The present invention relates to the design and use of a slip joint for mounting of an offshore wind turbine tower onto an offshore foundation.

The tower may or may not include a transition piece.

The foundation may be of a type that is fixed to the seabed, e.g. a monopile, a jacket, etc. Fixed foundation offshore wind turbines are effective in relatively shallow waters. In deeper waters it is envisaged that floating foundation wind turbines will be most effective, e.g. economical. For example, the document WO2009/131826 discloses several designs of floating foundation wind turbines. A spar-type floating foundation or a tension-leg type floating foundation may also be envisaged as examples.

Nowadays, offshore wind turbines are enormous. Power ratings of over 9 MW, e.g. of about 15 MW, are rather common. This requires a very tall and larger diameter tower that is mounted onto the foundation.

In order to establish a connection between the tower and the foundation, it is known to provide a slip joint mounting. For example, the document W02018/070868 discloses a slip- joint for this purpose.

An aim of the present invention is to provide an improved slip joint for mounting an offshore wind turbine tower, possibly including a transition piece, to the foundation.

The present invention provides a slip joint according to claim 1.

As will be explained herein, the provision of the first and second flanges and bolts may be advantageous when the slip joint is mated and as it later absorbs the enormous forces during operation of the wind turbine at sea. As explained herein, the inventive slip joint may also be of advantage in view of the de-installation of the wind turbine tower, for example in the de- installation of an entire offshore wind turbine, so including not only the tower but also nacelle and rotor blades as a unit from the (floating) foundation. For installation and, possibly, de-installation of an offshore wind turbine provided with the inventive slip joint use may be made of a vessel and/or methods as described in non- prepublished patent applications NL2027279, NL2027489, NL2027788, NL2027280 and NL2027786, which are incorporated herein by reference.

Advantageously, the spaced apart first and second flanges provide two internally protruding structural elements which each belong to a respective barrel of the slip joint, are internally accessible, e.g. as personnel can enter the into this part of the foundation and/or into the tower, e.g. for mounting and tightening of the bolts, possibly also spacer bushing, and/or for the removal thereof and the placement of jacks.

As explained herein, the placement of jacks, e.g. single action hydraulic jacks, allows to move the first and second flanges (very) forcefully apart from each other and thereby cause or at least assist in the release of the frictional engagement between the barrels of the slip joint.

Advantageous embodiments of the slip joint are described in the subclaims.

The barrels may frictionally contact one another at one interface, but multiple interfaces vertically spaced from one another, e.g. having different cone angles, are also envisaged in the context of the present invention.

The invention furthermore relates to an offshore wind turbine comprising an offshore foundation and an offshore wind turbine tower, e.g. the tower including a transition piece, wherein the tower is mounted or mountable to the foundation by means of a slip joint as described herein.

The invention furthermore relates to a combination of an offshore wind turbine tower and the outer barrel of a slip joint as described herein which is configured to be mated to an inner barrel of the slip joint as described herein, which inner barrel is connected to an offshore foundation, e.g. a floating foundation.

The invention furthermore relates to a method for installing an offshore wind turbine tower on an offshore foundation, wherein use is made of the slip joint as described herein, the method comprising the steps of: • mating the inner barrel of a slip joint with the outer barrel of the slip joint, e.g. by lowering the wind turbine tower provided with the outer barrel by means of a lifting device over the inner barrel that is connected to the foundation,

• arranging bolts through the bolt holes of the first and second flanges and tightening said bolts.

In an embodiment, the installation method further comprising the step of arranging spacer bushings between the first and second flanges, with at least one bolt extending through a spacer bushing.

In an embodiment, the installation method comprises lowering the wind turbine tower provided with the outer barrel by means of a lifting device, e.g. as described in one or more of the mentioned non-prepublished applications, over the inner barrel that is connected to the foundation, which lifting involves connecting at least one attachment member, e.g. multiple attachment members distributed along the outer circumference, of the upper section with the lifting device, e.g. with a hoisting cable thereof.

For example, in an embodiment of the method according to the invention the outer barrel of the slip joint and the tower are transferred by means of a vessel to the offshore location at which the offshore foundation is already installed, the foundation already being provided with the inner barrel.

