Theoretical and numerical analysis of bending behavior of unbonded flexible risers

Zhang, Mengmeng; Chen, Xiqia; Fu, Shixiao; Guo, Yousong; Ma, Leixin

doi:10.1016/j.marstruc.2015.10.001

Cited by 27 publications

(8 citation statements)

References 8 publications

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“…The stress at critical point of helical wire is highly concerned in the fatigue assessment of flexible riser. Figure 7 demonstrates the comparison of the axial stress calculated from the present model and the detailed finite element model under initial contact pressure [23]. It can be seen that under the bending moment of 140 NÁm, the helical wires keep full-sticking, and the result shows good agreement.…”

Section: Riser-seabed Interaction Modelmentioning

confidence: 86%

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

Wang

Xue

et al. 2017

Journal of Offshore Mechanics and Arctic Engineering

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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.

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Section: Riser-seabed Interaction Modelmentioning

confidence: 86%

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

Wang

Xue

et al. 2017

Journal of Offshore Mechanics and Arctic Engineering

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“…Since the geometric nonlinearity is strong within the riser model, the explicit solution method is applied to avoid non-convergence in the numerical simulation. Since it is difficult to define the accurate contact between and within interlayers, general contact is thus applied to define the nonlinear contact, where the tangential behavior is simulated by using the Coulomb friction model [25] and the corresponding frictional coefficient is 0.1 [37], while the normal contact is set to hard contact. Two reference points, RP1 and RP2 (see Figure 4), are set at the geometric center of the top and end cross-section and all the layer's nodes at the edge of the cross-section are kinematically and rigidly bounded onto the two reference points, thus applying the boundary conditions and external loads.…”

Section: Model Verificationmentioning

confidence: 99%

“…The proposed numerical can effectively simulate the inter-layer and intra-layer contact and friction, and therefore can be used to calculate the cross-sectional mechanical properties of an unbonded flexible riser under axisymmetric and bending loads. Zhang et al [25] also developed a numerical model containing detailed geometries, and all layers of the structure were also simulated using body cells to analyze the structural response under the combined effects of external pressure and bending moments. Yoo et al [26,27] established a carcass layer with detailed geometrical characteristics through ANSYS software (https://www.ansys.com/, accessed on 7 May 2024), and simplified it into an equivalent cylindrical shell layer by analyzing the mechanical failure characteristics of the carcass layer under the action of axial force, and established an equivalent simplified eight-layer numerical model and a five-layer numerical model; at the same time, they considered the effect of the shell unit or the body unit in simulating the cylindrical shell layer, and investigated the axial load-bearing capacity of the unbonded flexible riser.…”

Section: Introductionmentioning

confidence: 99%

Axial Tensile Ultimate Strength of an Unbonded Flexible Riser Based on a Numerical Method

Li,

Jiang,

Xing

et al. 2024

Materials

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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 to harsh marine environments, and is a key equipment in transporting oil and gas resources from Ultra Deep Waters (UDWs) to offshore platforms. The helical interlayer of an unbonded flexible riser makes the structural behavior difficult to predict. In this paper, the axial tensile behavior and the axial tensile ultimate strength of an unbonded flexible riser are studied based on a typical 2.5-inch eight-layer unbonded flexible riser model, and verified through a theoretical method considering the contact between adjacent layers. First, the balance equation of separate layers is deduced by a functional principle, and then the overall theoretical model of an unbonded flexible riser is established considering the geometric relationship between adjacent layers. Then, the numerical model considering the detailed geometric properties of an unbonded flexible riser is established to simulate the axial tensile behavior. Finally, after being verified through the experimental results, the axial tensile stiffness and axial tensile strength of an unboned flexible riser considering the elasticity of the tensile armor layer are studied using the proposed two methods. Additionally, the effect of frictional coefficients is conducted. The numerical and theoretical results show good agreement with the test results, and the friction between adjacent layers would increase the axial tensile stiffness of an unbonded flexible riser.

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“…Few studies have carried out research on the cross-sectional mechanical properties of unbonded flexible risers under bending loads using this method. Among them, Zhang et al [ 121 ] took a class of eight-layer 2.5-inch unbonded flexible risers as the object of study, and simplified the cross-sectional mechanical structure of the carcass layer and the pressure armor layer to some extent (shown in Figure 19 ). The mesh of the regular cylindrical shell layers was divided into large elements.…”

Section: Development Of Cross-sectional Properties Of An Unbonded Fle...mentioning

confidence: 99%

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

Liu,

Qu,

Chen

et al. 2024

Materials

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Unbonded flexible risers consist of several helical and cylindrical layers, which can undergo large bending deformation and can be installed in different configurations to adapt to harsh marine environments; thus, they can be applied to transport oil and gas resources from ultra-deep waters (UDW). Due to their special geometric characteristics, they can ensure sufficient axial tensile stiffness while having small bending stiffness, which can undergo large deflection bending deformation. In recent years, the development of unbonded flexible risers has been moving in an intelligent, integrated direction. This paper presents a review of unbonded flexible risers. Firstly, the form and properties of each interlayer of an unbonded flexible riser are introduced, as well as the corresponding performance and configuration characteristics. In recent years, the development of unbonded flexible risers has been evolving, and the development of machine learning on unbonded flexible risers is discussed. Finally, with emphasis on exploring the design characteristics and working principles, three new types of unbonded flexible risers, an integrated production bundle, an unbonded flexible riser with an anti-H2S layer, and an unbonded flexible riser with a composite armor layer, are presented. The research results show that: (1) the analytical methods of cross-sectional properties of unbonded flexible risers are solved based on ideal assumptions, and the computational accuracy needs to be improved. (2) Numerical methods have evolved from equivalent simplified models to models that account for detailed geometric properties. (3) Compared with ordinary steel risers, the unbonded flexible riser is more suitable for deep-sea resource development, and the structure of each layer can be designed according to the requirements of the actual environment.

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Theoretical and numerical analysis of bending behavior of unbonded flexible risers

Cited by 27 publications

References 8 publications

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

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

Axial Tensile Ultimate Strength of an Unbonded Flexible Riser Based on a Numerical Method

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

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