TECHNICAL FIELD

The invention relates to schemes for improving the torsional capacity of tubular subterranean support structures for above-ground structures such as wind turbines.

BACKGROUND

Various approaches are known to anchor an offshore wind turbine installation, and the type of approach that is used in a particular installation depends to a large extent on the water depth at the installation site.

For deeper waters of more than about 35m, tripod and jacket foundations tend to be preferred. Such foundations typically take the form of large lattice structures (e.g. of welded steel constriction) which are anchored to the seabed using suitable piles. Such jacket-style foundations tend to be more robust for deep water locations, although the cost of such structures is very high due to their steel mass and complexity.

For shallower water, the current preference is to use a monopile anchoring system in which a single tubule structure, the monopile, is driven vertically into the seabed to a depth such that the upper end of the monopile provides a mounting platform for a wind turbine. Monopiles are generally a reliable technology which are compact pre-installation so that they take up less room on shipdeck. However, with the ever-growing scale of offshore wind turbines, there is a need to push the technical boundary of monopile design. Larger wind turbines means larger monopiles, which are more costly to fabricate and challenging to transport to site and drive into the seabed.

For multi-rotor wind turbines of the type described in WO 2019/114896 A1 a torsional moment is applied to the tower in the event of unbalanced operation of the wind turbine rotors. Therefore, monopiles for multi-rotor wind turbines tend to be susceptible to torsional forces.

It is against this background that the invention has been devised. SUMMARY OF THE INVENTION

An object of this invention is to lower the required forces necessary to drive the monopile. The effect of this will be that the stresses on the monopile becomes lower and therefore it becomes less susceptible to buckling. Another object of the invention is to improve the structure of a monopile to make it more resistant to buckling.

According to a first aspect of the invention, there is provided a support structure, such as a monopile, for a wind turbine, comprising a tubular body having a wall extending about a body axis, the wall defining a leading edge and an outer circumferential surface extending away from the leading edge and a plurality of fins supported on the wall near to the leading edge of the tubular body and extending in an axial direction. Each fin comprises a first fin portion that extends from the wall in a radial outwards direction with respect to the body axis and a second fin portion that extends from the wall in a radial inwards direction with respect to the body axis.

Beneficially the arrangement of fins on the head of the support structure provides structural reinforcement to the pile wall so the pile is less likely to buckle during installation. Having fin portions that extends radially inwards and outwards from the pile wall means that the pile wall is supported evenly.

In some embodiments, longitudinal edges of the finds may extend in a direction that is parallel to the pile wall. However, in some embodiments, the fin edges may extend at an angle to the pile wall. For example, the first fin portion may define an outer fin edge that is inclined in the axial direction with respect to the wall of the tubular body. Similarly, second fin portion may define an outer fin edge that is inclined in the axial direction with respect to the wall of the tubular body.

To provide more control over pile end buckling, the edges of the fins may be inclined at different angles. For example, the outer edge of the second fin portion (that is, the radially inner fin portion) may be inclined to the wall of the tubular body at a greater angle than the outer edge of the first fin portion. This means that as the pile is driven into the ground, a greater force is applied to the inner surface of the pile end by the more steeply angled fin portions, which guards against the leading edge of the fin buckling in a radially inwards direction. Tips of the fins may in some examples terminate at or around the level of the leading edge of the pile. However, in some examples the fins may be configured so that the fin tips extend beyond the leading edge of the pile, and they may even define a sharp point. This configuration helps the pile end to ‘bite’ into the ground during installation and reduces the force required to drive the initial part of the pile into the ground

In some examples the leading edge may simply be flat and circular in form with a uniform profile. However, in other examples, the leading edge may be configured to have a wavy, toothed or sawtoothed profile. This measure may also promote the way the pile end engages with the ground during installation. In a further modification, the fins may be arranged so that the fin tip coincide with teeth of the leading edge. In such an arrangement, therefore, portions of the leading edge that extend between adjacent ones of the plurality of fins, may be shaped to define a V-shaped or a U-shaped profile.

The fins may be fitted to the pile wall in various ways. For example, the fin portions may be attached to the pile wall, e.g. by welding directly onto the underlying wall. Another option is for the fins to be received in a respective slot defined in the wall of the tubular body. The fin can then be welded in place in the slot.

