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Research Papers: Ocean Space Utilization

Wave-Induced Accelerations of a Fish-Farm Elastic Floater: Experimental and Numerical Studies

[+] Author and Article Information
Peng Li

Department of Marine Technology,
Norwegian University of Science
and Technology (NTNU),
Trondheim NO-7491, Norway
e-mail: peng.li@ntnu.no

Odd M. Faltinsen

Department of Marine Technology,
Centre for Autonomous Marine Operations
and Systems (AMOS),
Norwegian University of Science
and Technology (NTNU),
Trondheim NO-7491, Norway
e-mail: Odd.faltinsen@ntnu.no

Marilena Greco

Department of Marine Technology,
Centre for Autonomous Marine Operations
and Systems (AMOS),
Norwegian University of Science
and Technology (NTNU),
Trondheim NO-7491, Norway;
CNR-INSEAN,
The Italian Model Basin,
Rome 00128, Italy
e-mail: marilena.greco@ntnu.no

1Corresponding author.

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received June 2, 2016; final manuscript received July 19, 2017; published online September 20, 2017. Assoc. Editor: Wei Qiu.

J. Offshore Mech. Arct. Eng 140(1), 011201 (Sep 20, 2017) (9 pages) Paper No: OMAE-16-1055; doi: 10.1115/1.4037488 History: Received June 02, 2016; Revised July 19, 2017

Numerical simulations and experiments of an elastic circular collar of a floating fish farm are reported. The floater model without netting structure is moored with nearly horizontal moorings and tested in regular deep-water waves of different steepnesses and periods without current. Local overtopping of waves was observed in steep waves. The focus here is on the vertical accelerations along the floater in the different conditions. The experiments show that higher-order harmonics of the accelerations matter. A three-dimensional (3D) weak-scatter model with partly nonlinear effects as well as a 3D linear frequency-domain method based on potential flow are used. From their comparison against the measurements, strong 3D and frequency dependency effects as well as flexible floater motions matter. The weak-scatter model can only partly explain the nonlinearities present in the measured accelerations.

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Figures

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Fig. 1

Experimental setup. Upper: top view; lower: side view.

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Fig. 2

Illustration of hydroelasticity of floater from side view during model test with wave steepness H/λ=1/15 and wave period T=1.6 s

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Fig. 3

Definitions of inertial Cartesian coordinate system and body-fixed Cartesian coordinate system. c is the torus radius and β is the angle.

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Fig. 4

Nondimensional retardation functions for heave, pitch, first elastic vertical and radial modes as a function of time. The enlarged view shows the detail of the behavior at small times intervals. K33′=K33/(ρgac), K55′=K55/(ρgac3), Kw1w1′=Kw1w1/(ρga), Kr1r1′=Kr1r1/(ρga). ρ, g, a, c are defined in Table 1 and text.

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Fig. 5

Nondimensional amplitudes of first-harmonics for experimental and numerical vertical acceleration w¨a(ω) at front, left and aft positions on the floater for different wave steepnesses and frequencies. The height of the experimental error bars is two times the estimated standard deviation.

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Fig. 6

Time series of the first harmonics of experimental vertical accelerations at the front of the floater with H/λ=1/15 and T=1.6 s

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Fig. 7

Nondimensional amplitudes of higher-order harmonics for experimental vertical acceleration at the front of the floater for different wave steepnesses and frequencies. w¨a(2ω), w¨a(3ω), and w¨a(4ω) are the amplitudes of the second, third, and fourth harmonics of vertical accelerations. The height of the experimental error bars is two times the estimated standard deviation.

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Fig. 8

Frequencies ω, 2ω, 3ω, and 4ω in rad/s as a function of νa=ω2a/g. The lower bond for the natural frequencies of vertical modes is indicated with the horizontal line.

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