Improved Turret Mooring System

Abstract

Disclosed herein is a turret mooring system for a tidal turbine assembly 150 that increases the ratio of torque exerted by the turret 100 to frictional forces between the turret 100 and an assembly 150 moored to the turret 100. In some examples, frictional forces are reduced by the turret exerting an upward force on the assembly, which also resists pitching moments. In other examples, the torque exerted by the turret 100 is increased through the use of a bifurcated mooring line 106, 108 that connects to two spaced apart attachment points 128 on the chain table 104 of the turret 100 and a single point 114 on the water bottom.

Description

Technical Field

[0001] The present invention relates to a turret mooring system, and more particularly to a turret mooring system adapted to use a turbine assembly to extract energy from tidal currents.

Background Art

[0002] Turret mooring is a mooring that allows a moored vessel to rotate (about the yaw axis) in response to local currents. Generally, turret mooring is used where there is no dominant flow direction, meaning that the moored vessel sways in response to local wind and wave forces. In principle, since these flows and forces can come from any direction, the turret mooring facility forms a substantially circular arrangement converging on a central point (the turret) and is fixed to the seabed to resist drift in all directions caused by wind and waves.

[0003] In a tidal power generation system, the flow is strong and bidirectional, i.e., the flow is strong in a first direction when the tide comes in, substantially zero at high tide, strong in a second direction (substantially opposite to the first direction) when the tide goes out, and substantially zero at low tide, at which point the cycle repeats. Therefore, turret mooring seems to be an ideal candidate for a tidal power generation system. This is because the power system can rotate by turret mooring to align with the current flow, and only two fixed points on the seabed, i.e., an upstream fixed point and a downstream fixed point, are required to resist the drift motion introduced by the flow in each of the two main directions that make up the bidirectional local flow.

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, typical turret mooring systems are usually installed in relatively deep sea locations. For example, in this case, the average water depth d M =(d H +dL ) / 2 is the change in depth δd = d H -d L is much larger than, where d M , δd, d H and d L are the average water depth, change in depth, depth at high tide, and depth at low tide, respectively. In such a case, the rise and fall of the moored ship are small compared to the general distance from the water bottom to the ship, and the form of the mooring system does not change significantly over the tidal cycle.

[0005] This generally does not apply to the location where the tidal power generation system is moored. Here, a relatively large tidal range is selected to maximize the power generation potential. This usually means fixing the assembly near the shore. This is because the coastline is related to the geographical features that tend to produce large tidal ranges and currents. Furthermore, installing the assembly near the shore simplifies the installation and also facilitates the transmission of the generated power to the shore. As a result, the ratio of δd to d M tends to be relatively large, which causes the mooring line to slack at low tide. It has been found that a slack line is insufficient to resist rotation and may not be able to keep the turret stationary with respect to the water bottom. As a whole result, as the tide changes, instead of the assembly rotating around the turret to match the local flow, the entire assembly including the turret yaws. Over several cycles, this may cause the mooring lines to become entangled or twisted. Due to the strong bidirectionality of the flow, increasing the number of mooring lines has little or no effect in resisting entanglement. In fact, apart from the increased cost and complexity of fixing additional mooring lines to the water bottom, more mooring lines may worsen the entanglement because more mooring lines are likely to easily entangle with each other, so more mooring lines are not desirable.

Means for Solving the Problem

[0006] The present invention aims to address the above problems.

[0007] A series of closely related solutions to the above problems are presented herein.

[0008] Disclosed herein is a turret mooring system for a tidal turbine assembly, the turret mooring system comprising a turret having a shaft for attachment to a tidal turbine assembly and for enabling relative movement between the turret and the tidal turbine assembly about a rotational axis, and a chain table fixed to a lower end of the shaft, an upstream mooring line for coupling a single upstream fixed point on the seabed to an upstream portion of the chain table, and a downstream mooring line for coupling a single downstream fixed point on the seabed to a downstream portion of the chain table, wherein the upstream mooring line is a bifurcated mooring line for coupling to two spaced upstream attachment points on the upstream portion of the chain table and / or the downstream mooring line is a bifurcated mooring line for coupling to two spaced downstream attachment points on the downstream portion of the chain table.

[0009] As used herein, the terms "upstream" and "downstream" broadly refer to the dominant flow directions in the rising and falling tides, which are in substantially opposite directions. It is clear that as the tide changes from ebb to flood (and vice versa), the upstream and downstream directions switch. However, there is always a clearly defined upstream (and downstream) direction. Further, the dominant flow direction is a horizontal axis that defines an upstream-downstream axis, which may also be referred to as the x-direction or the x-axis. In the following description, the terms "front", "fore", "forward", "bow" and related terms shall refer to the upstream direction and the positive x-direction. Similarly, the terms "rear", "aft", "rearward", "backward", "stern" and related terms shall refer to the downstream direction and the negative x-direction.

[0010] The spaced attachment points are spaced in a direction (also referred to as the spacing axis, y - direction, or y - axis) that traverses the horizontal and upstream - downstream axis. For example, the spacing may be perpendicular to the upstream - downstream axis. The positive and negative directions on the y - axis shall be referred to as “right” or “starboard” and “left” or “port,” respectively. Thus, the x - axis and the y - axis collectively form a horizontal plane (wherein, horizontal is broadly meant to be perpendicular to the local gravitational field). In this coordinate system, the turret shaft extends in a vertical direction (also sometimes referred to as the z - axis), where vertical is broadly meant to be parallel to the local gravitational field). The positive direction of the z - axis is referred to by terms such as “up,” “top,” “upper,” “high,” “higher,” etc. Similarly, the negative z - axis direction is referred to as “down,” “bottom,” “lower,” “low,” etc., and clearly, in accordance with common usage, the part that extends out of the water is higher than the part that is immersed in the water (when the assembly is fixed in place). Thus, the three axes (x, y, and z) form a convenient and coherent coordinate system for consistently describing how the various parts of the system are spatially related to each other.

[0011] When the assembly is installed, the turret is attached to the assembly such that the coupling is arranged to allow relative rotation between the turret and the tidal turbine assembly. Further, the upstream and downstream mooring lines are coupled to respective attachment points on the chain table, and the spacing (in the y - direction) between the attachment points on the chain table increases the lever arm of the mooring system on the turret. To rotate the turret in both directions with respect to the seabed, the turret needs to act against the tension of one of the two branch strands of the bifurcated mooring line. Thereby, the tension in the branch strand resists the rotation of the turret. This addresses the problem that only two fixed points on the seabed provide insufficient torque between the turret and the assembly to overcome internal friction and cause relative rotational movement between the turret and the assembly by using a bifurcated mooring line to separate the attachment points, and to increase the torque applied by the turret such that the tension in the mooring line exists to resist rotation in both directions. In other words, the separated attachment points and the bifurcated mooring line hold the turret in a stable state and can prevent the turret from rotating with respect to the seabed (meaning the turbine assembly rotates with respect to the turret and the seabed).

[0012] Furthermore, the use of a bifurcated mooring line enables this effect to be achieved with only a single fixed point on the seabed for each mooring line, thus reducing the installation cost without compromising the stability of the turret against rotation with respect to the seabed. This helps to reduce the likelihood of the mooring system getting entangled.

