Constitutive Behavior of Tensile Armor Wires in Unbonded Flexible Pipes

The use of unbonded flexible pipes in the offshore industry is continuously moving towards deeper waters, leading to a demand for flexible pipes with ever-increasing structural ca-pacity. Detailed design tools which capture the true mechanical behavior of the individual pipe layers enable improvement of the cross-sectional pipe design and at the same time enhances the pipe operation reliability. The defining feature of unbonded flexible pipes is the relative movement between the individual layers. The relative movement reduces the internal loading state of the armor layers, leading to improved fatigue performance. The relative sliding between the layers introduces a hysteresis behavior during bending, where the loading condition of the armor wires is dependent on the load history. This thesis aims to provide a general design tool to asses the tensile armor wire loading and the bending hysteresis effect for the dynamic operational conditions of the unbonded flexible pipes. The presented thesis is organized in five parts that asses the critical loading condition related to different sections along flexible riser systems. The model configuration related to the loading condition in each section is presented to enable analysis of the axial loading and bending hysteresis effect of flexible pipes as well as the associated tensile armor wire stresses.
The first part of this thesis presents a large-scale finite element (FE) model suitable for analysis of the tensile armor wire loading near the end fitting anchor points. An experimental in-plane bending test of a flexible pipe that is interacting with a bellmouth is analyzed. The tensile armor loading determined by the large-scale FE model is compared to stresses obtained experimentally and a good correlation is found.
The second part of the thesis describes the implementation of an computational efficient repeated unit cell FE model. It is only applicable for analysis of flexible pipes sections unaffected by end terminations and interaction with ancillary components. The method-ology for analyzing flexible pipes by the repeated unit cell approach, where a single armor wire represents each helical armor layer with periodic boundary conditions, is outlined.
The third part of the thesis focuses on the analysis of flexible pipes subjected to various tension-bending loads. The tensile armor stress and the global flexible pipe behavior predicted with the small-scale FE model are compared to well-known analytical models for axial tension and static tension-bending load cases. Good agreement is found between results obtained with the approaches. Furthermore, the bending hysteresis effect and tensile armor loading corresponding to flexible pipes subjected to symmetric and non-symmetric cyclic bending are studied. The study shows that the loading condition of the tensile armor wires and the bending response of the flexible pipe become symmetric around the mean cyclic bending curvature corresponding to the equilibrium state with frictionless contact interaction.
The fourth part of the thesis presents a homogenization procedure to represent the me-chanical behavior of helical armor layers by a continuum layer with equivalent orthotropic material properties. This homogenization procedure has been used to reduce the complex-ity of the above-mentioned small-scale FE model.
The fifth part concerns the prediction of lateral buckling capacity of tensile armor wires in a flexible pipe using the methods mentioned above. The study shows that reliable prediction of lateral buckling can only be achieved if friction interaction and lateral contact between individual armor wires in the armor layers are accounted for.
The sum of the work presented in this thesis provides an insight improving the under-standing of the tensile armor wires constitutive behavior in flexible pipes, hereby paving the way to enhanced cross-sectional designs and increased reliability of future unbonded flexible pipes.