Hydrodynamic and Experimental Analysis on a Novel Hybrid Offshore Renewable Structure

Since the Kyoto Protocol in Japan in 1997, countries have been on a drive to reduce their carbon emissions but recently this drive has been given a new thrust. Security of energy supplies is now a high priority and one of the possible methods is by harnessing offshore renewable energy. It is proposed, that the use of a hybrid or a combination of two sources of renewable energy that utilizes a single floating structure can effectively reduce the cost of delivering renewable energy. Cost reduction is of key importance, however, it will not be the focus of this paper as it is a well proven concept. However, the motion characteristics of such a novel structure as the one that is presented here has never been fully examined and the literature is very limited on the subject. A scaled model of a conceptual structure was fabricated and subjected to regular waves for a range of wave heights and periods. The effect of wave-current interaction was noted and mooring line forces were measured. Conventional hydrodynamic analysis and rules used in ship structures and semi-submersible design were merged into one cohesive analysis and a comparison was made with the experimental results. There are two operational states that are mentioned, the ‘hybrid mode’ i.e. acting as a barge and a semisubmersible by virtue of the volume of the structure below the Mean Water Level (MWL). The second state is the ‘semisubmersible mode’, which has the pontoon and 75% column of the structure only below the MWL.