Application of analytical model in the prediction of dynamic responses and fatigue damage of flexible risers: Part II – Dynamic analysis of flexible risers in large-scale domain using a direct moment correction method. (September 2021)

• Record Type: Journal Article
• Authors: Kim, Jeong Du; Jang, Beom-Seon; Kim, Hyeon-Jin
• Is Part Of: Marine structures. Volume 79(2021)
• Publication Date: 2021-09
• Subjects: Flexible riser -- Dynamic analysis -- Fatigue damage of helical wires -- Shear deformation -- Varying tension; Naval architecture -- Periodicals; Offshore structures -- Periodicals; Architecture navale -- Périodiques; Structures offshore -- Périodiques; Naval architecture; Offshore structures; Periodicals; 620.4162
• DOI: 10.1016/j.marstruc.2021.103051
• Languages: English
• ISSNs: 0951-8339

Abstract

Abstract: In recent years, the dynamic responses of flexible risers have been the focus of many researchers. Most flexible risers undergo a substantial level of irregular motion from environmental loadings, which involves a continuous slip of helical wires. The slip of helical wires especially leads to a hysteretic effect by reducing the bending stiffness, making it hard to predict the dynamic responses of flexible risers. The current study, as an extension to Part I, presents a new large-scale dynamic analysis method for flexible risers. The suggested method creates a large-scale model for the dynamic analysis that considers a geometric and bending nonlinearity of flexible risers. The kinematics of each beam element is formulated based on a Green-Lagrangian strain and the interaction with the seabed, providing a realistic analysis of flexible risers. In particular, the current study introduces a direct moment correction method that modifies the internal force vector using an improved analytical model. The improved analytical model is assigned at each node of the large-scale model and estimates an accurate bending hysteresis curve considering the effect of shear deformation and varying tension. The suggested method corrects the bending moment and shear force of all beam elements based on the bending hysteresis curves obtained from the improved analytical model, by which a complex bending behavior of flexible risers is reflected in a large-scale domain. As a result, this study achieves a more accurate prediction of the dynamic responses and fatigue damage of flexible risers. A new dynamic analysis program, called OPFLEX, is developed herein based on the suggested analysis method. Using the developed program, the current study conducts several numerical investigations to identify the effect of the shear deformation and varying tension. Consequently, it is confirmed that the shear deformation of internal layers reduces the fatigue damage of helical wires by delaying the increase of internal stress. It is also identified that the effect of varying tension deteriorates the fatigue life of flexible risers through a continuous change of contact pressure during bending. Highlights: This study proposes introduces a new dynamic analysis method of flexible risers in a large-scale domain. The suggested method utilizes an improved analytical model presented in Part I, which produces a realistic nonlinear bending moment during irregular motion of flexible risers. The current study proposes a direct moment correction method to consider the effect of varying tension. The direct correction method produces the nonlinear bending moment through the control of the internal force vector, allowing to incorporate a complex variation of bending stiffness in the dynamic analysis. The current study carries out a numerical investigation to identify how the interaction between layers of flexible risers affects the fatigue damage of helical wires. Consequently, it is figured out that the shear deformation of internal layers reduces the fatigue damage. It is further confirmed that the change of contact pressure from varying tension deteriorates the fatigue life of flexible risers.