Flexible Pipe Hysteretic Bending Behavior: Comparison With Experimental Characterization and Finite Element Method

Abstract

The flexible riser global behavior is of major importance during the design phase of a flexible riser system configuration. Flexible riser design is an optimization loop involving several iterations. The flexible riser main mechanical characteristics are used as inputs into the global dynamic analysis. The dynamic analysis is performed to derive global loads that are then used for the flexible riser design. At each step of the riser design optimization loop, from the global configuration to flexible pipe cross-section, accurate knowledge of the flexible pipe mechanical characteristics is essential. One of the main mechanical characteristics of a flexible riser that drives its global behavior is its bending stiffness. Considering the flexible riser composition, the bending stiffness is much lower than its torsional and axial stiffnesses and therefore has a larger influence on its static and dynamic behavior. In order to simplify the global analysis, the first approach considered uses a linear bending stiffness. In this case, only the contribution of the polymer layers of the flexible is accounted for. The linear bending stiffness is therefore only impacted by thermal loads. In reality, because of the friction that can occur between the armor layers, the bending behavior is non-linear and the relation between bending moment versus curvature is hysteretic. This hysteretic behavior induces energy dissipation and therefore, when accounted in the global modeling of the riser, reduces the curvature response of the flexible riser. Technip and IFPEN previously developed a model to describe mechanical flexible pipe characteristics. This model is based on the finite difference method. To ensure the accuracy of stress prediction of this method in response to external loads (tension, curvature, internal pressure and external pressure), results are validated with both experimental and finite element methods. This paper presents the theory of bending behavior of a flexible riser and explains how the model is used to derive a hysteric bending model. The matrix of experimental tests campaigns performed on flexible pipe to characterize bending behavior of flexible risers, and comparisons with hysteretic behavior predictions from the FEA and finite difference models are presented. Finally results from experimental tests, FEA and calculated hysteretic bending models are implemented into the dynamic analysis model of a flexible riser system. Then the global behaviors of the flexible pipe configuration, with the different properties, are compared.