BETWEEN A NACELLE AND A SEABED SUPPORT STRUCTURE AND A METHOD

RELATED THERETO

Technical Field

The present disclosure relates to a tidal turbine including a nacelle and a seabed support structure, and in particular, but not limited thereto, a tidal turbine with a progressively couplable structural interface between the nacelle and the seabed support structure.

Background

Known tidal turbines include tidal blades connected a nacelle which comprises a generator. The tidal turbine is placed underwater such that the tidal blades may be rotated by currents in the water so as to produce electricity by the generator.

In certain tidal turbines, the tidal turbine is placed on the seabed and anchored thereto by a seabed support structure. The seabed support structure extends away from the seabed and the nacelle is connected thereto.

However, in known tidal turbines, the seabed support structure and the nacelle are not lowered to the seabed in a connected configuration. Instead, the seabed support structure is lowered to the seabed, and thereafter the nacelle is lowered and connected to the seabed support structure whilst underwater. Connection of the nacelle to the seabed support structure may pose significant problems due to the currents in the water which may cause unwanted relative motion between the seabed support structure and the nacelle.

In view of the above, there is a need for a tidal turbine in which the nacelle may be connected to the seabed support structure in a more reliable manner. There is also a need for a method of connecting the nacelle to the seabed support structure in a more reliable manner.

Summary of the Disclosure Accordingly, it is an object of the present disclosure to provide a tidal turbine in which the nacelle may be connected to the seabed support structure in a more reliable manner. It is also an object of the present disclosure to provide a method of connecting the nacelle to the seabed support structure in a more reliable manner.

These objectives and others are achieved with the device of Claim 1 and the method of Claim 23. Preferred embodiments and implementations are recited in the dependent claims.

In an aspect of the present disclosure, there is provided a tidal turbine comprising: a seabed support structure for supporting the tidal turbine on the seabed, the seabed support structure configured to extend, in use, upwardly away from the seabed along a longitudinal axis of the seabed support structure; and a nacelle configured to be removably attachable to the seabed support structure via a structural interface upon lowering of the nacelle onto the seabed support structure along the longitudinal axis, the structural interface comprising: (i) a spigot portion of the seabed support structure located at an upper end thereof; and (ii) a nacelle socket of the nacelle for receiving the spigot portion therein, wherein the spigot portion comprises at least one spigot portion bearing surface and the nacelle socket comprises at least one nacelle socket bearing surface, the at least one spigot portion bearing surface and the at least one nacelle socket bearing surface being complementary bearing surfaces for progressive coupling and decoupling of the structural interface during respective attachment and detachment of the nacelle to the seabed support structure.

As used herein, the term 'bearing surface’ of an object relates to a surface of the object for contacting a corresponding 'bearing surface’ of another object. Once the two corresponding bearing surfaces are in contact, forces may be transferred therebetween.

As used herein, the term 'progressive coupling’ of complementary bearing surfaces relates to the gradual engagement of two corresponding bearing surfaces such that the contact surface area between the two corresponding bearing surface is gradually increased. As used herein, the term 'progressive decoupling’ of complementary bearing surfaces relates to the gradual disengagement of two corresponding bearing surfaces such that the contact surface area between the two corresponding bearing surface is gradually decreased.

With the above configurations, the progressive coupling of the complementary bearing surfaces of the nacelle and seabed support structure allow the nacelle to be lowered onto the seabed support structure to a reliable, determined connected position. In particular, due to the progressive coupling between the complementary bearing surfaces, any offsets in position of the nacelle during lowering (e.g. caused by currents in the water) are reliably countered due to the progressive coupling of the bearing surfaces such that the final lowered position of the nacelle is the same regardless of small deviations in position during lowering.

Therefore, these configurations provide for a tidal turbine in which the nacelle may be connected to the seabed support structure in a more reliable manner.

In certain embodiments, the spigot portion is elongate, and its longitudinal axis is generally parallel to the longitudinal axis of the seabed support structure. In certain embodiments, the longitudinal axis of the spigot portion and the longitudinal axis of the seabed support structure are coincident.

In certain embodiments, said at least one spigot portion bearing surface is inclined towards the longitudinal axis such that an uppermost edge thereof is closer to the longitudinal axis than a corresponding lowermost edge.

In certain embodiments, said at least one spigot portion bearing surface is inclined inwardly towards the longitudinal axis when moving up along the spigot portion.

With the above configurations, the inclination of the spigot portion bearing surface allows for the progressive/gradual limitation of the longitudinal location of the nacelle relative to the seabed support structure. Therefore, these configurations provide for a tidal turbine in which the nacelle may be connected to the seabed support structure in a more reliable manner. In certain embodiments, said at least one spigot portion bearing surface is inclined towards the longitudinal axis such that a lowermost edge thereof is closer to the longitudinal axis than a corresponding uppermost edge.

