The present invention relates to transition pieces for use with a monopile structure, such as a driven monopile used in off-shore structures.

Off-shore structures, such as wind-turbines , are often supported by a pile structure, for example, a monopile structure. Wind turbines mounted on a monopile structure have a single cylindrical pile that is driven into the seabed. A hollow cylindrical transition piece sits on top of the pile in order to provide a level platform on which to mount the wind turbine . The transition piece is usually levelled if needed, and then the transition piece is fixed to the pile with grout. The grout may be supported by an annular grout seal attached to the transition piece or the pile. The wind turbine is then attached to, for example, the top of the level transition piece.

However, the monopile is normally subjected to a number of stresses after installation, such as the force of waves, the wind and the tide. The stress on the monopile structure can cause cracking of the grout and the grouted connection may deteriorate and eventually fail.

The present invention aims to reduce damage to a monopile structure after installation.

At its most general, the present invention provides a hollow cylindrical transition piece having one or more rigid spacers with a contact portion for abutting a cylindrical pile.

According to a first aspect, the invention provides a transition piece for fixing to a cylindrical pile, the transition piece comprising a cylindrical transition piece body having a cylindrical cavity for accommodating the pile, the cavity having an opening at a first end of the transition piece body, the transition piece further comprising a plurality of rigid spacers projecting from an inner cylindrical wall of the transition piece body into the cylindrical cavity, each rigid spacer having a contact portion to abut against an outer wall of the pile, and the spacer being adjustably attached to the transition piece body so that the distance between the contact face and the inner wall of the transition piece body is adjustably fixed.

The rigid spacers of the first aspect may be adjusted so that the contact portion abuts the cylindrical pile, when installed. As a result, the rigid spacer fixes a space between the inner wall of the transition piece body and the outer wall of the cylindrical pile. In this way, the rigid spacers help the transition piece body and

cylindrical pile to work together as a composite and reduce the stresses in the grout which are caused by distortion (or ovalling) of the transition piece body.

Equally, some of the stress applied to the transition piece body may be transferred to the monopile via the rigid spacers rather than the grout, and so the force applied to the grout may be reduced. So, the transition piece of the first aspect provides a component of a monopile structure whereby the grout (once added to the monopile structure) is less likely to suffer damage as a result of stress caused by force acting on the exterior of the transition piece.

The rigid spacer is rigid along at least part of the axis extending from the inner wall of the transition piece body to the contact portion. In this way, the rigid spacer fixes a space between the inner wall of the transition piece body and the outer wall of the

cylindrical pile.

In preferred embodiments, the rigid spacer includes a rigid rod having an elongate axis extending from the contact portion to the inner wall of the transition piece body and being rigid with respect to its elongate axis . The contact portion is typically rigid along the elongate axis of the rigid rod. Alternatively, the contact portion is deformable in the elongate axis of the rigid rod.

The rigid rod may be threaded to engage with an adjusting nut to allow the distance from the contact portion to the inner wall of the transition piece body to be adjusted. The adjusting nut has a complementary threaded aperture for receiving the threaded rigid rod.

Typically, the adjusting nut is fixed to the transition piece wall with nut fixtures. In this way, the rigid spacer is attached to the transition piece wall. The adjusting nut may be located on the inner wall of the transition piece body or on an outer wall of the

transition piece body. The rigid spacer may also include a fixing means for fixing the rigid rod relative to the transition piece. In some embodiments, the adjusting nut may include the fixing means. In this way, the adjusting nut allows the distance between the contact portion and the inner wall of the transition piece body to be adjusted to an appropriate distance, and further allows the distance to be fixed so that rigid spacer may hold the inner wall of the transition piece body at a fixed minimum distance from the outer wall of the cylindrical pile.

