The effect of catch weight on trawl behaviour

Measurements at sea during fishing trials on bottom trawl, have revealed that the geometry of the trawl is affected by the catch. A series of 27 hauls of around 3h have been carried out during the EFFICHALUT project. This project aimed to find out improvements on the fishing gear in order to reduce the energy consumption. For this project numerous sensors have been used, for bridles tension, doors spread, vertical opening, doors attitude. The measurements show quite clearly that the doors spread decreases and the bridles tension increases along most of the hauls. The doors spread decreases by 1.35m per hour, with a standard deviation of 1.98m/h when the top bridle tension increases of 47kg/h, with a standard deviation of 59kg/h. The mean catch per haul is around 1.48ton. The modelling of the trawl gear and the catch with a FEM allows to explain the variation of the doors spread and the bridle tension: The model explains a mean decrease of 0.77m/h for the door spread and a mean increase of 55Kg/h for the top bridle tension

Rapport de mission à Trondheim du 15 au 20 août 1998

1/ Séminaire sur les technologies liées aux activités de la pêche et de l' offshore pétrolier 2/ Visite du salon de la pêche à Trondhei

Modelling axisymmetric codends made of hexagonal mesh types

Codends are the rear parts of trawls, which collect the catch and where most of the selectivity process occurs. Selectivity is the process by which the large sh are retained while the small ones are released. The codends applied in many sheries often consist of only one type of mesh. Therefore it is reason- able to consider these codends as being axisymmetric. Their shapes depend mainly on the volume of catch, on the shape of meshes (diamond, square, hexagonal) and on the number of meshes along and around the codend. The shape of the codends is of prime importance in order to understand the se- lectivity process. This paper presents a model of deformation of codends made up of hexagonal meshes. Two types of hexagonal meshes have been investigated: the T0 codend where two sides of the hexagons are in axial planes and the T90 codend where two sides are perpendicular to the codend axis. The forces involved in this model are twine tension and catch pressure. A Newton-Raphson scheme has been used to calculate the equilibrium

Genèse des zones de cisaillement : Application de la méthode des éléments finis à la simulation numérique de la déformation des roches.

Mémoires et Documents du Centre Armoricain d'Etude Structurale des Socles n°4, 157 p. ISBN : 2-905532-03-3Cette thèse est une étude de la naissance des zones de cisaillement. La modélisation théorique de leur amplification permet de définir les trois paramètres influençant leur développement : la déformation globale du matériau, l'hétérogénéité initiant la zone de cisaillement, et l'instabilité rhéologique du matériau. Les simulations numériques fondées* sur la méthode des éléments finis quantifient ces trois paramètres. Différents cas d'instabilité ont été traités : amollissement avec la déformation, anisotropie et échauffement lié à la déformation. Enfin, l'étude de la propagation d'une zone de cisaillement permet de préciser une relation entre sa largeur, sa vitesse de propagation et la rhéologie de la roche.No abstrac

Twines equilibrium in a finite element dedicated to hexagonal mesh netting

The equilibrium of hexagonal mesh netting is calculated by a finite element method. A triangular element has been devolopped for such hexagonal meshes[1] [2]. The hexagonal mesh is made of three families of twines. The basic hypothesis of the triangular element imposes that the three families of twines be parallel inside the element. The twine tension are calculated to deduce the forces on the vertex of the triangular element.
The calculation of twine tension needs the concurent resolution of 6 equations with as many unknowns. In a first step these equations are reduced to 2 with 2 unknowns. Then they are solved by the Newton-Raphson method

Theoretical study of the effect of round straps on the selectivity in a diamond mesh cod-end

FEMNET, a numerical tool based on the finite element method, was applied to estimate the shapes of various diamond-mesh cod-end designs during fishing. The only design differences rest in the use of round straps of different lengths, positions and numbers. The cod-end shape estimates were then entered in the selectivity simulation tool PRESEMO to simulate the selectivity processes of the various cod-end designs under the same varying fishing conditions. This enabled us to demonstrate how one or two round straps along the cod-end axis may change the selectivity of the cod-end compared with a reference cod-end, without round straps. We predict that in cod-end designs, which comply with the EU legislation, the 50% retention length (L50) may be reduced by up to 1.5 cm (5%) for haddock. (c) 2006 Elsevier B.V. All rights reserved

Optimisation of trawl energy efficiency under fishing effort constraint

Trawls energy efficiency is greatly affected by the drag, as well as by the swept area. The drag results in an increase of the energy consumption and the sweeping influences the catch. Methods of optimisation of the trawl design have been developed in order to reduce the volume of carburant per kg of caught fish and consequently the drag per swept area of the trawl. Based on a finite element method model for flexible netting structures, the tool modifies step by step a reference design. For each step the best-modified design, in terms of drag per swept area is kept. Such optimisation can lead to a decrease of the drag or an increase of the swept area. In the second case, that could lead to an increase of fishing effort. To avoid such increase, which could be not welcome in some fisheries; a tool of homothetic transformation of fishing gears has been developed. Such global tool (optimisation and homothetic) has shown potential saving of 39% in fuel cost without increasing the fishing effort

Simulation-based study of the combined effect on cod-end size selection of turning meshes by 90 degrees and reducing the number of meshes in the circumference for round fish

FEMNET, a numerical tool based on the finite element method, was used to estimate the shapes of four different designs of trawl cod-ends during fishing operations. Compared to a traditional diamond-mesh cod-end the design differences were the following: (i) the netting orientation was turned by 90 degrees (T90), (ii) the number of meshes in the circumference was reduced by 50% or (i) and (ii) were combined. The cod-end shape estimates were then entered into the simulation tool PRESEMO, to estimate their influence on the selectivity processes in the cod-end. This enabled us to predict how these design alterations - alone or combined - may act on the selectivity of each cod-end under identical fishing conditions. For instance, we predict that for a I 10 mm diamond-mesh cod-end the 50% retention length (1-50) is increased by nearly 12 cm by both turning the mesh orientation and reducing the number of meshes in the circumference. Of this combined effect we predict that 24% of it is caused by only turning the mesh orientation whereas 71 % of the effect stems from only reducing the number of meshes in circumference. The remaining 5% is due to the interaction between the two factors. (c) 2006 Elsevier B.V. All rights reserved