CA2916763A1

CA2916763A1 - Platform for tidal turbines

Info

Publication number: CA2916763A1
Application number: CA2916763A
Authority: CA
Inventors: Wee Keong Khoo
Original assignee: TIDAL HARNESS Ltd
Current assignee: TIDAL HARNESS Ltd
Priority date: 2013-06-28
Filing date: 2013-06-28
Publication date: 2014-12-31
Also published as: GB201600717D0; WO2014205603A1; CN105339651A; GB2531460A

Abstract

A platform (20, 80, 100) for supporting a tidal turbine (4) is disclosed as including at least one hull (22, 41, 42, 61, 62) to which at least one tidal turbine (4) is mountable, a number of ballast compartments (2, 92) within the hull (22, 41, 42, 61, 62), and a ballast structure (3) within the hull (22, 41, 42, 61, 62) and underneath the ballast compartments (2, 92), the platform (20, 80, 100) being floatable on seawater when the ballast compartments (2, 92) are de-ballasted, and the platform (20, 80, 100) being adapted to descend to the bottom of the sea when the ballast compartments (2, 92) are ballasted.

Description

PLATFORM FOR TIDAL TURBINES
TECHNICAL FIELD
[0001] This invention relates to tidal turbines and particularly, though not exclusively, relates to platforms or bases for supporting a tidal turbine so as to facilitate, and thus reduce the cost of deployment of an offshore tidal turbine. Hereafter the terms "gravity base", "platform", and "foundation" are used interchangeably to mean a structure for mounting and supporting at least one tidal turbine.
BACKGROUND OF THE INVENTION

[0002] Tidal renewable energy is an emerging market compared to the more mature wind energy counterpart. Given that tidal flow is predictable and that the global tidal energy resource is vast, tidal energy is expected to become an important contribution to the global renewable energy mix. The cost of energy of tidal renewable energy is still very high compared to that of wind energy. This is primarily due to the inherent high cost associated with the offshore installation, maintenance and retrieval of tidal turbines.

[0003] The foundations, platforms or bases of a tidal turbine (which functions as an ocean power generator) can be broadly categorized into two groups, namely, fully submerged and floating. Turbines having monopiles, piled jackets (tripod or quad) or gravity bases are fully submerged underwater whereas floating turbines or pontoons are usually moored to the seabed. Monopiles and piled jackets require subsea drilling to fix their foundations on the seabed but they can be made of lightweight structures. Gravity bases, on the other hand, do not require subsea drilling, which implies that they are potentially more straightforward to install. However, they are relatively large and heavy in order to withstand the overturning moment due to the tidal flow. Seabed works are not needed for floating turbines but clump weight anchors are required to prevent the turbine from being swept away by tides or waves.

[0004] One inconvenient attribute common to most fully submerged tidal turbines is that they require an offshore crane to lift the structure from the installation vessel to the seabed. The limited number of crane-equipped vessels to meet the demand of offshore installation has inevitably increased the cost of installation of fully submerged tidal turbines.
It is common to find that offshore installation accounts for the largest breakdown of cost in such tidal turbines. Some turbines are deployed using custom-built vessels but since such vessels deploy only one turbine at a time, the installation costs is still expensive.
CONFIRMATION COPY

[0005] It is thus an object of the present invention to provide a new platform for tidal turbines and a tidal turbine assembly in which the aforesaid shortcomings are mitigated, or at least to provide useful alternatives to the trade and public.
SUMMARY OF THE INVENTION

[0006] According to the preferred embodiments of the present invention, there is provided a gravity base for supporting a tidal turbine which can be deployed offshore without needing a crane to lift the turbine and the base from the vessel to the sea.
Drilling into the seabed is not required and the deployment of the base may be accomplished quicker and easier.

[0007] A gravity base according to the present invention may have ballast tanks or compartments that are strengthened by structural means. The ballast tanks or compartments may have empty content (i.e. filled with air) initially so that the gravity base may be floated at the harbour or shipyard. It may then be towed to its offshore destination by a conventional tugboat. Once the deployment site is reached, the ballast compartments may be ballasted by pumping in seawater appropriately controlled so that the gravity base descends to the targeted seabed location in a stable manner. The air within the ballast compartments is displaced via the vent conduit at the top as seawater fills the compartments.

[0008] The present invention also seeks to provide a turbine foundation which facilitates maintenance without needing a crane and is also easier to decommission. For retrieval of the turbine, the seawater inside the ballast compartments may be pumped out via the conduits. The extraction of seawater is appropriately coordinated between the ballast compartments so that the foundation can ascend in a controlled and stable manner. Air is drawn into the ballast compartments via the conduits as seawater is pumped out. When most of the seawater has been pumped out of the ballast compartments, the whole foundation floats and exposes the turbine where maintenance work may be undertaken. The floating foundation may also be towed by a tugboat to the harbour or shipyard for decommissioning.
There are not any piles or structures left on the seabed during decommissioning, which is also an advantage.

[0009] The present invention also seeks to provide a platform where its centre of gravity is below its centre of buoyancy so that the platform may descend in a stable manner.
This may be achieved by having a ballast structure, which accounts for the major proportion of the requisite weight, underneath the air-filled ballast compartments.
Unlike methods in prior art that use separable and lightweight apparatus for installing the turbine, the present invention advantageously combines the weight of both the structural construction of the ballast compartments and the ballast structure to provide the necessary weight to function as a gravity base. Thus, the attachment and detachment of lifting apparatus before and after installation are not required.

