API Bull. 5C2 only quote the structural and not the sealing capacity of a connection.

4 Effect of bending loads

In a deviated hole, the casing will tend to take the same bend as the hole. However, due to the couplings, the pipe will stand off from the wall. The clearance, created by the couplings or centralisers, allows tension to pull the midspan of the pipe inward toward the wall, while the end of the pipe is still offset by the couplings.

This causes the pipe to have a different curvature than that of the hole. This difference in curvature increases the bending stress at the ends of the pipe, just outside the couplings, far beyond the stress normally associated with bending through a dogleg. This is called the Bending Stress Magnification (BSM) effect. This BSM effect does not require pipe body contact to occur before the stress is magnified, it begins as soon as tension or compression changes the curve of the pipe. This leads to the advice to try to keep the casing as centralised as possible with centralisers, equally spaced between the couplings. As the distance between couplings and/or centralizers decreases, the size of the BSM effect decreases. With flush connections, the BSM effect will not occur.

The influence of bending on the stresses in a casing and its connections can be evaluated with computer programs which calculate the yield pressure envelope of casing based on axial load, torque and stress from bending through a dogleg. Operating outside this envelope will cause yielding the pipe body.

5 Failure mechanisms

A threaded connection may fail under one or more load conditions. The failure could be a leakage failure or a structural failure.

The following failure mechanisms are common:

  • Plastic deformation of the sealing area can be the result of excessive make-up torque or a result from external loads on the casing. Once the sealing area has been plastically deformed and the load conditions are changed again, the seal area will not return to its original state. This will give a reduction in the effectiveness of the seal.
  • Belling out of the coupling under high tensile or compressive loads, while using an API round thread, the box area may start to bell out. The first thread on the pin stays engaged while the box starts to yield, also the last engaged thread stays engaged. The threads in the middle do not grip into each other any more. For buttress threads this may occur under compression due to the wedging action of the stabbing flank.
  • Jumping out or "unzipping" of the thread. This unzipping of the thread is caused by high axial loads and external pressures and is generally only seen for thick walled connections. The use of too much thread compound might increase the risk of jump out occurring.
  • Shear of the threads This sort of failure is not very common, certainly not when API buttress thread or modified buttress thread connections are used.
  • Splitting of the box This can occur when a tapered pin is screwed into a tapered box. It may happen by over-torquing of API connections (no torque shoulder) and when the coupling is made of high grade (more brittle) steel.
  • Circumferential fracture of the pipe end or coupling. High strength connections, e.g. provided with a buttress thread or a modified buttress thread, may fail either by fracture of the pipe end or the coupling, both occurring near the last engaged thread.

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