Turret mooring system
Disclosed herein is a turret mooring system for a tidal turbine assembly 150 , which increases the ratio of the torque exerted by the turret 100 relative to frictional forces between the turret 100 and the assembly 150 moored to the turret 100 . In some examples, the frictional forces are reduced by the turret providing an upward force to the assembly, which also resists pitching moments. In other examples, the torque exerted by the turret 100 is increased by the use of a bifurcated mooring line 106, 108 which couples to two spaced apart attachment points 128 on a chain table 104 of the turret 100 and to a single point 114 on the water bed.
1. A turret mooring system for a tidal turbine assembly, the turret mooring system comprising:
a turret having:
a shaft for mounting to the tidal turbine assembly and for allowing relative motion between the turret and the tidal turbine assembly about a rotational axis; and
a chain table secured to a lower end of the shaft;
an upstream mooring line for coupling a single upstream anchoring point on a water bed to an upstream portion of the chain table; and
a downstream mooring line for coupling a single downstream anchoring point on the water bed to a downstream portion of the chain table; wherein
the upstream mooring line is a bifurcated mooring line for coupling to two spaced apart upstream attachment points on the upstream portion of the chain table; and/or wherein
the downstream mooring line is a bifurcated mooring line for coupling to two spaced apart downstream attachment points on the downstream portion of the chain table; wherein:
the turret is arranged to provide an upward force when the chain table is submerged in water; and
the upward force is provided by a hydrodynamic fairing on the chain table.
2. The turret mooring system of claim 1 , wherein the spaced apart upstream and/or downstream attachment points on the chain table are spaced apart by at least 1 metre.
3. The turret mooring system of claim 1 , wherein the upstream mooring line has a first portion for coupling the upstream anchoring point on the sea bed to a bridle and a second portion for coupling the bridle to two upstream attachment points on the upstream portion of the chain table; and/or wherein
the downstream mooring line has a first portion for coupling the downstream anchoring point on the sea bed to a bridle and a second portion for coupling the bridle to two downstream attachment points on the downstream portion of the chain table.
4. The turret mooring system of claim 3 wherein the first and second portions of the upstream or downstream mooring line are made from different materials.
5. The turret mooring system of claim 4 , wherein the first portion of the upstream or downstream mooring line is a chain and/or the second portion of the upstream or downstream mooring line is a pair of synthetic cables.
6. The turret mooring system of claim 1 , wherein the hydrodynamic fairing has a variable buoyancy.
7. The turret mooring system of claim 1 , wherein the hydrodynamic fairing is bi-directional.
8. The turret mooring system of claim 1 , wherein the hydrodynamic fairing is symmetric in an upstream-downstream direction.
9. The turret mooring system of claim 1 , wherein the hydrodynamic fairing is shaped to counteract expected pitching moments in the tidal turbine assembly due to drag on turbines of the tidal turbine assembly.
10. The turret mooring system of preceding claim 1 , wherein the shaft includes an upper radial bearing and a lower radial bearing for engaging with the tidal turbine assembly.
11. The turret mooring system of claim 1 , wherein the turret is mounted into the tidal turbine assembly.
12. The turret mooring system of claim 11 wherein the turret is slidable in a vertical direction relative to the tidal turbine assembly.
13. A method of mooring a tidal turbine assembly to a water bed using the turret mooring system of claim 1 , the method comprising:
(i) transporting the tidal turbine assembly to an installation site;
(ii) coupling an upstream portion of the chain table of the turret to the water bed at a single upstream anchoring point and coupling a downstream portion of the chain table of the turret to the water bed at a single downstream anchoring point; and
(iii) securing the turret into the tidal turbine assembly.
14. The method of claim 13 , wherein step (iii) is performed prior to steps (i) and (ii), and step (iii) is performed in a dock or on land.
15. The method of claim 14 , wherein once step (iii) has been completed, the turret is slidable in a vertical direction relative to the tidal turbine assembly between a raised position and a lowered position such that a draft of the tidal turbine assembly is greater when the turret is in the lowered position than when the turret it is in the raised position.
16. The method of claim 15 , wherein the turret is in the raised position during step (i).
17. The method of claim 13 , wherein the tidal turbine assembly includes one or more turbines which are configurable in a deployed arrangement in which they are below an expected water line, and also in a raised arrangement in which the turbines are entirely above the expected water line; and wherein the turbines are further configured in the raised arrangement during step (i).