a two-dimensional approximation of a floating fish … bardestani.pdf · a two-dimensional...
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31.07.2013
www.cesos.ntnu.no CeSOS – Centre for Ships and Ocean Structures
A two-dimensional approximation of a floating fish farm in
waves and current with the effect of snap loads
Mohsen Bardestani
Odd M. Faltinsen
Outline
Introduction
Floater in regular waves Net panel in uniform current
Floater, net and sinker in regular waves Snap load prediction
Floater, net and sinker in regular waves
and uniform current Summary
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Introduction
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• Main challenges for offshore aquaculture industry - Fish escape from fish farms: mainly structural failure - Fish diseases - Environmental pollution www.forskningsradet.no
• Hydrodynamic effects - Wave forces - Current forces - Wave and current combination Section of cage selected: simplified problem
Experimentally and numerically analyzed
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Floater in regular waves- Experiments
• Model test conditions: - Wave flume (2D flow conditions), MTS, NTNU - Deep water (h=1.0 m) - 10 wave periods - Wave steepness (H/λ=1/14) - Model scale 1:10 - Rigid floater • Measurements: - Accelerations of the floater - Forces in mooring lines - wave elevation in the flume
From D. Kristiansen PhD Thesis, 2008
From D. Kristiansen PhD Thesis, 2008
31.07.2013
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• Sway and Heave Response Amplitudes; comparison with linear potential flow theory
- Sway instability in both experiments at T=0.544 sec
Floater in regular waves- Experiments
Effects of viscous damping and pulley friction damping
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Net panel in uniform current - Screen Force Model (T.Kristiansen, O.M.Faltinsen (2012)): Constant solidity ratio, Constraint: length of the truss remains constant - Hydrodynamic forces on the elements and tension in the elements Small twine diameter; large KC
From T.Kristiansen, O.M.Faltinsen (2012)
weights
Cross-flow velocity at twines:
Reynolds number:
Sn=projected area of the net twines / total area of the net panel
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Net panel in uniform current: Truss Model vs. Catenary Eq.
• Model verification in uniform current - Net shape - Tension in net twines
Net panel connected to sinker tube and fixed bar
Sinker tube in uniform current
• Morison equation is used.
• Experiments by James and Truong (1972) for spanwise tripping wire on a cylinder: - Ratio between net twine diameter and sinker tube diameter (d/D = 3.2 %, 6.3 %) - Angle between net connection relative to the flow for angle 65 to 90 degree, Cd = 1.6 to 1.8
• Unsteady lift force on the sinker tube due to vortex shedding - 3D effects of sinker tube and net connection can affect Strouhal number
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fv
2fv
U=0.2 m/s, Sn=0.19, Ws=1.6, fv=0.66 Hz
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www.cesos.ntnu.no CeSOS – Centre for Ships and Ocean Structures
• Tension in net connection points to the bar
Ws=1.2 kg
Ws=1.4 kg Ws=1.6 kg
Net panel in uniform current- Experiments
- Ws=1.2, 1.4, 1.6 kg - U=0.1, 0.2, 0.3 m/s - Sn=0.16, 0.19, 0.23
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Floater, net and sinker in regular waves: Experiments
• Model test conditions: - 10 wave periods - Wave steepness (H/λ=1/14) - Solidity ratios (Sn=0.16, 0.19, 0.23) - Sinker weights (Ws=1.2, 1.4, 1.6 kg) • Measurements: - Accelerations of the floater - Forces in mooring lines - wave elevation in the flume - Net deflection - Tensions in net connecting points to the floater Sinker Tube
Net
Floater
End Plate
Mooring Line
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• Net deflection in a wave period Time domain simulations
- Sn=0.23; Ws=1.6 kg - T=0.761 sec; H=0.065 m
Slack net
Very large snap loads in very short time; similar as risers, marine cables, equipment lowering devices, …
VIDEO
Floater, net and sinker in regular waves- Experiments
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www.cesos.ntnu.no CeSOS – Centre for Ships and Ocean Structures