In certain embodiments, said at least one spigot portion bearing surface is inclined outwardly away from the longitudinal axis when moving up along the spigot portion.

With the above configurations, the inclination of the spigot portion bearing surface allows for the progressive/gradual limitation of the longitudinal location of the nacelle relative to the seabed support structure. Therefore, these configurations provide for a tidal turbine in which the nacelle may be connected to the seabed support structure in a more reliable manner.

In certain embodiments, said at least one spigot portion bearing surface has a height and width, and wherein its width extends perpendicularly with respect to the longitudinal axis. In certain embodiments, its width diminishes with increasing height from the seabed.

In certain embodiments, the width is greater than the height. In certain embodiments, the at least one spigot portion bearing surface is generally elongate/rectangular. In certain embodiments, the at least one spigot portion bearing surface defines a longitudinal axis, and wherein the longitudinal axis of the spigot portion bearing surface is generally perpendicular to the longitudinal axis of the seabed support structure. In certain embodiments, the at least one spigot portion bearing surface comprises or consists of a generally planar surface.

In certain embodiments, the at least one spigot portion bearing surface lies in a plane which is parallel with respect to the longitudinal axis.

In certain embodiments, the at least one spigot portion bearing surface lies in a plane which is non-parallel with respect to the longitudinal axis. With such configurations, the orientation of the spigot portion bearing surface allows for the progressive/gradual limitation of the longitudinal location of the nacelle relative to the seabed support structure. Therefore, these configurations provide for a tidal turbine in which the nacelle may be connected to the seabed support structure in a more reliable manner.

In certain embodiments, the spigot portion comprises a plurality of spigot portion bearing surfaces about the longitudinal axis. In certain embodiments, any one of / each one of the plurality of spigot portion bearing surfaces may have any/all of the features of the "at least one spigot portion bearing surface” referred to herein.

With such configurations, due to the plurality of spigot portion bearing surfaces being disposed around the longitudinal axis, the structural interface allows for the progressive/gradual limitation of the angular orientation (i.e. the yaw / orientation in a plane perpendicular to the longitudinal axis of the seabed support structure) of the nacelle relative to the seabed support structure. Therefore, these configurations provide for a tidal turbine in which the nacelle may be connected to the seabed support structure in a more reliable manner.

In certain embodiments, three spigot portion bearing surfaces are provided about the longitudinal axis. In other words, exactly three spigot portion bearing surfaces are provided about the longitudinal axis.

With such configurations, exactly three spigot portion bearing surfaces provided about the longitudinal axis provides optimum, reliable yaw restriction as three-point contact is less influenced by any dimensional inaccuracies.

In certain embodiments, each spigot portion bearing surface defines one side of a substantially triangular shape when viewed in a cross-sectional plane extending perpendicularly with respect to the longitudinal axis.

In certain embodiments, the lowermost edges of the three spigot portion bearing surfaces respectively define one side of a substantially triangular shape when viewed in a cross- sectional plane extending perpendicularly with respect to the longitudinal axis. Additionally/alternatively, in certain embodiments, the uppermost edges of the three spigot portion bearing surfaces respectively define one side of a substantially triangular shape when viewed in a cross-sectional plane extending perpendicularly with respect to the longitudinal axis.

With such configurations, the arrangement of the spigot portion bearing surfaces in a generally triangular shape provides reliable yaw restriction.

In certain embodiments, the, or each, spigot portion bearing surface is a surface of a spigot portion bearing pad of the spigot portion.

With such configurations, the material of the spigot portion bearing pad may be made of a different material that the rest of the material of the spigot portion, thereby providing for more suitable bearing surfaces.

In certain embodiments, said at least one nacelle socket bearing surface is inclined towards the longitudinal axis when the nacelle is attached to the seabed support structure such that an uppermost edge thereof is closer to the longitudinal axis than a corresponding lowermost edge.

In certain embodiments, said at least one nacelle socket bearing surface is inclined inwardly towards the longitudinal axis when moving up along the longitudinal axis when the nacelle is attached to the seabed support structure.

With the above configurations, the inclination of the nacelle socket bearing surface allows for the progressive/gradual limitation of the longitudinal location of the nacelle relative to the seabed support structure. Therefore, these configurations provide for a tidal turbine in which the nacelle may be connected to the seabed support structure in a more reliable manner.

In certain embodiments, said at least one nacelle socket bearing surface is inclined towards the longitudinal axis when the nacelle is attached to the seabed support structure such that a lowermost edge thereof is closer to the longitudinal axis than a corresponding uppermost edge.

