Strength analysis of net structures

Abstract

The main goal of this PhD project was to develop a method for non-linear strength analysis of net structures applied in the aquaculture and fishing industries, e.g. net cages and trawls. The work focused on the aquaculture net cage, which was applied in experiments and analyses, and included research to establish knowledge within material properties and failure modes of traditional netting materials for aquaculture. It was chosen to focus on tensile properties and to study elastic and plastic behaviour, fracture, creep behaviour and cod bite damage. The project consisted of three main activities: tensile testing of netting materials, studies and testing of cod bite damage and resistance, and structural analysis of aquaculture net cages.

Net cages are built as a system of ropes and netting. They are designed to transfer and carry all major forces through the ropes. Loads from current, waves, weights and handling induce forces in the net cage, which must be dimensioned to withstand this. Analyzing an aquaculture net cage is both complex and time consuming due to non-linear effects, detailed geometry and dynamic loads. The behaviour of the net cage is dominated by very large deformations and displacements, and materials with non-linear properties. There is a need for verified analysis methods and specific guidance on how to reduce the complexity of a net cage analysis.

In order to perform a strength analysis of a net cage, it is crucial to know the material properties of the netting material. Traditionally, the material property of major interest for the aquaculture industry has been the tensile breaking strength of netting (mesh) and ropes, and comparatively little focus has been on their detailed stiffness properties and general behaviour prior to fracture. A new test method was established to determine the uniaxial tensile properties of knotless netting materials. It was applied on a variety of netting materials and stress-strain relations were developed. Data on temporary creep properties, recovery of strain post creep and post creep tensile properties of a selection of Raschel knitted netting materials was presented. Post creep tensile testing showed that the length and force at break were not significantly affected by the creep load history.

Cod farmers have reported that cod interact with the cage netting through biting and thereby create wear and tear. The nature of the cod bite attack on traditional, multifilament netting materials was described based on studies of cod interaction with traditional knotless netting and resulting fracture damage on netting fibres. Field experiments were performed, subjecting panels of netting to cod bite in commercial cod cages. In addition, a test method was developed to simulate damage on traditional netting from cod bite, and a prototype bite-jig was designed and assembled. It was concluded that netting materials for cod aquaculture must be resistant to cod bite or be repellent or uninteresting for cod.

A method for numerical analysis of net cages in constant uniform current was developed and verified for limited solidity, deformations and current velocities. Various new designs for aquaculture net cages were presented and compared to a traditional net cage with regard to stresses in the netting material and deformation of the net cage.