The rigid rod is typically longer in length than the annular gap created by the gap between the transition piece body and the cylindrical pile. In this way, the distance between the contact portion of the rigid spacer and the inner wall of the transition piece body may be adjusted so that the contact portion can abut the cylindrical pile. Accordingly, an adjusting end of the rigid rod may protrude through an aperture in the transition piece body to allow for variance of the annular gap. The adjusting end of the rigid rod may be used to adjust the distance between the contact portion and the inner wall of the transition body, for example, by rotating a threaded rigid rod clockwise or

counter-clockwise. The adjusting end of the rigid rod may be shaped for ease of adjustment, e.g. like a nut.

The annular gap may vary around the circumference of the transition piece body. This may occur if the cylindrical pile is not vertical and the transition piece body is levelled, so that the inner cylindrical wall of the transition piece body and an outer cylindrical wall of the cylindrical pile are not parallel. The number of rigid spacers may be varied to suit, for example, the diameter of the cylindrical pile. Preferably, the transition piece includes 16 rigid spacers. 16 rigid spacers helps to distribute force exerted on the exterior of the transition piece body evenly, thereby reducing deformation of the transition piece body.

The rigid spacers may be located anywhere on the

transition piece body. Preferably, at least two of the rigid spacers are located in a single plane perpendicular to the cylindrical axis of the cylindrical transition piece body. The rigid spacers may be distributed

substantially equally around the circumference created by a plane perpendicular to the cylindrical axis of the cylindrical transition piece body. In this way, the beneficial effect produced by the rigid spacer is provided evenly around the circumference of the

transition piece body. In particular, deformation of the cross-sectional shape of the transition piece body may be greatly reduced when a plurality of substantially equally spaced rigid spacers are used in the transition piece body of the invention. Preferably the transition piece of the present invention includes an annular grout seal attached to the inner wall of the transition piece body. Annular grout seals are known and provide support for grout provided in the annular gap between the inner wall of the transition piece body and the outer wall of the cylindrical pile.

Preferably the transition piece includes an annular grout seal attached to the inner wall of the transition piece body for supporting grout. When a grout seal is present, the transition piece may include an array of rigid spacers, the array having two or more of the rigid spacers of the transition piece, and the array is positioned within 20 % of a groutable length of the transition piece body from the annular grout seal, the groutable length being the distance along the cylindrical axis of the transition piece body from the grout seal to end of the transition piece body in the direction of the array of rigid spacers from the annular grout seal .

Cracking of the grout is more likely to occur at a lower end of the grout in the vicinity of the annular grout seal. The positioning of the array of rigid spacers close to the annular grout seal (i.e. within a percentage of the groutable length of the transition piece body) , each rigid spacer of the array further assists to reduce the damage to the grout . The array of rigid spacers may be positioned within about 2 meters or less, about

1.5 meters or less, or about 1 meter or less from the annular grout seal .

Preferably the transition piece of present invention includes a reinforcement cage attached to the inner cylindrical wall of the transition piece body, wherein the reinforcement cage includes a first and a second reinforcing wire loop connected by one or more deformable stirrups, the first and second reinforcing wire loops extending circumferentially within the inner wall of the transition piece body, the deformable stirrup having a buffering portion for buffering against the cylindrical pile and a fixing portion for fixing the reinforcement cage to the inner wall of the transition piece body, and a resiliently deformable portion connecting the buffering portion and the fixing portion of the deformable stirrup so that the distance between the buffering portion and the fixing portion is variable, and wherein the first reinforcing wire loop is attached to the buffering portion of the deformable stirrup, and the second reinforcing wire is attached to the fixing portion of the deformable stirrup. In this way, the first reinforcing wire loop provides an inner reinforcing wire loop in vicinity of the outer wall of the cylindrical pile and the second reinforcing wire loop provides an outer reinforcing wire loop in the vicinity of the inner wall of the transition piece body. This configuration provides additional reinforcement to the grout (when provided in the annular gap) . The resiliently deformable portion of the stirrup allows the first and second reinforcement wire loops to be connected while allowing variance in the distance between the first and second reinforcement wire loops to accommodate variation in the annular gap.

