The objective of this Project was to design a flexible mooring arrangement for the CoRMaT turbine. The mooring was to achieve the following key requirements: station-keeping, minimising excursion and footprint, stability in turbulent tidal currents and cost effectiveness. The mooring is tensioned by buoyancy or lift forces at the top of the mooring system and consideration of how these forces can be generated was a major part of the Project. Two particular options were considered: a Buoy, providing static buoyancy forces; or a Hydrofoil, providing dynamic lift forces.
The Hydrofoil was found to offer the distinct advantage over the Buoy of increasing the lift at the top of the mooring system, by introducing dynamic lift to assist the static buoyancy. The additional lift results in an increased mooring angle (between the seabed and the mooring line), and therefore a reduced footprint. However, there are also a number of disadvantages of the Hydrofoil compared with the Buoy, which resulted in the conclusion that for this Project the Buoy is a more viable solution than the Hydrofoil.
HydrofoilA number of technical concerns were encountered, including marine growth significantly affecting the lift and drag characteristics, as well as concerns regarding stability of the Hydrofoil and its dependency on the correct angle of attack.
The Hydrofoil would have high manufacturing costs, due to its complex structure and profile. Composite materials may offer improved structural performance, but at further increased cost. An asymmetrical Hydrofoil profile would provide an improved lift to drag ratio compared with the symmetrical profile studied in this Project. However, this would further add to manufacturing costs. Marine growth would also result in increased maintenance costs (e.g. regular application of anti-fouling).
BuoyThe Buoy offers a simpler and more robust solution compared with the Hydrofoil. The Buoy will be a more cost effective solution compared with the Hydrofoil, since sub-surface modular buoyancy can be procured from existing suppliers to the offshore industry. This would avoid the high costs of the bespoke manufacture of the Hydrofoil.
A number of mooring options were also investigated for the Buoy solution: Option 1 has a single mooring line, Option 2 has multiple lines, and Option 3 has multiple mooring lines and a Spar buoy. Options 2 and 3 offer the advantage of allowing for more densely packed arrays. For constrained sites in narrow tidal channels, this increased array density may significantly improve the economic viability of developing the site.