טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentMilich Michael
SubjectHydrodynamic-Structural Analysis for the Design of Flexible
Systems for Open Sea Aquaculture
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Nitai Drimer
Full Thesis textFull thesis text - English Version


Abstract

Aquaculture is the fastest growing animal food-production sector and has doubled its production since about 1990. The fish are cultivated at sea, enclosed by a net. A structure holds the internal volume of the cage-net and a mooring system holds the gages array at sea.

Due to harsh environment of the open sea, such as high waves, most designs of fish farm are limited to sheltered waters. In the last years, it has been suggested to move the aquaculture further offshore into unsheltered waters. At open sea, submergibility of the fish farm is necessary.  Submerging reduces the wave loads and also provides a safer environment to the fish to survive storms.

However, the submersion of a fish farm is not stable in maintaining depth. Deepening compresses the components and reduces floatation, while surfacing increases floatation. A number of designs were proposed to stabilize the submersion of the fish farms, but they limited the fish farm to areas with up to ocean depth of 80 meters.

This thesis presents and analyses a new submersible single point mooring fish farm concept, capable of operating in unsheltered ocean waters, without limitation of the ocean depth. A parametric model is developed for the new concept, using a commercial Finite Element program, tailor made for the highly nonlinear hydro-elastic behavior of fish farms.  After verifying the model against existing results in the literature, it is applied to analyze the proposed structure, at the design environmental conditions. The thesis also compares the results of the Finite Element simulations to simplified design calculations.

A new concept for stabilization of submerged structure is proposes and analyzed. The complete design validated, by checking the usage factors of all the structural components, at extreme environmental loads for the operational state and the submerged state.

This thesis also examines a configuration of the structure with square shaped cages, for which there is little operational experience.

The results of the nonlinear hydro-elastic analysis confirm that the preliminary design by the simplified calculations is adequate to the design conditions.  The cages always retain above 70% of the volume.

This thesis presents valuable information of a practical design, such as the method of the analysis, the environmental design criteria, the scantlings of the structure and the obtained usage factors at extreme sea conditions. 

This thesis was carried out in the frame of a research for the European Union and Gili Ocean Technology Ltd. The design of the new concept was modified in view of the simulations and during the summer of 2017, the system was constructed, launched and anchored at open sea offshore Ashdod. After sea trials, the cages were cultivated. On January 2018 the system was exposed to an extreme storm, of about 10 years return period (significant wave height of 7 meters) at the submerged State. There was no damage at all to the structure or the fish, while in other systems in the area there was always a loss of fish or failure of structural components following such extreme storms.