Cheslav Balash

The 'W' prawn-trawl with emphasised drag-force transfer to its centre line to reduce overall system drag

For prawn trawling systems, drag reduction is a high priority as the trawling process is energy intensive. Large benefits have occurred through the use of multiple-net rigs and thin twine in the netting. An additional positive effect of these successful twine-area reduction strategies is the reduced amount of otter board area required to spread the trawl systems, which leads to further drag reduction. The present work investigated the potential of redirecting the drag-strain within a prawn trawl away from the wings and the otter boards to the centre line of the trawl, where top and bottom tongues have been installed, with an aim to minimise the loading/size of the otter boards required to spread the trawl. In the system containing the new ‘W’ trawl, the drag redirected to the centre-line tongues is transferred forward through a connected sled and towing wires to the trawler. To establish the extent of drag redirection to the centre-line tongues and the relative drag benefits of the new trawl system, conventional and ‘W’ trawls of 3.65 m headline length were tested firstly over a range of spread ratios in the flume tank, and subsequently at optimum spread ratio in the field. The developed ‘W’ trawl effectively directed 64% of netting-drag off the wings and onto the centre tongues, which resulted in drag savings in the field of ∼20% for the associated ‘W’ trawl/otter-board/sled system compared to the traditional trawl/otter-board arrangement in a single trawl or twin rig configuration. Furthermore, based on previously published data, the new trawl when used in a twin rig system is expected to provide approximately 12% drag reduction compared to quad rig. The twin ‘W’ trawl system also has benefits over quad rig in that a reduced number of cod-end/By-catch Reduction Device units need to be installed and attended each tow.

Hydrodynamic Evaluation of a Generic Sail Used in an Innovative Prawn-Trawl Otter Board

In prawn-trawling operations, otter boards provide the horizontal force required to maintain net openings, and are typically low aspect ratio (∼0.5) flat plates operating on the seabed at high angles of attack (AOA; 35–40°). Such characteristics cause otter boards to account for up to 30% of the total trawling resistance, including that from the vessel. A recent innovation is the batwing otter board, which is designed to spread trawls with substantially less towing resistance and benthic impacts. A key design feature is the use of a sail, instead of a flat plate, as the hydrodynamic foil. The superior drag and benthic performance of the batwing is achieved by (i) successful operation at an AOA of ∼20° and (ii) having the heavy sea floor contact shoe in line with the direction of tow. This study investigated the hydrodynamic characteristics of a generic sail by varying its twist and camber, to identify optimal settings for maximum spreading efficiency and stability. Loads in six degrees of freedom were measured at AOAs between 0 and 40° in a flume tank at a constant flow velocity, and with five combinations of twist and camber. The results showed that for the studied sail, the design AOA (20°) provides a suitable compromise between greater efficiency (occurring at lower AOAs) and greater effectiveness (occurring at higher AOAs). At optimum settings (20°, medium camber and twist), a lift-to-drag ratio >3 was achieved, which is ∼3 times more than that of contemporary prawn-trawling otter boards. Such a result implies relative drag reductions of 10–20% for trawling systems, depending on the rig configuration.Copyright

Harmonic wave and steady current effects on plane fishnets

Individual and interaction effects of parallel current and waves on three plane nets were empirically examined. A current opposing the direction of the waves was shown to shorten the wavelength while a current in-line with the direction of the waves stretched the wave length. Due to these wave modifications, the combined loads produced by a current and waves were significantly less than the sum of current and wave forces applied individually. Applying a vector approach, the unsteady loads were split into drag forces and inertia components. Both components contributed considerably to the hydrodynamic loads for wave-only cases. For combinations of waves and current, the inertia force was significantly greater than the drag force. Further insights were also provided into the concepts of effective thickness and the modified Keulegan-Carpenter number as parameters quantifying inertia force and drag force for fishnets.

Vortex-induced motion of a free-standing riser below the critical mass ratio

The presented work studied the vortex-induced motion (VIM) response of a free-standing riser (FSR) with varied riser length and buoyancy can (BC) mass with an ultimate aim to find a combination that would reduce the motion of the system. Specifically, four model configurations were experimentally tested in a flume tank over a range of flow velocities, with the BC motion recorded by a submersible camera positioned directly above the model; consequently, inline (IL) and crossflow (CF) amplitudes were estimated with a motion tracking software. In the pre-resonant flow regime, non-dimensionally, minimal differences were observed between the CF amplitudes, and the IL motion was reduced with a longer riser. Given the extreme length of full-scale FSRs and inherent low natural frequency, it is impractical to increase the riser tension to a point where VIM would not occur under normal environmental conditions. Alternatively, increasing the mass ratio of the BC so that it is above the critical mass ratio of 0.54 (the ratio of the mass of the body to the mass of the fluid) would limit the resonant flow velocities to a finite range, but a larger BC may not be an economically viable solution, and because of the increased diameter, it would experience a larger CF amplitude during resonance. Further study into the prevention of VIM of an FSR by varying the riser length and BC mass is unlikely to be beneficial.

Subsea-asset protection from falling objects using multi-layered shielding: a preliminary study

Protection solutions for pipelines, umbilicals, and cables from accidentally dropped objects are generally implemented with concrete mattresses, though concrete does not effectively dissipate shock loading. The presented work investigated relative absorption properties of two materials (concrete and polystyrene), singly and in combination, with an aim to ultimately advance the protection of subsea assets from falling objects. A series of experiments were undertaken to measure the impact force from dropped objects of varied mass and height on single and stacked plates of varied thickness. It was concluded that the combination of absorptive and non-absorptive materials could be beneficial; specifically, a protection shield for a subsea asset could comprise concrete at the base, polystyrene through the middle, and a thin shell layer of concrete on the outer surface. The proposed next phase will seek the combination of concrete strength and polystyrene compression to provide optimum levels of absorption.

High Porosity Net Drag at a Low Angle of Attack in Application to a Representative Prawn Trawl

The presented work investigated the extent by which Reynolds number determines the drag coefficient for high porosity nets at a low angle of attack. A simple prawn trawl model that incorporates the main design features of prawn trawls employed in Australia was developed. Four trawl models of various high porosities were tested in a flume tank with respect to drag and shape over a range of flow velocities. The physical trawl model was analysed as a system of independent plane net sheets, each with an orientation to the flow estimated from analysis of stereo-vision data. The main finding was that the drag coefficient was weakly dependent on the Reynolds number in the range typical for prawn trawl operations, 1000 Re 1700 . These combined findings imply that trawler operators can approximately estimate prawn net drag from a function containing trawl twine area, towing speed and spread ratio.