[0013] Optionally, the bifurcation occurs closer to the turret than to the seabed (or where the mooring cable is located). For example, the bifurcation (or mooring cable) is located at 90% or more along the length of the mooring line as measured from the seabed.

[0014] In some cases, depending on the local topography and tidal current patterns, for example, when the tidal current is asymmetric, it may be possible to use only a single bifurcated mooring line. As a result, the above entanglement problem is most likely to occur in one flow direction, but is very unlikely to occur in the other flow direction. As an example, when the tide begins to ebb, the mooring line tends to be relatively taut, so the turret is held moderately firmly and the entanglement problem is not so serious. In contrast, when the tide comes in, both lines slacken and the problem is at its worst. However, due to the direction of the flow, the tidal turbine assembly is pushed in the downstream direction and the upstream line becomes taut. In this case, in order to achieve the desired effect, it may be necessary to bifurcate only the mooring line on the upstream side when the tide comes in. In other cases, a suitable design may have a bifurcation only in the downstream mooring line (when the tide comes in). Of course, in many applications, bifurcating both mooring lines provides a good level of resistance to the rotating turret against the seabed (thus preventing entanglement).

[0015] Optionally, the spaced upstream and / or downstream attachment points on the chain table are at least 1 meter apart. A relatively large spacing increases the leverage in the sense that an increase in the tension of one of the two branched strands of the bifurcated mooring line due to a given rotation angle of the turret increases with an increase in the mooring line spacing, thereby improving the rotational resistance of the turret against the seabed.

[0016] Optionally, the upstream mooring line has a first portion for coupling an upstream fixed point on the sea floor to the mooring cable and a second portion for coupling the mooring cable to two upstream attachment points on the upstream portion of the chain table, and / or the downstream mooring line has a first portion for coupling a downstream fixed point on the sea floor to the mooring cable and a second portion for coupling the mooring cable to two downstream attachment points on the downstream portion of the chain table. In other words, the second portion of the mooring line comprises a pair of lines providing a parallel load path, one of which is connected to each of the spaced attachment points on the chain table while branching away from each other. The mooring cable has three attachment points, one for connecting to the first portion of the mooring line and two for the second portion. The mooring cable is the portion that allows the mooring line to split into two branches. Optionally, the first and second portions of the upstream or downstream mooring line are formed from different materials. This allows the lower portion of the mooring line (connected to the fixed point) and the upper portion of the mooring line (connected to the chain table) to be designed in a manner specific to the roles they are each to play.

[0017] For example, the first portion of the upstream or downstream mooring line may be a chain and / or the second portion of the upstream or downstream mooring line may be a pair of low-mass synthetic cables. The chain connection forms a strong connection, which can be useful when abrasion resistance is important, for example when the line may come into contact with the water bottom, while the low-mass synthetic cables provide a lightweight and adaptable mooring system that improves handling characteristics.

[0018] Optionally, the turret is configured to provide an upward force when the chain table is immersed in water. The upward force can resist the downward force caused by the weight of the turret (resulting from the turret mass) and the downward component of the mooring line tension. This helps to reduce rotational friction and thus enables the tidal turbine assembly to rotate relative to the turret, while also reducing the likelihood of entanglement.

[0019] Optionally, the upward force is provided by an element that has buoyancy in water. This buoyancy provides an upward force that exists regardless of the water flow. In particular, the worst point for entanglement problems is the slack low tide when the tidal flow is 0 and no other local flows can exist. Providing buoyancy elements (such as air pockets, foams, filled regions, etc.) ensures that the aforementioned friction reduction effect exists even when there is no flow. The buoyancy element may be housed within the chain table, thereby enabling an upward force to be applied to the base of the shaft to lift the shaft vertically from below and avoid introducing torsional effects. In other examples, the buoyancy element may be located on the lower portion of the shaft. The buoyancy element can be selected to have the desired buoyancy by choosing a material suitable for the application. Additionally, the expected density of the water at the installation location can be included in the determination.

[0020] The element may have variable buoyancy. For example, the buoyancy can be changed to vary the attitude and / or orientation of the tidal turbine assembly in the water. This change can be cycled, for example, to adapt to known, predicted, regular, or periodic effects. In other examples, the buoyancy can be adaptively changed, in which case the attitude and / or orientation of the tidal turbine assembly is measured and the buoyancy of the element can be adjusted to cause the desired change in attitude and / or orientation. The buoyancy of the element may be variable through the shaft, for example, by pumping air into a buoyancy element, such as into the chain table. The use of air is a convenient option as air is abundant and easily accessible. The shaft provides a convenient way to deliver air to the chain table.

[0021] In addition to or instead of this, the upward force may be provided by a hydrodynamic fairing on the chain table. The fairing may cover only all or part of the chain table. The use of the hydrodynamic fairing provides the strongest upward force when the flow is strongest, and in this case, the exact relationship depends on the shape of the hydrodynamic fairing. This can help prevent the entanglement problems identified above, but can also help keep the tidal turbine assembly in a stable state during power generation. This is because the turbine that is to be driven produces a large time-varying thrust (time-varying in the sense of depending on the time-dependent flow velocity). This thrust generates a pitching moment. The hydrodynamic surface enables the turret to resist the pitching moment. Since both the hydrodynamic lift and the turbine thrust depend on the flow velocity, the hydrodynamic fairing provides a convenient passive stabilization effect. The hydrodynamic fairing may be further shaped to resist the expected pitching moment in the tidal turbine assembly due to the drag force acting on the turbine. In other words, the fairing may be formed to provide an upward force profile that is an upward force profile with respect to the flow velocity and that substantially matches the profile of the thrust force generated by the turbine with respect to the flow velocity.

[0022] In combination with the fixed buoyancy, the upward force from the buoyancy and the predetermined form of the hydrodynamic fairing have the form of F Z = B + L(v), where B is a constant upward force due to a predetermined buoyancy and L(v) represents a variable lift that is a function of at least the flow velocity v and the shape of the fairing. If the buoyancy is variable, the equation becomes F(t) = B(t) + L(v), and the total upward force and the buoyancy depend on time (t). The lift from the hydrodynamic fairing also depends on time, for example, through the time-dependence of the flow velocity caused by tidal changes and local flows. These basic equations can be used to guide the design of both the buoyancy element and the hydrodynamic fairing to achieve the desired effect.

[0023] In particular, it can be difficult to achieve an exact cancellation of the pitching moment due to turbine thrust (with respect to the magnitude of the force and / or the force profile with respect to flow velocity) using only hydrodynamic fairings. In such cases, a predetermined buoyancy can be used to bring the hydrodynamic lift closer to a complete reaction to the pitching moment.

[0024] Furthermore, if the profile of the hydrodynamic lift (the flow velocity dependence of the lift) does not match the profile of the turbine thrust, variable buoyancy can be used to improve the match by conforming to the expected flow conditions or by conforming based on live measurements.

