Axial Cyclic Behavior of Unbonded Flexible Riser Under Cyclic Axial Loads

Abstract Unbonded flexible risers consist of several helical and cylindrical layers, which can undergo large bending deformation and can be installed to different configurations to adapt harsh marine environment, thus can be applied to transport oil and gas resources from Ultra Deep Waters (UDW). The special construction of the structure results the complicated cross-sectional properties and has always been the research hotspot in the field of ocean engineering, and the structural behavior under long-term marine loads is the research challenges. In this paper, different from the previous studies on the cross-sectional mechanical properties of unbonded flexible riser, the cyclic tensile and compressive behavior is studied. Considering the deformation characteristics of each layer in unbonded flexible riser model and the contact deformation between adjacent layers, a theoretical method is established, and a corresponding numerical method in consideration of the detailed geometric properties of unbonded flexible riser is built up for verification. Firstly, based on a typical 2.5-inches 8-layer unbonded flexible riser model and taking the deformation characteristics of helical layer and cylindrical layer under axisymmetric loading into account, the balance equation of each layer considering the axial, radial, circumferential strains and torsional angle is established by functional principle, and the corresponding geometric relationship between adjacent layers is also included to build up the overall theoretical model of unbonded flexible riser. Then the corresponding numerical model containing all the geometric properties of unbonded flexible riser is modeled within Abaqus software and the ratio of kinetic energy to internal energy is very small to ensure the calculation accuracy. After verified through experimental results, the behavior of unbonded flexible riser under cyclic axial tension and cyclic axial compression is analyzed by the proposed theoretical and numerical methods. The theoretical and numerical results are in good agreement and the results show that: unbonded flexible riser model presents some hysteresis characteristics under cyclic axial forces and the axial tensile and compressive stiffness varies within a certain range, albeit insignificantly. Besides, the external pressure would enhance the axial stiffness of unbonded flexible riser, especially for the axial compression, where the axial compressive stiffness has significant strengthening.