In certain embodiments, said at least one nacelle socket bearing surface is inclined outwardly away from the longitudinal axis when moving up along the longitudinal axis when the nacelle is attached to the seabed support structure.

With the above configurations, the inclination of the nacelle socket bearing surface allows for the progressive/gradual limitation of the longitudinal location of the nacelle relative to the seabed support structure. Therefore, these configurations provide for a tidal turbine in which the nacelle may be connected to the seabed support structure in a more reliable manner.

In certain embodiments, said at least one nacelle socket bearing surface has a height and width, and wherein, when the nacelle is attached to the seabed support structure, its width extends perpendicularly with respect to the longitudinal axis. In certain embodiments, its width diminishes with increasing height from the seabed.

In certain embodiments, the width is greater than the height. In certain embodiments, the at least one nacelle socket bearing surface is generally elongate/rectangular. In certain embodiments, the at least one nacelle socket bearing surface defines a longitudinal axis, and wherein the longitudinal axis of the nacelle socket bearing surface is generally perpendicular to the longitudinal axis of the seabed support structure when in use. In certain embodiments, the at least one nacelle socket bearing surface comprises or consists of a generally planar surface.

In certain embodiments, at least one nacelle socket bearing surface lies in a plane which is parallel with respect to the longitudinal axis.

In certain embodiments, at least one nacelle socket bearing surface lies in a plane which is non-parallel with respect to the longitudinal axis. With such configurations, the orientation of the nacelle socket bearing surface allows for the progressive/gradual limitation of the longitudinal location of the nacelle relative to the seabed support structure. Therefore, these configurations provide for a tidal turbine in which the nacelle may be connected to the seabed support structure in a more reliable manner.

In certain embodiments, the nacelle socket comprises a plurality of nacelle socket bearing surfaces about the longitudinal axis. In certain embodiments, any one of / each one of the plurality of nacelle socket bearing surfaces may have any/all of the features of the "at least one nacelle socket bearing surface” referred to herein.

With such configurations, due to the plurality of nacelle socket bearing surfaces being disposed around the longitudinal axis, the structural interface allows for the progressive/gradual limitation of the angular orientation (i.e. the yaw / orientation in a plane perpendicular to the longitudinal axis of the seabed support structure) of the nacelle relative to the seabed support structure. Therefore, these configurations provide for a tidal turbine in which the nacelle may be connected to the seabed support structure in a more reliable manner.

In certain embodiments, three nacelle socket bearing surfaces are provided about the longitudinal axis. In other words, exactly three nacelle socket bearing surfaces are provided about the longitudinal axis.

With such configurations, exactly three nacelle socket bearing surfaces provided about the longitudinal axis provides optimum, reliable yaw restriction as three-point contact is less influenced by any dimensional inaccuracies.

In certain embodiments, each nacelle socket bearing surface defines one side of a substantially triangular shape when viewed in a cross-sectional plane extending perpendicularly with respect to the longitudinal axis.

In certain embodiments, the lowermost edges of the three nacelle socket bearing surfaces respectively define one side of a substantially triangular shape when viewed in a cross- sectional plane extending perpendicularly with respect to the longitudinal axis. Additionally/alternatively, in certain embodiments, the uppermost edges of the three nacelle socket bearing surfaces respectively define one side of a substantially triangular shape when viewed in a cross-sectional plane extending perpendicularly with respect to the longitudinal axis.

With such configurations, the arrangement of the nacelle socket bearing surfaces in a generally triangular shape provides reliable yaw restriction.

In certain embodiments, the, or each, nacelle socket bearing surface is a surface of a nacelle socket bearing pad of the nacelle socket.

With such configurations, the material of the nacelle socket bearing pad may be made of a different material that the rest of the material of the nacelle socket, thereby providing for more suitable bearing surfaces.

In certain embodiments, the spigot portion further comprises at least one secondary spigot portion bearing surface and the nacelle socket further comprises at least one secondary nacelle socket bearing surface, the at least one secondary spigot portion bearing surface and the at least one secondary nacelle socket bearing surface being complementary bearing surfaces for progressive coupling and decoupling of the structural interface during respective attachment and detachment of the nacelle to the seabed support structure.

Such secondary bearing surfaces allow such configurations to resist relative movement along another direction. Therefore, these configurations provide for a tidal turbine in which the nacelle may be connected to the seabed support structure in a more reliable manner.

