An Experimental Study on the Wave-Induced Hydroelastic Response of a Floating Solar Island
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- Institutt for marin teknikk 
Future energy demand are expected to increase substantially in the next decades, and there is a real need to consider new ways of supplying the energy market with cost-efficient, dependable and green sources of energy. The ocean is vast and large proportions of it are continually exposed to sunlight. Floating solar islands that are capable of carrying photovoltaic solar panels in offshore sea conditions could supply solar power directly onto power grids or could be used in an electrochemical process that produces liquid methanol from CO2 in sea water, an energy source that can be stored for later use. A few designs of floating solar islands exists already, but none that can operate in offshore sea conditions. There are considerable challenges in designing such a structure, and this thesis investigates the potential of a large floating structure consisting of multiple slender floating tori elastically connected. For this purpose, an experimental study into the governing behavior and response of the proposed structure have been performed. The deck has been omitted, and the structure in this study consists of five elastic tori enclosing each other that are connected through trusses. The trusses are elastic, creating motions that are hard to predict but that avoids snap loads and jerky motions, and enables the structure to move with the waves. Previous theoretical and experimental studies involving a single semi-submerged slender torus have been used as a basis for this study. A model with multiple tori and one with a single torus have therefore been built and tested in scale 1:50. The testing was conducted at the Small Towing Tank at the Centre of Marine Technology at NTNU. During the experiments the structures were moored to the walls of the tank and the wave elevation, mooring-line tensions and the vertical and horizontal responses were measured. The models were tested in regular waves with full scale wave period varying from 2.0-14s and wave steepness varying from 1/100-1/10. The multi-torus was tested in irregular waves as well, with peak wave periods of 8-12s and significant wave heights of 2-5m. The experimental results from the multi-torus were compared to those from the single-torus in addition to a low-frequency linear slender-body theory for vertical radiation loads on an elastic semi-submerged torus and a zero-frequency theory. The experiments with the single-torus showed good agreement with the theoretically predicted linear responses. The experiments with the multi-torus also approximated the theoretical response for very long wave periods. However, for smaller wave periods the vertical motion was lower relative to the wave amplitude than predicted by the theoretical response for a single-torus. This was expected as a results of the trusses that are connecting the tori. The mass and added mass of each torus will affect the motion of the tori in question in addition to all the other tori. These interacting forces seems to be larger relative to the wave force for smaller waves, than when the waves are large. Overtopping waves were identified as the main concern for the structure and the solar panels, threatening the integrity of both. Overtopping in regular waves occurred predominantly at the aft of the outermost tori for longer wave periods with relatively low wave steepness, and at the fore for high wave steepnesses and shorter wave periods. In irregular waves, overtopping waves occured at random locations on the multi-torus, with a tendency of occuring more often at the fore of the structure and on the two outer tori. However, it did not follow the pattern observed for regular waves, where there was a clear tendency for where overtopping would occur. Overtopping was registered at relatively low peak wave periods and significant wave heights compared to regular waves. The results from the multi-torus shows potential, but more work and research are needed for further development of a functioning floating solar island.