Analysis of Ocean Farming's Steel Cage Concept in Very Exposed Waters
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- Institutt for marin teknikk 
The behaviour of the offshore fish farm concept developed by Ocean Farming, a subsidiary of the salmon company SalMar ASA, has been investigated for very exposed waters. A finite element model created by Bore and Fossan during their master thesis in 2015 has been used as a starting point to develop a modified model applicable for stochastic response analyses with a significant wave height of 10 meters. The modified design can be described as an enlargement of the original model, with focus on sufficient freeboard and draft to avoid wave impact on the deck and preserve fish welfare during extreme storms. Ultimate limit state analyses of the modified model have been performed in the time domain by using the nonlinear finite element program USFOS. 40 3-hour sea states have been simulated for three different headings of the environmental forces, where the load contribution from the ocean waves, current and the wind are included. A significant wave height and spectral peak period equal to 10 meters and 13 seconds have been used in the stochastic generation of the ocean waves. An elastic utilization ratio of the hull structure has been calculated according the API-WSD standard, where a ratio below 1 is defined as sufficient structural capacity. The maximal elastic utilization from the 40 simulated sea states have been plotted in a Gumble probability paper, where the ULS utilization corresponding to a 0.9 percent fractile have been identified. The obtained results indicate that the structural capacity of the modified model is sufficient for each environmental heading according to the API-WSD standard.Characteristic load effects of the mooring lines corresponding to the ultimate limit state have been evaluated according to the DNV-GL standard. The mooring lines consist of a combination between steel chains and fibre material, where the fibre material is used on the upper part of the mooring line. A calculated utilization factor for both parts based on the line tension illustrates sufficient structural capacity for the heaviest loaded line at each environmental heading. Even though the modified model provides satisfactorily structural capacity according to the authorized standards, changes in the obtained results compared to the original model has been identified. The main difference is related to the environmental heading providing the largest response, where it has changed from 315 degrees to 360 degrees. An exact reason for this change is difficult to identify. However, it is most likely related to the increased dimension of the modified model and the resulting alternation of the global load distribution, in addition to the different time length of the ULS analyses performed with the two models. The results obtained in this master thesis indicates that a modified design of the Ocean Farming concept is suitable for exposed waters with significant wave height equal to 10 meters. Further development of the modified model and validation of the obtained results should be performed before a final conclusion regarding sufficient structural capacity can be made.