In certain embodiments, the at least one secondary spigot portion bearing surface is disposed, in use, above the at least one spigot portion bearing surface. Additionally/alternatively, the at least one secondary nacelle socket bearing surface is disposed, in use, above the at least one nacelle socket bearing surface. Such secondary bearing surfaces allow optimum pitch restriction of the nacelle relative to the seabed support structure. Therefore, these configurations provide for a tidal turbine in which the nacelle may be connected to the seabed support structure in a more reliable manner.

In certain embodiments, the at least one spigot portion bearing surface has a greater radial extent relative to the longitudinal axis than the at least one secondary spigot portion bearing surface. Additionally/alternatively, the at least one nacelle socket bearing surface has a greater radial extent relative to the longitudinal axis than the at least one secondary nacelle socket bearing surface.

With such configurations, the lower bearing surfaces may be conveniently engaged before the upper bearing surfaces, thereby providing a more reliable connection. Therefore, these configurations provide for a tidal turbine in which the nacelle may be connected to the seabed support structure in a more reliable manner.

In certain embodiments, the at least one secondary spigot portion bearing surface is generally cylindrical. Alternatively, the at least one secondary spigot portion bearing surface is generally in the shape of part of a cylinder.

Additionally/alternatively, the at least one secondary nacelle socket bearing surface is generally in the shape of at least part of a cylinder. In certain embodiments, the at least one secondary nacelle socket bearing surface is generally cylindrical.

With such configurations, the upper bearing surfaces act to restrict pitching of the nacelle in all directions. Therefore, these configurations provide for a tidal turbine in which the nacelle may be connected to the seabed support structure in a more reliable manner.

In certain embodiments, the, or each, secondary spigot portion bearing surface is a surface of a secondary spigot portion bearing pad of the spigot portion. Additionally/alternatively, the, or each, secondary nacelle socket bearing surface is a surface of a secondary nacelle socket bearing pad of the nacelle socket. With such configurations, the material of the secondary nacelle socket bearing pad/secondaiy spigot portion bearing pad may be made of a different material that the rest of the material of the nacelle socket/spigot portion, thereby providing for more suitable bearing surfaces.

In certain embodiments, the downstream side of the secondary spigot portion bearing surface is inclined away from the longitudinal axis of the spigot portion such that an uppermost edge thereof is further away from the longitudinal axis than a corresponding lowermost edge.

In certain embodiments, the upstream side of the secondary spigot portion bearing surface is inclined towards the longitudinal axis of the spigot portion such that an uppermost edge thereof is closer to the longitudinal axis than a corresponding lowermost edge.

In certain embodiments, the downstream side of the secondary nacelle socket bearing surface is inclined away from the longitudinal axis of the nacelle socket such that an uppermost edge thereof is further away from the longitudinal axis than a corresponding lowermost edge.

In certain embodiments, the upstream side of the secondary nacelle socket bearing surface is inclined towards the longitudinal axis of the nacelle socket such that an uppermost edge thereof is closer to the longitudinal axis than a corresponding lowermost edge.

In another aspect of the present disclosure, there is provided a method of removably attaching a nacelle of a tidal turbine to a seabed support structure of the tidal turbine, the seabed support structure comprising a spigot portion located at an upper end thereof, the nacelle comprising a nacelle socket for receiving the spigot portion therein, the method comprising: lowering the nacelle onto the seabed support structure along a longitudinal axis of the seabed support structure; progressively coupling a structural interface of the nacelle and seabed support structure upon the lowering of the nacelle onto the seabed support structure, wherein the progressive coupling includes progressively contacting complementary bearing surfaces of each of the spigot portion and the nacelle socket. With the above implementations, the progressive coupling of the complementary bearing surfaces of the nacelle and seabed support structure allow the nacelle to be lowered onto the seabed support structure in a reliable, determined connected position. In particular, due to the progressive coupling between the complementary bearing surfaces, any offsets in position of the nacelle during lowering (e.g. caused by currents in the water) are reliably countered due to the progressive coupling of the bearing surfaces such that the final lowered position of the nacelle is the same regardless of small deviations in position during lowering.

Therefore, these implementations provide for a method of connecting the nacelle to the seabed support structure in a more reliable manner.

In certain implementations, the nacelle is lowered onto the seabed support structure with the nacelle being angled with the rotor/generator of the tidal turbine being raised. In certain implementations, the nacelle is angled upwardly from the horizontal by between about 7 degrees to about 8 degrees, preferably about 7.5 degrees.

With these implementations, the chance of incorrect final positioning due to the stick-slip phenomenon is greatly reduced. Therefore, these implementations provide for a method of connecting the nacelle to the seabed support structure in a more reliable manner.

In certain implementations, after the nacelle is lowered in the angled orientation and the structural interface is partially coupled, the nacelle is levelled such that the nacelle is generally horizontal.

