Fatigue Damage Study of Helical Wires in Catenary Unbonded Flexible Riser Near Touchdown Point

Abstract

This study presents an analytical model of flexible riser and implements it into finite-element software abaqus to investigate the fatigue damage of helical wires near touchdown point (TDP). In the analytical model, the interlayer contact pressure is simulated by setting up springs between adjacent interlayers. The spring stiffness is iteratively updated based on the interlayer penetration and separation conditions in the axisymmetric analysis. During the bending behavior, the axial stress of helical wire along the circumferential direction is traced to determine whether the axial force overcomes the interlayer friction force and thus lead to sliding. Based on the experimental data in the literature, the model is verified. The present study implements this model into abaqus to carry out the global analysis of the catenary flexible riser. In the global analysis, the riser–seabed interaction is simulated by using a hysteretic seabed model in the literature. The effect of the seabed stiffness and interlayer friction on the fatigue damage of helical wire near touchdown point is parametrically studied, and the results indicate that these two aspects significantly affect the helical wire fatigue damage, and the sliding of helical wires should be taken into account in the global analysis for accurate prediction of fatigue damage. Meanwhile, different from the steel catenary riser, high seabed stiffness may not correspond to high fatigue damage of helical wires.

Review of the Development of an Unbonded Flexible Riser: New Material, Types of Layers, and Cross-Sectional Mechanical Properties

“…At the same time, they provided the explicit expression of bending stiffness in the partially slipping phase, which greatly simplified the calculation process. In addition, Wang et al [104,105] analyzed the variation of stresses in helical belts under the action of irregularly varying bending moments and curvatures based on the theoretical models of Kebadze et al [61] and Dong et al [103]. In addition, some other research has developed equivalent mathematical and analytical models based on the nonlinear bending characteristics of unbonded flexible risers.…”

Nonlinear Slippage of Tensile Armor Layers of Unbonded Flexible Riser Subjected to Irregular Loads

“…For theoretical methods under axisymmetric loads, early studies generally only focused on the overall response of the riser under a certain load alone, and the response of each interlayer could not be decoupled [1]; subsequent studies established the equilibrium equations of each interlayer, and gradually considered the thickness deformation of different layers and established the geometrical relationship between layers, so as to establish the overall stiffness matrix of the unbonded flexible riser [2][3][4][5]. The understanding about the slippage of helical layers in unbonded flexible risers has gone through a process from simple to complex, and the core idea is to consider the helical tendon from the non-slip phase to the full slip phase, which gradually considers the bending moment-curvature relationship in the partial slip phase to improve the understanding of the slippage of the helical tendon [6][7][8][9][10]. Féret and Bournazel [11] were among the early investigators of the structural response of the unbonded flexible riser; they presented an analytical method to quickly assess the stress of the helical tendon, while ignoring the effects of internal and The analytical methods for assessing the cross-sectional mechanical properties of unbonded flexible risers typically include two categories, theoretical analysis and numerical simulation, since the cost of unbonded flexible riser specimens is high and the test conditions are harsh.…”