A primary design requirement for onshore pipelines is a need to demonstrate that hoop stress associated with the internal pressure loading is less than a prescribed allowable value. In addition to this management of the hoop stress (design factor) there is also a requirement to manage the magnitude of other stress components that may arise due to the interactive effects of soil friction and thermal gradients. It is quite a straight forward matter to determine the magnitude of such stress components using non-linear finite element techniques with the pipeline represented as a series of beam elements. A complexity arises when the pipeline contains fittings, such as bends and tees, since these components give rise to stress concentrations that cannot be addressed by simple beam elements. In some situations, it is possible to apply standard (handbook) values of the stress concentrations to the nominal stresses obtained from the beam element analyses and to assess the resulting stress levels against allowable values. However, stress concentration factors are not available for all components and in these situations the stresses can only be determined using 3D finite element analyses. The computational methods associated with these procedures have previously been undertaken in two stages. The first stage involved the determination of forces and moments, using beam elements, that act at the location of the fittings. The second stage involved the 3D finite element analysis with the previously computed forces and moments applied to the fitting. The process involved considerable effort and some accuracy was lost due to the approximate nature of the applied boundary conditions as described above. In order to mitigate both of these effects, AFAA have recently developed a method in which the two stages are integrated giving both a more accurate result, thus saving time and cost. The first part of this paper describes the modelling approach adopted by AFAA in some detail. Considerable focus is then given to the application of the technique. Working in close collaboration with Laing O’Rourke (UK), the technique was successfully applied to demonstrate the fitness-for-purpose of a number of hot-tap encirclement split-tees that were to be installed in close proximity on an operational pipeline. Both the installation procedure and subsequent operation were assessed in detail, taking account of the installation loads, pilot drill holes and the operational loads that would be encountered in the future. It was shown that installation process would not pose a threat to integrity of the existing pipeline and nearby components and that the system could be safely operated over the proposed design life period.

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