The fins may take various dimensions, both in terms of their lengths and also their radial dimension or ‘width’. It is envisaged that fins having an axially shorter length could benefit from being wider in order to maintain a useful resistance against torsional moments on the monopile. Conversely, longer fins could benefit from being made narrower whilst still retaining sufficient torsional resistance.

In some examples, the axial length of each fin is greater than approximately 50% of the diameter of the tubular body but below than about 150% of the tubular body. Although in theory the upper limit to the fin length is the length of the monopile itself, preferably the fin length should be equal to the proposed depth that the monopile will be sunk into the seabed.

In other examples, the fin length is greater than approximately 200% of the diameter of the tubular body.

In another aspect, there is provided a support structure for a wind turbine, comprising a tubular body having a wall extending about a body axis, the wall defining a leading edge and an outer circumferential surface extending away from the leading edge and a plurality of fins attached to the wall near to the leading edge of the tubular body and extending in an axial direction. At least one of the plurality of fins is configured to define a fluid channel extending along a length of the fin, wherein the fluid channel terminates at an open fin tip so as to allow fluid to pass through the fluid channel and out of the tip.

Beneficially, in this configuration of monopile structure the fins act as a hydraulic means to disturb and loosen seabed material at the leading edge of the structure as it is driven into the ground. This can lower the force required to perform the pile driving process which, in turn, can reduce the likelihood of buckling. The fins therefore form part of a fluid delivery system through which means fluid at high pressure, for example sweater or drilling ‘mud’ can be injected at the tips of the fins so as to loosen the bed of soil/sand into which the monopile is driven.

The fin tip may terminate approximately in line with an adjacent part of the leading edge of the tubular body, or may terminate at a point that extends axially beyond the leading edge of the tubular body. Extending beyond the leading edge may confer a benefit such that it makes it easier for the monopile to ‘dig in’ to the sea bed and reduce the initial force required in the pile driving process.

The at least one fin may comprise a radially outer fin portion that extends in a radial outwards direction from the wall with respect to the body axis, wherein the radially outer fin portion defines the fluid channel at least in part with an adjacent part of the wall. There may also be a radially inner fin portion that defines the fluid channel at least in part with an adjacent part of the wall. In an alternative configuration the radially outer fin portion and the radially inner fin portion cooperate together to define the fluid channel.

To allow high pressure fluid to be delivered to the fluid channel, the fin includes a fluid connector or any suitable type.

In another aspect, the invention provides a support structure for a wind turbine, comprising a tubular body having a wall extending about a body axis, the wall defining a leading edge and an outer circumferential surface extending away from the leading edge; wherein the leading edge of the tubular body is irregularly shaped so as to define a plurality of leading edge teeth. The support structure further comprises a plurality of fins supported on the wall near to the leading edge of the tubular body and extending in an axial direction and wherein the plurality of fins define respective fin tips, each fin tip being positioned at a respective one of the leading edge teeth.

The inventors have found that embodiments of the invention with fluid channels in combination with a fluid delivery system can be considered an invention of its own independently of the fins extending internally and externally on the monopile.

A separate invention has been found in a support structure for a wind turbine, comprising: a tubular body having a wall extending about a body axis, the wall defining a leading edge and an outer circumferential surface extending away from the leading edge; a plurality of fins attached to the wall near to the leading edge of the tubular body and extending in an axial direction; wherein at least one of the plurality of fins is configured to define a fluid channel extending along a length of the fin, wherein the fluid channel terminates at an open fin tip so as to allow fluid to pass through the fluid channel and out of the tip.

It is herewith achieved that the required forces to apply to the monopile while driving it through the seabed is reduced.

In a first embodiment of the separate invention, the first fin portion defines an outer fin edge that is inclined in the axial direction with respect to the wall of the tubular body.

In a second embodiment of the separate invention, the second fin portion defines an outer fin edge that is inclined in the axial direction with respect to the wall of the tubular body.

In a third embodiment of the separate invention according to the second embodiment of the separate invention, the outer edge of the second fin portion is inclined to the wall of the tubular body at a greater angle than the outer edge of the first fin portion.

