Analysis of a single point tensioned mooring system for station keeping of a contra-rotating marine current turbine

The Energy Systems Research Unit within the Department of Mechanical Engineering at the University of Strathclyde has developed a novel contra-rotating tidal turbine (CoRMaT). Novel aspects of this turbine include two contra-rotating sets of rotor blades directly driving an open-to-sea permanent magnet generator (PMG). The balancing of reactive forces by the use of contra-rotation enables the use of a single-point compliant mooring system for station keeping. A series of tank and sea tests have led to the deployment and demonstration of a small stand-alone next generation tidal turbine. The stability of a single-point mooring system is examined and power quality from the direct drive generator is evaluated. It is noted that good stability from a single-point mooring can be achieved within a real tidal stream; however from sea testing of the turbine off the west coast of Scotland in the Sound of Islay, it is shown that some instabilities in device station keeping may have an effect on the output electrical power quality. Finally, the scaling up of the power take-off and delivery options for a 250 kW production prototype are described and assessed. It was concluded that the most promising option was an enlarged version of the system already tested, namely a direct-drive contra-rotating PMG.