POSITION MOORING OF WAVE ENERGY CONVERTERS - An engineering study into the mooring of structures in a highly exposed shallow ocean regime, within the context of the economics of renewable energy conversion

Doctoral thesis, 2009

The principles of systems engineering are applied in order to establish the requirements for mooring wave energy conversion devices. These requirements capture both the function of maintaining position in energetic wave climates as well as the need to enable an economic lifecycle for the development of a viable wave energy conversion industry. As such, material costs, marine operations costs and concerns captured by existing industry regulatory bodies gain relative importance in the supporting discussion, though technical concerns remain central to the presented scientific methods. Requirements that would be unique to a particular method of wave energy conversion are avoided as the aim is to contribute to a general body of knowledge to support the wide variety of proposed solutions in this area. Engineering and scientific methods are applied to investigate which kind of mooring systems are most likely to fulfil these holistic requirements. Cable mooring solutions are studied in some detail. In addition the relative merits of fixed and articulated tower structures are assessed. Consideration is given to different scales of structures and different water depths. Previously established analytical methods are applied in conjunction with experimental work carried out at 1:60 scale. The overall conclusions show that mooring system design should be an integral part of wave power system designs and not a postscript to the design process. There are significant challenges to the positioning of structures in this exposed, shallow-water regime, especially structures that must fundamentally have a wave absorbing wet surface. The choice of mooring design can significantly influence how wave power is extracted and how such systems are operated and maintained. The results and supporting discussion establish that technological breakthroughs in vertically loaded anchors will be required before point absorbing arrays can be viably used to generate electricity for the grid. Such anchors, in conjunction with articulated towers or elastic tendons are the most promising line of investigation. Such solutions could theoretically satisfy low material costs and enable acceptable seabed footprints. However, the lack of a versatile and inexpensive vertically loaded anchor or elastic cables with acceptable fatigue life, mean that these solutions are not yet commercially available.