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Research Papers: Ocean Engineering

Deformation of Nets With Bending Stiffness Normal To Uniform Currents

[+] Author and Article Information
Lars Gansel

SINTEF Fisheries and Aquaculture,
Trondheim 7010, Norway
e-mail: lars.gansel@sintef.no

Østen Jensen

SINTEF Fisheries and Aquaculture,
Trondheim 7010, Norway
e-mail: osten.jensen@sintef.no

Per Christian Endresen

SINTEF Fisheries and Aquaculture,
Trondheim 7010, Norway
e-mail: per.christian.endresen@sintef.no

Martin Føre

SINTEF Fisheries and Aquaculture,
Trondheim 7010, Norway
e-mail: martin.fore@sintef.no

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 24, 2013; final manuscript received June 26, 2014; published online August 22, 2014. Assoc. Editor: Hideyuki Suzuki.

J. Offshore Mech. Arct. Eng 136(4), 041102 (Aug 22, 2014) (8 pages) Paper No: OMAE-13-1060; doi: 10.1115/1.4028224 History: Received June 24, 2013; Revised June 26, 2014

The tremendous growth of the fish farming industry in Norway over the past decades was supported by new designs and materials for fish farms, enabling bigger fish cages to be positioned in more exposed sea areas. Today, the nets of most fish cages in Norway are made from nylon, but also new net materials are proposed to better prevent escapes, protect fish from predator attacks, improve the stability of fish cages, and reduce biofouling. Some of these materials are stiff in at least one direction, and there still is a lack of knowledge about the behavior of nets with bending stiffness in currents and waves. The aim of this study was to determine how nets with bending stiffness deform in different currents and how the deformation influences the drag on the nets and to compare the results with predictions from a numerical model. Three types of net (PET, copper, and steel) were clamped to a solid steel bar on the top side but were otherwise unrestricted. The nets were subjected to several flow speeds between 0.1 and 0.9 m/s. The net deformation was determined with an optical tracking system, and the forces on the net panels were measured with a multi-axis force/torque sensor system. It is shown that bending stiffness and density of nets affect net deformation, as both parameters impact the balance between drag and gravitational forces on the nets. Net deformation leads to a decrease of the projected net area. As the rate of deformation with current speed varies greatly between different net types, the discrepancy between measured drag and drag values normalized to the projected net area at different current speeds follows different relationships for different nets. A numerical model, FhSim, was able to predict net deformation of nets with bending stiffness well, and it is shown that FhSim could not only account for the effect of bending stiffness on net deformation, but also that the model captures the structural dynamics of nets with bending stiffness in a current.

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References

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Figures

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Fig. 1

Setup of the experiments. The net panels were clamped in between rigid metal bars as shown in the perspective view to the right. In the drawings to the left, the metal bars are red, the multi-axis force/torque measurement system is green, and the velocity meter is yellow. The dark gray bar at the lower end of the net panel in the image to the right indicates the position of a steel bar, and the red circles indicate the positions of reflective markers for control of the three-dimensional orientation of the frame using a Qualisys motion tracking system. It should be noted that the number and the positions of the reflectors were different from the positions shown in this figure. In the present study, four reflectors were mounted evenly spaced along the vertical edges of the nets and only two reflectors were mounted on the vertical centerline of the nets. The reflectors on the mounting bars and on the rod extruding from above the force measurement system were positioned as shown in this figure.

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Fig. 2

Outlines of all nets in the orientation during the tests. Image (a) steel net, (b) copper net, and (c) PET net. Back lighted photos of the nets were reduced to two colors.

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Fig. 3

Illustration of how an element based model in FhSim is used to define net panels made from materials with bending stiffness. The 10 model nodes (black circles), 8 net elements (meshed triangles), 10 constraints elements (white bars), and rigid bar element (black bar) comprising such a panel are highlighted in the figure.

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Fig. 4

Deformation of the steel net in the x-direction at different flow speeds. Markers depict the positions of reflectors in the x-z-plane

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Fig. 5

Deformation of the copper net in the x-direction at different flow speeds. Markers depict the positions of reflectors in the x-z-plane

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Fig. 6

Deformation of the PET net in the x-direction at different flow speeds. Markers depict the positions of reflectors in the x-z-plane

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Fig. 7

Projected area of and measured and normalized forces on the steel net panel at different flow speeds. Squares, triangles, and circles mark the projected area, drag force on the net panel, and drag forces normalized by projected area, respectively. The drag forces (triangles) were calculated by dividing measured drag with the projected area of the panel (m2) at 0 ms−1 flow speed, i.e., the drag is per m2 nondeformed net.

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Fig. 8

Projected area of and measured and normalized forces on the copper net panel at different flow speeds. Squares, triangles, and circles mark the projected area, drag force on the net panel, and drag forces normalized by projected area, respectively. The drag forces (triangles) were calculated by dividing measured drag with the projected area of the panel (m2) at 0 ms−1 flow speed, i.e., the drag is per m2 nondeformed net.

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Fig. 9

Projected area of and measured and normalized forces on the PET net panel at different flow speeds. Squares, triangles, and circles mark the projected area, drag force on the net panel, and drag forces normalized by projected area, respectively. The drag forces (triangles) were calculated by dividing measured drag with the projected area of the panel (m2) at 0 ms−1 flow speed, i.e., the drag is per m2 nondeformed net.

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Fig. 10

Projected area of the PET net with and without a metal rod attached to the lower edge of the net at different flow speeds

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Fig. 11

Comparison between measured and estimated deformation of PET net panel when subjected to currents of 0–0.63 ms−1. Different symbols represent different current speeds. Filled symbols mark measurements and open symbols mark model estimates.

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Fig. 12

Comparison between measured and estimated deformation of copper net panel when subjected to currents of 0–0.9 ms−1. Different symbols represent different current speeds. Filled symbols mark measurements and open symbols mark model estimates.

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