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Received Dec 8, 2017; Accepted Jan 23, 2018
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1. Introduction
Unbonded flexible pipes (or, simply, flexible pipes), such as the one presented in Figure 1, are largely employed in offshore oil and gas exploitation. These multilayered composite structures combine high axial strength and stiffness with low bending stiffness, resulting in highly compliant pipes. Each layer of these pipes has a specific function and may be either metallic or polymeric. Whereas the metallic layers provide structural resistance, the polymeric layers are used to seal the pipe and/or to mitigate wear and friction between layers. In a typical flexible pipe, three different metallic layers are found [1]: inner carcass, which is made from profiled steel strips wound at angles close to 90 degrees with respect to the pipe axis that mainly resists radial inward forces; pressure armors, which are usually Z-shaped steel wires also wound at angles close to 90 degrees with the main function of supporting the system internal pressure and also radial inward forces; and tensile armors, which are constituted of several approximately rectangular steel wires laid in two or four layers, cross-wound at angles between 20 degrees and 55 degrees, which resist tension, torque, and pressure end cap effects.
[figure omitted; refer to PDF]The structural responses of these pipes to bending and axial loads have been extensively studied since the mid-seventies. Several analytical and numerical models, as reviewed and summarized by Tang et al. [2], are available in the public literature, but some aspects of their response remain challenging and the computation of the stresses in the tensile armors due to bending loads, as stated by Dai et al. [3] and Zhou and Vaz [4], is among them.
The bending response of a flexible pipe depends on the curvature imposed on the pipe. Several authors [5–7] describe this response as a stick-slip mechanism (Figure 2), which is activated by the contact pressures between layers due to the applied axisymmetric loads.
[figure omitted; refer to PDF]For small curvatures, friction between the armors and the adjacent layers prevent their relative slip....