The outer barrel is connected, e.g. on-board the vessel or already prior to said transfer, to the tower via the respective upper connection member. Subsequently, the combination of the outer barrel and the tower, possibly fitted with nacelle and rotor blades, is engaged by a lifting device, e.g. of a crane vessel, for example involving engaging and connecting to the attachment member described in claim 6.

The installation may involve the step of firstly upending the tower, e.g. already provided with the outer barrel, into an upright orientation by means of the lifting device.

The installation may involve, lowering the outer barrel onto the inner barrel as the lifting device lowers the tower, e.g. the entire wind turbine. During this mating process, the guide blocks described in claim 4 may be used for guiding the inner surface of the outer barrel into correct engagement with the outer surface of the inner barrel. The bolts are then arranged through the aligned bolt holes in the first and second flanges. However, it may for example also be possible to arrange the bolts into the bolt holes of the second flange already prior to the mating process.

It is noted that due to the presence of a vertical spacing between the first and second flanges, some misalignment between the first and second flange bolt holes may be less critical compared to a situation wherein flanges are directly in contact with one another upon installation.

In case the spacer bushings are provided, these are preferably provided prior to the arrangement of the bolts through the bolt holes. The spacer bushings may, apart from defining the relative vertical spacing between the first and second flanges, advantageously also prevent the frictional engagement between the two barrels from becoming too tight, e.g. due to the weight, possibly aided by vibrations, of the wind turbine causing the upper barrel to be pushed further downwards onto the lower barrel over time. Therewith the spacer bushings may advantageously facilitate a later controlled release of the slip joint.

The invention furthermore relates to a slip joint as described herein for mounting or actually mounting an offshore wind turbine tower to an offshore foundation.

The invention also relates to a method for de-installation of an offshore wind turbine tower from an offshore foundation, e.g. when repair of the wind turbine is required, wherein the tower is mounted to the foundation by means of a slip joint as described herein, wherein the de-installation method comprises:

• removal of the bolts connecting the first and second flanges, possibly also removal of the spacer bushings when present,

• placement of one or more jacks, e.g. hydraulic cylinders, between the first and second flanges, e.g. the jacks being angularly spaced from one another, and driving the first and second flanges apart by extension of the one or more jacks, thereby releasing the frictional engagement between the inner surface of the outer barrel and the outer surface of the inner barrel.

The method allows for the removal of a tower, or entire wind turbine, from an offshore foundation by release of the slip joint as described herein. Such method may further comprise a placement of one or more jacks, e.g. hydraulic cylinders, vertically between the flanges e.g. angularly spaced from one another, such as to enable driving the first and second flanges apart upon extension of the jacks, increasing the vertical spacing between the flanges and therewith releasing the engagement at the interface between the inner surface of the upper, outer barrel and the outer surface of the lower, inner barrel.

The placement of the jacks may e.g. be performed prior to prolonged use of the tower on the foundation, for later release of the joint after subsequent prolonged use of the tower on the foundation, for example for releasing the joint in order to perform subsequent servicing or replacement the tower. Alternatively, the placement may be performed after prolonged operation of the wind turbine, for example for releasing the slip joint in order to perform subsequent servicing or replacement the wind turbine or wind turbine tower.

The invention will hereinafter be described with reference to the appended figures. In the figures: figure 1 shows a slip joint according to the invention interconnecting a foundation and a tower, and a combination of the tower with the upper barrel of the slip joint, figure 2 shows in a side view onto a vertical cross-section, the slip joint with the barrels being engaged, figure 3 shows in the same side view onto a vertical cross-section, the upper barrel of the slip joint, figure 4 shows in a perspective view the lower and upper barrel and the tower being disengaged and positioned above one another, figure 5 shows in a top, side, and front view a slip joint according to the invention, figures 6A-C illustrate approaching and guiding of the upper barrel towards engagement with the lower barrel, figures 7A-C illustrate release of the engagement between the barrels by moving apart the first and second flanges, and thus release of the joint, figure 8 illustrates engagement of the barrels with bolts arranged through the first and second flange bolt holes, the bolts being fastened to secure the slip joint, figures 9A-B illustrate extension of jacks between the first and second circumferential flanges, moving apart the first and second flanges, and releasing the slip joint.

The figures illustrate a slip joint 20,30 for mounting an offshore wind turbine tower 40 to an offshore foundation 10, e.g. a floating foundation. The slip joint 20,30 comprises a lower, inner barrel 20, and an upper, outer barrel 30. See figure 1.