In a fourth embodiment of the separate invention, each fin terminates in a fin tip. In a fifth embodiment of the separate invention according to the fourth embodiment of the separate invention, the fin tip of at least some of the fins extends axially beyond the leading edge of the tubular body.

In a sixth embodiment of the separate invention according to the fourth or fifth embodiment of the separate invention, the fin tip defines a sharp point.

In a seventh embodiment of the separate invention according to any of the fourth to sixth embodiment of the separate invention, the leading edge of the tubular body is irregularly shaped so as to define a plurality of teeth, and wherein a respective one of the plurality of fins is positioned at a respective one of the teeth.

In an eighth embodiment of the separate invention according to the seventh embodiment of the separate invention, portions of the leading edge that extend between adjacent ones of the plurality of fins, is shaped to define a V-shaped or a U-shaped profile.

In a ninth embodiment of the separate invention or any embodiments of the separate invention, each of the fins is received in a respective slot defined in the wall of the tubular body.

In a tenth embodiment of the separate invention or any embodiments of the separate invention, the axial length of each fin is greater than approximately 50% of the diameter of the tubular body.

In an eleventh embodiment of the separate invention or any embodiments of the separate invention, the axial length of each fin is less than 150% of the diameter of the tubular body.

In an twelfth embodiment of the separate invention or any embodiments from the first to the ninth embodiment of the separate invention, the axial length of each fin is greater than approximately 200% of the diameter of the tubular body.

In the main invention the internal and external fins together with the fluid that exits at the head of the monopile provides a complimentary and synergistic effect that lowers the stresses and thereby enable the construction of a monopile of a thinner gauge material and thereby will reduce cost. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a view of an offshore wind turbine mounted on a monopile foundation to provide context to the invention;

Figure 2 is a view of a first example of a monopile with fins or ribs in accordance with the invention;

Figure 3 is a cross section through part of the finned monopile in Figure 2;

Figures 4 and 5 are schematic views of the monopile example, showing an associated fluid delivery system;

Figure 6 and 7 are cross section views through two more example of monopiles in accordance with the invention;

Figure 8 and 9 are views of two different examples of finned monopiles illustrating how fins may be attached toa wall of the monopile;

Figure 10 is a further example of a monopile in accordance with the invention, showing a monopile leading edge having a sawtooth profile; and Figure 11 and 12 are two further examples of monopile design, based on that shown in Figure 10, but which show relatively short and long fins, respectively.

Note that features that are the same or similar in different drawings are denoted by like reference signs.

SPECIFIC DESCRIPTION

A specific embodiment of the invention will now be described in which numerous features will be discussed in detail in order to provide a thorough understanding of the inventive concept as defined in the claims. However, it will be apparent to the skilled person that the invention may be put into effect without the specific details and that in some instances, well known methods, techniques and structures have not been described in detail in order not to obscure the invention unnecessarily.

In overview the invention provides a design of monopile foundation for a wind turbine, particularly though not exclusively to an offshore wind turbine, that is more robust and effective than conventional designs. In particular, the improvements in monopile design illustrated here in the examples of the invention provide a monopile that is particularly resistant to torsional forces meaning that the monopile design is well suited to foundation applications where high torsional forces are expected in use, such as when used in conjunction with multirotor wind turbines. Examples of the invention are also more robust to the pile driving process and less prone to end buckling.

Referring first to Figure 1 , a wind turbine 10 is shown in an offshore setting. The wind turbine shown here is conventional in the sense that it is a horizontal axis wind turbine or HAWT, comprising a tower 12 supporting a nacelle 14 to which a rotor is attached. The rotor comprises a hub 16 which carries a set of blades 18, three in this case. The operation of the wind turbine 10 is conventional and so no further discussion will be provided.

