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Technical Briefs

Second-Order Low-Frequency Wave Forces on a SPM Offloading Tanker in Shallow Water

[+] Author and Article Information
S. Ma

Deepwater Engineering Research Center, Harbin Engineering University, Harbin, Heilongjiang 150001, China

M. H. Kim

Ocean Engineering Program and Department of Civil Engineering, Texas A&M University, College Station, TX 77843

S. Shi

Houston Offshore Eng. Inc. and Deepwater Engineering Research Center, Harbin Engineering University, Harbin, Heilongjiang 150001, China

J. Offshore Mech. Arct. Eng 131(4), 044502 (Oct 01, 2009) (6 pages) doi:10.1115/1.3160519 History: Received June 24, 2008; Revised March 17, 2009; Published October 01, 2009

This paper studies the influence of three different calculation methods of the second-order low-frequency (LF) wave-force quadratic transfer functions (QTFs) for a single point mooring (SPM) tanker system in relatively shallow water region. The multivessel-mooring hawser coupled dynamic analysis is used to simulate the floater relative motions and mooring and hawser tensions. Because the SPM tanker is deployed in shallow water region and the slowly varying drift motions are to be dominant in typical operational conditions, the accurate calculation of LF wave-force QTFs become important especially for mooring and hawser-tension prediction. The practically popular Newman’s approximation and another approximation excluding complicated free-surface integrals are used to calculate the LF QTFs on the offloading tanker and they are compared with the complete QTF results. Further comparison is carried out by calculating the resulting LF wave-force spectra and motion time histories and analyzing their impacts on hawser and mooring line tensions. Through the example studies, the limitation of the Newman’s approximation in the case of shallow water and longer period irregular waves is underscored.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

Analysis model of single point mooring buoy and tanker system

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Figure 2

The amplitude of the surge-wave force LF QTF for frequency difference 0.05 rad/s

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Figure 3

The amplitude of the surge-wave force LF QTF for frequency difference 0.1 rad/s

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Figure 4

The amplitude of the surge-wave force LF QTF for frequency difference 0.05 rad/s for 200 m water depth case

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Figure 5

The LF surge-wave force spectrum comparison for Tp=10 s

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Figure 6

The LF surge-wave force spectrum comparison for Tp=15.7 s

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