[0010] The present invention further seeks to provide a turbine platform constructed from cost effective materials without compromising its overall structural integrity. The structure may be built from standardized cast metal and reinforcement concrete may be used as ballast to save cost.

[0011] The present invention also seeks to provide a passive gravity base, in which the absence of any active moving mechanical components increases the reliability of the foundation.

[0012] The present invention further seeks to provide a turbine platform which does not need a custom-built vessel for deployment of the turbine. The platform may be towed to offshore for deployment and onshore for maintenance or decommissioning by a conventional tugboat. Supplying seawater to and extracting seawater from the ballast compartments may be carried out by a bi-directional water pump system onboard the vessel.

[0013] The present invention also seeks to provide a method of linking two or more platforms so that they may be towed together to their deployment site by, for e.g. a tug vessel.
This is advantageous in that multiple installations may be accomplished in one trip and therefore saving travel time and cost.

[0014] The present invention further seeks to provide a foundation that may be floated and moored at seabed. As 75% of tidal energy is found in the top half of the water depth, it is advantageous to position the turbine at this region to maximize energy extraction. This floating foundation or platform may be pontoons supporting a turbine which may be lowered beneath water surface for harvesting tidal energy or raised for maintenance.
This is particularly useful for prototype testing. With the turbine lowered underwater, the pontoon may also be towed by a vessel at various speeds to simulate different tidal velocities. Floating platforms are also necessary at sites where the cost of fixing the turbine on the seabed becomes prohibitively expensive.

[0015] According to a first aspect of the present invention, there is provided a platform for supporting a tidal turbine, said platform including (a) at least one hull to which at least one tidal turbine is mountable, (b) a plurality of ballast compartments within said hull, and (c) a ballast structure within said hull and underneath said plurality of ballast compartments; wherein said platform is floatable on a body of water when said ballast compartments are de-ballasted, and said platform is adapted to descend to the bottom of said body of water when said ballast compartments are ballasted.

[0016] According to a second aspect of the present invention, there is provided a tidal turbine assembly including at least one tidal turbine mounted to a platform for supporting a tidal turbine, said platform including (a) at least one hull to which at least one tidal turbine is mountable, (b) a plurality of ballast compartments within said hull, and (c) a ballast structure within said hull and underneath said plurality of ballast compartments;
wherein said platform is floatable on a body of water when said ballast compartments are de-ballasted, and said platform is adapted to descend to the bottom of said body of water when said ballast compartments are ballasted.

[0017] According to a third aspect of the present invention, there is provided a platform assembly including at least two platforms for supporting a tidal turbine, each said platform including (a) at least one hull to which at least one tidal turbine is mountable, (b) a plurality of ballast compartments within said hull, and (c) a ballast structure within said hull and underneath said plurality of ballast compartments; wherein said platform is floatable on a body of water when said ballast compartments are de-ballasted, and said platform is adapted to descend to the bottom of said body of water when said ballast compartments are ballasted, and wherein said hulls of said at least two platforms are releasably connected with each other by at least one elongate connecting member.

[0018] According to a fourth aspect of the present invention, there is provided a tidal turbine assembly including at least one tidal turbine mounted to a platform assembly, said platform assembly including at least two platforms for supporting a tidal turbine, each said platform including (a) at least one hull to which at least one tidal turbine is mountable, (b) a plurality of ballast compartments within said hull, and (c) a ballast structure within said hull and underneath said plurality of ballast compartments; wherein said platform is floatable on a body of water when said ballast compartments are de-ballasted, and said platform is adapted to descend to the bottom of said body of water when said ballast compartments are ballasted, and wherein said hulls of said at least two platforms are releasably connected with each other by at least one connecting member.

[0019] According to a fifth aspect of the present invention, there is provided a platform for supporting a tidal turbine, said platform including (a) at least one pontoon, (b) at least two columns constrained to said pontoon; and (c) a ballast compartment mounted at a bottom end of and between said columns; wherein at least one tidal turbine is mountable between said columns and above said ballast compartment, and wherein said columns are adapted to move, when said ballast compartment is ballasted, relative to said pontoon to a first position to submerge said tidal turbine into said body of water along a pre-defined path and are adapted to move, when said ballast compartment is de-ballasted, relative to said pontoon to a second position to raise said tidal turbine to the surface of said body of water along said pre-defined path.

[0020] According to a sixth aspect of the present invention, there is provided tidal turbine assembly including at least one tidal turbine mounted to a platform for supporting a tidal turbine, said platform including (a) at least one pontoon, (b) at least two columns constrained to said pontoon; and (c) a ballast compartment mounted at a bottom end of and between said columns; wherein at least one tidal turbine is mountable between said columns and above said ballast compartment, and wherein said columns are adapted to move, when said ballast compartment is ballasted, relative to said pontoon to a first position to submerge said tidal turbine into said body of water along a pre-defined path and are adapted to move, when said ballast compartment is de-ballasted, relative to said pontoon to a second position to raise said tidal turbine to the surface of said body of water along said pre-defined path.

