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Research Papers: Structures and Safety Reliability

Numerical Study on the Heading Misalignment and Current Velocity Reduction of a Vessel-Shaped Offshore Fish Farm

[+] Author and Article Information
Lin Li, Jungao Wang, Muk Chen Ong

Department of Mechanical and
Structural Engineering and Materials Science,
University of Stavanger,
Stavanger, 4036, Norway

Zhiyu Jiang

Department of Engineering Sciences,
University of Agder,
Grimstad, 4879, Norway;
Department of Marine Technology,
Norwegian University of
Science and Technology,
Trondheim, 7491, Norway
e-mail: zhiyu.jiang@uia.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 March 23, 2018; final manuscript received December 3, 2018; published online February 15, 2019. Assoc. Editor: Nilanjan Saha.

J. Offshore Mech. Arct. Eng 141(5), 051602 (Feb 15, 2019) (9 pages) Paper No: OMAE-18-1028; doi: 10.1115/1.4042266 History: Received March 23, 2018; Revised December 03, 2018

Recently, the concept of a vessel-shaped fish farm was proposed for open sea applications. The fish farm comprises a vessel-shaped floater, five fish cages, and a single-point mooring system. Such a system weathervanes, and this feature increases the spread area of fish waste. Still, the downstream cages may experience decreased exchange of water flow when the vessel heading is aligned with the current direction, and fish welfare may be jeopardized. To ameliorate the flow conditions, a dynamic positioning (DP) system may be required, and its power consumption should relate to the heading misalignment. This paper proposes an integrated method for predicting the heading misalignment between the vessel-shaped fish farm and the currents under combined waves and currents. A numerical model is first established for the fish farm system with flexible nets. Current reduction factors are included to address the reduction in flow velocity between net panels. The vessel heading is obtained by finding the equilibrium condition of the whole system under each combined wave and current condition. Then, the Kriging metamodel is applied to capture the relation between the misalignment angle and environmental variables, and the probability distribution of this misalignment angle is estimated for a reference site. Finally, the requirement for the DP system to improve the flow condition in the fish cages is discussed.

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Figures

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Fig. 1

Overview of the vessel-shaped fish farm concept [5]

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Fig. 2

Main geometry of the submerged part of the vessel-shaped fish farm

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Fig. 3

Illustration of the coupled fish farm model with global coordinate system and mooring line numbers

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Fig. 4

The flexible fish net model using representative bar elements

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Fig. 5

Illustration of the calculation of i values in Eq. (6) for reduced current velocity under two conditions: (a) vessel aligned with the currents and (b) vessel misaligned with the currents

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Fig. 6

Flowchart for the integrated method using metamodels

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Fig. 7

Validation of the Kriging metamodel using additional simulation points

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Fig. 8

Map of the reference site area

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Fig. 9

Wave and current roses for the reference site (‘N’, ‘S’, ‘W’ and ‘E’ refer to North, South, West and East, respectively): (a) wave rose and (b) current rose

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Fig. 10

Equilibrium condition of the fish farm structure (net solidity ratio of Sn = 0.2) under uniform currents (Uc = 1.28 m/s, Dirc = 180 deg) and waves (Hs = 6.29 m, Tp = 10.21 s, Dirw = 261 deg): (a) top view and (b) side view

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Fig. 11

Probability of occurrence and exceedance for the misalignment angle during 1-year

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Fig. 12

Probability of occurrence and exceedance for the current velocity in cage 5 during 1-year

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