In certain implementations, as the nacelle is levelled, the bearing pads progressively engage/couple to a final engagement/coupling level.

In certain implementations, as the nacelle is levelled, the secondary bearing pads progressively engage/couple to a final engagement/coupling level. As used throughout herein, the terms 'upwardly’, 'downwardly’, 'below’, 'above’, 'horizontal’ and 'vertical’ refer to their usual meanings understood by the skilled person when the tidal turbine is in use and being anchored to the seabed.

As used throughout herein, the terms 'downstream’ and 'upstream’ have their usual meanings understood by the skilled person. Specifically, 'downstream’ refers to the end of the tidal turbine which is furthest away from the tidal blades and 'upstream’ refers to the end of the tidal turbine on which tidal turbines are disposed.

Brief Description of the Drawings

For a better understanding of the present disclosure and to show how the same may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, in which:

Figure 1 shows a perspective view of the top of a tidal turbine including a nacelle and seabed support structure;

Figure 2 shows a side view of the top of the tidal turbine;

Figure 3 shows a perspective view of the top of the tidal turbine with the nacelle removed;

Figure 4 shows a perspective view of a spigot portion of the seabed support structure;

Figures 5 and 6 show different side views of the spigot portion;

Figure 7 shows a top view of the spigot portion;

Figure 8 shows a perspective view of a nacelle socket of the nacelle;

Figure 9 shows a bottom view of the nacelle socket; Figure 10 shows a cross section of the nacelle socket; and

Figures 11 to 13 show the configuration in which the nacelle socket has been lowered onto the spigot portion.

Detailed Description

Figure 1 shows a perspective view of the top of a tidal turbine 100. The tidal turbine 100 is for being placed underwater such that the tidal turbine 100 is able to produce electricity due to the currents in the water.

The tidal turbine 100 comprises a nacelle 110 which has, amongst other things, a generator therein (not shown). Two tidal blades 120a, 120b are rotationally coupled to the generator such that rotation of the two tidal blades around a common rotation axis generates electricity via the generator. The two tidal blades 120a, 120b are each configured such that a current flowing in a direction intersecting the face of the respective blade effects a force on the respective blade such that it rotates about the rotation axis.

In certain embodiments, the two tidal blades 120a, 120b are configured such that they rotate around the common rotation axis in the same direction regardless of the direction of current. In other words, both ebb and flow currents result in rotation of the two tidal blades 120a, 120b in the same direction/sense around the common rotation axis. Accordingly, in certain embodiments, the tidal turbine 100 is a unidirectional tidal turbine.

The tidal turbine 100 further comprises a seabed support structure (not shown) which is configured to support the tidal turbine on the seabed. The seabed support structure comprises a central tower 130 and a support arm structure(not shown).

The central tower 130 extends downwardly from the nacelle 110. The central tower 130 defines a longitudinal axis which is coincident with the longitudinal axis of the seabed support structure. As shown in Figure 1, the central tower 130 is elongate and generally cylindrical. The longitudinal axis of the central tower 130 is configured to be generally perpendicular to the seabed when the tidal turbine 100 is in use. The longitudinal axis of the central tower 130 is configured to be generally parallel to the tidal blade rotation plane. Figure 2 shows a side view of the top of the tidal turbine 100. As can be seen in Figure 2, the nacelle 110 comprises a nacelle socket 111. The nacelle socket 111 is disposed at a downstream end of the nacelle 110 (i.e. on the opposite end to the two tidal blades 120a, 120b.

Figure 3 shows a perspective view of the top of the tidal turbine 100 with the nacelle 110 removed from the central tower 130. As can be seen in Figure 3, the central tower 130 comprises a spigot portion 131 disposed at an uppermost end thereof.

The nacelle socket 111 is configured to receive the spigot portion 131 of the central tower

130 therein. In particular, the nacelle socket 111 is configured to receive the spigot portion

131 upon lowering of the nacelle socket 111 onto the spigot portion 131 (as described in detail below).

The spigot portion 131 and the nacelle socket 111 define a structural interface therebetween which allows for the progressive coupling and decoupling of the spigot portion 131 and the nacelle socket 111.

Figure 4 shows a perspective view of the spigot portion 131. The spigot portion 131 defines a longitudinal axis Lsp. The spigot portion 131 forms part of the central tower 130 such that the longitudinal axis of the central tower and the longitudinal axis L _SP of the spigot portion are coincident.

As can be seen in Figure 4, the spigot portion 131 is generally elongate. The spigot portion 131 generally tapers in width when moving upwardly along its longitudinal axis Lsp.