[0021] According to a seventh aspect of the present invention, there is provided a platform for supporting a tidal turbine, said platform including (a) at least one pontoon, (b) at least two columns fixed to said pontoon, and (c) a ballast compartment constrained between said columns; wherein at least one tidal turbine is constrainable between said columns and mountable above said ballast compartment; and wherein said tidal turbine and said ballast compartment are adapted to move, when said ballast compartment is ballasted, relative to said pontoon to a first position to submerge said tidal turbine into a body of water along a pre-defined path and are adapted to move, when said ballast compartment is de-ballasted, relative to said pontoon to a second position to raise said tidal turbine to the surface of said body of water along said pre-defined path.
BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Preferred embodiments of the invention will now be described, by way of examples only and not in any limitative sense, with reference to the accompanying drawings, in which:-FIG. 1(a) shows a triangular-shaped gravity base according to a preferred embodiment of the present invention, with ballast compartments spread out at three locations within the base;
FIG. 1(b) shows a tidal turbine mounted on the base of FIG. 1(a) with water conduits and vent conduits attached to a respective buoy;
FIG. 2 shows a tidal turbine mounted on a triangular base where its water and vent conduits are routed to exit at sideways of the turbine;
FIG. 3 shows a tidal turbine mounted on a triangular gravity base with water and vent conduits grouped together and floated at a distance away from the base;
FIG. 4(a) shows a catamaran-like gravity base with a tidal turbine mounted on the base, forming a turbine and gravity base assembly;
FIG. 4(b) shows ballast compartments within the base of FIG. 4(a);
FIG. 5(a) shows the turbine and gravity base assembly of FIG. 4(a) being towed by a vessel to be deployed to an installation site;
FIG. 5(b) shows the turbine and gravity base assembly of FIG. 5(a) sinking to the seabed as seawater is pumped into the ballast compartments;
FIG. 5(c) shows the turbine and gravity base assembly of FIG. 5(a) after installation, having sunk to the seabed;
FIG. 6 shows a second vessel assisting with the stabilising and positioning of the turbine and gravity base assembly as the base descends to the seabed;

FIG. 7 shows a second turbine and gravity base assembly providing additional stability during deployment of the first turbine and gravity base assembly;
FIG. 8 shows a method of orientating the gravity base with an active thruster;
FIG. 9 shows a method of orientating the gravity base with an ROV;
FIG. 10 shows the water and vent conduits of the catamaran-like turbine and gravity base assembly of FIG. 5(a) floated at a distance away from the base;
FIG. 11(a) shows a trimaran-like gravity base with a tidal turbine mounted thereon to form a turbine and gravity base assembly;
FIG. 11(b) shows the ballast compartments within the gravity base of FIG.
11(a);
FIG. 12(a) shows an alternative arrangement of ballast compartments in which venturi-like structures are provided above a gravity base;
FIG. 12(b) shows the ballast compartments within the venturi-like structures of FIG. 12(a);
FIG. 13 shows adaptation of the gravity base to support a twin turbine;
FIG. 14 shows adaptation of the gravity base to support a non-ducted, towered tidal turbine;
FIG. 15(a) shows a floating platform having a ballast compartment for raising and lowering a tidal turbine, in which the turbine is raised to the floating platform;
FIG. 15(b) shows the tidal turbine and platform assembly of FIG. 15(a) with the turbine descended beneath the floating platform;
FIG. 16(a) shows a turbine and floating platform assembly, with the platform having a rotatable support and ballast compartment for raising and lowering the tidal turbine, in which the turbine is descended beneath the floating platform;
FIG. 16(b) shows the turbine in the assembly of FIG. 16(a) raised to the floating platform;
FIG. 17(a) shows a turbine and floating platform assembly, with the platform having a rotatable support actuatable by a mechanical pusher for raising and lowering the tidal turbine, with the turbine descended beneath the floating platform;
FIG. 17(b) shows the turbine in the assembly of FIG. 17(a) raised to the floating platform;
FIG. 18(a) shows a turbine and floating platform assembly, with the floating platform having a winch system for raising and lowering the tidal turbine, with the turbine raised to the floating platform;
FIG. 18(b) shows the turbine in the assembly of FIG. 18(a) descended beneath the floating platform; and FIG. 19 shows adaptation of the floating platform to support a twin turbine.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0023] The present invention seeks to provide a gravity base which serves the purpose of supporting at least one tidal turbine and may be floated and towed to its deployment site.
The present invention also provides a tidal turbine assembly with a tidal turbine mounted to a platform.

[0024] FIG. 1(a) illustrates a triangular-shaped gravity base 1 having three ballast compartments 2 disposed at each corner and at the upper half of the base 1.
The ballast compartments 2 are shown here as being of a triangular shape but other shapes may also be used. More importantly the ballast compartments 2 are designed to be of a sufficiently large volume so that when they are empty (i.e. filled with air) the gravity base 1 may be floated on a body of water, such as seawater. The internal walls of the ballast compartments are strengthened structurally, rigid, impermeable to the surrounding seawater and immune to the external hydrostatic pressures imposed by water depth, in particular when the gravity base 1 descends to the seabed. The gravity base 1 may be ballasted by pumping liquid into the ballast compartments 2 or de-ballasted by pumping liquid out of the ballast compartments 2.
Preferably liquids having specific gravity of at least equivalent to that of seawater are used.
Seawater may be the natural choice as it is the cheapest and the most conveniently available source.

