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Title: Experimental investigation of the interaction of bottom towed fishing gears and the seabed
Author: Casanovas Revilla, Clara Ines
ISNI:       0000 0004 7432 0293
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 2018
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Bottom towed fishing gears are commonly used to catch target species that live on or nearby the ground. These fishing gears include several components that act in direct contact with the seabed. The physical interaction between these components and the seabed is found to leave permanent marks on the seabed surface. Also it is reported that these interactions damage the benthos, generate ground resistant forces directly related with the vessel fuel consumption and represent a risk to subsea infrastructure such as cables and pipelines. Therefore, a better understanding of this interaction is of great importance to assess the environmental impact that these types of fishing methods have on the benthos and the risk they represent to subsea infrastructure. The interaction between individual components of bottom towed fishing gears and the seabed has been investigated by means of 1g scaled modelling. During the experiments, the models were towed at a constant towing speed along dry and saturated sand beds prepared at the desired relative density. The penetration of the scaled gear components, ground forces, water pore pressures and characteristic dimensions of the deformed sand were measured during the experiments. The geotechnical processes associated with the interaction of bottom towed fishing gears and the seabed have been identified for sandy type of sediments. The penetration of the bottom towed fishing gear model tested is found to be greatly influenced by the weight of the moving object and the degree of consolidation achieved by the saturated sand. This implies the penetration is rate dependent, decreasing with the towing velocity, and is limited by an upper bound defined by the case where fully drained conditions of the sand are permitted. The ground force acting on partially embedded objects towed along saturated sands is found to be composed by the sum of a passive earth pressure and frictional component and a rate dependent component. While the passive and frictional component is predominant for very slow towing velocities, the rate dependent component becomes the main ground force when the objects are towed at greater velocities. The experimental data revealed that the relationship between the penetration of towed fishing gear objects, their weight and the ground forces acting on them are dependent on the model geometry, the towing velocity and the sand properties. In particular, the relationship between the ground forces acting on towed objects and the penetration, which is defined by a linear function, is found to be especially useful. This relationship represents a useful tool to predict the penetration of a towed object provided that the ground forces acting on it are known (or vice versa). This relationship was used in this study to back-calculate penetrations from ground forces measured during sea trials and realistic values were obtained.
Supervisor: Ivanović, Ana Sponsor: EU FP Project BENTHIS
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Ocean bottom ; Fisheries