The spigot portion 131 comprises a spigot portion body 132. The spigot portion body 132 is generally elongate. The spigot portion body 132 generally tapers in width when moving upwardly along the longitudinal axis L _SP of the spigot portion. The spigot portion body 132 provides the structural backbone of the spigot portion 131. The spigot portion 131 further comprises three spigot portion bearing pads 133a, 133b, 133c (only two spigot portion bearing pads 133a, 133b being visible in Figure 4) attached to the spigot portion body 132. Each of the three spigot portion bearing pads 133a, 133b, 133c defines a respective spigot portion bearing surface for progressive contact with the corresponding nacelle socket bearing surfaces (described below). In certain embodiments, at least part / all of the outer surface of the three spigot portion bearing pads 133a, 133b, 133c defines a respective spigot portion bearing surface.

The arrangement of the three spigot portion bearing pads 133a, 133b, 133c is described with reference to Figures 5 and 6. Figures 5 and 6 show different side views of the spigot portion 131. In particular, reference is made to the side profile of the third spigot portion bearing pad 133c.

As can be seen in Figure 5, the spigot portion bearing surface of the third spigot portion bearing pad 133c is inclined towards the longitudinal axis LSP such that an uppermost edge 133c’ thereof is closer to the longitudinal axis LSP than a corresponding lowermost edge 133c”. Accordingly, the third spigot portion bearing surface is generally inclined inwardly towards the longitudinal axis LSP when moving up along the spigot portion 131.

The spigot portion bearing surface of the third spigot portion bearing pad 133c is generally elongate. The third spigot portion bearing surface defines a longitudinal axis which is generally perpendicular to the longitudinal axis Lsp.

As can be seen in Figure 5, the third spigot portion bearing surface comprises two generally planar surfaces connected to each other. The upper planar surface is more inclined than the lower planar surface. Both the upper planar surface and the lower planar surface are non parallel with respect to the longitudinal axis Lsp.

Corresponding features to those described above in relation to the third spigot portion bearing pad 133c are present for each of the first spigot portion bearing pad 133a and the second spigot portion bearing pad 133b. Figure 7 shows a top view of the spigot portion 131. As can be seen from Figure 7, the three spigot portion bearing pads 133a, 133b, 133c are disposed about/around the longitudinal axis LSP of the spigot portion 131. As shown in Figure 7, the three spigot portion bearing pads 133a, 133b, 133c are disposed about/around the longitudinal axis LSP in a rotationally symmetric manner.

When viewed in a cross-sectional plane extending perpendicularly with respect to the longitudinal axis (such as the view shown in Figure 7), each spigot portion bearing pad 133a, 133b, 133c defines one side of a substantially triangular shape. The substantially triangular shape being in a plane which is perpendicular to the longitudinal axis LSP of the spigot portion 131. The triangular shape is a substantially equilateral triangular shape.

The lowermost edges of the three spigot portion bearing surfaces of the three spigot portion bearing pads 133a, 133b, 133c respectively define one side of a substantially triangular shape when viewed in a cross-sectional plane extending perpendicularly with respect to the longitudinal axis LSP of the spigot portion 131.

The uppermost edges of the three spigot portion bearing surfaces of the three spigot portion bearing pads 133a, 133b, 133c respectively define one side of a substantially triangular shape when viewed in a cross-sectional plane extending perpendicularly with respect to the longitudinal axis LSP of the spigot portion 131.

The substantially triangular shape defined by the lowermost edges of the three spigot portion bearing surfaces is larger than the substantially triangular shape defined by the uppermost edges of the three spigot portion bearing surfaces. In particular, the length of each side of the substantially triangular shape defined by the lowermost edges of the three spigot portion bearing surfaces is larger than the length of each side of the substantially triangular shape defined by the uppermost edges of the three spigot portion bearing surfaces.

Figures 4 to 7 also show that the spigot portion 131 further comprises a secondary spigot portion bearing pad 134 attached to the spigot portion body 132. The secondary spigot portion bearing pad 134 defines a secondary spigot portion bearing surface for progressive contact with the corresponding secondary nacelle socket bearing surface (described below). In certain embodiments, at least part / all of the outer surface of the secondary spigot portion bearing pad 134 defines the secondary spigot portion bearing surface.

As can be seen in Figures 4 to 6, the secondary spigot portion bearing pad 134 is disposed above the three spigot portion bearing pads 133a, 133b, 133c. The three spigot portion bearing pads 133a, 133b, 133c have a greater radial extent relative to the longitudinal axis LSP than the secondary spigot portion bearing pad 134. Referring to Figure 7, the secondary spigot portion bearing pad 134 is confined entirely within the generally triangular shape formed by the three spigot portion bearing pads 133a, 133b, 133c when viewed in a cross- sectional plane extending perpendicularly with respect to the longitudinal axis LSP of the spigot portion 131.