[0025] The lower half of the gravity base 1 is a ballast structure 3 which accounts for the major proportion of the weight of the gravity base 1. The ballast structure 3 is below the ballast compartments 2. The purpose of the ballast structure 3 is twofold.
First, the ballast arrangement is akin to a heavy keel in a conventional monohull ship where the centre of buoyancy is directly above its centre of gravity when the base 1 is floating.
The centre of buoyancy, which is defined by the submerged shape, acts upwards (upthrust) whereas the centre of gravity acts downwards. This essentially creates a stable equilibrium for the structure to float where minor heeling would result in a restoring force. With such a structure, the gravity base 1 can also descend to the seabed with ease, without the tendency to roll over.
Second, the ballast structure 3 keeps the gravity base 1 and turbine 4 in upright position on the seabed and resists undesirable dislodgement and overturning moment caused by the tidal flow.
The structural build-up of the ballast compartments 2 and the ballast structure 3 combine to provide the necessary weight to function as a gravity base 1.

[0026] For stable descent during deployment or ascent during retrieval, a minimum of three independently controlled ballast compartments 2 are preferred so as to prevent the base 1 from rolling excessively from its equilibrium position. Each ballast compartment 2 can be ballasted and de-ballasted independently. Seawater may be pumped into or out of each ballast compartment 2 via a respective water conduit 5. Each ballast compartment 2 has a vent conduit 6 to allow the air inside to be displaced as the air volume is replaced by the inflowing water. When water is pumped out of the ballast compartment 2, air is drawn in via the vent conduit 6 to replace the out-flowing water. Such an arrangement allows the buoyancy of the base 1 and descent and ascent of the base 1 to be controlled. The water conduit 5 and vent conduit 6 may have a valve at the other end and may be attached to a buoy 7 and left floating after installation. The buoy 7 is advantageous in that the water conduit 5 and vent conduit 6 would be easy to locate and retrieve when the turbine 4 is required to be serviced.

[0027] The ascent or descent of the gravity base us controlled by three independent water pumps which may be carried onboard a vessel. Each pump supplies seawater to and draws seawater from a ballast compartment 2. Adjustment of the volume flow rate of the pump is necessary to prevent the turbine 4 and the gravity base 1 from rolling beyond limits in the longitudinal axis or pitching excessively in the lateral axis.
Additional stability and alignment of the turbine 4 to the tidal flows may be provided by securing winch cables to the gravity base 1 and gradually wound out as the gravity base 1 descends to the seabed. The tilting, acceleration or positioning of the gravity base 1 may be monitored using, for example, incline angle sensors, acceleration sensors and imaging sonar, and the pump flow rate to or from the ballast compartments 2 may then be adjusted accordingly.

[0028] The gravity base 1 may be built from standardised elements, including cast steel, and appropriately assembled to give structural strength to the base 1.
Reinforcement concrete may also be used for the construction of the base 1 and as a ballast structure 3. The gravity base 1 spreads substantially in the horizontal direction and has the requisite weight to keep the turbine 4 upright. For a horizontal axis turbine, the axis of rotation of the turbine may be mounted in alignment to the base longitudinal axis, as shown in FIG.
1(b). A rim driven or ducted tidal turbine 4 may be mounted to the gravity base 1 at three locations. For example, the 6 o'clock segment of the turbine 4 is mounted directly to the base 1 itself whereas the 3 o'clock and 9 o'clock segments are mounted to vertical columns which are fixed to the base 1, such as lattice tower structures 8 shown in FIG. 1(b).
The longitudinal length of the gravity base 1 may be made relatively long compared to its lateral length in order to better overcome the axial thrust and overturning moment caused by tidal flows.

[0029] As shown in FIG. 1(b), the gravity base 1 has a number of feet 9 constructed to support and elevate the structure 1 above the seabed. Preferably, at least three feet 9 (one in each corner) are provided in the triangular shape gravity base 1. The length of each foot 9 may be different from each other to suit the uneven seabed at the installation site.
Additionally, the feet 9 of the gravity base 1 may have anti-scour means for preventing scouring of sand by current flow around the feet.

[0030] Depending on the depth of the sea, the water conduits 5 and vent conduits 6 shown in FIG. 1(b) may be made rigid for the length of at least the height of turbine 4 so that the conduits 5, 6 do not entangle each other. The rest of the length of the conduits 5, 6 may be flexible and floated at the other end. Although the obstruction of tidal flow by the conduits 5, 6 in FIG. 1(b) may be very minimal, it is nevertheless preferable not to have any object present in the direction of tidal flow toward the turbine 4. FIG. 2 shows an example where the water conduits 5 and vent conduits 6 are routed to exit at sideways of the turbine 4 so that they do not obstruct the useful tidal flow. FIG. 3 shows a further example of how the conduits 5, 6 may be located clear from obstructing the tidal flow toward the turbine 4. The water conduits 5 and vent conduits 6 are flexible throughout and are held down by a small clump of weight 10 or a submerged buoy at an appropriate distance away from the gravity base 1. The free ends of each water conduit 5 and vent conduit 6 are then attached to a buoy 7.

[0031] FIGS. 4(a) and 4(b) show a gravity base 20 constructed in the form of a catamaran mounted with a tidal turbine 4, in which the 6 o'clock segment of the tidal turbine 4 is mounted on a bridge 21 connecting between two hull-like structures 22.
The 3 o'clock and 9 o'clock segments of the turbine 4 are mounted on lattice towers 8 which are in turn fixed to the hull-like structures 22. FIG. 4(b) shows four independent ballast compartments 2 in this catamaran-like gravity base 20 for stabilizing control, i.e. two compartments per hull-like structure 22. A fifth ballast compartment may be constructed within the bridge 21 if required. Water conduits 5 and vent conduits 6 are connected to ballast compartments 2. The conduits 5, 6 may be rigid for the length of up to the height of the turbine 4 and these rigid portions may be fastened to the lattice towers 8 so that they are immovable by the influence of the tides. The remaining length of the water conduits 5 and vent conduits 6 may be flexible, attached to a buoy 7 and left floating at the water surface.

