Field of the Invention

This invention relates to a method of forming and installing a J- tube system on an offshore structure, e.g. a wind turbine foundation.

It is an object of the present invention to provide an improved method of forming and installing a J-tube system on an offshore structure.

Summary of the Invention

According to a first aspect of the present invention there is provided a method of forming a J-tube system in which the J-tube is formed from an upper section of pipe, a curved section at the bottom of the upper section and one or more lower sections extending below the heel of the J-tube.

The upper section and the curved section are preferably of tubular form and the inner spaces of the upper section and the curved section are preferably continuous and unobstructed as this is the space within which it is intended that the cables and draw-wires, etc are to be inserted and installed.

The lower section or sections may also be of tubular form or formed from any other sectional shape. Any inner space within the lower section(s), however, need not be integral with that of the upper section and the curved section.

Prior to installation, the J-tube assembly is held in an elevated position above the seabed. The upper section of the J-tube is preferably restrained by and/or enclosed within a top support or supports that is or are firmly attached to the main structure. The top support or supports is or are preferably from a concentric tube or tubes. At this point, the upper section only has a close-fit or loose-fit within the top support or supports, i.e. the J-tube assembly is free to slide up and down relative to the top support(s).

The main structure can comprise any offshore structure, e.g. a monopile, a lattice structure, or a concrete gravity structure, etc. However, it is envisaged that the present invention will be most applicable to the large diameter vertical steel cylinders, commonly known as monopile type foundations, which are associated with offshore wind turbines. With a typical offshore wind turbine foundation, the main structure often comprises an upper transition piece and a lower monopile that extends into the seabed.

The whole J-tube assembly is lowered to the seabed and then preferably driven, vibrated, pushed, socketed or otherwise inserted into the seabed. Application of the necessary forces to insert the lower section or sections into the seabed would ideally be applied at the top of the upper section using conventional and readily available (i.e. onshore type) piling equipment. Vertical installation of the J-tube assembly preferably ceases when the toe of the lower section or sections reaches the desired embedment depth, and/or the level of the curved section reaches the desired level for cable entry, etc. The embedment of the lower section or sections into the seabed then provides the horizontal and vertical support to the bottom of the J-tube assembly required throughout its service life.

The actual embedment depth of the lower section or sections will normally depend on the environmental loading, the size of the J-tube, and ground conditions, etc, and whether the support for the bottom of the J-tube is to be designed as a 'pinned' or 'fully fixed' feature.

If a scour hole has been allowed to develop adjacent to the main structure, the minimum embedment of the lower section or sections will be governed by the bottom of the scour hole, whilst the level of the curved section will be governed by the level of the surrounding seabed or cable trench, etc.

According to another aspect of the present invention there is provided a method of installing a J-tube system, particularly one formed by the method described above, in which, prior to installation, the J-tube assembly is held in an elevated position above the seabed and the upper section of the J-tube is enclosed within or restrained by a top support or supports that is or are restrained by a top support or supports that is or are firmly attached to a main structure.

According to a further aspect of the present invention there is provided a method of installing a J-tube system, particularly one formed by the method described above, in which the upper section is secured in position by filling the space between the outside of the upper section and the inside of a top support or supports with an in-fill material, for example, grout.

If a permanent sliding joint is required, a slip layer (or other suitable element that permits sliding) can be incorporated within the top support or supports. In this case, the grouting of the joint is still used as a convenient way of securing the connection. The grouted connection then provides the required horizontal and/or vertical support to the top of the J-tube assembly throughout its service life.

In order to contain the grout, it is preferred that the upper section is enclosed by the top support or supports in a manner that prevents grout loss. The preferred method of doing this is to use two concentric pipes or tubes. In addition, the bottom of the space that contains the grout (and the top of the space if required) is sealed to prevent loss of grout, or the components are a sufficiently close fit to keep the loss of grout to a minimum.

Of particular note is the fact that the whole J-tube installation generally does not require the use of divers. Thus the operations can be carried out remotely and underwater if necessary.

Draw-wires and cables, etc are preferably installed in the normal way through the upper section and the curved section. The end of the curved section can terminate in the normal manner as would be used for conventional J-tubes, e.g. a bell-mouth, horizontal extension pieces, or a flexible section of hose, etc. The support of the lower section or sections in the seabed and the pile installation aspect of J-tube fixing system of the present invention can be used with other methods of supporting the top of the J-tube assembly. Similarly, the grouted top support of the improved J- tube fixing system can be used without necessarily the need to install the lower section or sections into the seabed. It is, however, anticipated that the greatest advantage will be realised by using both aspects of the improved J-tube fixing system.

