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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

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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Copyright © 2008 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Block diagram depicting complex interactions

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Figure 2

Schematic diagram of simplified system

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Figure 3

Type 2 coupling illustrated from linearized equations of motion

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Figure 4

Plot of the wave spectrum

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Figure 5

Time history of x and y from time domain simulation

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Figure 6

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

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Figure 7

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

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Figure 8

Cubic springs in series

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Figure 9

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

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