The outer barrel 30 is configured to slidingly receive therein the inner barrel 20 from below, and to engage a conical outer surface 21 of the inner barrel 20 by a conical inner surface 31 of the outer barrel at an interface along a lower section 30L thereof that vertically overlaps with the received inner barrel, such that the outer barrel 30 is frictionally supported by the interface with an upper section 30U thereof still protruding above the inner barrel 20. See figures 2 and 8.

The inner barrel 20 comprises a first inwardly protruding circumferential flange 25 at an upper end thereof, comprising first angularly spaced bolt holes 24 for receiving bolts in a substantially vertical direction. See figures 2 and 4.

The upper section 30U of the outer barrel 30 comprises a second inwardly protruding circumferential flange 35, comprising second angularly spaced bolt holes 34 for receiving bolts in a substantially vertical direction. See figures 2 and 4.

In figures 2 and 8 it is best visible, that the second circumferential flange 35 is arranged above and vertically spaced from the first circumferential flange 21 of the received and engaged inner barrel 20, such, that bolts 50 can be arranged through the first and second bolt holes 24,34 with lower ends 51 thereof being received through the first bolt holes and with upper ends 52 thereof through the second bolt holes, with a center section 50C of the bolts 50 extending between the first and second circumferential flanges 25,35.

The inner barrel 20 further comprises, at a lower end thereof, a lower connection member 22,23 for connection to a top of the offshore foundation 10.

Figure 1 shows the interconnection, and figure 2 shows that the lower connection member is formed by a third inwardly protruding circumferential flange 22 at a lower end of the outer barrel 20, comprising third angularly spaced bolt holes 23 for receiving bolts in a substantially vertical direction, for joining the flange 22 with a circumferential flange of the foundation 10 at an upper end thereof.

The outer barrel 30 further comprises, at an upper end thereof, an upper connection member 32,33 for connection to a bottom 42 of the offshore wind turbine tower 40. As discussed, the tower may or may not include a transition piece. In an embodiment, the outer barrel is part of the lower end of a transition piece. In another embodiment, a transition piece is absent. In yet another embodiment, a transition piece is present, with a first slip joint as described herein between the transition piece and the foundation and a second slip joint between the transition piece and the rest of the tower. In yet another embodiment of the invention, the transition piece is considered part of the foundation, e.g. provided at its upper end with an inner barrel of an inventive slip joint.

Figure 1 shows the interconnection, and figure 2 shows that the upper connection member is formed by a fourth inwardly protruding circumferential flange 32 at an upper end of the inner barrel 30, comprising fourth angularly spaced bolt holes 33 for receiving bolts in a substantially vertical direction, for joining the flange 32 with a circumferential flange of the offshore wind turbine tower 40 at a lower end thereof.

It is shown best in the magnified detail in figure 2, that the illustrated slip joint further comprises spacer bushings 53 through which bolts 50 extend, so as to circumferentially enclose respective ones of the bolts 50 along the vertical spacing between the first and second circumferential flanges 25,35, e.g. for maintaining a fixed distance therebetween.

Figures 6 - 9 show that guide blocks 60 are provided on the upper end of the lower, inner barrel 20, e.g. on top of the first inwardly protruding flange 25, of which an outer surface 61 forms an upward extension of the outer surface 21 of the inner barrel 20, slanting inwardly in the upward direction.

As is illustrated in figures 6A-6C, therewith, the outer surface 61 is configured to engage the inner surface 31 of the outer barrel 30 when it approaches the inner barrel 20 from above, and to guide the inner surface 31 into engagement with the outer surface 21 as the outer barrel 30 slides over the inner barrel 20.

Figures 6B-C show that when the barrels 20, 30 are vertically out of alignment, that is, the central longitudinal axes 30A and 20A of the respective barrels 20, 30 are not in line with each other, the guide member 60 shown leftmost in the figures, guides the inner surface 31 of the upper, outer barrel 30 over the outer surface 61 into alignment with the outer surface 21 of the lower, inner barrel 20.

The first and second circumferential flanges 25,35 are dimensioned and mutually vertically spaced, to enable placement of one or more jacks, in the form of hydraulic cylinders, vertically between the flanges 23,35. The jacks are shown placed between the flanges 25,35, and engaging the flanges, in figures 9A - B. The hydraulic cylinders drive the flanges 25,35 apart upon extension thereof, increasing the vertical spacing between the flanges 25,35, and therewith releasing the engagement at the interface between the inner surface 31 of the upper, outer barrel 30 and the outer surface 21 of the lower, inner barrel 20.