Typically the first and second reinforcing wire loops extend in parallel with the circumference of the

cylindrical transition piece body. So, the reinforcing wire loop may be circular or elliptical in shape.

Preferably the reinforcement cage includes a third reinforcing wire loop attached to the buffering portion of the deformable stirrup and/or a fourth reinforcing wire loop attached to the fixing portion of the

deformable stirrup. In some embodiments, the first and second reinforcing wire loops or the third and fourth reinforcing wire loops are connected by a further deformable portion of the deformable stirrup.

The resiliently deformable portions may independently be made from any compressible or bendable material to allow for the variance of the annular gap. In preferred embodiments, the deformable portion is a bendable curved wire. In particularly preferred embodiments, the fixing portion, buffering portion, deformable portion and further deformable portion (if present) are made from a single piece of wire, with the deformable portions including a bendable curved piece of the wire. The wire is typically made of a deformable metal . According to a second aspect, the present invention provides a monopile structure comprising a cylindrical pile and a hollow cylindrical transition piece of the first aspect. Preferably the monopile structure of the second aspect includes grout located in the annular gap between an outer wall of the cylindrical pile and an inner wall of the transition piece body, whereby the grout is supported by an annular grout seal attached to the inner wall of the transition piece body.

The monopole structure may include a wind- turbine fixed to a mounting surface of the transition piece. According to a third aspect, the present invention provides a method of installing a transition piece on a cylindrical pile, the transition piece comprising a cylindrical transition piece body having a cylindrical cavity for accommodating the pile, the cavity having an opening at one end of the transition piece body for receiving the pile, the method comprising the steps of:

(i) positioning the cylindrical pile into the cavity of the transition piece body;

(ii) optionally levelling the transition piece body that a mounting portion of the transition piece body is horizontally level; and

(iii) adjusting one or more rigid spacers projecting from an inner cylindrical wall of the transition piece body so that a contact portion of the rigid spacer abuts an outer wall of the cylindrical pile; and then

(iv) fixing each rigid spacer relative to the inner wall of the transition piece body so that the distance from the inner wall of the transition piece body to the contact portion is fixed.

According to a fourth aspect, the present invention provides use of a rigid spacer in a monopole structure having a cylindrical pile and a transition piece, the transition piece having a hollow cylindrical transition piece body, wherein the rigid spacer has a contact portion to abut against an outer wall of the pile, and the spacer being adjustably attachable to the transition piece body so that the distance between the contact face and the inner wall of the transition piece body is adjustably fixable.

According to a fifth aspect, the present invention provides a kit for use with a monopole structure having a cylindrical pile and transition piece, the transition piece having a hollow cylindrical transition piece body, wherein the kit comprises: a rigid spacer having a contact portion to abut against an outer wall of the pile, and the rigid spacer being adjustably attachable to the transition piece body so that the distance between the contact face and the inner wall of the transition piece body is adjustably fixable; and

an annular grout seal for attaching to an inner wall of the transition piece body.

The above optional and preferred features of any of the aspects may be combined with any of the other optional or preferred features of any of the aspects.

For the avoidance of doubt, the cylindrical axis of the transition piece body is the axis around which the cylinder has rotational symmetry. Typically the

transition piece body will have a circular or elliptical cross-section with respect to a plane perpendicular to the cylindrical axis and an oblong or quadrilateral cross-section with respect to a plane parallel to the cylindrical axis.

The present invention will now be described with

reference to the following drawings:

Fig. 1 shows a transition piece of the present invention installed on a cylindrical pile.

Fig. 2 shows an example of the rigid spacer of the transition piece of the present invention.

Fig. 3 shows a reinforcement cage of the present invention.

Figs. 4A and 4B show the rigid spacer, reinforcement cage and grout seal arrangement of the installed

transition piece of Fig. 1. Figure 1 shows a monopile structure 8 having a transition piece 10 installed on cylindrical pile 12. In this embodiment, the cylindrical pile is about 4600 mm in diameter and is suitable for supporting a wind-turbine structure. The diameter of the transition piece is about 4900 mm.