The wind turbine 10 is supported on the seabed 19 by a monopile foundation 20, or more simply just ‘monopile’. As is known, a monopile is an established approach for providing a foundation for a wind turbine particularly in relatively shallow water depths of around 10 to 20 metres. Typically a monopile is a large steel tube that may be between 5 and 10 metres in diameter, although other sizes are possible. Monopiles are installed in an installation process in which the monopile is held in a vertical orientation and their leading edge or ‘head’ is lowered towards the seabed. Once the head of the monopile is rested on the seabed, force is applied to the top of the monopile to drive the tubular structure down into the bed material until an appropriate depth is reached. A common approach to drive the monopile includes what is sometimes known as ‘hydraulic impact piling’ or ‘hammering’ using a large mass suspended by a crane or similar machine which is used to repeatedly strike the top of the monopile to create the appropriate driving force. An alternative, and less aggressive, approach is to vibrate the monopile with a predetermined frequency which provide a more ‘gentle’ process which generates less noise, and has less risk of damaging the monopile.

Generally, monopiles have been a successful technique for installing wind turbines in relatively shallow waters. However, the conventional monopile designs have some limitations in their ability to deal with torsional loads, which in extreme circumstances can compromise the frictional bond between the outside surface of the monopile and the seabed material in which it is embedded. The emergence of multirotor wind turbine systems means that high torsional loads are more likely, so there is a need to improve the design of conventional monopiles to address this issue.

The following discussion explains various improved features relating to the design of monopile structures.

With reference firstly to Figure 2, a wind turbine support structure in the form of a monopile 30 comprises a tubular body 32 being defined by a wall 34 that extends about a longitudinal axis X. The wall 34 and defines radially inner and outer faces 36,38. Conventionally, a suitable material for the monopile is steel, and the tubular structures are typically manufactured by way of a rolling process.

The top of the monopile 30 cannot be seen in Figure 2. However, the lower end of the monopile 30 will be referred to as a ‘head’ 39 of the monopile as is defined by a lowermost edge 40 of the wall 34. The lowermost edge 40 therefore is a leading edge as the monopile is driven into the seabed.

As can be seen from the Figure, the head 39 of the monopile 30 features a torsional resistance arrangement 42 comprising a plurality of fins or ribs 44 attached to the wall 34 near to the leading edge 40. The illustrated example has four fins 44 which are circumferentially arranged in mutually orthogonal positions. It is envisaged that this arrangement would provide a balanced resistance to torsional loads around the head 39 of the monopile 30, but it should be appreciated that a fewer or greater number of fins 44 could be provided.

Each of the fins 44 extends axially in this example, in parallel to the longitudinal axis X of the monopile 30, although strict axial alignment is optional and the fins 44 could in practice extend in a slanted manner although that is not currently preferred.

Figure 3 shows a cross section view through one of the fins 44 in Figure 2, where it can be seen that the fin 44 is configured to define a fluid channel 46. The fin 44 is shown here having a radially inner edge 48 and a radially outer edge 50, wherein the radially inner edge 48 is attached to the outer surface 38 of the wall 34. The fin 44 has a U-shaped cross section in this example so that it resembles a deep trough, so that when the fin 44 is attached to the wall 34, the fluid channel 46 is defined partly by the fin 44 and also partly the outer surface 38 of the wall 34. Other embodiments are envisaged, however, in which the fluid channel 46 may be defined entirely by the fin 44 and not by the wall 34.

The fin 44 may be attached to the wall 34 by any suitable technique such as welding or even a bolted flanged connection. As shown here, the inner edge 48 of the fin walls 54 are welded to the wall. The weld beads are indicated at ‘56’.

Figure 4 shows the fin 44 in a side view where it can be seen that the fin 44 has a generally rectangular shape in side profile, with a bottom edge 58 being approximately aligned with the leading edge 40 of the wall 34. In other examples, however, the bottom edge 58 of at least some of the fins 44 protrude beyond the leading edge 40 of the wall 34.

The outer edge 50 of the fin 44 is straight in this example, and is generally parallel to the outer surface 38 of the wall 34 and, thus, to the longitudinal axis X of the monopile 30. As can be seen in Figure 4, the fluid channel 46 runs along the length of the fin 44 so as to define openings at its upper and lower extremities.

The fluid channel 46 forms part of a fluid delivery system 60 of the monopile 30. The fluid delivery system 60 is configured to inject high pressure fluid from the bottom of the fin 44 near to the leading edge 40 of the monopile 30 which assists the pile driving process. The fluid channel 46 has a top end 62 which includes a connector or inlet valve 64 to permit fluid, for example high pressure water or a type of drilling slurry or ‘mud’, to be injected into the top of the fluid channel 46 where it can flow down the fluid channel 46 and out through a fluid outlet or nozzle 66 defined at the bottom end 54 or ‘tip’ of the fin 44.

