Connections Testing and qualification

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The general accepted method for the qualification of casing connection designs is, at the moment, the conducting of tests on a number of full scale specimens. The aim of such an investigation is to assess the following primary characteristics:

  • the galling tendency;
  • the sealing capacity,
  • the structural load bearing capacity.

Qualification Tests

The tests to be performed simulate a number of load conditions which can be imposed on connections during service. In general the test programmes comprise:

  • 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.

API RP 5C5 specifies four test classes and a relationship is suggested between the test classes and the service applications. Class I connection tests are intended for the most severe field applications, whilst Class IV connection tests are intended for the least severe applications.

It is recommended that the Class I test procedures should be replaced by an even more stringent set of procedures (Class A), including internal pressure sealing tests under bending and anti-clockwise torsion and an extension of the thermal cycle tests which, in the past, have proved to be very informative.

At present, there is no recognised way of generalising test results for a particular connection to the same type of connection but having a different size, weight, grade/alloy composition or surface treatment. the scaling of results to different sizes or grades are not accepted. Therefore, each connection will have to be tested for approval. However, the following trends may be observed:

Size Typically, smaller sizes are more susceptible to over-torquing and hence correct make-up torque becomes more important.

Weight Lighter weights of tubing are also more susceptible to over-torquing and tend to be more susceptible to leakage problems because of connection deformation, especially valid for Premium connections with a limited seal lip thickness.

Grade/alloy composition The effects of plasticity in more highly alloyed steels may be more damaging owing to reduced work-hardening. Such steels also tend to be more prone to galling and therefore connection design, surface treatment and handling become more critical.

Surface treatment Generally these treatments have beneficial effects on both the galling resistance and the sealing performance of a connection [331]. Consequentially, successful sealing tests on connections with a typical surface treatment should not be extrapolated to connections with a different surface treatment.

It is to be mentioned that connection failures are still frequently encountered, both during field application and during connection tests. Therefore it is recommended that:

All production casing connections to be used in critical applications are to be qualified prior to use. For drilling casing connections which may be subjected to gas, it is strongly recommended that qualification according to above procedure is also followed.

Other evaluation techniques

So far qualification tests were performed with full scale connections under actual loads. There are also other ways of evaluating the connection, although not regarded as a qualification, if used in isolation.

These methods are:

Strain gauge measurements , which provide information on surface strains and thus stresses in the connection at applied load conditions, such as make-up and combined axial and pressure loads.

Finite Element Analysis (FEA) , which is a calculation method for the determination of the deformation and stresses in a connection, in particular contact stresses at the sealing surfaces. It can give a fundamental engineering understanding of the connection design and its behaviour and provide the ability to make detailed comparisons between designs. However, at the moment there are still some difficulties in the use of FEA: so far the models used in the computer are axi-symmetric, which means that for instance the effect of ovality cannot yet be assessed. It has not been proven possible yet, to input a model which is identical with the three-dimensional connection configuration; modelling contact problems with friction is difficult, there is no unique solution and the model may converge to the wrong solution; it is difficult to assess the accuracy of the models; there are no sealing criteria yet, i.e. what should be the contact pressure between the sealing surfaces, the contact width, the surface finish, the surface treatment and what is the exact role of the thread lubricant.

Nevertheless there are some software packages available on the market to perform FEA on connections which can predict the stresses of the connection under certain load conditions. It should be kept in mind however, that at the moment these FEA cannot replace the actual laboratory testing. So far they should be considered as an assistance to the actual testing.

Some Operators have developed an evaluation procedure based on the combination of full scale tests and FEA. They have also used FEA to develop a new make-up method for API round connections.

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