A number of rigid spacers 14 are attached to a hollow cylindrical transition piece body 16 of the transition piece 10. The transition piece body has a cylindrical axis 17 running along its elongate length. In Fig. 1, there are sixteen rigid spacers 14. The rigid spacers 14 are equally spaced around the circumference of the cylindrical transition piece body 16.

An annular grout seal 18 is positioned in the annular gap 19 formed by the transition piece body 16 and the cylindrical pile 12 , and positioned at a lower end of the transition piece body 16. A reinforcement cage 20 is fixed to the inner wall of the transition piece body 16 and positioned along the elongate cylindrical length of the transition piece body 16 between the rigid spacers 14 and the annular grout seal 18. An expanded view of the rigid spacer, reinforcement cage and annular grout seal are shown in Figs. 4A and 4B (marked as A and B in

Fig. 1, respectively).

One of the rigid spacers 14 can be seen in more detail in Fig. 2. Each rigid spacer 14 has a rigid cylindrical bolt 22. A contact plate 24 for abutting the cylindrical pile 12 is attached to an abutting end 26 of the bolt 22. The cross-sectional shape of the rigid bolt 22 in a plane perpendicular to the elongate axis of the rigid bolt 22 is not particularly limited. The cross-sectional-shape may be, for example, square, triangular, rectangular, elliptical or circular. The cross-sectional shape of the rigid bolt 22 in this embodiment is circular.

The rigid spacer 14 also has a spacer nut 28 surrounding the rigid cylindrical bolt 22 and acting as an adjusting nut. The spacer nut 28 is fixed to the inner cylindrical wall of the transition piece body 16 and the bolt 22 is adjustably engaged with an aperture in the nut 28. The spacer nut 28 may be fixed to the outer wall of the transition piece body 16. The way in which the spacer nut 28 is fixed to the transition piece body 16 is not particularly limited. In this embodiment, the spacer nut 28 is fixed to the transition piece body 16 by two fixing studs 30 that penetrate the spacer nut 28 and the transition piece body 16.

Typically, the spacer bolt 22 will have a helical thread on its exterior surface to engage with a complementary groove on the inner surface of the aperture of the spacer nut 28. The distance between the contact plate 24 and the inner wall of the transition piece body 16 can be

adjusted using the spacer nut 28 and spacer bolt 22.

The spacer nut further includes a clamping means for preventing further rotation of the spacer nut. In this way, the distance from the contact plate to the inner wall of the transition piece body is fixed. Additionally, the grout (when set) prevents the rotation of the spacer nut so as to fix the distance of the contact plate from the inner wall of the transition piece body. In some embodiments, the grout alone may prevent rotation of the spacer nut . The number of rigid spacers 16 is not particularly limited, and the skilled person will recognise that the number of rigid spacers 16 may be tailored to the

specification of the transition piece body 16 and

cylindrical pile 12. So, for example, the transition piece 10 may include 2, 3, 4, 5, 6, 8, 10, 12, 14, 16, 20 or 24 rigid spacers 16. In this embodiment, the

transition piece 10 has 16 rigid spacers 16.

The diameter of the rigid bolt 22 should be large enough to withstand force exerted on it. In some embodiments, the rigid bolt 22 is at least 60 mm in diameter. The upper limit of the diameter of the rigid bolt 22 is not particularly limited. In some embodiments, the diameter of the rigid bolt 22 is in the range of 80 to 200 mm, for example, about 80 mm, 100 mm, 120mm, 150 mm or 200 mm.

The diameter of the rigid bolt 22 in this embodiment is about 100 mm.

