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Influence of the trawling gear on the drag force

e-fishing ? 18-20 May 2010, Vigo,Spain. Outline. Background Research undertaken by UoA Finite Element modellingFluid modellingOtter Door AnalysisRoller Clump AnalysisConclusion . e-fishing ? 18-20 May 2010, Vigo,Spain. Background. High percentage of fuel (60%) is used to tow the net itse

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Influence of the trawling gear on the drag force

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    1. Influence of the trawling gear on the drag force A.Ivanovic R.D.Neilson School of Engineering University of Aberdeen Aberdeen, UK

    2. e-fishing – 18-20 May 2010, Vigo,Spain Outline Background Research undertaken by UoA Finite Element modelling Fluid modelling Otter Door Analysis Roller Clump Analysis Conclusion

    3. e-fishing – 18-20 May 2010, Vigo,Spain Background High percentage of fuel (60%) is used to tow the net itself followed by the otter door and roller clump (approx 20%) and smaller percentage from the bridles, warps and foot ropes. [Parente et al., 2008] Single and twin trawls were compared in terms of their influence on fuel consumption and single trawl was deemed to be more fuel efficient. [Rihan D., 2005] Otter trawling gear has the least effect on the fauna but the otter doors have the greatest impact on the habitat [Kaiser et al, 2002]

    4. e-fishing – 18-20 May 2010, Vigo,Spain Background - DEGREE As part of DEGREE project the influence of trawling gear components on the seabed was investigated. Physical interaction between the seabed and gear components was examined by both physical and numerical modelling approaches. FE Model of the gear component/sediment interface (Abaqus) Model the dynamics of a complete trawl system (Matlab) Model the fluids influence on a trawl component (Cosmos FloWORKS) Laboratory testing of scaled trawl components This work is being taken forward in the project with SINTEF and Marine Scotland - ScienceThis work is being taken forward in the project with SINTEF and Marine Scotland - Science

    5. Drag force For each gear component two sources of drag force are considered Due to the contact between the seabed and gear components Due to the fluid influence FE models were generated to investigate the contact between seabed and trawl component using Abaqus software. Cosmo FLOWorks package was used to investigate drag force. Simulations were carried out for both the roller and otter door. e-fishing – 18-20 May 2010, Vigo,Spain

    6. Validation Fluid drag results were compared with data provided by Morgere (CD=0.95 and CL=1.45). The values from the models are lower than that calculated from the coefficients, but the influence of the shackles etc are not taken in consideration in fluid analysis. FE models of both trawl elements were validated against sea-trials where profiles of the trenches we observed. Good comparison was found. e-fishing – 18-20 May 2010, Vigo, Spain

    7. e-fishing – 18-20 May 2010, Vigo,Spain Soil model Soil is represented as a simple elastic – perfectly plastic material This model attempts to find a simple but general method to simulate the physical disturbance Soil Properties: density 1900kg/m3; Young’s modulus 2MPa, Yield stress 18kPa and Poisson’s ratio 0.49, friction coefficient between the shaft and the roller 0.1.

    8. e-fishing – 18-20 May 2010, Vigo,Spain Roller clump FE model 3D models of the roller clump and the sea bed were built in the FE analysis The roller and shaft are modelled as discrete rigid bodies while the sea bed is modelled as a flexible part Boundary conditions – boundary conditions were applied at the sides while the bottom was constrained. The roller is towed on its own rather than being part of the whole trawl system The roller dimensions: 600mm dia, 500mm width

    9. e-fishing – 18-20 May 2010, Vigo,Spain FE results

    10. e-fishing – 18-20 May 2010, Vigo,Spain Comparison between sea trials and FE model (roller clump)

    11. Influence of speed and soil Speed Speeds examined influence the fluid drag more than the drag from the contact with the seabed. Soil Two components: base friction and passive pressure from the build up of soil in front. Softer sediment results in higher drag force due to the build up of sediment in front of the gear element. e-fishing – 18-20 May 2010, Vigo,Spain

    12. Otter Door The shoe of the otter door only is modelled as it is in direct contact with the seabed. Uneven condition of the seabed and vessel motion were not taken into consideration. e-fishing – 18-20 May 2010, Vigo,Spain

    13. Influence of attitude Angle of attack The angles of angle of attack varied 0-40?; beyond 15? an almost linear progression is obtained. The doors used in the sea-trials work in the range 25-40? which falls within a near linear relationship. e-fishing – 18-20 May 2010, Vigo,Spain

    14. Influence of attitude Angle of attack vs spreading force With an increase of angle of attack the spreading force increases until it reaches 30?. This confirms that 30? degrees is an optimum angle of attack where max spreading force can be obtained. e-fishing – 18-20 May 2010, Vigo,Spain

    15. Influence of attitude Beyond that point the spreading force decreases – due to a stall condition with separation of flow CFD analysis show similar effect where large eddies were formed behind the door at 40?. e-fishing – 18-20 May 2010, Vigo,Spain

    16. Influence of attitude Pitch angle In the CFD analysis the pitch angle was varied from -20? to 20?. A similar effect was obtained regardless of the direction of the pitch rotation of the door e-fishing – 18-20 May 2010, Vigo,Spain

    17. Influence of attitude Pitch angle For FE analysis pitch angle varied 0-10?. The pitch angle of 5 generates a lower drag force than either 0 or 10, which is similar to the findings observed with fluids simulations. e-fishing – 18-20 May 2010, Vigo,Spain

    18. e-fishing – 18-20 May 2010, Vigo,Spain Observations – drag forces Pitch angle has an influence on the drag forces 0 degrees – the door performs more like a sledge (cause: large contact results in a large drag force) Both the penetration and the drag force at 5 pitch angle are less than with 0 angle (cause: reduced contact with the seabed) Drag force higher for 10 pitch angle (cause: the heel of the door penetrates more and builds up the sand in front of the door)

    19. Influence of speed Drag force due to soil interaction does not fluctuate greatly with speed Drag due to fluid is substantial according to the scaling laws FD = 0.5CD?Av2 1m/s – 648N ; 2m/s – 2592N If the two analyses were combined and the speeds are high the changes of drag force of saturated sediment are postulated to be more prominent. e-fishing – 18-20 May 2010, Vigo,Spain

    20. Influence of type of soil Both firm and soft soils are examined. The changes in soil properties results in changes in penetration; if the soil is stiffer the penetration is lower and the soil in front is pushed away by the door without making a significant build up of soil. e-fishing – 18-20 May 2010, Vigo,Spain

    21. e-fishing – 18-20 May 2010, Vigo,Spain Conclusions This study highlights the importance of both fluid and contact drag forces for a particular gear components For the door the fluid drag component is more important as the speed increases whereas for the roller clump it is the contact forces. The type of sediment influences the magnitude of the drag force which governs the fuel efficiency

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