Time domain simulations: Floater Free-decay tests • Accuracy of model by doing heave free-decay tests • Time-domain equation of motion by including retardation function - Non-dimensional displacement and velocity of cylinder with radius R 0 0
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• Snap loading prediction by considering net elasticity: Net modulus of elasticity is constant for 10% elongation
From H. Moe PhD Thesis, 2009
linear
Negative Tension T=0
5~6 times T0
Floater, net and sinker in regular waves- Comparisons
Tension≠0
Tension=0
Floater
Sinker
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• Snap loading prediction by considering net elasticity - T=0.878 sec, amp=0.043 m
6~7 times T0
Floater, net and sinker in regular waves- Comparisons
Large floater heave acceleration due to the snap loads
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• Net tensions for Sn=0.23 and Ws=1.6 kg - Increase in maximum tensions for periods higher than T=0.601 sec - Elasticity, length of net, sinker tube weight, wave excitation frequency and amplitude have effects on the loading
Floater, net and sinker in regular waves- Comparisons
Start of net slack
Oscillating about static tensions T=T0
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• Sway and heave response amplitude operator, Sn=0.23, Ws=1.6 kg - No instability in sway - Less heave motion relative to floater only
0,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
0 0,2 0,4 0,6 0,8 1 1,2
Ya/0
.5H
T [sec]
Heave RAO
Floater_Only Numerical_snap Experiments
0,0
1,0
2,0
3,0
4,0
5,0
6,0
0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4
Xa/0
.5H
T [sec]
Sway RAO
Experiments Truss_LPFT time_domain_truss
Floater, net and sinker in regular waves- Comparisons
31.07.2013
www.cesos.ntnu.no CeSOS – Centre for Ships and Ocean Structures
• Net tensions for Sn=0.16 and Ws=1.6 kg - Opposite phasing of hydrodynamic forces on the floater and tensions due to netting on the floater cancellation of forces in the mooring lines - Calculation of Cd for sinker tube is not straight forward
Floater, net and sinker in regular waves and uniform current
Summary
- Screen force model predicts hydrodynamic forces on the netting structure with very good accuracy,
- Time domain formulation used to solve floater motions, coupled with net and sinker tube,
- Snap forces in the netting predicted with good accuracy and should
be of concern for net structure design,
- Floater, net and sinker tube have the same importance for the forcing on the model,
- Forces in mooring lines are measured and calculated for the wave
and current combinations.
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www.cesos.ntnu.no CeSOS – Centre for Ships and Ocean Structures
References
- Kristiansen, T. and Faltinsen, O.M., 2012, “Modelling of Current Loads on Aquaculture Net Cages’’, Journal of Fluids and Structures, Vol. 34, pp. 218-235.
- Kristiansen, D., 2010, “Wave Induced Effects on Floaters of Aquaculture Plants”, PhD Thesis, Norwegian University of Science and Technology, Trondheim, Norway.
- Kristiansen, D. and Faltinsen, O.M., 2009, “Non-linear Wave-induced Motions of Cylindrical-shaped Floaters of Fish Farms”, Journal of Engineering for the Maritime Environment, Vol. 223(3), pp. 361-375.
- Moe, H., Fredheim, A. and Hopperstad, O. S., 2010, “Structural Analysis of Aquaculture Net Cages in Current”, Journal of Fluids and Structures, Vol. 26, pp. 503-516.
- Le Bris, F. and Marichal, D., 1998, “Numerical and Experimental Study of Submerged Supple Nets: Applications to Fish Farms”, Journal of Marine Science Technology, Vol. 3, pp. 161-170.
- Salvesen, N., Tuck, E.O., Faltinsen, O.M., 1970, “Ship Motions and Sea Loads”, SNAME, 78, pp. 250-287.
- Sarpkaya, T., Bakmis, C., and Storm M.A., 1984, “Hydrodynamic Forces from Combined Wave and Current Flow on Smooth and Rough Circular Cylinders at High Reynolds Numbers”, Offshore Technology Conference, Houston, Texas.
- Faltinsen, O.M., 1990, “Sea Loads on Ships and Offshore Structures”, Cambridge University Press.
31.07.2013
www.cesos.ntnu.no CeSOS – Centre for Ships and Ocean Structures
31.07.2013
www.cesos.ntnu.no CeSOS – Centre for Ships and Ocean Structures
Thank You