Stress Assessment in Armour Layers of Flexible Risers

Abstract

The accurate modeling of offshore flexible risers behaviour remains a great challenge because of (i) their complex internal structure, (ii) the variable nature of the loads along the pipe (tension, curvature, internal and external pressures), (iii) and the interactions with structures used to limit the pipe curvature. Technip and IFP have been codeveloping for many years models dedicated to stresses calculation in the armour wires, to assess the flexible lifetime. These models must account for a large number of potential inner contacts (contacts between upper and lower layers, lateral contacts between adjacent armour wires in the same layer) as well as external contacts (bend stiffener, arch, bellmouth or other curvature limitation setup). The paper presents a comparison between 3 models with different level of complexity and realism. The first one is a in house model, whereas the two others were developed on the basis of the commercial FE code Abaqus. The first model, Life6, is based on some simplified assumptions in particular the fact that periodic solutions are assumed (given constant curvature in the pipe) and uses analytical solutions of equilibrium of wires on a torus (the bend pipe). The effect of non uniform curvature (in particular end-fitting proximity) is not considered in this model. The second model, namely 3D/Periodic model, is still based on a periodic assumption, but can cope with severe loadings (such as large curvature of the pipe or compression) leading to specific wires contact interactions. Abaqus Standard (Implicit scheme) is used. The third model, called 3D/Explicit model, is a full length model, as it includes end fittings effects, outer structure (like stiffener) interactions and any curvatures variations along the pipe. All contacts interactions are considered. The number of DOFs involved in the analysis requires the use of an explicit integration scheme (Abaqus Explicit) running on a parallel platform. These models are cross validated on a dedicated case study that consisted of a pressurized pipe cyclically bent with constant curvature. The validation of the model results is very satisfying even when lateral contacts between wires occur. Finally, a comparison of the 3D/Explicit model results with experimental data is presented. This model provides a very good estimation of the flexible behavior and of the end fitting effects.