The contact plate 24 may have a contact face for abutting the cylindrical pile 12 that is larger than the

cross-section of the abutting end 16 of the rigid

bolt 22. Where the contact plate 24 has a circular contact face, the diameter of the contact face may be larger than cross-section of the abutting end of the rigid rod by 1.1 times or more, 1.2 times or more, 1.3 times or more, 1.5 times or more, 1.75 times or more, 2 times or more, 2.5 times or more, or 3 times or more. The contact face of the contact plate 24 of this embodiment has a surface area of about 3 times the area of the cross-section of the abutting end 26 of the rigid

bolt 22. The contact plate 24 may be attached to the rigid bolt 22 by any means. The contact plate 24 may be formed in a single piece with the rigid bolt 22 or may be attached with, for example, a screw. In this embodiment, the contact plate 24 is attached to the rigid bolt 22 with a spherical bearing.

The reinforcement cage 20 is shown in more detail in Fig. 3. The cage 20 has four reinforcing loops 32 extend circumferentially inside the annular gap 19. There are two inner reinforcing loops 34 in the vicinity of the outer wall of the cylindrical pile 12 (not shown) and two outer reinforcing wire loops 36 in the vicinity of the inner wall of the transition piece body 16. The

reinforcing wire loops 32 are substantially rigid

A wire stirrup 38 connects the four reinforcing wire loops 32. The stirrup 38 is formed from a single piece of wire. The stirrup 38 has a bendable curved part 40 of wire at the upper end of the stirrup 38 connecting two straight parts 42, 44 of the wire. The reinforcing wire loops 32 are attached to the straight parts 42, 44 of the wire. A further part 46 of the wire at the lower end of the stirrup 38 is angled with respect to the straight parts 42, 44 and has a bendable curved part.

As can be seen form Figs. 4A and 4B, the stirrups 38 are independently deformable so that variation of the annular gap 19 between the outer wall of the cylindrical pile 12 and the inner wall of the transitional piece body 16 can be accommodated. The upper and lower parts of the stirrup bend in Fig. 4A when the annular gap 19 is small relative to the annular gap 19 on the other side of the monopile structure 8 (shown in Fig. 4B) .

The number of deformable stirrups 38 is not particularly- limited. The stirrups may be spaced in the range of 15 mm to 450 mm apart around the circumference of the inner transition piece body. In this way, the number of stirrups may be determined by the circumference of the transition piece body.

In this embodiment, there are 16 stirrups 38. Each stirrup 38 is attached to the transition piece body 16 between each rigid spacer 14 and the annular grout seal 18. In this way, each stirrup 38 provides

additionally reinforcement around each rigid spacer to reduce damage to the grout after installation of the monopile structure 8.

The reinforcement cage 20 ties together the grout seal 18 and the rigid spacers 14, and provides shear resistance to help reduce the formation of cracks in the grout located in the annular gap 19 formed between the

cylindrical pile 12 and the inner wall of the transition piece body 16 (when installed on the cylindrical

pile 12) .

The transition piece 10 of the present invention is particularly useful for monopile structures 8 with a large diameter. The diameter of a typical cylindrical pile 12 of an off-shore structure is around 2 to 8 1

16

meters. The diameter of the inner wall of the transition piece body 16 is typically around 0.1 to 0.4 meters greater than the diameter of the cylindrical pile 12, so, about 2.0 to 8.5 meters. The elongate cylindrical length of the transition piece body 16 is not particularly limited, and may be in the range of 4 to 15 meters.

The transition piece 10 of the present invention may include one or more levelling jacks 48 fixed to the inner wall of the transition piece body 16 for levelling the transition piece body 16. Typically the levelling

jacks 48 are located at an upper end of the transition piece body 16. In this way, the levelling jacks 48 may engage with an upper surface of the cylindrical pile 12. The levelling jacks 48 provide a convenient way of levelling a mounting surface 50 of the transition piece body 16.

In Fig. 1, a number of levelling jacks 48 are fixed to the inner cylindrical wall of the transition piece body

16. The levelling jacks 48 have a levelling face 52 for contacting with the cylindrical pile 12. Each levelling face 52 may sit on an upper end of the cylindrical pile 12. The distance between the levelling face 52 and the point at which the levelling jack 48 is fixed to the inner cylindrical wall of the transition piece body 16 is adjustable independently for each levelling jack 48 so that the vertical orientation of the transition piece body 16 with respect to the cylindrical pile 12 can be adjusted. In this way, the levelling jacks 48 allow the transition piece 10 to provide a horizontally level mounting surface 50, even if the cylindrical pile 12 is not vertical along its cylindrical axis. A wind-turbine may then be mounted on the horizontally level mounting surface 52.

