Argyll Tidal Demonstrator Project

Technology

The CoRMaT device comprised a neutrally buoyant cylindrical nacelle with contra-rotating rotors (upstream – 3 blades, downstream – 4 blades) in close proximity around a common axis of rotation. This arrangement encouraged stability during operation and eliminated the requirement for a rigid support structure to transmit operational loads to the seabed. CoRMaT generated at variable speed via a submersible, contra-rotating permanent magnet generator with the rotor and stator driven directly by the upstream and downstream rotors respectively. For the planned demonstration deployment at the Mull of Kintyre, a minimum clearance of 10m below chart datum was intended to be maintained above the hydrobuoy to allow safe passage of vessels and to distance the buoy from wave action, the loading from which is most severe close to the surface. The lower tips of the rotors would be at least 7m from the seabed in order to operate above the highly sheared, lower resource region of the turbulent bottom boundary layer. A rigid ‘stinger’ with float surround would connect the nacelle to the yaw limiter. The nacelle was approximately 2.5m in diameter and 9.5m in length. Initially, 10m diameter rotor blades would have been fitted with scope for future upgrade to a maximum of 14m diameter. The overall system design would allow the nacelle to self-align with the predominant flow direction following periods of slack water. Power generation would be governed by site-specific flow conditions.

The device had a cut-in speed of 1 m/s, a rated output in flows greater than 2.5m/s and a maximum rotational speed (for each rotor) of 12 rpm.
 

Moorings

The device and mooring system would be attached to 3 cylindrical can foundations anchored to the seabed via drilled and grouted pins. The can dimensions were not expected to exceed 1.4m in height and 0.8m in diameter. The spread and relative orientation of the foundation locations to the tidal flow would be determined by a number of design factors including deployment depth, the morphology and bathymetry at the deployment location and local resource variations. Drilling depths were not expected to be greater than 12m and the diameter of holes would likely to have been in the 140 – 250mm range. Cuttings - rock fragments - would be flushed away. The anchors would be solid cylindrical steel pins of diameter in the range 75-150mm. A non-shrink, high strength grout would secure the anchor pins. All grouting operations would be sealed to prevent loss of grout to the environment.

The mooring system would be comprised of a ‘platform’ of 3 steel wire lines. A single line would be connected to each of the foundation cans and these would meet approximately mid-water column and connect to the yaw limiter. The ‘platform’ mooring line inclinations would be dependent on design factors including deployment depth, foundation specification and local resource level. Operational tilt resulting from thrust and drag loading would be up to 30 degrees from the upright position. The mechanical yaw limiter acted as the connection point for the CoRMaT system via a rigid ‘stinger’. Its operational function was 2-fold: 
•    To allow alignment with prevailing tidal flow direction; and 
•    To prevent coiling of the electrical and control cable passing through its centre. 

The hydro buoy would be foam-filled, constructed from steel with suitable corrosion and anti-fouling protection and be tethered to the upper plate of the yaw limiter using steel mooring lines. To avoid disturbance to the flow upstream of the rotor swept area, the lower surface of the hydro buoy would be located above rotor tip height in the water column. The hydrofoil profile has been selected to offer stable performance through a wide range of angles of attack. It was expected that the maximum dimensions of the hydro buoy would be a 6.5m chord length and an 8m span. As with the final specification of the mooring system and foundations, the design of this element would be determined in light of detailed information on the local flow environment.  

Export Cable

Power would be delivered to shore via an electrical export cable (3 phase, 3.3kV) protected in a single 4” to 6” diameter drill pipe run.  Also enclosed within the drill pipe would be a 230V electrical supply cable and fibre bundle for the CoRMaT control and braking systems. The connection for electrical export, electrical supply and communications cabling would most likely be dry-mate. Detailed design activity may have concluded that additional means of securing the pipeline to the seabed would be required at exposed sections of the route e.g. use of subsea cable mattress.

The subsea export cable landing j-tube would bound the offshore and onshore elements of the project. Landward of this point, a short tail of the subsea cable-in-pipe (approximately 25-50m in length) would protrude from the wider diameter j-tube following a winched installation. It was anticipated that conduits would be buried in a trench of up to 1m depth along this section. An isolator / earth switch would interface the onshore electrical equipment within the container compound from the generator. It was expected that the isolator would be containerised and located within the compound. The container compound would house both project and Distribution Network Operator (Scottish Hydro Electric Power Distribution) equipment. Project-side equipment would include:
•    An isolator / earth switch bridging onshore electrical equipment and the generator
•    Converters, transformers and switchgear housed in a number (provisionally 3) of compact containers

The DNO equipment comprised switchgear on a concrete plinth housed within a GRP enclosure.
 

Vessel spread