The connector 64 is shown here schematically as being attached to a fluid line 68 and a fluid source 70. The fluid source 70 may be any suitable device, machine or system that can supply fluid to the fluid channel 46 in the fin 44. For example, it could be a water pump that is associated with a wind turbine installation vessel that can pump sea water or fresh water through the fluid line 68 and into the fluid channel 46.

Fluid that flows down the fluid channel 46 in the fin 44 is ejected from the nozzle 66 as a jet which has the effect of disturbing and loosening the sandy material of the seabed which reduces the resistance to the motion of the monopile 30 as it is driven downwards.

The example of Figure 4 includes a fin 44 that extends radially outwards from the wall 34. However, in other examples, fins 44 may be included that extend radially inwards from the wall 34. One such example is shown in Figure 5. Here, it will be appreciated that two fins 44 are provided, one of which extends in a radially outwards direction from the wall 34 and the other of which extends radially inwards. What is more, here the fins 44 are shown as being oriented generally in the same vertical plane.

As shown here, both fins 44 can be considered to be identical but it is envisaged that they may differ in certain features. However, both of the fins 44 have a respective fluid channel 46 with a respective connector 64 at the top end thereof which connects to a fluid delivery source 70 by way of respective branches of a fluid delivery line 58.

Providing fins 44 that extend outwards and inwardly from the wall 34 provides a greater resistance to torsional loading and, in this case, the pair of fins 44 located at each circumferential station around the leading edge 40 of the monopile 30 can provide liquefaction of the sea bed more evenly around the leading edge 40 during driving of the monopile 30.

In the illustrated embodiments discussed above, it will be noted that the fins 44 are configured so as to be attached to the outer surface 38 of the wall 34, as in the example of Figure 4, and also attached to the inner surface 36 of the wall 34, as in the example of Figure 5. Other configurations of fin 44 are envisaged, however, and two of these are shown in Figures 6 and 7, in which the same reference numerals will be used to refer to the same components or features where appropriate.

Referring firstly to Figure 6, firstly it will be noted that a total of eight fins 44 are provided rather than four in the previous embodiment, and that the fins 44 are equi-spaced around the circumference of the wall 34. However, in contrast to the previous examples, each of the fins 44 is embedded in the wall 34 rather than being attached to the outer surface 38 of the wall 34. Therefore, each of the fins 44 bridges the thickness of the wall 34 and extends radially outwards from the outer wall surface 38 and also radially inwards from the inner surface 36 of the wall 34.

Considered another way, each of the fins 44 has a first wall portion 44a that extends radially outwards from the wall 34 and a second wall portion 44b that extends radially inwards from the wall 34. This is similar in configuration to the example in Figure 5 above which has inner and outer fins, such that those separate fins can be considered to define first and second fin portions in the same way as the example shown in Figure 6.

In contrast to the previous examples, however, each fin 44 has a box-shaped cross section profile so it is the structure of the fin 44 itself that defines its internal fluid channel 46. That is, the first or ‘outer’ fin portion 44a cooperates with the second or ‘inner’ fin portion 44b to define the fluid channel 46. Here, the fins 44 are approximately square in cross section, although other profiles are envisaged, such as an elongated rhombus. Still further, cross section profiles of the fins 44 may be configured so that the radially outer fin portions 44a have a deeper section than the radially inner fin portion 44b, such that the fin portions extend away from the wall 34 by different extents.

Although not shown in Figures 6 and 7, each fin 44 may be associated with a fluid delivery system 60 as in the previous examples.

As can be seen in Figure 6, each of the fins 44 bridges or penetrates the structure of the wall 34 and so may be received in a slot or opening (not shown) defined by the wall and then secured in place by a suitable method, for example being welded at edges of the fins 44. Weld points 41 are shown in Figure 6 by way of example on the inner surface 36 and the outer surface 38 of the wall 30. Turning to Figure 7, there is shown a further fin configuration in which each fin has a boxlike cross section like that in Figure 6, but each fin 44 is now equipped with a further radially extending fin portion 72 that extends radially outwards from the respective fin 44. In effect, therefore, the further fin portion 72 increases the radial dimension of each fin 44 which may increase the torsional resistance of the monopile 30, if that is required for a particular application.