[0032] Similar to the construction principles as the triangular gravity base 1 of FIGS.
1(a) to 3, the catamaran-like gravity base 20 has a ballast structure 3 underneath the ballast compartments 2. If the platform is constructed to float at all times then the ballast structure would not be needed, because by maintaining the separation of the two floating hull-like structures, the platform may float stably like a conventional catamaran. In the present invention, however, the base 20 is not only required to float stably but must also be able to descend to the seabed without rolling over. With the ballast structure 3 accounting for the major proportion in weight of the gravity base 20, the centre of gravity of the base 20 and tidal turbine 4 combined is below their combined centre of buoyancy, thereby enabling the base 20 to submerge and descend in stable equilibrium. An exemplary method for deploying the aforementioned tidal turbine 4 will now be described with reference to FIGS. 5(a) to 9.

[0033] FIG. 5(a) depicts a catamaran-like gravity base 20 being towed to its deployment location by a conventional tug vessel 23. The vessel 23 has a winch cable 24 secured to the front end of the floating base 20. The ballast compartments 2 within the gravity base 20 are not ballasted, thus floating the whole gravity base 20.
Once the vessel 23 arrives at the deployment location, the winch cable 24 is detached from the front end of the gravity base 20 and secured to top end of the lattice tower structures 8 or other parts as practicable. The winch cable 24 is unwound gradually as the gravity base 20 descends to the seabed 200, but the cable 24 is preferably maintained taut so as to guide and provide additional stability to the descending gravity base 20. Minor yaw adjustment of the gravity base 20 with respect to its vertical axis to align the turbine 4 to the prevailing tidal flows may also be provided by the winch cable 24.

[0034] FIG. 5(b) shows the turbine 4 and the gravity base 20 in the course of descent to the seabed 200. The flexible water conduits 5 are connected to the water pumps onboard the vessel 23 while the valves of the vent conduits 6 are opened. The sinking of the gravity base 20 is accomplished by actively controlling the volume flow rate of seawater into the four independent ballast compartments 2. If required, water may also be pumped out of the compartments 2 to compensate for the imbalance. A first terminal of a power cable 25 is connected to the turbine 4 and the other terminal for connection to the next turbine in succession or to the subsea power equipment or to the offshore substation or to the onshore substation is held at the vessel 23. This power cable 25 is wound out as the turbine 4 is lowered to the seabed 200 to avoid stressing the power cable 25. When the connection to the next turbine in succession is made, the power cable 25 may be placed on the seabed 200.
Subsea inclinometer and acceleration sensors may be used to monitor the tilting movement and acceleration of the gravity base 20 as it descends to the seabed 200. The positioning of the gravity base 20 may also be monitored by using imaging sonar.

[0035] FIG. 5(c) shows a deployed tidal turbine 4 sitting on the seabed 200.
The water conduit 5, vent conduit 6 and winch cable 24 from each side of the hull 22 are secured to a buoy 7. Alternatively, all the water conduits 5, vent conduits 6 and winch cables 24 may be secured together and held down by a clump of weight 10 at a distance away from the turbine 4, similar to the case of the gravity base 1 of FIG. 3. The winch cables 24 may also be disconnected from the lattice tower structures 8 after installation. It may be preferable to leave the cables 24 connected to the lattice tower structures 8 for conveniently retrieving the turbine 4 later. Alternatively a remotely operated vehicle (ROV) may be employed to re-connect the winch cables 24 to the gravity base 20 when retrieval of the turbine 4 is required.
The power cable 25 may be left lying on the seabed 200 after connecting to the next turbine in succession or to subsea power equipment or to the offshore substation or to the onshore substation. Details of such connections are not shown in FIGS. 5(a) to 5(c) since these are outside the scope of the present invention.

[0036] The turbine 4 may be required to be aligned to an intended orientation during installation. As shown in FIG. 6, a second vessel 26 may be used to assist with the stabilising and positioning of the gravity base 20. A winch cable 27 connecting between this second vessel 26 and the rear end of the base 20 is gradually unwound but kept taut as the gravity base 20 descends. The first vessel 23 or the second vessel 26 may be manoeuvred sideways to yaw the gravity base 20 (and the turbine 4) to face into the tidal currents.

[0037] Alternatively, as shown in FIG. 7, the stability of the gravity base 20 may be provided by both the tug vessel 23 and the next gravity base 20A in succession deployment.
Multiple gravity bases 20, 20A, 20B are tied by means of a mechanical linkage 28 and towed together to the deployment site. A winch system 29 may be temporarily installed on the second platform 20A where its cable 30 is connected to the rear end of the first platform 20 to be installed. The front end of the platform 20 is connected to the tug vessel 23 via a winch cable 24. As the first platform 20 descends to the seabed 200, the winch cables 30, 24 from the second platform 20A and tug vessel 23 are unwound and held taut without exceeding their load capacity. When the installation of the first platform 20 is done, the temporary winch system 29 on the second platform 20A may be removed.