[0025] In some examples, it may be desirable to use a predetermined buoyancy to precisely balance the turret mass and the downward component of the mooring line tension at low tide such that the vertical frictional force is substantially zero. However, this, in combination with the hydrodynamic lift when the tidal current is flowing, can cause the upward force to exceed the downward force acting on the turret due to the mass of the turret and the downward component of the mooring line tension. This can lift the turret locally from the tidal turbine assembly, canceling out the pitching moment due to turbine thrust and, in some cases, completely canceling it. This introduces rotational friction into the relative rotational movement between the turret and the tidal turbine assembly. However, generally, the state where the upward force cancels the pitching moment is a state where relative rotation is not substantially expected because the flow usually does not change direction when the tide is rising or falling. In practice, the increased friction can help keep the turbine assembly stable in the flow by resisting rotation while allowing for many of the desired pitch balancing lift and buoyancy to be supplied. In this way, the buoyancy and lift can be matched to the expected pitching moment from the turbine as a function of flow velocity.

[0026] The hydrodynamic fairing may be bidirectional in the sense that it is configured to provide lift when water flows over the fairing in each of two main flow directions. In some cases, the hydrodynamic fairing is symmetric in the upstream-downstream direction. Generally, a symmetric fairing has lower performance (less lift generated) than a unidirectional fairing, but in this case where the flow is expected to be along two dominant directions, a stable system of bidirectional flow can exceed the greater lift that can be generated in only one direction. If a fairing is provided with a predetermined or variable buoyancy element, the buoyancy element can be housed inside the fairing.

[0027] Also disclosed herein is a turret mooring system for a tidal turbine assembly, comprising a turret, the turret having a shaft for attachment to the tidal turbine assembly and for enabling relative movement between the turret and the tidal turbine assembly about a rotational axis, and a chain table fixed to a lower end of the shaft, the turret being configured to provide an upward force when the chain table is immersed in water. The upward force can resist the downward force due to the weight of the turret (resulting from the turret mass) and the downward component of the mooring line tension. This helps to reduce rotational friction and thus enables the tidal turbine assembly to rotate relative to the turret, and also reduces the likelihood of entanglement.

[0028] This addresses the problem that only two fixed points on the seabed provide insufficient torque between the turret and the assembly to overcome internal friction and result in relative rotational movement between the turret and the tidal turbine assembly by reducing the internal friction in the operation between the turret and the tidal turbine assembly. This is caused by the upward force provided by the turret reducing the frictional component in the vertical direction. Since the frictional force that hinders the relative rotation of the turret and the turbine assembly is reduced, the relative rotational movement becomes easier, and the turbine assembly can rotate relative to the turret and the seabed.

[0029] Optionally, the upward force is provided by an element that has buoyancy in water. This buoyancy provides an upward force that exists regardless of the water flow. In particular, the worst point for entanglement problems is the slack low tide when the tidal current is zero and no other local flow can exist. Providing buoyancy elements (e.g., air pockets, foams, filled regions, etc.) allows the aforementioned friction reduction effect to exist even in the absence of flow. The buoyancy elements may be housed within the chain table, whereby an upward force can be applied to the base of the shaft to lift the shaft straight up and avoid the introduction of torsional action. In other examples, the buoyancy elements may be located on the lower portion of the shaft. The buoyancy elements can be selected to have the desired buoyancy by choosing a material that is suitable for the application. Additionally, the expected density of the water at the installation location can be included in the determination.

[0030] The element may have variable buoyancy. For example, the buoyancy can be varied to change the attitude and / or orientation of a tidal turbine assembly in water. This change can be made cyclically, for example, to adapt to known, predicted, regular, or periodic effects. In other examples, the buoyancy can be changed adaptively, in which case the attitude and / or orientation of the tidal turbine assembly is measured and the buoyancy of the element can be adjusted to cause the desired change in attitude and / or orientation. The buoyancy of the element may be variable through the shaft, for example, by pumping air into a buoyancy element, such as into a chain table. The use of air is a convenient option because air is abundant and readily available. The shaft provides a convenient way to deliver the air to the chain table.

[0031] In addition to or instead of this, the upward force may be provided by a hydrodynamic fairing on the chain table. The fairing may cover only all or part of the chain table. The use of the hydrodynamic fairing provides the strongest upward force when the flow is strongest, in which case the exact relationship depends on the shape of the hydrodynamic fairing. This can help prevent the entanglement problems identified above, but can also help keep the tidal turbine assembly in a stable state during power generation. This is because the turbine that is to be driven produces a large time-varying thrust (time-varying in the sense of depending on the time-dependent flow velocity). This thrust generates a pitching moment. The hydrodynamic surface allows the turret to resist the pitching moment. Since both the hydrodynamic lift and the turbine thrust depend on the flow velocity, the hydrodynamic fairing provides a convenient passive stabilization effect. The hydrodynamic fairing may be further shaped to resist the expected pitching moment in the tidal turbine assembly due to the drag force acting on the turbine. In other words, the fairing may be formed to provide an upward force profile that is an upward force profile with respect to the flow velocity and that substantially matches the profile of the thrust force generated by the turbine with respect to the flow velocity.

[0032] In combination with a fixed buoyancy, the upward force from the buoyancy and the predetermined form of the hydrodynamic fairing have the form of F Z = B + L(v), where B is a constant upward force due to a predetermined buoyancy and L(v) represents a variable lift that is a function of at least the flow velocity v and the shape of the fairing. If the buoyancy is variable, the equation becomes F(t) = B(t) + L(v), and the total upward force and the buoyancy depend on time (t). The lift from the hydrodynamic fairing also depends on time, for example, through the time-dependence of the flow velocity due to tidal changes and local flows. These basic equations can be used to guide the design of both the buoyancy element and the hydrodynamic fairing to achieve the desired effect.

[0033] In particular, it can be difficult to achieve an exact cancellation of the pitching moment due to turbine thrust (with respect to the magnitude of the force and / or the force profile with respect to the flow velocity) using only hydrodynamic fairings. In such cases, a given buoyancy can be used to bring the hydrodynamic lift closer to a complete reaction to the pitching moment. Further, if the profile of the hydrodynamic lift (the flow velocity dependence of the lift) does not match the profile of the turbine thrust, variable buoyancy can be used to improve the match by conforming to the expected flow conditions or by conforming based on live measurements.

[0034] In some examples, it may be desirable to use a given buoyancy to exactly balance the turret mass and the downward component of the mooring line tension at low tide so that the vertical frictional force is substantially zero. However, this, in combination with the hydrodynamic lift when the tidal current is flowing, can cause the upward force to exceed the downward force acting on the turret due to the mass of the turret and the downward component of the mooring line tension. This causes the turret to locally lift the tidal turbine assembly, canceling out the pitching moment due to turbine thrust and, in some cases, completely canceling it. This introduces rotational friction into the relative rotational movement between the turret and the tidal turbine assembly. Generally, however, the state where the upward force cancels the pitching moment is a state where relative rotation is not substantially expected because the flow usually does not change direction when the tide is rising or falling. In practice, the increased friction can help keep the turbine assembly stable in the flow by resisting rotation while allowing for many of the desired pitch equilibrium lift and buoyancy to be supplied. In this way, the buoyancy and lift can be matched to the expected pitching moment from the turbine as a function of the flow velocity.

[0035] The hydrodynamic fairing may be bidirectional in the sense that it is configured to provide lift when water flows over the fairing in each of two main flow directions. In some cases, the hydrodynamic fairing is symmetric in the upstream-downstream direction. Generally, a symmetric fairing has lower performance (less lift generated) than a unidirectional fairing, but in this case where the flow is expected to be along two dominant directions, a stable system of bidirectional flow can exceed the greater lift that can be generated in only one direction. If a fairing is provided with a predetermined or variable buoyancy element, the buoyancy element can be housed inside the fairing.

