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. The proposed formulation, which comprises equilibrium conditions, constitutive equations and geometrical relations, leads to the solution of the problem in terms of the loads supported by the layers and deformation components of a flexible riser subjected to the above mentioned combined loads. All the modeling hypotheses are highlighted in the work. 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.

Flexible risers are complex structures composed of several concentric polymeric and steel armor layers that withstand static and dynamic loads applied by the floating production vessel and by the ocean environment. Determining the response of these structures when subjected to axisymmetric loadings (i.e., any combination of traction, torsion, and internal or external pressures) is an important task for the local structural analysis since it provides probable values for the loading distribution along the layers and, thus, allowing estimating the expected life of a riser using fatigue tools. Although finite element models have been increasingly used to accomplish this task in the last years, the simplicity and the reasonable accuracy provided by analytical models can be seen as reasons that justify their continued use, at least in the initial cycles of the design. However, any analytical model proposed for such a task must be checked with well-conducted experimental results in order to be considered as an acceptable analysis tool. The aims of this article are twofold: (i) to present the main results of experimental tests involving both internal pressure and traction loadings on a 63.5 mm (2.5 in.) flexible riser, carried out at the Institute for Technological Research of São Paulo (IPT), which can be used as a means of checking finite element or analytical models proposed by other researchers, and (ii) to compare some results obtained experimentally with those predicted by an analytical model which can also include any combination of axisymmetric loadings. Besides presenting full data concerning the internal structure of the riser, the experimental procedures used to perform the tests and the main results (e.g., Force × Displacement curves) are also presented. A brief discussion about the validity of some hypotheses that are usually assumed by analytical models found in the technical literature is made.

The structure of an autonomous underwater vehicle (AUV), usually composed of a cylindrical shell, may be exposed to high hydrostatic pressures where buckling collapse occurs before yield stress failure. In conventional submarines, welded stiffeners increase the buckling resistance, however, in small AUVs, they reduce the inner space and cause residual stresses. This work presents an innovative concept for the structural design of an AUV, proposing the use of sliding stiffeners that are part of the structure used to accommodate the electronics inside it. Since the sliding stiffeners are not welded to the shell, there are no residual stresses due to welding, the AUV fabrication process is simplified, enabling a reduction of the manufacturing cost, and the inner space is available to accommodate the equipment needed for the AUV mission. Moreover, they provide a higher buckling resistance when compared to that of an unstiffened cylindrical shell. A comparative analysis of the critical buckling loads for different shell designs was carried out considering the following: (i) the unstiffened shell, (ii) the shell with ring stiffeners, and (iii) the shell with sliding stiffeners. Results evidenced that major advantages were obtained by using the latter alternative against buckling.

Risers anchored in a floating system and excited by a moderate sea state are often subjected to dynamic compression near the touchdown point, since the static tension in this region is usually very small. An analytical expression for the critical load of dynamically compressed risers was recently derived by Aranha et al. [1], whose results were compared with numerical simulations, showing a good agreement in several cases. This paper extends Aranha’s results by showing that the well-known Greenhill formula can be used to predict stability conditions in cables/risers having an initial curvature and subjected simultaneously to dynamic compression and twisting. This is done by extending Aranha’s reasoning, of a “local buckling length scale,” a little further, therefore creating a general procedure that enables one to obtain the buckling number n appearing in the classical Greenhill formula. An application to rigid risers is presented and discussed.