The secondary spigot portion bearing pad 134 and its corresponding secondary spigot portion bearing surface is generally cylindrical. The axis of the cylinder being generally parallel to the longitudinal axis LSP of the spigot portion 131.

Figure 8 shows a perspective view of the nacelle socket 111. The nacelle socket 111 defines a longitudinal axis LNS. In use (i.e. when the nacelle socket 111 is connected to the spigot portion 131), the longitudinal axis LNS of the nacelle socket 111 is coincident with the longitudinal axis LSP of the spigot portion 131.

The nacelle socket 111 comprises a nacelle socket body 112. The nacelle socket body 112 provides the structural backbone of the nacelle socket 111.

Figure 9 shows a bottom view of the nacelle socket 111. As can be seen, the nacelle socket 111 further comprises three nacelle socket bearing pads 113a, 113b, 113c attached to the nacelle socket body 112. Each of the three nacelle socket bearing pads 113a, 113b, 113c defines a respective nacelle socket bearing surface for progressive contact with the corresponding spigot portion bearing surfaces (described above). In certain embodiments, at least part / all of the inner surface of the three nacelle socket bearing pads 113a, 113b, 113c defines a respective nacelle socket bearing surface. As can be seen from Figure 9, the three nacelle socket bearing pads 113a, 113b, 113c are disposed about/around the longitudinal axis LNS of the nacelle socket 111. As shown in Figure 9, the three nacelle socket bearing pads 113a, 113b, 113c are disposed about/around the longitudinal axis LNS in a rotationally symmetric manner.

When viewed in a cross-sectional plane extending perpendicularly with respect to the longitudinal axis (such as the view shown in Figure 9), each nacelle socket bearing pad 113a, 113b, 113c defines one side of a substantially triangular shape. The substantially triangular shape being in a plane which is perpendicular to the longitudinal axis LNS of the nacelle socket 111. The triangular shape is a substantially equilateral triangular shape.

The lowermost edges of the three spigot portion bearing surfaces of the three nacelle socket bearing pads 113a, 113b, 113c respectively define one side of a substantially triangular shape when viewed in a cross-sectional plane extending perpendicularly with respect to the longitudinal axis LNS of the nacelle socket 111.

The uppermost edges of the three spigot portion bearing surfaces of the three nacelle socket bearing pads 113a, 113b, 113c respectively define one side of a substantially triangular shape when viewed in a cross-sectional plane extending perpendicularly with respect to the longitudinal axis LNS of the nacelle socket 111.

The substantially triangular shape defined by the lowermost edges of the three nacelle socket bearing surfaces is larger than the substantially triangular shape defined by the uppermost edges of the three nacelle socket bearing surfaces. In particular, the length of each side of the substantially triangular shape defined by the lowermost edges of the three nacelle socket bearing surfaces is larger than the length of each side of the substantially triangular shape defined by the uppermost edges of the three nacelle socket bearing surfaces.

Figure 10 shows a cross section of the nacelle socket 111. A side profile of the third nacelle socket bearing pad 113c is shown in Figure 10. The nacelle socket bearing surface of the third nacelle socket bearing pad 113c is inclined towards the longitudinal axis LNS such that an uppermost edge 113c’ thereof is closer to the longitudinal axis LNS than a corresponding lowermost edge 113c”. Accordingly, the third nacelle socket bearing surface is generally inclined inwardly towards the longitudinal axis LNS when moving up along the nacelle socket 111.

The nacelle socket bearing surface of the third nacelle socket bearing pad 113c is generally elongate. The third nacelle socket bearing surface defines a longitudinal axis which is generally perpendicular to the longitudinal axis LNS.

As can be seen in Figure 10, the third nacelle socket bearing surface comprises two generally planar surfaces connected to each other. The upper planar surface is less inclined than the lower planar surface. Both the upper planar surface and the lower planar surface are non-parallel with respect to the longitudinal axis LNS.

Corresponding features to those described above in relation to the third nacelle socket bearing pad 113c are present for each of the first nacelle socket bearing pad 113a and the second nacelle socket bearing pad 113b.

Figures 8 to 10 also show that the nacelle socket 111 further comprises a secondary nacelle socket bearing pad 114 attached to the nacelle socket body 112. The secondary nacelle socket bearing pad 114 defines a secondary nacelle socket bearing surface for progressive contact with the corresponding secondary spigot portion bearing surface (described above). In certain embodiments, at least part / all of the inner surface of the secondary nacelle socket bearing pad 114 defines the secondary nacelle socket bearing surface.