[0036] Optionally, the upstream portion of the chain table is configured to be coupled to a single upstream fixed point on the bottom of the water via an upstream mooring line, and the downstream portion of the chain table is configured to be coupled to a single downstream fixed point on the bottom of the water via a downstream mooring line. This example may further include upstream and downstream mooring lines. Thereby, a complete mooring system is provided.

[0037] Optionally, 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 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. To rotate the turret in both directions with respect to the bottom of the water, the turret needs to act against the tension of one of the two branch strands of the bifurcated mooring line. Thereby, the tension of the branch strand resists the rotation of the turret, and as a result, reduces the possibility of entanglement.

[0038] Optionally, the bifurcation occurs closer to the turret than to the bottom of the water (or where the mooring cable is located). For example, the bifurcation (or mooring cable) is located at 90% or more along the length of the mooring line as measured from the bottom of the water.

[0039] Optionally, the spaced-apart upstream and / or downstream attachment points on the chain table are at least 1 meter apart. This increases the lever action in the sense that an increase in the tension of one of the two branched strands of the bifurcated mooring line due to a given rotation angle of the turret increases with an increase in the mooring line spacing, thereby improving the rotational resistance of the turret to the seabed. In combination with a reduction in rotational friction, this provides a system that is less prone to entanglement.

[0040] Optionally, the upstream mooring line has a first portion for connecting an upstream fixed point on the seabed to the mooring cable and a second portion for connecting the mooring cable to two upstream attachment points on the upstream portion of the chain table, and / or the downstream mooring line has a first portion for connecting a downstream fixed point on the seabed to the mooring cable and a second portion for connecting the mooring cable to two downstream attachment points on the downstream portion of the chain table. In other words, the second portion of the mooring line comprises a pair of lines providing a parallel load path, one of which is connected to each of the spaced-apart attachment points on the chain table while branching from each other. The mooring cable has three attachment points, one for connecting to the first portion of the mooring line and two for the second portion. The mooring cable is the portion that allows the mooring line to bifurcate. Optionally, the first and second portions of the upstream or downstream mooring line are formed from different materials. This allows the lower portion of the mooring line (connected to the fixed point) and the upper portion of the mooring line (connected to the chain table) to be designed in a manner specific to the roles they are each to fulfill.

[0041] For example, the first portion of the upstream or downstream mooring line may be a chain, and / or the second portion of the upstream or downstream mooring line may be a pair of low-mass synthetic cables. The chain connection forms a strong connection, which can be useful when abrasion resistance is important, for example, when the line may contact the water bottom, while the low-mass synthetic cables provide a lightweight and adaptable mooring system that improves handling characteristics.

[0042] The following optional features may be included in any variation of the mooring system described above.

[0043] Optionally, the shaft includes an upper radial bearing and a lower radial bearing for engaging the tidal turbine assembly. This can help provide support against torsional or overturning motions and ensure smooth rotation.

[0044] Optionally, the turret is attached to the tidal turbine assembly. This provides a complete device ready for installation and operation.

[0045] Optionally, the turret is slidable vertically relative to the tidal turbine assembly. For example, the turret may be configured to slide along a portion of the length of the shaft to raise or lower the chain table relative to the tidal turbine assembly. In some cases, this can reduce the draft for transporting the assembly.

[0046] Optionally, the shaft is a group of pipes. In other words, the shaft comprises a plurality of pipes held together to form a composite structure. The composite structure can include an outer protective casing. Forming the shaft in this way is a simple and inexpensive way to form a strong and robust shaft. Further, the pipe lumen can be used to transmit control signals between the turbine assembly and the chain table or even to the shore, convey the power generated by the assembly to the shore, and supply air to the chain table to adjust buoyancy or any other purpose function. Multiple pipes can be used to keep these roles separate in order to reduce damage to electrical cables by pumping air or to reduce crosstalk and interference between control lines or between control lines and high-capacity power lines. The hollow core of the pipe can also be used to provide buoyancy to the lower end of the shaft.

[0047] Also disclosed herein is a method of mooring a tidal turbine assembly underwater using a turret mooring system according to any one of the preceding claims, the method comprising: (i) transporting the tidal turbine assembly to an installation location; (ii) coupling an upstream portion of the chain table of the turret to the underwater at a single upstream fixed point and coupling a downstream portion of the chain table of the turret to the underwater at a single downstream fixed point; and (iii) fixing the turret to the tidal turbine assembly. Thereby, a complete assembly installed at a desired position is provided.

[0048] Optionally, step (iii) is performed before step (i) and step (ii), and step (iii) is performed at a dock or on land. Thereby, the operator can have the sometimes complex mounting process performed in a controlled environment so that safety checks can be carried out.

[0049] Optionally, when step (iii) is completed, the turret is vertically slidable relative to the tidal turbine assembly between a raised position and a lowered position, such that the draft of the tidal turbine assembly is greater when the turret is in the lowered position than when it is in the raised position. Optionally, the turret is in the raised position during step (i). This can reduce the draft of the assembly during transportation, and as a result, the transportation process can be made easier and more efficient. When the assembly reaches its destination, the turret can be lowered so that the chain table is sufficiently low below the water surface to avoid the turbine when the turbine rotates in the flow. During installation, it may also be advantageous to adjust the height of the turret, for example to adapt to changes in the water state.

[0050] Optionally, the turret is configured to apply an upward force when the chain table is immersed in water and the turret is vertically slidable, and the vertical position of the turret is adjusted before or during step (i) to control the pitch of the tidal turbine assembly. This can help to provide stability to the tidal turbine assembly during transportation.

[0051] Optionally, the tidal turbine assembly includes one or more turbines, and the turbines can be configured in a deployed configuration where they are below their expected waterline (and optionally below the hull of the turbine assembly) and in a raised configuration where the turbines are generally above their expected waterline (and optionally supported on the tidal power turbine device, such as on a deck). The turbines can be further configured in the raised configuration during step (i). This can raise the turbines during transportation and reduce the drag during towing the assembly to the installation location, thereby improving the installation efficiency.

[0052] Next, with reference to the drawings, examples and embodiments will be described in detail.

Brief Description of the Drawings

[0053]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

DETAILED DESCRIPTION OF THE INVENTION

[0054] FIG. 1 shows a tidal turbine assembly 150 moored to the seabed (not shown). The tidal turbine assembly 150 includes a buoyant, substantially rigid body 152 enclosed by a hull to enable the assembly to float on the water surface. The assembly 150 is shown from a side view having a conveniently marked x-axis and z-axis, where in this case the arrows point in the positive direction of the axes. The y-axis is perpendicular to both the x-axis and the z-axis (and thus also perpendicular to the plane of the image). Toward the rear or stern (negative x-direction) of the assembly 150, a turbine deployment module 154 is arranged to raise and lower one or more turbines 158 from or into the water. The turbine 158 is attached to the distal end of a beam 156, and the beam 156 is driven by the turbine deployment module 154 to swing upward from the water to raise the turbine 158 out of the water. This can help reduce the draft of the assembly 150 and facilitate the transport of the assembly 150 to or from the installation site. In other examples, the turbine 158 can be raised out of the water to prevent damage from rough seas or to repair or inspect the turbine 158. In either case, the turbine deployment module 154 can be caused to act in reverse to swing the turbine 158 back into the water, for example, under the hull, so that power generation can be started (or in some cases, restarted).

