A number of alternative arrangements were considered for the buoy, and the effects on mooring line lengths, mooring system footprint and mooring line tension loads were investigated.
Option 1: Shallow Buoy - Single Line (Base Case)The first option, shown in the image below, consists of a single anchor and mooring line, with the buoy located above the CoRMaT turbine in relatively shallow water. This is the simplest option investigated, and is therefore considered as the Base Case.
Option 2: Shallow Buoy - Multiple LinesThe second option, shown in the image below, consists of multiple anchors and mooring lines, with the buoy located above the CoRMaT turbine in relatively shallow water. The objective of this option was to reduce the footprint of the mooring system, by reducing the motion of the mooring and turbine as the current direction and velocity changes over the diurnal tidal cycle.
Option 3: Deep Buoy - 4 Lines (Spar Buoy)The third option, shown in the image below, consists of multiple anchors and mooring lines, with the buoy located below the CoRMaT turbine, in relatively deep water. The turbine is fixed to a column rising above the buoy, in order to raise the turbine above the turbulent wake behind the buoy. The resulting moment on the buoy is balanced by a ballast weight fixed to a column below the buoy. The addition of this counterweight required a larger buoy.
Evaluation of OptionsThe table below summarises the results of this investigation. The results show that Options 2 and 3 (with multiple mooring lines) reduce the mooring system footprint compared with Option 1 (single mooring line). Options 2 and 3 also reduce the mooring line tension loads compared with Option 1. However, Options 2 and 3 would be costly compared with Option 1, due to the increased mooring line length, and the increased buoy size required for Option 3.
Note that the results in the table above are for the turbine located at 50 m above the seabed in each case. This is different from the Vertical Velocity Profile section, where the turbine height is different for each of the 3 options, in order to investigate the change in tidal current velocity.