As can be seen in Figures 8 to 10, the secondary nacelle socket bearing pad 114 is disposed above the three nacelle socket bearing pads 113a, 113b, 113c. The three nacelle socket bearing pads 113a, 113b, 113c have a greater radial extent relative to the longitudinal axis LNS than the secondary nacelle socket bearing pad 114. Referring to Figure 9, the secondary nacelle socket bearing pad 114 is confined entirely within the generally triangular shape formed by the three nacelle socket bearing pads 113a, 113b, 113c when viewed in a cross- sectional plane extending perpendicularly with respect to the longitudinal axis L _NS of the nacelle socket 111.

The secondary nacelle socket bearing pad 114 and its corresponding secondary nacelle socket bearing surface defines part of the curved surface of a notional cylinder (as visible in Figures 8 and 9). The centre of the notional cylinder lines on the longitudinal axis L _NS of the nacelle socket 111. When in use, the centre of the notional cylinder lines on the longitudinal axis L _SP of the spigot portion 131. The axis of the notional cylinder being generally parallel to the longitudinal axis L _NS of the nacelle socket 111.

Figures 11 to 13 show the configuration in which the nacelle socket 111 has been lowered onto the spigot portion 131 and is in its final position suitable for when the tidal turbine 100 is in use.

In the attached configuration shown in Figures 11 to 13, the corresponding bearing surfaces of the nacelle socket 111 and the spigot portion 131 are coupled / in contact with each other.

Specifically, the first nacelle socket bearing pad 113a and the first spigot portion bearing pad 133a are coupled to each other such that the first nacelle socket bearing surface and the first spigot portion bearing surface abut each other.

Similarly, the second nacelle socket bearing pad 113b and the second spigot portion bearing pad 133b are coupled to each other such that the second nacelle socket bearing surface and the second spigot portion bearing surface abut each other.

Similarly, the third nacelle socket bearing pad 113c and the third spigot portion bearing pad 133c are coupled to each other such that the third nacelle socket bearing surface and the third spigot portion bearing surface abut each other.

Similarly, the secondary nacelle socket bearing pad 114 and the secondary spigot portion bearing pad 134 are coupled to each other such that the secondary nacelle socket bearing surface and the secondary spigot portion bearing surface abut each other. A method of removably attaching the nacelle 110 of the tidal turbine 100 to the seabed support structure of the tidal turbine 100 will now be described.

The nacelle 110 is initially angled such that the rotor/ generator of the tidal turbine 100 is raised above the horizontal. In other words, the nacelle 110 is pivoted about the nacelle socket 111 such that the upstream end of the nacelle 110 is raised above the horizontal.

Thereafter, the nacelle 110 in this angled orientation is lowered onto the spigot portion 131 such that the nacelle socket 111 receives the spigot portion 131. During this lowering, at least some / all of the nacelle socket bearing pads 113a, 113b, 113c progressively engage/couple with the respective spigot portion bearing pads 133a, 133b, 133c.

After the nacelle 110 is lowered, the nacelle socket bearing pads 113a, 113b, 113c and the respective spigot portion bearing pads 133a, 133b, 133c are only partially engaged/coupled to each other.

Thereafter, the nacelle 110 pivoted such that it is horizontal thereby progressively engaging/coupling the secondary nacelle socket bearing pad 114 and the secondary spigot portion bearing pad 134. In other words, the nacelle 110 is pivoted about the nacelle socket 111 such that the nacelle 110 is horizontal thereby progressively engaging/coupling the secondary nacelle socket bearing pad 114 and the secondary spigot portion bearing pad 134.

In this final position, the nacelle socket 111 and the spigot portion 131 are fully engaged/coupled.

Although particular embodiments of the disclosure have been disclosed herein in detail, this has been done by way of example and for the purposes of illustration only. The aforementioned embodiments are not intended to be limiting with respect to the scope of the appended claims. For example, even though the above embodiment refers to three nacelle socket bearing pads 113a, 113b, 113c and respective spigot portion bearing pads 133a, 133b, 133c, any number of bearing pads may be implemented, so long as at least one spigot portion bearing surface and at least one nacelle socket bearing surface are complementary bearing surfaces for progressive coupling and decoupling during respective attachment and detachment of the nacelle socket 111 to the spigot portion 131.

Furthermore, the secondary nacelle socket bearing pad 114 and the secondary spigot portion bearing pad 134 may be omitted, and, optionally, other connection means be used instead.

It is contemplated by the inventors that various substitutions, alterations, and modifications may be made to the invention without departing from the scope of the invention as defined by the appended claims.