Analysis of Ocean Farming's Steel Cage Concept Subjected to Environmental Loads
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- Institutt for marin teknikk 
For marine structures with significant dynamics, the extreme responses are determined by time domain analyses of extreme sea states. These are based on the short-term extreme sea state selected from environmental contour lines for the actual location. Three methods for integration of the wave spectrum has been investigated by measuring mooring line forces for the Ocean Farming concept; Fast Fourier Transform, Fast Fourier Transform with increased randomization of the wave frequency and the Equal Area Principle (EAP). The number of harmonic wave components in the spectrum integration has also been studied. The EAP method with 50 wave components was concluded to be sufficient for stochastic ultimate limit state analysis of the Ocean Farming concept in USFOS.The ultimate limit state analysis (ULS) of the Ocean Farming concept has been carried out using three different sea states, coming from three different headings. 3-hour time domain simulations of the sea states were conducted with 40 random seeds. The extreme structural utilizations were checked against API-WSD rules, while mooring line loads were checked against DNVGL-OS-E301. The maximum loads were plotted Gumbel diagrams, and the 90-percentile value of the Gumbel line was then used to estimate the characteristic 100-year ULS utilizations. Heading 0° gave the highest utilization for structural elements, while for mooring lines, the highest utilization was found from a heading of 315°. Neither of the structural elements nor mooring lines exceeded the allowable criteria, thus leading to the conclusion that the structure is properly dimensioned.Simplified design wave analyses have been carried out to obtain the characteristic ULS-loads coming from direction 315°. Stoke s 5th and Airy waves were applied through dynamic analysis, ramping up the waves to the maximum height. A Stoke s 5th wave with height 8.56 metres and period 6.25 seconds was able to determine the ULS mooring line loads adequately, but some discrepancies were found for the element utilization. This design wave method was concluded to be used with care for dynamic structures.