The annular grout seal 18 is positioned at a lower end of the transition piece body 16. The annular grout seal 18 has an attachment portion 54 and a grout supporting portion 56. The attachment portion 54 attaches to the transition piece body 16 and holds the grout seal 18 in a fixed position relative to the transition piece body 16. The grout supporting portion 56 supports the grout. The grout supporting portion 56 is deformable so that variance in annular gap 19 around the circumference of the cylindrical pile 12 may be tolerated. Once the transition piece 10 has been placed over the top of the cylindrical pile 12 and levelled (if necessary) , the rigid spacers 14 are adjusted so that the contact plate 24 is abutting the cylindrical pile 12. As can be seen from Figs. 4A and 4B, the distance from the contact plate 24 to the inner cylindrical wall of the transition piece body 16 is adjusted independently for each rigid spacer 14. The distance between the contact plate 24 and the inner wall of the transition piece body 16 reflects the annular gap distance at that part of the transition piece 10.

The grout is then added to the annular gap 19 between the inner wall of the transition piece body 16 and outer wall of the cylindrical pile 12. The grout is set in order to fix the transition piece 10 to the cylindrical pile 12.

Typically, the grout fills the annular gap 19 from the annular grout seal 18 to around the top of the

cylindrical pile 12. In the embodiment of Fig. 1, the grouted length 58 along the elongate axis of the

transition piece body 16 is around 8300 mm.

The rigid spacer 14 is positioned around 900 mm from the annular grout seal . This distance is around 11% of the grouted length 58. As a result the support of the rigid spacer 14 is at a lower end of the grout, where cracking is more likely to occur. The area of the transition piece body 16 adjacent to each rigid spacer 14 is held at a fixed distance from the cylindrical pile 12 by the rigid spacer 14. In this way, deformation of the transition piece body 16 is

substantially reduced. A reduction in the annular gap 19 under stress of use is also reduced. As a result, stress exerted on the grout is distributed evenly and cracking of the grout is reduced.

In particular, deformation of the cross-section with respect to an elongate axis of the transition piece body 16 may be reduced. In this way, stress on the transition piece may be more evenly distributed over the length and circumference of the transition piece body 16, and the force exerted on the grout is distributed

relatively evenly.

Example

To show a reduction in the stress shown by the present invention, finite element analysis (FEA) was performed on a monopile structure similar to the embodiment shown in Fig. 1. The test was performed in the absence of a reinforcement cage . The testing was performed on a transition piece with 16 equally spaced rigid spacers. Each rigid spacer is positioned approximately 900 mm above the grout seal. The diameter of the monopile is 4600 mm, and the monopile is offset from the vertical by 0.5'. The maximum annular gap is 180 mm, and the grouted length is about 8300 mm. The rigid spacer has a machined contact plate on a spherical bearing mounted on a 100 mm diameter rigid bolt, with an 88 mm inside thread. A 300 mm diameter adjusting nut is fixed to the transition piece wall with two fixing studs.

The same monopile was also tested with a transition piece without the rigid spacers.

The results of the analysis show that the rigid spacers reduce the hot spot stresses at the bottom of the grouted connection from greater than 5 N/sq mm to around

3 N/sq mm on both the compression and tension side of the combined section. This is a major reduction in stress which would, in itself, greatly reduce the possibility of grout crushing.

The major reduction in stress is the result of the spacers causing the combined pile, grout, and transition piece to operate as a combined section. Without the spacers, the transition piece tends to 'oval' with the result that the maximum stresses tend to be concentrated over a limited arc. With the combined section, the stresses are spread over a much wider arc. As a result the stress on the grout is greatly reduced.