Keywords: mechanical behaviour of pipes, steel pipes, axisymmetric structure, numerical modelling of pipes, cylindrical thin structure, combined actions.

The active growth of the national oil industry and its maintenance has required the evaluation of the existing infrastructure, as well as the creation of new and major facilities to provide for the operation, exploration, production, storage and distribution of the petroleum and its derivatives. Therefore, in connection with the distribution of these products in recent years, the construction of the new pipelines and the retrofit of others already in service has been required. For this reason this research is focused on the study of the mechanical behaviour of the pipes subject to combined loads.

Consequently, in this paper steel pipes used in the oil industry are analyzed, examining the linear-elastic behaviour and non-linear analysis for different actions. Mechanical models of the steel material such as the Ramberg-Osgood law are used with the aim of studying the structural behaviour under internal pressure, axial tension, and bending moment. The research is carried out using two types of analysis, elastic analysis with shell theory and numerical analysis by finite element methods in which the shell finite elements are used in the modelling of the walls of the pipes. The structure studied is a typical steel pipe of 20" diameter, API 5L X52 used in the petroleum industry; the geometric and mechanical characteristics of the numerical model are representative of the steel pipes studied, and the non-linear analysis takes into account the plasticity of the steel material.

Finally, the numerical results provided different states of collapse for the pipes subject to combined loads (internal pressure, axial tension force, and bending moment), and their combinations in two and three-dimensional interaction diagrams. By means of the non-linear simulation approach the structural response of the pipe was obtained; in almost all cases it reproduced the pattern of the input curve material, except for the bending moment result where an increase in the carrying capacity is observed. With respect to the three collapse interaction diagrams, for the (T+p_i) case a maximal increase of the axial tension force of about 0.6 p_i is observed. For the combined load (M+p_i) the carrying capacity was reduced when p_i appeared. The interaction diagram of M+T shows an increase of the moment at about 0.8 T. Thereafter the non-linear approach simulation developed in this work allowed a collapse surface for the three complex combined actions (p_i+T+M) to be obtained.

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