It should be noted here, that the box-like profiles of the fins 44 shown in Figures 6 and 7 may also be achieved by attaching a pair of half sections to either side of the wall 34, much like as shown in Figure 3.

The discussion will now turn to Figures 8 and 9 which show further examples of fin structures for monopiles in accordance with the invention.

Referring firstly to Figure 8, a small portion of a wall 34 of a monopile 30 is shown in cross section, to which a fin 44 is attached as in the previous examples. Note that the lower part of the Figure shows the fin 44 from the side, where the wall 34 is shown in section, whereas the upper part of the Figure shows the fin from above, with the fin and the wall in section.

It will be appreciated in this example that the fin 44 does not include an internal fluid channel. However, it will be noted that the fin 44 has a radially outer fin portion 44a and a radially inner fin portion 44b. Each of the fin portions 44a, 44b are aligned such that they lay in a common plane. The two portions of the fin 44 are suitably attached to the wall 34, as by welding, so that together they form a completed fin that appears to traverse the wall 34 in the thickness direction. As shown, the lowermost end of the fin 44 extends beyond the leading edge 40 of the wall 34 and so here the two fin portion 44a, 44b abut one another and are preferred to be welded together, as can be seen at join line 79.

Unlike the example in Figure 5, the two fin portions 44a, 44b are not identical. Each of the fin portions 44a, 44b is generally triangular in this example such that each fin portion 44a, 44b defines an outer edge, labelled as 80 and 82, respectively, that extends at an angle to the wall 34. The angled edges 80,82 converge at a point below and generally in line with the leading edge 40 of the wall 34 to define a fin tip 81 that is a relatively sharp point. Each of the outer edges 80,82 may define approximately the same angle with the wall 34, but in this example the triangular shape of each of the fin portions 44a, 44b is different such that the outer edge 80 of the outer fin portion 44a defines a shallower angle with the wall 34 when compared with the outer edge 82 of the inner fin portion 44b. More specifically the outer edge 80 defines an angle with the wall 34 of approximately 10 degrees, whilst the inner edge 82 defines an angle of approximately 30 degrees. The effect of the unequal edge angles of the two fin portions 44a, 44b can be appreciated by considering the forces that are exerted on the head 39 of the monopile as it is driven into the seabed. As the leading edge 40 is hammered downwards into the ground, the ground exerts pressure on the parts of the monopile which it contacts. Due to the angles of inclination defined by the edges of the outer and inner fin portions 44a, 44b, the ground exerts an unequal force on the wall 34 via those fin portions. Due to the greater angle of inclination of the inner fin portion 44b, the ground ‘pushes’ on the wall 34 with a larger force than the wall is experiencing from the outer fin portion 44a. This means that the wall 34 is supported strongly from the inside which is a benefit because it discourages the leading edge 40 of the monopile 30 from buckling during the pile driving process.

Figure 9 shows another example of a finned monopile structure which is similar to that in Figure 8. However, the fin 44 is constructed differently and is attached to the wall 34 in a different manner.

In Figure 9, the fin 44 is of single part construction for example a single piece of sheet steel. The geometry of the fin 44, when considered in side profile, is in essence the same as the fin shown in Figure 8. However, rather than being formed of two parts which are welded onto the wall 34, the fin 44 in the example of Figure 9 is provided with a linear slot 83 that extends downwardly from an upper edge 84 of the fin 44. The axial length of the slot 83 is approximately half the vertical height of the fin 44, in the orientation shown in the figures.

The width of the slot 83 is approximately the same as the thickness of the wall 34 so that the fin 44 can be received onto the leading edge 40 of the wall 34. Once in position, the fin 44 can be secured in place by a suitable technique such as welding for a high strength attachment.

It will be appreciated that in both the examples of Figure 8 and 9, the tip 81 of the fin 44 extends beyond the leading edge 40 of the wall 34. When considered across a plurality of such fins 44 distributed about the head of the monopile, the fins provide what can be considered as a toothed leading edge, which helps the monopile penetrate into the ground.