The upper section 30U of the upper, outer barrel 30 further comprises, at an upper end of the outer barrel 30, multiple attachment members 37 in the form of eyelets distributed along the outer circumference. The eyelets 37 are configured for engagement by, and connection with, a lifting device, e.g. with a hoisting cable or object suspension device, e.g. a hook, thereof, for suspending at least the upper, outer barrel 30 from the lifting device by means of the attachment member 37.

The eyelets 37 are formed on an annular outward collar 36 from the upper section 30U, e.g. extending vertically along the outer side wall 36S thereof and protruding upwardly from the annular outward collar 36.

The outward collar is hollow with an internal space being provided between walls 36B, 36S, 36T thereof and an outer surface of the upper, outer barrel 30. A horizontal bottom wall 36B of the walls forms an extension of the second inwardly protruding circumferential flange 35 of the upper, outer barrel 30 and a horizontal top wall 36T of the walls forms an extension of the fourth inwardly protruding circumferential flange 32.

The shown slip joint 20,30 enables a method for installing the wind turbine tower 40 on the offshore foundation 10.

The method comprises slidingly inserting a lower, inner barrel 20 of a slip joint 20,30 in an upper, outer barrel 30 of the slip joint 20,30, by receiving inner barrel 20 from below in the outer barrel 30. This is illustrated in figures 6A-C.

The method comprises after the insertion, engaging by the outer barrel 30 the conical outer surface 21 of the inner barrel 20 by the conical inner surface 31 of the outer barrel at an interface along a lower section 30L thereof that vertically overlaps with the received inner barrel, such that the outer barrel 30 is frictionally supported by the interface with an upper section 30U thereof still protruding above the inner barrel 20. This engagement is shown in figures 2 and 8. Reaching the engagement is furthermore illustrated by the advancement from figure 7C to figure 7B and subsequently to figure 7A.

The method comprises arranging bolts 50 through the second bolt holes 33 with upper ends 52 thereof being received in the second bolt holes and arranging the bolts 50 through the first bolt holes 23 with lower ends thereof being received in the first bolt holes. The bolts are shown in this arrangement in figures 2 and 8. These show that the arranging of the bolts 50 through the first and second bolt holes 23,33 has been done such that the bolts 50 are arranged with a center section 50C of the bolts 50 extending between the first and second circumferential flanges 25,35 defining the vertical spacing therebetween.

The method comprises connecting the lower, inner barrel 20 at a lower end thereof to a top of the offshore foundation 10 by means of a lower connection member 22,23 of the inner barrel 20, and connecting the upper, outer barrel 30 at an upper end thereof to a bottom 42 of the offshore wind turbine tower 40 by means of an upper connection member 32,33 of the outer barrel 30.

The method may further comprise, prior to the step of arranging the bolts through the second bolt holes and/or the step of arranging the bolts through the first bolt holes, the step of arranging the spacer bushings 53 indicated in the magnified detail of figure 2 in the vertical spacing between the first and second circumferential flanges 25,35, and engaging the circumferential flanges at respective ends thereof around the bolt holes 23,33, for maintaining a fixed or defined (minimum) distance between the flanges 25,35.

The de-installation method may comprise the placement of one or more jacks, e.g. hydraulic cylinders, vertically between the flanges 23,35, e.g. angularly spaced from one another, such as to enable driving the flanges 25,35 apart upon extension of the jacks, increasing the vertical spacing between the flanges 25,35, and therewith releasing the engagement at the interface between the inner surface 31 of the upper, outer barrel 30 and the outer surface 21 of the lower, inner barrel 20. This is illustrated in figures 9A-B as explained before.

The installation and de-installation methods may further comprise connecting the eyelets 37 or the like with a lifting device, e.g. with a hoisting cable thereof, so as to suspend at least the upper, outer barrel 30 from the lifting device by means of the attachment member 37. The lifting may e.g. be done after the outer barrel 30 is connected to the tower 40 via the upper connection member 32, so that by said suspension, the combination of the outer barrel 30 and the tower 40 is suspended from the lifting device, e.g. a vessel mounted crane.