[0038] An active thruster 31 for better yawing the gravity base assembly 20 with respect to its vertical axis may also be provided, as shown in FIG. 8. Such a thruster 31 is latched between two hulls 22 at the rear side of the gravity base 20 and is powered and controlled remotely via a power cable 32 from the same vessel 23 or from a second vessel 33.
When the turbine 4 has been deployed, the thruster 31 may be disengaged from the base 20 and retrieved by the vessel 33 or 23. Alternatively, an ROV 34 may be used to pull and guide the gravity base 20 to its intended position as it lowers onto the seabed 200, as illustrated in FIG. 9. The triangular-shaped gravity base 1 of FIGS. 1(a) to 3 may be deployed using the exemplary method disclosed in FIGS. 5(a) to 9.

[0039] Similar to the gravity base 1 shown in FIG. 3, the water conduits 5 and vent conduits 6 of the catamaran-like gravity base 20 may be flexible so that they may be placed at a distance away from the gravity base 20. As shown in FIG. 10, the conduits 5, 6 connecting to the ballast compartments are routed and retained at the seabed 200 by a small clump of weight 10 (or an anchor) or held down with a submerged buoy. The other end of the conduits 5, 6 are attached to a buoy 7 for convenient retrieval and access.

[0040] As shown in FIG. 11(a), a tidal turbine 4 may also be supported by a trimaran-like gravity base 40. In FIG. 11(a), the gravity base 40 has a central hull 41 linking to two smaller lateral hulls 42 via a bridge 21. Making the central hull 41 relatively long compared to the height of the turbine 4 decreases the tendency of the gravity base 40 to overturn. The turbine 4 is mounted on the central hull 41 and on the bridges 21 through lattice tower structures 8. It is possible to have narrower bridges 21 so that the lattice tower structures 8 are fixed at the lateral hulls 42, but widening the span of the lateral hulls 42 increases the stability for floating, ascending to the surface and descending to the seabed.
FIG. 11(b) shows a total of four ballast compartments 2 within the upper half of the gravity base 40 (i.e.
two compartments at the central hull 41 and one compartment at each lateral hull 42) which may be de-ballasted or ballasted with water to float and sink respectively.
The lower half of each hull 41, 42 has a ballast structure 3.

[0041] The ballast compartments 2 may also be constructed or assembled external to a gravity base 50 according to the present invention. An example is shown in FIG. 12(a) where four rigid, vertical structures 51 are mounted on the gravity base 50. The structures 51 may advantageously be shaped like a venturi duct to accelerate the incoming flows to the tidal turbine 4. FIG. 12(b) shows that each venturi duct structure 51 has a ballast compartment 2 embedded within and the buoyancy of the gravity base 50 may be controlled by pumping water in or out of the ballast compartment 2.

[0042] The gravity base according to the present invention may be extended to an assembly where multiple turbines may be supported. FIG. 13 shows an example of a pair of turbines 4 mounted on a trimaran-like gravity base 60. The width and length of a central hull 61 and lateral hulls 62 may be different. Each hull 61, 62 has two ballast compartments 2 and a lattice tower structure 8 for mounting one segment of the turbine 4. Both tidal turbines 4 have the same power rating but they twist in opposite directions. Such a contra rotary twin turbine 4 has the advantage that the rotary moments in effect cancel each other.

[0043] The gravity base according to the present invention may also be used to support a ductless, towered tidal turbine 70, as illustrated in FIG. 14. A
pedestal 71 of the turbine 70 is mounted on a bridge 21 linking two hull-like structures 22. The water conduit 5 and vent conduit 6 is held down by an anchor 10 or a submerged buoy at an appropriate distance away from the base 20 so that they would not be entangled to the rotating turbine blades 72.

[0044] According to a further preferred embodiment of the present invention, the platform disclosed above may be adapted to always float on a body of water.
FIGS. 15(a) and 15(b) show an example of a catamaran-like floating platform 80 for supporting a tidal turbine 4, having two pontoons 81. Since the floating platform 80 is not required to be sunk on the seabed and to resist the overturning moment, the ballast material is not required and the structural elements for construction of the pontoons 81 may be significantly reduced.

[0045] The absence of the ballast means that the centre of gravity of the platform 80 is directly above the centre of buoyancy. The separation of the pontoons 81 by a bridge 82 gives good static stability akin to a conventional catamaran. When the platform 80 heels (due to disturbances or turbulences from the wind or wave) the low side of the pontoon 81 is substantially submerged compared to the high side, but the total submersion volume to float the platform 80 remains the same. The centre of buoyancy shifts substantially towards the low side and the gravity and buoyancy forces are no longer aligned. The mismatch of the centre of gravity and the centre of buoyancy exerts to rotate and tends to restore the platform 80 to equilibrium.

[0046] The floating platform 80 of the present invention may be moored 83 to the seabed for stability and for prevention of being swept away by the tidal currents. Many conventional mooring methods may be used, including tension-leg or catenary systems. The mooring systems 83 may or may not be rigidly fixed to the seabed. Anchoring systems which literally rest on the seabed may be advantageous in that they can be deployed and retrieved at ease. The floating platform 80 may be constrained or allowed to swivel and align the turbine 4 to the tidal flow. If the platform 80 is constrained, at least three points of the platform 80 may be moored to the seabed to prevent rolling and pitching. For swivelling to align with the tidal flows, a single mooring cable connecting the platform 80 to the seabed may suffice.