[0055] Toward the front or bow (positive x-direction) of the turbine assembly 150, there is a turret 100 that forms part of the mooring system. The turret mooring system includes a shaft 102 fixed to the tidal turbine assembly 150. The shaft 102 is attached to the tidal turbine assembly 150 to enable relative rotational movement between the turret 100 and the tidal turbine assembly 150. A chain table 104 is fixed to the lower end (negative z-axis direction) of the shaft 102. The operations of the chain table 104 and the shaft 102 are both of substantially rigid structures and are coupled to each other such that rotation and vertical movement of one of the chain table 104 and the shaft 102 cause the same movement of the other of the chain table 104 and the shaft 102.

[0056] The upstream mooring line 106 and the downstream mooring line 108 couple the chain table 104 to the water bottom at respective upstream and downstream fixed points (not shown). The chain table 104 is fixed to the water bottom such that the mooring lines 106, 108 resist the rotation of the chain table 104 with respect to the water bottom. This means that the shaft 102, and in fact the entire turret 100, is held in place above the water bottom and is prevented from rotating by the tension of the mooring lines 106, 108. By holding the turret 100 firmly in this way, the tidal turbine assembly 150 can be rotated with respect to the water bottom, for example to adapt to changing local flows.

[0057] In particular, the assembly 150 yaws by approximately 180° (rotates about the z-axis) as the tide changes direction. Also, the turret mooring system 100 allows the tidal turbine assembly 150 to adjust its orientation so that the turbine 158 is better aligned with the flow vector of the local flow, for example due to local flows, when the direction of the tidal flow is not the same in each cycle. This is an improvement over fixed mooring systems that can hold the assembly 150 firmly in a fixed orientation or attitude, but doing so necessarily sacrifices the ability to adapt to changes in the flow direction.

[0058] The tidal turbine assembly 150 is shown in use such that the flow (from the positive x-direction to the negative x-direction) drives the turbine 158. The figure shows the various forces involved in this process. In particular, the drag from the hull, the drag from the beam, and the turbine thrust combine to form the net drag load F D The net drag load acts from the effective drag center that is very close to the turbine 158 due to the turbine thrust dominating the drag.

[0059] The center of gravity is marked as CoG, and the total mass M of the tidal turbine assembly 150 acts downward through the CoG. Similarly, the buoyancy B of the tidal turbine assembly Aacts upward through the longitudinal center of buoyancy LCB. Note that generally CoG and LCB are not located at the same point.

[0060] The force on the turret is the net vertical mooring load T due to the downward force applied by the tensions in the mooring lines 106, 108 Z and. Further, due to the horizontal components of the tensions in the mooring lines 106, 108, there is a net horizontal mooring load T X exists. Clearly, if the tidal turbine device 150 remains at a given position and does not sink, B A = M, and T X = F D which can be easily achieved by careful design of the tidal turbine assembly 150, the turret 100, and the mooring lines 106, 108. However, this constraint only satisfies static conditions. When there is water flow, if only the above forces act, the moments generated by each of these forces can be calculated. Since the forces generally do not pass through the longitudinal center of buoyancy, there is a tendency to cause rotation around the longitudinal center of buoyancy.

[0061] Taking the clockwise direction as positive (the direction in which the positive z - direction maps to the positive x - direction along the shortest arc), these moments are as follows. The net drag force F D causes a moment F D ·z2, where z2 is the vertical offset between the effective point of action of the net drag force and the longitudinal center of buoyancy. The mass M of the tidal turbine assembly 150 acts to give a moment of - M·x2, where x2 is the horizontal (in the x - direction) separation between the center of gravity and the longitudinal center of buoyancy.

[0062] Finally, the turret gives a moment of T Z· x1 - T X z1, where x1 is the horizontal (in the x - direction) separation between the turret 100 and the longitudinal center of buoyancy, and z1 is the vertical separation between the chain table (specifically, the position of the connection between the chain table 104 and the mooring lines 106, 108) and the longitudinal center of buoyancy.

[0063] A stable arrangement (an arrangement that does not rotate due to forces) is [Number] the arrangement where m i represents the moment from each of the above sound sources. Otherwise, the tidal turbine assembly 150 rotates due to a force acting along a line that does not pass through the longitudinal center of buoyancy. The tidal turbine assembly 150 rotates clockwise when Σm i is positive and counterclockwise when this sum is negative. This changes the orientation of the assembly 150 in water and also changes the values of x1, x2, z1, z2. The rotation continues until a stable form is found. For example, the assembly 150 may be in a stable form when there is no flowing tidal current, which means that F D and T x are zero. As the current flow rate increases, the forces F D and T x increase, but due to the dynamics of the mooring lines, they are not necessarily equal (for example, it means that the assembly 150 drifts slightly backward until equilibrium is restored). Similarly, generally, z1≠z2, and the net result is that the assembly 150 rotates because the moments introduced due to the flow of the current do not match. Usually, the result is a clockwise rotation, driving the bow (positive x - direction) of the assembly 150 into the water while raising the stern (negative x - direction) and the turbine(s) 158. Since it affects power generation, it is clearly a problem when a significant portion of the turbine 158 rises out of the water. Even if the turbine 158 remains fully submerged, it is important to resist this movement because the turbine 158 may no longer be in the optimal part of the flow for generating power.

[0064] The present invention addresses this problem by applying an additional moment to the turret 100. Specifically, the turret 100 is configured to apply an upward force to the tidal turbine assembly 150. In some examples, this is a predetermined buoyancy, and in other examples it is a variable buoyancy. In yet further examples, it is a hydrodynamic fairing on the chain table 104, and in some examples it is a combination of two or more of these. Applying an upward force with the turret 100 changes the moment equation to Σ i m i =F D z2+(T z -L-B)x1T x z1-Mx2, where in this case, L and B are the hydrodynamic lift and buoyancy, respectively. These can be selected to resist pitching (rotation about the y-axis) caused by the turbine 158. In particular, since both L and F D vary with the flow velocity, the hydrodynamic surface can be designed to provide an L that dynamically cancels the pitching moment caused by the turbine thrust. The buoyancy can be variable to provide an additional time-varying force to fine-tune the moment balance. In other words, by applying an upward force with the turret 100, the orientation of the tidal turbine assembly 150 can be changed, resulting in a stable configuration. In particular, the stable configuration can be brought closer to a flat orientation (the bow-stern line is broadly horizontal).

[0065] Although not shown in detail in FIG. 1, the turret 100 may be configured to slide vertically with respect to the tidal turbine device 150. This can enable further adjustment of the stable position of the device 150 by changing the z1 distance and thus the magnitude of the moment due to the x - component of the mooring load generated by the turret 100. Also, this vertical movement can be useful for reducing the draft of the assembly 150 while transporting the assembly 150 to the installation site by raising the turret 100 while transporting the assembly 150 to the installation site. This enables the turret 100 to be installed on the assembly 150 in dry land or in relatively calm waters of a dock or harbor without affecting the efficiency of the transport process. When the assembly 150 arrives at the installation site, the mooring lines 106, 108 can be used to fix the turret 100 to the seabed. In other examples, the turret 100 can be fixed to the seabed before the assembly 150 arrives at the installation site.

