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Safety and Reliability

Linear and Nonlinear Approach of Hydropneumatic Tensioner Modeling for Spar Global Performance

[+] Author and Article Information
Chan K. Yang

 FloaTEC, LLC, Houston, TX 77079

M. H. Kim

 Texas A&M University, College Station, TX 77843

J. Offshore Mech. Arct. Eng 132(1), 011601 (Dec 21, 2009) (9 pages) doi:10.1115/1.3160468 History: Received January 11, 2008; Revised May 07, 2009; Published December 21, 2009; Online December 21, 2009

This paper deals with a numerical model of top tension risers with hydropneumatic tensioner for Spar application in the Gulf of Mexico environment. The nonlinearity of the stiffness and the friction characteristics of the tensioner combined with stick-slip behavior of the riser keel joint are investigated. The relationship between tensions and strokes for the hydropneumatic tensioner is based on the ideal gas equation where the isotropic gas constant can be varied to achieve an optimum stroke design based on the tensioner stiffness. Challenges of modeling the coupling effects in the finite element (FE) method between the tensioner and hull motion are also presented. This new FE model is implemented into a fully-coupled time-domain coupled-dynamics-analysis program for floating bodies. The effect of nonlinearity of tensioner curve, tensioner friction, and riser keel friction is intensively investigated. The resultant global motion, TTR stroke, and tensions are systematically compared with those of a simple engineering approach, in which the nonlinear coupling effect is captured by linearization.

Copyright © 2010 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Panel for hydrodynamic calculation—a total of 691 panels were used for the quarter of the hull

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Figure 2

Schematic of the coordinate system and free body diagram of a thin rod

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Figure 3

Fully coupled Spar and mooring/riser model

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Figure 4

Configuration of linear and nonlinear hydropneumatic tensioner and keel guide model

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Figure 7

Static vertical force and heave relation for linear and nonlinear model obtained from the static heave test

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Figure 8

Static tension and the stroke relation obtained from the static heave test compared with the original curve

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Figure 9

Stick-slip effect of the keel joint compared with the slip-only case

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Figure 10

Heave free-decay time history—comparison between the linear spring model and the nonlinear tensioner model

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Figure 11

Pitch free-decay time history—comparison between the linear spring model and the nonlinear tensioner model

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Figure 12

Comparison of the damping ratio—linear and nonlinear model of the tensioner

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Figure 13

Heave motion for the 100 year hurricane: (a) linear spring model and (b) nonlinear pneumatic tensioner

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Figure 14

Heave motion for the 1000 year hurricane: (a) linear spring and (b) nonlinear pneumatic tensioner

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Figure 15

Motion statistics for the 100 year hurricane: (a) surge (in %WD), (b) heave (in meters), and (c) pitch (in degrees)

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Figure 16

Motion statistics for the 1000 year hurricane: (a) surge (in %WD) (b) heave (in meters), and (c) pitch (in degrees)

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Figure 17

Time history of stroke of the piston—upstroke positive: (a) linear spring-dashpot model and (b) nonlinear model

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Figure 18

Stroke statistics—linear and nonlinear models (a) 100 year hurricane and (b) 1000 year hurricane

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Figure 19

Time history of stroke and friction force relationship—exemplified around the time interval where the stroke exceeds its limit

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Figure 20

Statistics of the top tension of a riser (a) 100 year hurricane and (b) 1000 year hurricane

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Figure 6

Sensitivity of the spring and TTR stretch to cubic spring stiffness modeled for the upper and lower stroke limit

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Figure 5

Tensioner curves for z0=7.6 m, T0=5249 kN, zdown=−3.8 m, and zup=3.8 m

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