Research Papers: Structures and Safety Reliability

An Experimental Study of Mooring Line Damping and Snap Load in Shallow Water

[+] Author and Article Information
Wen-Yang Hsu

Green Energy and Environment
Research Laboratories,
Industrial Technology Research Institute,
Tainan 701, Taiwan
e-mail: wyhsu26@gmail.com

Tzu-Ching Chuang

Department of Hydraulic and Ocean Engineering,
National Cheng Kung University,
Tainan 701, Taiwan
e-mail: fxm30342@gmail.com

Ray-Yeng Yang

Department of Hydraulic and Ocean Engineering,
National Cheng Kung University,
Tainan 701, Taiwan
e-mail: ryyang@mail.ncku.edu.tw

Wei-Ting Hsu

Energo Engineering,
KBR Company,
Houston, TX 77002
e-mail: Wei-Ting.Hsu@kbr.com

Krish P. Thiagarajan

Department of Mechanical and
Industrial Engineering,
University of Massachusetts,
Amherst, MA 01003
e-mail: kthiagarajan@umass.edu

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received May 28, 2018; final manuscript received January 2, 2019; published online February 15, 2019. Assoc. Editor: Amy Robertson.

J. Offshore Mech. Arct. Eng 141(5), 051603 (Feb 15, 2019) (9 pages) Paper No: OMAE-18-1065; doi: 10.1115/1.4042535 History: Received May 28, 2018; Revised January 02, 2019

The aim of this paper is to establish a simple approach to experimentally study the mooring line damping in shallow water, where snap loading may occur more frequently. Experimental measurements were conducted in a wave basin at a scale of 1:50, which corresponds to a full scale of 28 m water depth. A chain made by stainless steel was used, and the tension force at the fairlead was measured by tension gages. Moreover, the line geometry, touchdown point speed, and mooring line velocity were derived from image processing techniques. Surge motions at fairlead were driven from a programmable wavemaker. Regular surge motions with different frequencies and pretensions were tested in this system in order to investigate the quasi-static and dynamic behaviors of the mooring chain. In the quasi-static test, the mooring line keeps a typical catenary shape, and its indicator diagram exhibits a smooth-closed curve. In the dynamic test, the mooring line is fully lifted from the seabed, and it cyclically goes through the stage of semitaut and fully taut. We successfully reproduced a snap event in the laboratory scale, and the resulting mooring line damping can considerably increase in this manner. Two criteria for snap event were examined, and both of them were verified by the experiments.

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Fig. 1

(a) Experimental setup for a single mooring line and (b) photo of a single mooring line test

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Fig. 2

The photo, shape, and dimensions of the mooring chain

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Fig. 3

Static test of the mooring system

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Fig. 4

Photos of the mooring line geometry at different offsets

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Fig. 5

Histogram of the gray photo for Fig. 1(b)

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Fig. 6

Example of the velocity calculation from image pair using the cross-correlation algorithm. The reference vector of 0.5 m/s is plotted at the top of figure.

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Fig. 7

Surge motion at fairlead (left column), horizontal tension (middle column), and indicator diagram (right column) under different pretension conditions shown in Table 2

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Fig. 8

(a) Comparison of energy dissipation from the mooring line under different pretensions for cases 1–4, (b) for cases 5 and 6, and (c) nondimensional damping coefficients

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Fig. 9

Comparison of touchdown point speed and transverse wave speed for selected cases

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Fig. 10

Mooring line velocities under different phases for case 6. The reference vector of 0.5 m/s is plotted at the top of figure.



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