Analysis of the Loads on and Dynamic Response of a Floating Flexible Tube in Waves and Currents

Abstract

Recently proposed offshore structure designs for wastewater treatment and algae biomass production may incorporate floating flexible tubes. In one configuration, the tubes float at the surface and contain a fluid and algae mixture. The purpose of this study is to analyze the loads on and dynamic response of such tubes in varying environmental conditions, specifically waves and currents. The approach includes both physical and numerical modeling techniques. Physical models at a scale of 1:4.21 (model to prototype) and two fill levels (50% and 95%) were tested in wave and towing tank facilities in the Hydromechanics Laboratory at the United States Naval Academy. The models were towed at a range of scaled speeds to represent currents and subjected to scaled wave conditions in one of the tanks. During these tests, attachment loads were measured and dynamic response was evaluated. In addition to the physical modeling approach, Computational Fluid Dynamics (CFD) simulations were performed to assess steady drag measurements at the 1:4.21 scale and the results were compared to the experimental tests. The CFD modeling results were comparable to those obtained from the physical modeling tests, at least at the higher fill levels that were analyzed. The results indicated that the model response tended to follow the wave forcing more at the middle wave frequencies. For both fill levels, the response of the forward end of model increased with wave frequency while the response of the rear end of the model remained rather consistent across the frequencies tested. The results did not indicate a significant difference in dynamic response when the model was tested in both waves and current. The average attachment loads, however, were higher when the model was tested in waves and a current than in waves only. The attachment loads were also higher in wave and current conditions that included a faster current or lower-frequency (larger) waves. The intent of the project, through both modeling methods, was to increase the understanding of the loads on and the response of a floating tube in waves and currents. Future work could include an increase number of replicates for better accuracy, additional test frequencies and tow speeds, alternate attachment techniques, and testing at larger scales for comparison.