[0066] Also not shown in detail in FIG. 1 is a power transmission device that enables the power generated by the turbine 158 to be transmitted ashore. This configuration includes slip rings between the turbine assembly 150 and the turret 100, thereby enabling power to be safely transmitted to the turret 100 via a rotary connection between the turret 100 and the assembly 150. The power is transmitted ashore by a riser cable (not shown) extending along the mooring lines 106, 108 and can extend along the seabed to the shore. One or more slip rings can also be used to control the transmission of control signals to the turret as needed. Alternatively, short - range wireless communication can be used for control signals between the assembly 150 and the turret 100.

[0067] Although FIG. 1 does not show this in detail, in some cases, the turret 100 may be slidable in a direction perpendicular (or z) to the assembly 150 over at least a part of the length of the shaft 102. In such a case, the sliding operation can be selectively locked so as to be able to maintain the turret 100 at a predetermined height with respect to the assembly 150. As described elsewhere in this specification, the turret 100 can be configured to apply an upward force to the assembly 150. The locking mechanism (not shown) has sufficient strength to resist relative movement between the turret 100 and the assembly 150 such that when the locking mechanism is locked, such an upward force is fully transmitted to the assembly 150 even when applied.

[0068] Referring now to FIG. 2, the turret 100 is shown in more detail while mounted on the tidal turbine assembly 150. As described above, the forces L and B acting on the turret 100 are shown, representing the hydrodynamic lift effect and the buoyancy of the turret 100, respectively. The force T1 represents the tension in the upstream mooring line 106, and T2 represents the tension in the downstream mooring line 108. These are resolved into x (horizontal) and z (vertical) components T 1x , T 1z , T 2X , T 2Z . The mooring lines are coupled to the chain table 104 at the attachment points 128. Also shown in the figure is the weight due to the mass M of the turret.

[0069] The tidal turbine assembly 150 has a cylindrical mounting opening for receiving the turret 100. The turret 100 has an upper radial bearing 110 and a lower radial bearing 112 for engaging the inner surface of the cylindrical mounting opening. Each radial bearing 110, 112 is arranged to allow relative rotational movement between the turret 100 and the tidal turbine assembly 150. However, the various forces described above result in frictional forces between the turret 100 and the turbine assembly 150. In particular, the upper radial bearing has a radial reaction force R 1x and a vertical reaction force R 1zhas, and the lower radial bearing has a radial reaction force R 2x These reaction forces have friction coefficients of μ 1x , μ 1z and μ 2x respectively, as expressed.

[0070] The net effect of the reaction forces is to provide a frictional torque represented as Q F in Figure 2. In practice, this is the torque that must be applied between the turret 100 and the turbine assembly 150 in order for the assembly 150 to rotate relative to the turret 100 and thereby achieve the intended yawing effect. For a turret 100 having a radius r (the effective radius at which the radial bearing contacts the turbine assembly 150), the frictional torque is Q F = r · (μ 1z R 1z + μ 1x R 1x + μ 2x R 2x) as fully described.

[0071] The radial bearings 110, 112 can be designed to reduce friction as much as possible, but this is a limited benefit for the system. This is due to the bidirectionality of the tidal turbine assembly 150 and the fact that yawing occurs when the tide is low and there is no net flow. In the slack neap tide of an ideal system, since T1 = T2, R 1x = R 2X = 0, which means that the dominant contribution to the frictional force is due to the vertical component. Specifically, assuming the lift force L is 0 when the tidal current is not flowing, the frictional torque is Q F = r · μ 1z · R 1z = r · μ 1z · (T 1z + T 2z-B). In other words, when a predetermined buoyancy generates an upward force equal to the sum of the vertical components of the tensions in the mooring lines 106, 108, the frictional torque at slack low tide is minimized, and the impedance to rotation between the turret 100 and the assembly 150 is also minimized. Thus, the predetermined buoyancy directly addresses the problem that the assembly 150 cannot rotate around the turret 100. Of course, the period during which there is no current flow during tidal changes is relatively short. In such cases, there is a hydrodynamic lift force L that can be considered in the analysis to minimize the resulting frictional force. The buoyancy and the lift force, of course, also have the effect of resisting the pitching force described above.

[0072] Also, FIG. 2 shows a hydrodynamic surface 126 for providing the lift force L. More specifically, the hydrodynamic surface 126 is formed as a fairing on the chain table 104. In other cases, the hydrodynamic surface can include, in addition to or instead of this, a portion extending beyond the chain table 104, for example, an elongated wing-shaped structure extending in the y direction from the chain table 104, to increase the lift force while holding a moderately small turret 100.

[0073] In FIG. 2, the hydrodynamic surface 126 is bidirectional in the sense that lift is generated when water flows over the surface in either direction. More specifically, the surface 126 is symmetric, meaning that the lift generated at a given flow rate is the same regardless of whether the water is flowing in the positive or negative x direction. This means that the fairing 126 can be arranged so that the above-described effect of resisting the pitching moment exists regardless of the ebb and flow of the tide. In other examples, it may be beneficial for the hydrodynamic surface to behave differently in response to flow in opposite directions. A buoyancy element (an element for providing the aforementioned predetermined or variable buoyancy) can be disposed within the fairing 126, which can protect the buoyancy element from damage, and when the buoyancy is based on an air pocket in the water, the fairing can provide an outer envelope for holding air in the water.

[0074] Consider Figure 3 here. Here, a detailed view of the turret can be seen with clearly marked x, y, and z axes. The chain table 104 is generally rigid and has four attachment points 128 for coupling mooring lines to the chain table 104, which are arranged at the four corners of the chain table 104 and will be described in more detail below. The rigid chain table 104 holds the attachment points 128 apart. In this case, the pairs of upstream and downstream attachment points 128 are held apart from each other in the x - direction. Further, each pair of attachment points 128 (the upstream pair and the downstream pair) includes two attachment points 128 that are spaced apart from each other in the y - direction.

[0075] The shaft 102 appears to be formed from a group of pipes 122 surrounded by an outer casing. This provides an inexpensive and easy way to form a strong shaft 102. The general configuration of the pipes 122 and the outer casing 124 can be arranged to reduce the drag of the turret 100. In the example shown, the outer casing 124 is broadly diamond - shaped in plan view at a narrow angle along the x - direction (i.e., along the direction of flow) to streamline the shaft 102. Also, the pipes 122 themselves provide a convenient means for communicating between the chain table 104 and the assembly 150. For example, if the chain table 104 includes variable buoyancy elements, the buoyancy can be adjusted by pumping air into the chain table 104 through one of the pipes 122 that can be adapted for this purpose.

[0076] In addition to or instead of this, a separate pipe 122 can be used to carry the power generated by the turbine 158 from the assembly 150 to the shore, and to carry communication messages from / to the chain table and / or forward to / from the shore to / from the turbine assembly 150. In some examples, the communication between the shore and the assembly can enable control of various aspects of the operation of the turbine assembly 150, for example, the turbine 158 can be raised out of the water to prevent damage in rough seas, which can be advantageous when the sea is very rough and it is impractical or unsafe to physically access the assembly 150 to change its settings or configuration by boat or air.

