Research Papers: Offshore Technology

Wave Response of Closed Flexible Bags

[+] Author and Article Information
Pål Lader

Trondheim 7465, Norway
e-mail: pal.lader@sintef.no

David W. Fredriksson

United States Naval Academy,
Annapolis, MD 21402
e-mail: fredriks@usna.edu

Zsolt Volent

Trondheim 7465, Norway
e-mail: zsolt.volent@sintef.no

Jud DeCew

Jere A. Chase Ocean Engineering Laboratory,
University of New Hampshire,
Durham, NH 03824
e-mail: jdecew@gmail.com

Trond Rosten

Trondheim 7465, Norway
e-mail: trond.rosten@sintef.no

Ida M. Strand

Department of Marine Technology,
Norwegian University of Science
and Technology,
Trondheim 7491, Norway
e-mail: ida.strand@ntnu.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 July 12, 2016; final manuscript received April 25, 2017; published online May 25, 2017. Assoc. Editor: Robert Seah.This work is in part a work of the U.S. Government. ASME disclaims all interest in the U.S. Government's contributions.

J. Offshore Mech. Arct. Eng 139(5), 051301 (May 25, 2017) (9 pages) Paper No: OMAE-16-1083; doi: 10.1115/1.4036676 History: Received July 12, 2016; Revised April 25, 2017

Recent environmental considerations, as salmon lice, escape of farmed fish and release of nutrients, have prompted the aquaculture industry to consider the use of closed fish production systems (CFPS). The use of such systems is considered as one potential way of expanding the salmon production in Norway. To better understand the response in waves of such bags, experiments were conducted with a series of 1:30 scaled models of closed flexible bags. The bags and floater were moored in a wave tank and subjected to series of regular waves (wave period between 0.5 and 1.5 s and wave steepness 1/15, 1/30, and 1/60). Three different geometries were investigated; cylindrical, spherical, and elliptical, and the models were both tested deflated (70% filling level) and inflated (100% filling level). Incident waves were measured together with the horizontal and vertical motion of the floater in two points (front and aft). Visual observations of the response were also done using cameras. The main finding from the experiments were that a deflated bag was more wave compliant than an inflated bag, and that the integrity (whether water entered or left the bag over the floater) was challenged for the inflated bags even for smaller waves (identified as wave condition B (1.0 m < H < 1.9 m) in Norwegian Standard NS 9415). A deflated bag is significantly more seaworthy than an inflated bag when it comes to integrity and motion of the floater.

Copyright © 2017 by ASME
Topics: Waves , Water
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Fig. 1

Overview over bag models used in the experiments. Each bag model was tested with both an inflated and a deflated states. The deflated state had a filling level corresponding to 70% of the theoretical bag volume (the theoretical volume is the volume given in the figure), while the inflated state had a filling level slightly more than the theoretical volume (100%+). The floater cross section is circular with a diameter of 2.4 cm, and for the deflated states the floater is approximately 10% submerged and 50% submerged for the inflated states. This means that the freeboard of the floater is approximately 2.1 cm (deflated states) and 1.2 cm (inflated states). It is assumed that these models are 1:30 representations of full-scale bags (in full scale, the diameter of the floater is 22.9 m with a circumference of 72 m, and the displace volume for the spherical bag is 3 152 m3). The expansion of the bag fabric over the free surface (as seen on the top view) does not represent a structural part and is purely due to experimental practicalities. By closing of the free surface water is prevented from entering or leaving the volume in the most severe wave cases. This is practical since it is important to have control of the amount of water inside the bag. Even though the fabric in the model experiments maintain the integrity of the model bag, it is easily observed when the integrity of the bag would have been challenged if the surface fabric was not present.

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Fig. 2

The towing tank facilities at the United States Naval Academy. The tank is equipped with a hinged wave maker and a wave absorbing beach.

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Fig. 3

Overview of the experimental setup. A schematic drawing with details is shown in Fig.4.

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Fig. 4

A schematic drawing of the experimental setup

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Fig. 5

Overview over combinations of models, filling levels, and wave cases that were run. The wave cases are identified by wave the nondimensional parameters wave steepness (s) and wave length normalized with respect to bag diameter (L/D). The areas indicated with the dashed and dotted lines are the model/wave combinations that are used in the analysis of the dynamic response.

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Fig. 6

Integrity of the bags at different wave cases. Integrity is understood as the ability to prevent any water exchange between the volume inside the closed bag and the outside water volume. The integrity of the bags in each case was established through visual observations of whether or not water could enter or leave the bag over the rim of the floater.

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Fig. 7

First harmonic RAO for the cases with wave steepness 1/30 (dotted area in Fig. 5). Four different response modes are shown; heave front, heave back, surge global, and flexible mode I. RAO is calculated as the square of the ration between the energy in the response mode and the energy in the wave. The energy is calculated fromthe PSD by integrating the PSD in the frequency area f = [0.9f1stharmonic, 1.1 f1stharmonic], thus isolating the energy in the first harmonic peak.

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Fig. 8

First harmonic RAO for the spherical bag (dashed area in Fig. 5). The calculation of RAO is explained in the caption of Fig. 7.

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Fig. 9

Mean wave drift force. The force is given as a percent of the displace volume (100%+ inflated state) times ρ g (water density and the acceleration of gravity). The mean wave drift force is calculated from the mean surge displacement (x-direction) using the spring constant of the mooring lines along the x-axis. The contribution from the mooring along the y-axis to the force in x-direction is neglected. This is justified by the mean displacement in surge being less than 10 cm, and for such a displacement, the force contribution from the y-direction mooring lines is less than 10% of the force in the x-direction mooring lines.




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