Keywords

1.1 Why Fishing Cages and Fishing Gears?

Fish cages and fishing gears are generally quite large, a few tens or hundred meters, and are very flexible. Engineers and users are still trying to improve their knowledge of these flexible marine structures. This flexibility leads these structures to have different behaviours depending on the environment. The classical questions that arise are the following:

  • What is the tension in the mooring line of the fish cage under certain wave, current, wind, and tide conditions?

  • What is the volume reduction of the fish cage in the current?

  • How dependant are the horizontal and vertical openings of the trawl on the towing speed?

  • Is that the cables length of the trawl is optimal in terms of fuel consumption?

Several means are available to help engineers and users: observation at full scale, tests in flume tanks, numerical modelling. Each has its own advantages and drawbacks:

  • Observations give real information, but the observation area is generally very limited; it is impossible to see the whole structure at the same time.

  • Tests in tanks give a lot of information, such as the behaviour of the structure in waves and in current. The main drawback is probably that the models used in tests are quite expensive, and this limits the number of tests that can be performed.

  • Numerical modellings also give a lot of information, such as the tension in cables and netting twines, but they do not cover all the phenomena involved in the behaviour of the structure, such as wearing between yarns in twines or the plastic deformation of the sea bed.

1.2 Why the Finite Element Method?

Several mechanical modellings of flexible structures have been developed during the last decades. They are generally based on a decomposition of the structure into small elements in which approximations can be done. The most well-known modelling using this technique is the finite element method. This method has been widely used for mechanical modelling since the 1970s.

1.3 Why for Netting and Cable?

Nettings and cables are the main components of fishing gears and fish cages. Mechanical modellings of the structures are required to assess the behaviour of these components. Cable modellings have been described in a few publications [5, 25], but the modelling of nettings has not been given much attention. For these reasons, we attempt to fully describe a netting modelling using the finite element method. Even if the modelling of cables has been largely described, their modelling is also described here in order to propose a coherent document.

1.4 Why a Book?

Information on this finite element method for netting structures is sparse. There are portions of books and articles in journals on the topic, but there is no document that tries to group all the main matter on this subject. This book is a tentative attempt at such a publication.