[0047] FIG. 15(a) shows an example of a catamaran-like platform 80 with a tidal turbine 4 mounted thereto and raised above the water level 300. The platform 80 is shown as being moored 83 to the seabed at four locations. The tidal turbine 4 may be lowered or raised along a pre-defined path with respect to the platform 80 for energy harvesting mode and maintenance respectively. The 3 o'clock and 9 o'clock segments of the turbine 4 are mounted to two pillars 84 and together the pillars 84 are constrained to slide vertically at collars 85, 86, 87 and 88 relative to the pontoons 81. Collars 85, 86 provide full circular sliding contacts for the pillars 84 whereas collars 87, 88 are semi-circular to clear the mechanical mounting of the turbine 4 segments to the pillars 84. Collars 85, 86 are secured to the upper end of lattice tower structures 89 which are in turn fixed to the pontoons 81. A horizontal lattice beam 90 connecting collars 85 and 86 provides additional structural strength and stability to the lattice towers 89 and collars 85 and 86. The top end of each pillar 84 has a solid disc 91 with a larger diameter than the bore of collars 85 and 86. These solid discs 91 serve as a sliding limit to which the pillar 84 and turbine 4 may descend. A locking mechanism to lock the pillars 84 may be provided at collars 85, 86, 87 and 88.

[0048] A ballast compartment 92 is mounted to the bottom end of the pillars 84 and the 6 o'clock segment of the turbine 4, as shown in FIG. 15(a). Since the turbine 4 and the pillars 84 are constrained to move vertically along a pre-defined path relative to the pontoons 81, ballasting and de-ballasting a compartment 92 would submerge and raise the turbine 4 respectively. The ballast compartment 92 has water conduits and vent conduits and seawater may be used as ballast. Details of the water conduit and vent conduit extending from the ballast compartment 92 and connecting to the pump on the platform 80 are not shown for clarity.

[0049] FIG. 15(b) shows a floating platform 80 with its turbine 4 submerged underwater for harnessing tidal energy. The pillars 84 and the turbine 4 are extended downward until the solid discs 91 and collars 85, 86 are in contact. Having both the pillars 84 and turbine 4 raised above water 300 together is advantageous in that there are not any protruding structures beneath the pontoons 81. The platform 80 may be towed away without potential contact or collision with objects on the seafloor or with the seabed itself, especially in areas of shallow water. Alternatively, the pillars 84 may be fixed on the platform 80 and only the turbine 4 is mounted to the ballast compartment 92, in which case, only the turbine 4 is lowered or raised with respect to the water level 300 and relative to the pontoons 81.

[0050] FIGS. 16(a) and 16(b) show that submerging and raising the turbine 4 along a pre-defined path relative to the pontoons 81 may also be accomplished by having a rotatable mount 93 on the pontoons 81. The turbine 4 is, in this case, attached between pillars 84, and the top end of each pillar 84 is constrained to rotate at a rotatable mount 93. The bottom end of each pillar 84 and the 6 o'clock segment of the turbine 4 are mounted to a ballast compartment 92. The buoyancy of the ballast compartment 92 is controlled by pumping seawater into or out of the ballast compartment 92. When the ballast compartment 92 is ballasted, the pillars 84 rotate downward relative to the pontoons 81, thereby submerging the turbine 4, and when the ballast compartment 92 is de-ballasted, the pillars 84 rotate upward relative to the pontoons 81, thus raising the turbine 4 to the water surface 300. A plurality of elongate members 94 including marine grade steel wires may be used to secure the submerged turbine 4 so that it may overcome thrust forces. Alternatively, the rotatable mount 93 at the pontoons 81 may have a locking means 95 to secure the submerged pillars 84 in position.

[0051] The turbine 4 may also be submerged and raised by using a mechanical actuation system 96 on board the platform 80 in conjunction with the rotatable mount 93.
FIGS. 17(a) and 17(b) show a mechanical actuation system 96 pushes or pulls the turbine 4 via a plurality of rigid elongate members 97 to submerge or raise the turbine 4 relative to the pontoons 81 respectively. A winch system 98 may be used in place of the mechanical actuation system 96 of FIGS. 17(a) and 17(b). FIG. 18 illustrates a floating platform 80 having fixed pillars 84 extending beneath pontoons 81 where a turbine 4 is constrained to slide vertically between and along the pillars 84. Lattice towers 89 and a horizontal beam 90 provide the structural support for the pillars 84. A winch system 98 is mounted on the horizontal beam 90 and its cables 99 connect to the 3 o'clock segment and 6 o'clock segment of the turbine 4. The winch cables 99 may be unwound or wound up so that the turbine 4 may be lowered into or raised above the water 300 respectively.

[0052] The floating platform in the present embodiment of the invention may be adapted where multiple turbines 4 may be supported. FIG. 19 shows an example of a twin turbine 4 mounted on a trimaran-like floating platform 100. Both turbines 4 have the same power rating but one turbine has blades twisting in the opposite direction to that of the other turbine so that the contra rotary moments cancel each other.

[0053] Floating platforms 80, 100 are advantageous because the turbine's power conversion equipment may be housed within the pontoons 81 or in a container mounted on the bridge 82. A submarine enclosure for housing the power conversion equipment would not be needed and periodical maintenance may also be done with ease. The turbine 4 may be linked to the next turbine in succession or connected to the offshore substation or connected to the onshore substation via a submarine cable but wet-mate connectors are not required since the connection may be accomplished inside the pontoons 81 or a container external to the pontoons 81.

