Recent developments in the offshore and deepwater industry have increased interest in the use of unbonded flexible risers for deep water applications, and in the development of improved large-scale models for global dynamic analysis. Flexible risers are slender composite structures consisting of multiple polymer layers and spiral wound steel layers with complex geometries. To better understand and capture the complex behavior of actual unbonded flexible risers, in which global response is influenced by the ability of layers to slide over each other, incorporating nonlinearity in global dynamic analysis of risers and employing a suitable and reliable constitutive model increase the accuracy of the riser motion and stresses predictions. In this work, a novel elasto-plastic constitutive model is developed to account for the macroscopic response of unbonded riser structures. Determination of the parameters of the constitutive model is achieved through calibration of riser's response plots obtained by numerical simulations of detailed Representative Volume Element model of the riser. Co-rotational hybrid beam elements are used to implement the developed constitutive law, created using a user-defined element subroutine in Abaqus. It is shown that the integration of hybrid beam elements and the proposed constitutive model effectively simulate the nonlinear response of unbonded flexible risers under various working conditions by accounting for effects of internal and external pressures, coupling of axial and twist deformations and slip between the different layers of the riser.

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