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Journal of Offshore Mechanics and Arctic Engineering | Volume 130 | Issue 3 | Research Paper
Research Papers

Understanding the Dynamic Coupling Effects in Deep Water Floating Structures Using a Simplified Model

[+] Author and Article Information
Ying Min Low

School of Civil & Environmental Engineering, Nanyang Technological University, Block N1, Nanyang Avenue, Singapore 639798, Singaporeymlow@ntu.edu.sg

Robin S. Langley

Department of Engineering, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, UKrsl21@cam.ac.uk

J. Offshore Mech. Arct. Eng 130(3), 031007 (Jul 16, 2008) (10 pages) doi:10.1115/1.2904951 History: Received July 12, 2007; Revised January 30, 2008; Published July 16, 2008
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Citing Articles

The global dynamic response of a deep water floating production system needs to be predicted with coupled analysis methods to ensure accuracy and reliability. Two types of coupling can be identified: one is between the floating platform and the mooring lines/risers, while the other is between the mean offset, the wave frequency, and the low frequency motions of the system. At present, it is unfeasible to employ fully coupled time domain analysis on a routine basis due to the prohibitive computational time. This has spurred the development of more efficient methods, including frequency domain approaches. A good understanding of the intricate coupling mechanisms is paramount for making appropriate approximations in an efficient method. To this end, a simplified two degree-of-freedom system representing the surge motion of a vessel and the fundamental vibration mode of the lines is studied for physical insight. Within this framework, the frequency domain equations are rigorously formulated, and the nonlinearities in the restoring forces and drag are statistically linearized. The model allows key coupling effects to be understood; among other things, the equations demonstrate how the wave frequency dynamics of the mooring lines are coupled to the low frequency motions of the vessel. Subsequently, the effects of making certain simplifications are investigated through a series of frequency domain analyses, and comparisons are made to simulations in the time domain. The work highlights the effect of some common approximations, and recommendations are made regarding the development of efficient modeling techniques.
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References

1
Ward, E. G., Haring, R. E., and Devlin, P. V., 1999, “Deepwater Mooring and Riser Analysis for Depths to 10,000Feet,” Proc. Offshore Tech. Conf., 2 , pp. 297–303.
2
Ormberg, H., and Larsen, K., 1998, “Coupled Analysis of Floater Motion and Mooring Dynamics for a Turret-Moored Ship,” Appl. Ocean. Res.  [CrossRef], 20 , pp. 55–67.
3
Chaudhury, G., and Ho, C. Y., 2000, “Coupled Dynamic Analysis of Platforms, Risers, and Moorings,” Proc. Offshore Tech. Conf., 2 , pp. 647–654.
4
Senra, S. F., Correa, F. N., Jacob, B. P., Mourelle, M. M., and Masetti, I. Q., 2002, “Towards the Integration of Analysis and Design of Mooring Systems and Risers, Part I: Studies on a Semisubmerslble Platform,” Proc. Conf. Offshore Mech. Arctic Eng., 1 , pp. 41–48.
5
Garrett, D. L., Gordon, R. B., and Chappell, J. F., 2002, “Mooring-and Riser-Induced Damping in Fatigue Seastates,” Proc. Conf. Offshore Mech. Arctic Eng., 1 , pp. 793–799.
6
Thompson, J. M. T., and Stewart, H. B., 2002, "Nonlinear Dynamics and Chaos", 2nd ed., Wiley, New York.
7
Faltinsen, O. M., 1990, "Sea Loads on Ships and Offshore Structures", Cambridge University Press, Cambridge.
8
Roberts, J. B., and Spanos, P. D., 1990, "Random Vibration and Statistical Linearisation", Wiley, New York.
9
Wu, S. C., and Tung, C. C., 1975, “Random Response of Offshore Structures to Wave and Current Forces,” Sea Grant Publication UNC-SG-75-22, Department of Civil Engineering, North Carolina State University, Rayleigh, NC 27650.
10
1998, "Floating Structures: A Guide for Design and Analysis", N.D. P.Barltrop, ed., Oilfield Publications, Ledbury, England.
11
Liu, Y., and Bergdahl, L., 1999, “On Combination Formulae for the Extremes of Wave-Frequency and Low-Frequency Responses,” Appl. Ocean. Res.  [CrossRef], 21 , pp. 41–46.
12
Det Norske Veritas, 2001, Offshore Standard DNV-OS-E301: Position Mooring.
13
American Petroleum Institute, 1997, API-RP-2SK: Recommended Practice for Design and Analysis of Station-Keeping Systems for Floating Structures.
14
Schellin, T. E., Scharrer, M., and Matthies, H. G., 1982, “Analysis of Vessel Moored in Shallow, Unprotected Waters,” Proc. Offshore Tech. Conf., 2 , pp. 161–176.
15
MSC International and Noble Denton Europe, 1999, Joint Industry Project: Integrated Mooring and Riser Design, Doc No: MCS/NDE/06 Rev 03.
16
Langley, R. S., 1986, “On the Time Domain Simulation of Second Order Wave Forces and Induced Responses,” Appl. Ocean. Res.  [CrossRef], 8 , pp. 134–143.
17
Mathworks, 2002, MATLAB , Release 13 product documentation.
18
Aranha, J. A. P., 1994, “A Formula for ‘Wave Damping’ in the Drift of a Floating Body,” J. Fluid Mech.  [CrossRef], 275 , pp. 147–145.
19
Low, Y. M., and Langley, R. S., 2006, “Time and Frequency Domain Coupled Analysis of Deepwater Floating Production Systems,” Appl. Ocean. Res., 28 , pp. 371–385.
20
Low, Y. M., and Langley, R. S., 2008, “A Hybrid Time/Frequency Domain Approach for Efficient Coupled Analysis of Vessel/Mooring/Riser Dynamics,” Ocean Eng.  [CrossRef], 35 , pp. 433–446.

Figures

Grahic Jump Location
+Plot of the wave spectrum
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Plot of the wave spectrum
Figure 4

Plot of the wave spectrum

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+Time history of x and y from time domain simulation
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Time history of x and y from time domain simulation
Figure 5

Time history of x and y from time domain simulation

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+Spectral density of vessel displacement (x) from (a) time domain analysis and (b) frequency domain analysis
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Spectral density of vessel displacement (x) from (a) time domain analysis and (b) frequency domain analysis
Figure 6

Spectral density of vessel displacement (x) from (a) time domain analysis and (b) frequency domain analysis

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+Cubic springs in series
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Cubic springs in series
Figure 8

Cubic springs in series

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+(a) Catenary configuration and (b) approximation of relative displacement profile
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(a) Catenary configuration and (b) approximation of relative displacement profile
Figure 9

(a) Catenary configuration and (b) approximation of relative displacement profile

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+Spectral density of line displacement (y) from time and frequency domain analyses
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Spectral density of line displacement (y) from time and frequency domain analyses
Figure 7

Spectral density of line displacement (y) from time and frequency domain analyses

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+Block diagram depicting complex interactions
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Block diagram depicting complex interactions
Figure 1

Block diagram depicting complex interactions

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+Schematic diagram of simplified system
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Schematic diagram of simplified system
Figure 2

Schematic diagram of simplified system

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+Type 2 coupling illustrated from linearized equations of motion
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Type 2 coupling illustrated from linearized equations of motion
Figure 3

Type 2 coupling illustrated from linearized equations of motion

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