A further development of this idea of a toothed leading edge will now be explained with reference to Figure 10.

The monopile 30 of Figure 10 has many similar features to previously illustrated examples. For example, the head 39 of the monopile 30 is equipped with a plurality of fins 44 that are distributed around the head 39 with equal separation between them.

Each of the fins 44 are configured so that their tips 81 extend between the leading edge 40 so as to create a toothed effect for improved pile driving. What is noticeable in the illustrated example, however, is that in addition to the ‘teeth’ provided by the fins 44, the leading edge 40 is shaped to provide an undulating, wavy or sawtooth profile around its circumference. The peaks of the profile of the leading edge 40 coincide with the positions of the fins 44, so that portions of the leading edge 40 that extend between adjacent ones of the plurality of fins 44, are shaped to define a V-shaped or a U-shaped profile.

The effect of the sawtooth leading edge 40 profile and the fins 44 is to improve the characteristics of the monopile as it is being driven into the seabed. The fin tips 81 will present a very small surface area initially to the seabed which will ease the passage of the monopile into the seabed. The V-shaped portions of the leading edge 40 between the fins 44 will have the effect of increasing resistance to pile driving relatively gradually compared to the situation where the leading edge 40 is straight.

It should be noted that various features described in the examples above may be combined with features of other components. For instance, the fluid delivery system 60 of Figures 4 and 5 may be combined with the finned structure of Figure 8, 9 and 10.

In terms of dimensions of the fins 44, it is currently preferred that the fins extend only a relatively short distance from the outer or inner surface of the wall 34, as applicable. Typically, the wall thickness of a monopile will be in the region of around 50mm to 110mm, depending on the intended application and the diameter. In this context, it is envisaged that a useful range of widths for the fins will be around 100mm to 300mm, when considered for the types of fins shown in Figure 10. Expressed another way, each of the outer and inner fin portions could extend from the wall by a distance of about 25mm to 100mm. Considered for a range of monopile diameters between about 3m and 10m, then it is envisaged that the fin widths will be between about 2% and about 5% of the monopile diameter. In this context, it will be noted that shorter fin lengths may require greater fin widths to maintain sufficient torsional resistance.

It should be appreciated that the geometry of the fins may be optimised to achieve certain objectives. For examples, from a manufacturing perspective a fewer number of axially shorter but radially larger fins would be more cost effective as this would mean a comparatively small interface between the tower wall and the fin that needs to be welded. However, soil conditions may dictate that a different geometry may be more optimal. One instance of this is that if the sea bed is very hard, it may be the case that axially larger and radially smaller fins may be more suitable. Such a configuration would tend to reduce the local piling which would reduce the likelihood of buckling. In softer substrates, it may be the case that a higher total fin area is required in order to achieve the necessary torsional capacity of the pile to soil interface, which may be achieved by larger fins or a higher number of fins. In general buckling strength is improved with a higher number of fins.

The fins can be various lengths. Figure 11 and 12 illustrate examples of this. The monopile 30 in Figure 11 illustrates an example of relatively short fins 44, which each have an axial dimension D1 , from their tips to their upper ends. The axial distance D1 can be appreciated as being comparable, or between about 80% to 120%, to the diameter of the monopile 30 which is marked on Figure 11 as D2.

The relatively short fins 44 in Figure 11 can be compared with Figure 12 which shows a monopile 30 with relatively long fins 44. In this case, the axial dimension of the fins D1 is about twice the diameter D2 of the monopile 30, or in the range of between 180% to 250% of the diameter D2.

Notably, it is envisaged that longer fins may be constructed so that their radial dimension is reduced compared to shorter fins, in order to achieve a comparable resistance to torsional moments.

The skilled person will appreciate that the illustrated examples of the invention discussed above are not essential to performing the invention and that those examples may be modified without departing from the invention as defined in the claims. Some variants have already discussed above. Others will now be mentioned below.

In the illustrated examples, the types of fins that have been provided on the monopiles have been shown as being of one ‘type’ of fin. However, it should be appreciated that the types of fin that are provided on a particular monopile may be different.