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Research Papers: Piper and Riser Technology

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

[+] Author and Article Information
Kunpeng Wang

School of Naval Architecture
and Ocean Engineering,
Jiangsu University of Science and Technology,
Zhenjiang 212003, Jiangsu, China
e-mail: jstuwk@sina.com

Chunyan Ji

School of Naval Architecture
and Ocean Engineering,
Jiangsu University of Science and Technology,
Zhenjiang 212003, Jiangsu, China

Hongxiang Xue, Wenyong Tang

State Key Laboratory of Ocean Engineering,
Shanghai Jiao Tong University,
Shanghai 200240, China

1Corresponding author.

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received June 22, 2016; final manuscript received April 20, 2017; published online May 25, 2017. Assoc. Editor: Luis V. S. Sagrilo.

J. Offshore Mech. Arct. Eng 139(5), 051701 (May 25, 2017) (10 pages) Paper No: OMAE-16-1066; doi: 10.1115/1.4036675 History: Received June 22, 2016; Revised April 20, 2017

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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Figures

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Fig. 1

Typical flexible riser components

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Fig. 2

Displacement symbols of a layer and the critical point of helical wire

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Fig. 3

Sketch of interlayer interaction model

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Fig. 4

Linear hysteretic riser–soil interaction model

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Fig. 5

Relationship between axial force and elongation

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Fig. 6

Relationship between bending moment and curvature

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Fig. 7

Axial stress obtained from the present model and FE result under different bending moment: (a) 140 N·m and (b) 420 N·m

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Fig. 8

Relationship between critical point stress and curvature

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Fig. 9

Sketch of catenary flexible riser configuration

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Fig. 10

Bending moment–curvature relationship at the tenth user-defined element of bending stiffness

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Fig. 11

Time history of curvature and critical point stress: (a) and (b) are curvatures of the tenth and 13th user-defined bending elements, respectively; (c) and (d) are critical point stress at θ = π/2 of the tenth and 13th user-defined bending elements, respectively

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Fig. 12

Resistance versus penetration at the tenth user-defined element of bending stiffness

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Fig. 13

Envelope curves of the maximum axial force and bending moment

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Fig. 14

Fatigue damage at along the circumferential direction of different layers

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Fig. 15

Fatigue damage versus seabed stiffness: (a) full-sliding, (b) full-sticking, and (c) sticking–sliding

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Fig. 16

Fatigue damage versus friction coefficient ffriction: (a) full-sliding, (b) full-sticking, and (c) sticking–sliding

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