[0077] Referring now to FIG. 4, the turret 100 of FIG. 3 coupled to the mooring line is shown in plan view. Note that FIG. 4 shows only one mooring line at the bottom of the figure, but a complete mooring system includes two mooring lines (as shown in FIGS. 1 and 2 for example). The mooring line shown in FIG. 4 corresponds to one of the mooring lines 106, 108 shown in FIGS. 1 and 2.

[0078] FIG. 4 shows a mooring line coupled to the chain table 104. The mooring line has a first portion 116 for coupling to a fixed point 114 above the water bottom at its lower end. The first portion has its upper end connected to a mooring cable 120. The mooring cable 120 has three attachment points, one of which is connected to the first portion 116. The remaining two attachment points on the mooring cable 120 are coupled to a second portion 118 of the mooring line. The second portion 118 of the mooring line includes two lines that provide parallel load paths, which branch from the mooring cable 120 and are coupled to two attachment points 128 on the chain table 104. In other words, the mooring line is a bifurcated mooring line in that it bifurcates into two attachment parts at its upper end but is fixed to a single point 114 above the water bottom.

[0079] The two attachment points 128 to which the second portion 118 of the mooring line is coupled are spaced apart in the y-direction. This allows a single mooring line to provide greater resistance to rotation of the turret 100 relative to the seabed than would be possible with a single attachment point 128 on the chain table 104. As an example, consider the rotation of the turret 100 shown in FIG. 4 relative to the seabed. Regardless of the direction in which the turret 100 rotates, one or the other of the two branched strands of the second portion 118 is under tension. This means that rotation of the turret 100 in both directions is resisted by the mooring system. As an example, the mooring line can be connected to the chain table 104 with a y-axis spacing between the attachment points 128 of about 1 meter, which has been found to be sufficient spacing in most cases to achieve the desired effect. In other examples, the spacing may be larger or smaller depending on the specific embodiment contemplated.

[0080] The first portion 116 of the mooring line is formed as a chain, thereby providing a strong connection. The second portion 118 of the mooring line is formed from a low-mass synthetic material such as aramid fiber line, e.g., Dyneema®. This allows the line to be made from a material suitable for withstanding the required tension loads without introducing excessive mass to the chain table 104. The position of the bifurcation (i.e., the mooring cable 120 in FIG. 4) is shown to be relatively close to the turret 100 (certainly, the mooring cable is closer to the turret 100 than the attachment point 114). In most cases, the mooring cable 120 or bifurcation should be located at a distance of 10% or less of the total length of the mooring line away from the turret 100, specifically from the attachment point 128 on the chain table 104 (i.e., more than 90% of the length of the mooring line measured from the fixed point 114 on the seabed).

[0081] More specifically, the first portion 116 of the mooring line is a chain, which is heavy and provides a restoring force to the turret 100. This is best understood by referring to FIG. 2, where the tensions T1 and T2 include the contributions from the weights of the respective mooring lines 106, 108. By providing a bifurcation in the mooring line, this tension is distributed between the two attachment points 128. As a result, there is an equilibrium position for the turret, and the tensions in each of the two branch strands of the second portion 118 of the mooring line create equal and opposite rotational moments about the axis of rotation of the turret 100 (due to the symmetry of the turret 100 as seen in FIG. 3), thus balancing the torque and not causing rotation. When the lengths of the two branch strands are equal, the equilibrium position is as shown in FIG. 4.

[0082] Specifically, the equilibrium position shown in FIG. 4 is the equilibrium position where a first line drawn in the direction of the first portion 116 of the mooring line (in the x - y plane) is extended until it reaches the chain table 140. This first line intersects at a right angle with a second line drawn between the two attachment points 128. In FIG. 4, it is clearly seen that when the turret 100 is rotated in either direction, the tension in one of the two branch strands increases, while the other strand slackens and the tension decreases (in some cases dramatically, even substantially to zero). For clarity, as shown in FIG. 4, a clockwise rotation of the turret 100 increases the tension in the left - hand side of the two strands, while the tension in the right - hand side strand decreases dramatically, and vice versa.

[0083] The bifurcated mooring line of FIG. 4 has advantages over a pair of independent single mooring lines each attached to a different respective point on the seabed, quite apart from the improved simplicity of installation (e.g., requiring half the number of anchor points on the seabed to be installed). Generally, with independent mooring lines, when the lines are moderately slack, different tensions are applied to each attachment point 128. This is because the slack of each line affects the tension of that line, and thus the position of the turret 100 relative to the fixed point 114 on the seabed (e.g., due to drift under the movement of waves or currents) can cause a tension difference between the two mooring lines and thus rotation of the turret 100 relative to the seabed. Thus, counterintuitively, using two independent mooring lines for this task can result in precisely the rotation that the mooring system is intended to prevent.

[0084] In contrast, the bifurcated mooring line of FIG. 4 applies the same tension up to the mooring cable 120, which means that only rotation of the turret 100 about an axis defined by a straight line from the position of the fixed point 114 and the mooring cable 120 causes a tension difference. As a result, the mooring system preferentially supports the turret 100 against rotation relative to the seabed, so the bifurcated mooring system is better at holding the turret 100 in its intended orientation.

[0085] Although not shown, the chain table 104 can be connected to a second mooring line via a further attachment point 128. The second mooring line can be of the same configuration as the first mooring line and have the same advantages. In other examples, the second mooring line may be in a simpler form having only a single attachment point 128 on the chain table 104. This is possible when the tidal flow is asymmetric, such that the aforementioned entanglement problem is most likely to occur in one flow direction but relatively unlikely to occur in the other flow direction. Also, such installation locations can utilize an asymmetric hydrodynamic fairing for much the same reason.

[0086] This mooring system having spaced attachment points 128 on the chain table 104 provides more resistance to the rotation of the turret 100 relative to the water bottom. Alone or in combination with the aforementioned system (where the upward force at the turret 100 reduces friction), this reduces the likelihood of entanglement of the mooring lines 106, 108 since the turret 100 is firmly held. In fact, combined, the effect of increasing the torque exerted between the turret 100 and the tidal turbine assembly 150 (due to the bifurcated mooring line configuration) and the effect of reducing friction (due to the upward force provided by the turret 100) both work towards the common goal of allowing the tidal turbine assembly 150 to rotate relative to the turret 100 when the flow changes direction, thereby reducing entanglement.

[0087] Figures 5 and 6 show another form of the turret 100 in perspective and plan views respectively. Figures 5 and 6 are generally equivalent to Figures 3 and 4 and common features will not be described in detail again. However, in Figures 5 and 6, there is no fairing 126 and the internal structure of the chain table 104 is visible. In particular, the comparison between Figures 3 and 4 and Figures 5 and 6 shows that most of the volume of the chain table 104 (the region inside the fairing 126) is available for storing buoyancy elements. Further, the form shown in Figures 5 and 6 can be used as shown as part of the disclosure herein. Specifically, the attachment points 128 for the mooring lines are spaced in the y - direction, which can help reduce the occurrence of entanglement as described above. In some cases, the effect provided by the mooring lines can be very strong, so no further effect provided by buoyancy or hydrodynamic lift is necessary.

[0088] As can be seen above, the present application describes a number of advantageous features generally summarized herein. First, the specific adaptation of the chain table to the turret moored tidal turbine device allows for a more stable system in the sense that the turret can resist rotation with respect to the seabed, thereby allowing the actual turbine assembly to rotate and align with the local tidal current flow.