[0054] Although a rim driven or ducted turbine 4 is shown in FIGS. 15(a) to 19, the floating platform 80, 100 may support a towered turbine where it is inverted and mounted underneath the platform 80, 100. The turbine may also be a vertical axis tidal turbine. The floating platform 80, 100 may also be adapted to support a wave energy conversion system.

[0055] It will be appreciated by persons skilled in the art that the above embodiments have been described by way of examples only, not in any limitative sense, and that various alterations and modifications in details of design, construction or operation are possible without departing from the scope of the invention as defined by the claims.

Claims

CLAIMES

1. A platform for supporting a tidal turbine, said platform including:
(a) at least one hull to which at least one tidal turbine is mountable, (b) a plurality of ballast compartments within said hull, and (c) a ballast structure within said hull and underneath said plurality of ballast compartments;
wherein said platform is floatable on a body of water when said ballast compartments are de-ballasted, and said platform is adapted to descend to the bottom of said body of water when said ballast compartments are ballasted.

2. A platform according to claim 1 wherein said ballast structure accounts for a major proportion of the total weight of said platform.

3. A platform according to claim 1 or 2 further including a plurality of feet under said hull, each said foot including anti-scour means.

4. A platform according to any of the preceding claims wherein said ballast compartments are adapted to be ballasted and de-ballasted independently to control the buoyancy of said platform and descent and ascent of said platform in said body of water.

5. A platform according to any of the preceding claims wherein said ballast compartments are rigid, impermeable to water and immune to the external hydrostatic pressures imposed by water depth.

6. A platform according to any of the preceding claims wherein said ballast compartments are within venturi-like structures.

7. A platform according to any of the preceding claims wherein a liquid is deliverable into said ballast compartment(s) to ballast said ballast compartment(s) and said liquid is deliverable out of said ballast compartment(s) to de-ballast said ballast compartment(s).

8. A platform according to claim 7 wherein each said ballast compartment is connected with a respective liquid conduit and a respective air conduit, wherein said liquid is deliverable into and out of said ballast compartment via said liquid conduit, and wherein air is removed from and drawn into said ballast compartment via said air conduit.

9. A platform according to claim 7 or 8 wherein said liquid has a density at least equalling to the density of seawater.

10. A platform according to claim 8 or 9 wherein said liquid conduits and said air conduits extend to the surface of the body of water for attachment to a buoy.

11. A platform according to claim 10 wherein said liquid conduits and said air conduits are held down by a clump of weight or a submerged buoy at a distance away from said platform prior to extending to the surface of said body of water for attachment to a buoy.

12. A platform according to any one of claims 8 to 11, wherein said liquid is deliverable into and out of each said ballast compartment via said liquid conduit by a liquid pump on board a vessel.

13. A platform according to any of the preceding claims wherein said hull is releasably connectable with at least one vessel by an elongate connecting member.

14. A platform according to any of the preceding claims, further including incline angle sensors, acceleration sensors and imaging sonar.

15. A tidal turbine assembly including at least one tidal turbine mounted to a platform according to any of the preceding claims.

16. An assembly according to claim 15 wherein the centre of gravity of said tidal turbine assembly is below the centre of buoyancy of said tidal turbine assembly.

17. A platform assembly including at least two platforms according to any one of claims 1 to 14, wherein said hulls of said at least two platforms are releasably connected with each other by at least one elongate connecting member.

18. A tidal turbine assembly including at least one tidal turbine mounted to a platform assembly according to claim 17.

19. A platform for supporting a tidal turbine, said platform including:
(a) at least one pontoon, (b) at least two columns constrained to said pontoon, and (c) a ballast compartment mounted at a bottom end of and between said columns;
wherein at least one tidal turbine is mountable between said columns and above said ballast compartment, and wherein said columns are adapted to move, when said ballast compartment is ballasted, relative to said pontoon to a first position to submerge said tidal turbine into said body of water along a pre-defined path and are adapted to move, when said ballast compartment is de-ballasted, relative to said pontoon to a second position to raise said tidal turbine to the surface of said body of water along said pre-defined path.

20. A platform according to claim 19 wherein said columns are movable downward and upward relative to said pontoon to move between said first and second positions.

21. A platform according to claim 19 wherein said columns are rotatable relative to said pontoon in a first position to move to said second position and are rotatable relative to said pontoon in a second position to move to said first position.

22. A platform according to any one of claims 19 to 21 wherein said platform is moorable or anchorable to the bottom of said body of water.

23. A platform according to any one of claims 19 to 22 wherein power conversion equipment is housed within said hull.

24. A platform according to any one of claims 19 to 23 including a plurality of pontoons which are spaced apart from each other.

25. A tidal turbine assembly including at least one tidal turbine mounted to a platform according to any one of claims 19 to 24.

26. A platform for supporting a tidal turbine, said platform including:
(a) at least one pontoon, (b) at least two columns fixed to said pontoon, and (c) a ballast compartment constrained between said columns;
wherein at least one tidal turbine is constrainable between said columns and mountable above said ballast compartment; and wherein said tidal turbine and said ballast compartment are adapted to move, when said ballast compartment is ballasted, relative to said pontoon to a first position to submerge said tidal turbine into a body of water along a pre-defined path and are adapted to move, when said ballast compartment is de-ballasted, relative to said pontoon to a second position to raise said tidal turbine to the surface of said body of water along said pre-defined path.