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Offshore and Structural Mechanics

Time Domain Simulation of the 3D Bending Hysteresis Behavior of an Unbonded Flexible Riser

[+] Author and Article Information
Zhimin Tan

 Wellstream International Ltd., 11210 Equity Drive, Suite 350, Houston, TX 77041

Peter Quiggin

 Orcina Ltd., Daltongate, Ulverston, Cumbria, LA12 7AJ, UK

Terry Sheldrake

 Wellstream International Ltd., Wellstream House, Wincomblee Road, Newcastle upon Tyne, NE6 3PF, UK

J. Offshore Mech. Arct. Eng 131(3), 031301 (May 29, 2009) (8 pages) doi:10.1115/1.3058698 History: Received November 09, 2007; Revised October 17, 2008; Published May 29, 2009

This paper presents a “state-of-the-art” development in time domain dynamic simulation of 3D bending hysteresis behavior of a flexible riser under offshore environment loading. The main technical challenge is to understand and model the riser tensile armor behavior under continuous changes in both the magnitude and direction of bending, and its subsequent impact on the riser’s bending hysteresis characteristics. On account of this technical obstacle, the current industry practice is to model the riser as a linear structure, with certain conservatism enforced, and then to extract the global dynamic loads to a detailed local model for stress and life assessment. Two 3D flexible riser bending hysteresis models developed by Wellstream and Orcina are introduced in this paper, with their calibrations against the bending hysteresis loops measured in full scale tests. Both models are implemented using the analysis program ORCAFLEX . The Wellstream model is a detailed model that calculates both the total bending moment and the stresses in the tensile armor, whereas the Orcina model is a simpler model that only calculates the total bending moment. The study presented illustrates the difference in riser dynamic responses with and without consideration of the bending hysteresis behavior and assesses the difference in dynamic responses between the Wellstream and Orcina 3D bending hysteresis models. This development permits the modeling of more realistic riser structural properties in the dynamic simulation and reports detailed time history stress or strain results for strength components of the riser, and so expands the current practice of riser fatigue analysis, which uses the regular wave approach only, to using an irregular wave approach employing the rainflow counting method.

Copyright © 2009 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Flexible pipe construction

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Figure 2

Bending moment hysteresis loop

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Figure 3

Axial equilibrium of a tensile wire element

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Figure 4

Response to changes in curvature amplitude

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Figure 5

Data for Orcina model

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Figure 6

Tensile wire hoop position

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Figure 7

Interaction between ORCAFLEX and WELLSTREAM external 3D bending hysteresis model

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Figure 8

Test setup: 4 in. (101.6 mm) ID pipe

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Figure 9

Typical bending hysteresis loops measured over 4 in. (101.6 mm) pipe test (normalized data)

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Figure 10

Comparison of the model prediction and test results at 200 bar internal pressure

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Figure 11

Comparison of the model prediction and test results at 100 bar internal pressure

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Figure 12

Comparison of the model prediction and test results at 7 bar internal pressure

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Figure 13

Case study riser configuration

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Figure 14

Comparison of moment-curvature trajectory at TDZ

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Figure 15

Comparison of curvature responses at TDZ

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Figure 16

Applied 3D curvature

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Figure 17

Calculated 3D bending moment

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