Theoretical and numerical methods for predicting the structural stiffness of unbonded flexible riser for deep-sea mining under axial tension and internal pressure

Composite materials have received great attention in the design of flexible risers for deep-sea mining due to their significant advantages of high strength and lightweight. However, under the influence of various nonlinear factors, the cross-sectional response of composite flexible risers is extremely complex, and the prediction of their structural stiffness is greatly challenged. This paper focuses on a new type of composite flexible riser applied in deep-sea mining. The nonlinearity of materials and detailed geometric features of structures are fully considered. Theoretical and numerical models for predicting structural stiffness under axisymmetric loads are constructed. By iteratively solving the equilibrium equation, the strain and displacement fields of the flexible riser are obtained. Firstly, the accuracy of theoretical and numerical methods is verified based on a 5-layer 8-inch unbonded flexible riser as a case study. Secondly, the influence of fiber volume fraction (FVF) on the structural stiffness and weight of flexible risers is discussed. Finally, the influence of structural design parameters of the reinforcement layer on its axial tensile stiffness and radial stiffness is explored. The research results can provide parameter inputs for the design and dynamic analysis of flexible risers.