[0089] This effect is particularly difficult to achieve in floating structures, especially floating turbine structures, due to the large draft and drag (since the turbine is below the surface). As described above, this can cause the turret to twist in the turbine device and cause twisting of the mooring lines. Furthermore, there is a risk of the turbine being swiveled into the mooring line itself, damaging the turbine, the mooring line, or both.

[0090] When applying a turret mooring system to a tidal turbine array, on the one hand, by reducing the complexity of the mooring system (especially by having fewer anchor points above the seabed), installation costs are saved, but a dilemma arises where there are fewer underwater obstacles that impede the movement of the turbine when the device swivels in changing tidal currents. On the other hand, if there are fewer mooring lines, the turret tends to be less able to fix itself rotatably with respect to the seabed, increasing the likelihood of entanglement and twisting of the mooring lines.

[0091] The present application presents a solution where spaced attachment points on the chain table serve to provide increased rotational resistance (with respect to the seabed). This effect is very significant and allows the tidal turbine assembly to be fixed in this way using only two bifurcated mooring lines, i.e., an upstream mooring line and a downstream mooring line, even when the mooring lines are connected to a relatively small area of the turret such as the chain table. This effect can be achieved using only cables or chains as part of the mooring system. In other words, apart from the chain table, the mooring system may contain hardly any rigid brace elements, which corresponds to further cost and complexity savings.

[0092] Another advantageous aspect is to provide a turret and / or a chain table that gives an upward force. This not only provides a position for supporting the mooring line (as is obvious for all turret moorings), but also gives an upward force to the turbine assembly itself. In other words, the equilibrium depth at which the turret floats alone (removed from the assembly) is higher than the depth at which the turret floats in equilibrium when attached to the turbine assembly.

[0093] Due to this feature, the turret can resist the pitching moment of the assembly caused by the large thrust received by the turbine when currently deployed. Furthermore, the upward force from the turret can be variable so as to resist the variable pitching motion caused by the changing thrust from the turbine. This change in the upward force can be achieved by adjusting buoyancy, the use of hydrodynamic surfaces, or both (or actually completely by other means). Thereby, the attitude of the entire assembly can be controlled and stability can be provided.

[0094] This is a problem very specific to tidal turbine assemblies because most turret mooring devices do not experience such a strong variable thrust. This means that most turret mooring systems do not require a turret that generates an upward force, let alone a variable force, and doing so actually makes those systems unstable or causes unwanted pitching or tilting of the assembly.

Claims

1. A turret mooring system for a tidal turbine assembly, comprising: a turret, a shaft for attaching to the tidal turbine assembly and enabling relative movement between the turret and the tidal turbine assembly about a rotational axis, and a chain table fixed to a lower end of the shaft, an upstream mooring line for coupling a single upstream fixed point on the seabed to an upstream portion of the chain table, and a downstream mooring line for coupling a single downstream fixed point on the seabed 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, 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, the turret is configured to provide an upward force when the chain table is immersed in water, a hydrodynamic fairing having an upwardly convex curved surface is provided on the chain table, the upward force is provided by the hydrodynamic fairing, the shaft includes an upper radial bearing and a lower radial bearing for engaging with the tidal turbine assembly, the turret is slidable vertically relative to the tidal turbine assembly turret mooring system.

2. The upstream mooring line has a first portion for coupling the upstream fixed point on the seabed to a mooring cable, and a second portion for coupling the mooring cable to two upstream attachment points on the upstream portion of the chain table, and / or, the downstream mooring line has a first portion for coupling the downstream fixed point on the seabed to a mooring cable, and a second portion for coupling the mooring cable to two downstream attachment points on the downstream portion of the chain table The turret mooring system according to claim 1.

3. The upward force is provided by an element having buoyancy in water The turret mooring system according to claim 1 or 2.

4. The element has variable buoyancy The turret mooring system according to claim 3.

5. The hydrodynamic fairing is shaped to resist an expected pitching moment in the tidal turbine assembly due to the drag force acting on the turbine The turret mooring system according to any one of claims 1 to 4 **Claim 6** A turret mooring system for a tidal turbine assembly, comprising a turret, the turret having a shaft for mounting to the tidal turbine assembly and for enabling relative movement between the turret and the tidal turbine assembly about a rotational axis, and a chain table fixed to a lower end of the shaft, the turret being configured to provide an upward force when the chain table is immersed in water, a hydrodynamic fairing having an upwardly convex curved surface is provided on the chain table, the upward force being provided by the hydrodynamic fairing Turret mooring system **Claim 7** The upward force is provided by an element having buoyancy in water The turret mooring system according to claim 6 **Claim 8** The element has variable buoyancy The turret mooring system according to claim 7 **Claim 9** The hydrodynamic fairing is shaped to resist an expected pitching moment in the tidal turbine assembly resulting from the drag force acting on the turbine The turret mooring system according to claim 8 **Claim 10** An upstream portion of the chain table is configured to be coupled to a single upstream fixed point on the seabed via an upstream mooring line, and a downstream portion of the chain table is configured to be coupled to a single downstream fixed point on the seabed via a downstream mooring line The turret mooring system according to any one of claims 6 to 9 **Claim 11** Further comprising the upstream and downstream mooring lines The turret mooring system according to claim 10 **Claim 12** 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, 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 The turret mooring system according to claim 10 or 11 **Claim 13** The upstream mooring line has a first portion for coupling the upstream fixed point on the seabed to the mooring cable and a second portion for coupling the mooring cable to two upstream attachment points on the upstream portion of the chain table, and / or The downstream mooring line has a first portion for coupling the downstream fixed point on the seabed to the mooring cable and a second portion for coupling the mooring cable to two downstream attachment points on the downstream portion of the chain table The turret mooring system according to claim 11 or 12.

14. The shaft includes an upper radial bearing and a lower radial bearing for engaging with the tidal turbine assembly The turret mooring system according to any one of claims 6 to 13.

15. The turret is mounted on the tidal turbine assembly, The turret is slidable in a direction perpendicular to the tidal turbine assembly. The turret mooring system according to any one of claims 1 to 14.

16. A method of mooring a tidal turbine assembly to the seabed using the turret mooring system according to any one of claims 1 to 15, comprising: (i) transporting the tidal turbine assembly to an installation location; (ii) coupling an upstream portion of the chain table of the turret to a single upstream fixed point with respect to the seabed and coupling a downstream portion of the chain table of the turret to a single downstream fixed point with respect to the seabed; (iii) fixing the turret to the tidal turbine assembly The method includes.

17. Step (iii) is performed before step (i) and step (ii), and step (iii) is performed at a dock or on land The method according to claim 16.

18. When step (iii) is completed, the turret is slidable in a direction perpendicular to the tidal turbine assembly between a raised position and a lowered position, whereby the draft of the tidal turbine assembly is greater when the turret is in the lowered position than when the turret is in the raised position. The method according to claim 17.

19. The turret is in the raised position during step (i) The method according to claim 18.

20. The turret is configured to apply an upward force when the chain table is immersed in water and the turret is slidable in the vertical direction. The vertical position of the turret is adjusted before or during step (i) to control the pitch of the tidal turbine assembly during transportation. The method according to any one of claims 16 to 19.

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