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Analysis of Threaded Connections

Rapid advances in the drilling technologies used in oil exploration (e.g. directional drilling, deep offshore drilling) have fostered the need for better connecting devices between pipes, tubes and casings. Premium threaded connections are abundantly used in such mission critical drilling processes where any malfunction in the connecting devices may result in significant economic and environmental losses. Figure 1 depicts samples of such connections.

In this Brief, we present finite element studies of these threaded connections using ADINA which were graciously provided by SIM&TEC S.A. They have been involved in R&D projects related to different aspects of tubular products used in the oil wells, and also pipes used in the pipeline applications (which are collectively called Oil Country Tubular Goods or OCTG).

Figure 2 depicts a representative axisymmetric finite element model of a premium connection. An elasto-plastic material model with the von Mises yield function is assumed and contact conditions are established between the bodies. Due to the large plastic strains that can develop in these connections, a large strain elasto-plasticity model needs to be used.



Figure 1  A semi-premium (left) and premium (right) threaded connection




Figure 2  Finite element model of a premium connection


Threaded connections can experience different failure modes:

  • Jump-out : Figure 3 shows the plastic strains in the connection for different values of the tensile forces and the jump-out phenomenon.

  • Localized plastic deformation: Figure 4 shows the necking phenomenon in a premium connection. The failure is localized outside the threaded region. The evolution of the plastic region due to the load variation and the eventual localization phenomenon are shown in the graph.

  • Rupture of the pipe material without development of large plastic strains.

In addition to the above structural failure modes of a threaded connection, it can also fail functionally by losing contact in the metal-to-metal sealing region (see Figure 2).




Figure 3  Jump-out phenomenon in an API 8R connection subjected to tensile load






Figure 4  Finite element simulation of necking phenomenon in a premium connection


The movie above shows the deformation of the threaded connection due to insertion of a conical expansion tool. As seen, it is important to use a robust contact algorithm to account for the large separation/sliding between the threads during the process.

Figure 5 shows the plastic strains induced in the connection due to the insertion of the expansion tool. Note the large plastic strains induced in the connection during the process.

Also, since these connections are subjected to high torque, compression and also thermal cyclic loads, fatigue is another important issue that needs to be considered in their design. Figure 6 shows the contour plot of the stress concentration factor, as defined in the figure, which is an indicator that determines what part of the connection is more prone to fatigue-related failures.




Figure 5 Contour plot of the effective plastic strain due to expansion tool






Figure 6 Contour plot of the stress concentration factor (SCF)


This study shows some of the advanced capabilities of ADINA for analyzing problems in the oil and gas industry. For some other applications, please see


Keywords:
Oil and gas industry, Oil Country Tubular Goods, OCTG, threaded connection, API connections, premium connections, fatigue, stress concentration factor, finite strain elasto-plasticity, contact, necking, localization


Courtesy of SIM&TEC S.A., Argentina

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