A Consistent Analytical Model to Predict the Structural Behaviour of Flexible Risers Subjected to Combined Loads

This work presents an analytical model for the structural analysis of flexible risers subjected to combined loads of bending, twisting and tension, as well as internal and external pressures applied to the riser. Flexible risers, either umbilical cables or flexible pipes, are complex structures used in offshore oil exploitation activities. Such structures are composed of several concentric polymeric and steel armour layers, which withstand static and dynamic loads applied by the floating production vessel and by the ocean environment. The complexity imposed mainly by geometry renders a finite element analysis of these structures practically unfeasible, even if we are to consider that all the materials obey a linear elastic behaviour. So, in order to calculate the stress distribution in the layers, as well as axial, torsional and flexural stiffness values of these structures, analytical methods have been proved to be, till now, a better choice. Using sets of equations, which comprise equilibrium conditions, constitutive equations and geometrical relations, it is possible to solve the problem for all the unknowns. This paper presents a consistent and comprehensive formulation leading to the solution in terms of stresses and deformation components in a flexible riser subjected to the above mentioned combined loads. It is based on the assumption of full-slip of the helically armoured layers after bending is imposed to the pipe. Other main modeling hypotheses are also highlighted in this work. The presented analytical model is, therefore, rather comprehensive and recovers, asymptotically, many results previously published in the technical literature. Comparisons between analytical results using the full-slip model and experimental results obtained in literature are shown and discussed. Some proposals leading to improvement of the presented model are drawn in the conclusions.