A summary of the advantages of the method of forming and installing a J-tube over other methods that have previously been employed is as follows:- a) Reduced overall foundation fabrication costs (in particular due to better fatigue characteristics imparted to the main structure). The only welded attachments required to the main structure tend to be higher up the structure in regions of lower stress, e.g. within the grouted joint or on the transition piece.

b) Reduced overall installation costs (in particular due to the improved J-tube system being quick, simple and reliable to install, but also due to the main structure being much lighter).

c) Quick and simple to install. Only two operations required. Time on site is reduced to a minimum (time on site is a major cost factor for offshore installations).

d) The J-tubes can be lowered into position. Easier for transportation and particular useful where seabed levels and/or installation levels of main structure vary. Large installation tolerances can be accommodated, if necessary (e.g. embedment depth of monopile type foundations in variable ground conditions).

e) J-tubes designed as simply supported or fixed end beam. The design can be further optimised by incorporation into a boat landing system. In addition, the top supports can easily be adapted to incorporate a permanent sliding joint if required.

f) Cable pulling forces can be resisted directly.

g) Little development costs required. Operations utilise well established technologies, e.g. conventional pile installation and grouting.

h) Reliable and relatively risk-free in terms of potential for things to go wrong or delays on site. Relatively non-weather dependent.

i) The use of divers is not required. The use of divers can add considerably to costs and delays, in addition to safety issues.

Brief Description of the Drawings

Figure 1 of the drawings shows a J-tube system attached to an offshore wind turbine foundation in relatively shallow water, and

Figure 2 of the drawings shows a J-tube system on an offshore wind turbine foundation in relatively deep water.

Description of the Preferred Embodiments

The following reference numerals are used in the drawings:-

1 = upper section of J-tube,

2 = curved section of J-tube,

3 = lower section or sections, 4 = top support or supports,

5a = main structure - transition piece,

5b = main structure - monopole,

6 = profile of scour hole,

7 = grout.

The seabed is indicated at 8 and a typical water level at 9.

The J-tube assembly is formed in part from a tubing arrangement similar to that of a conventional J-tube. Thus, there is an upper section 1 and a curved section 2. The upper section 1 is formed from a section of pipe that is substantially vertical. The curved section 2 is also formed from a section of pipe, with its upper end substantially vertical and continuous with the upper section 1 , while the lower end of the curved section 2 is substantially parallel to the seabed 8.

Unlike conventional J-tubes, there is a lower section 3 that extends below the heel of the J-tube. This lower section 3 is also formed from a section of pipe and is substantially vertical. The lower section 3 is either in line with the upper section 1 or offset from the upper section 1 by a relatively small amount. The lower section 3 may be either a single element or a plurality of elements.

The internal space within the upper section 1 and the curved section 2 is continuous and unobstructed as this is the space within which the cables, draw-wires, etc. will be inserted and installed. The internal space within the lower section 3 need not, however, be integral with that of the upper and curved sections 1 and 2. For a typical offshore wind turbine foundation, the main structure comprises a transition piece 5a and a monopile 5b. The monopole 5b is typically driven into the seabed 8 and the transition piece 5a is typically grouted onto the top of the monopile 5b. Both the monopile 5b and the transition piece 5a are large diameter steel cylinders.

Typical diameters for the upper section 1 , the curved section 2 and the lower section 3 are between 300 and 350 mm. This is normally the minimum size required for cable pulling, etc. A typical diameter for the monopole 5b is between 4.8 and 6.5 metres, depending on the water depth and the size of the wind turbine. The transition piece 5a typically has a diameter 0.3 to 0.4 metres greater than that of the monopole 5b. The distance between the bottom of the transition piece 5a and the seabed is typically 10 to 20 metres and this roughly equates to the required span of the J-tube.

The top support or supports 4 is or are used to support the top part of the J-tube. The top support or supports 4 is or are firmly attached to the main structure. The J-tube or J-tube assembly is composed of sections 1 , 2 and 3. Prior to installation, the J-tube is held in an elevated position above the seabed 8 and is free to slide (under control) within the top support or supports 4.

Figure 1 shows a typical installation in which the top support 4 is formed from one relatively long attachment whereas Figure 2 shows a typical installation which includes two top supports 4 in the form of two relatively short attachments. Figure 1 is indicative of a typical installation in which part of an existing appurtenance, e.g. a boat landing, double up as the top support. Figure 2 shows two independent supports. There may be one top support, two top supports or more than two top supports.

With a typical offshore wind turbine, the top supports 4 are attached to the transition piece 5a by design - in order to avoid attachments on the monopole 5b - and by necessity - as the transition piece 5a and the monopole 5b are two separate components. It is usual for the transition piece 5a to be pre-installed before it arrives on site with all its appurtenances, such as boat landings, grout skirt and platforms. Thus, in the same way, the transition piece 5a can come pre-installed with the J-tube and its attendant top support(s).

As one function of the top supports 4 is to permit sliding and as they will ultimately end up being grouted in order to secure the connections for permanent use, the preferred shape of the top support(s) 4 is a concentric tube or pipe of larger diameter than the J- tube. In this way, the upper section 1 can be enclosed within the top supports 4 which, in turn, are firmly attached to the transition piece 5a. Prior to installation, the upper section 1 only has a close or loose fit within the top supports 4.

The top supports 4 are positioned on the transition piece 5a to optimise the span of the J-tubes. Tubular members with smaller diameter/thickness ratios are generally more efficient at resisting wave loading. The distance between the lowest top support 4 and the seabed 8 needs to be greater than the length of the lower section 3 plus the height of the curved section 2. For the shallowest water depths with a deep scour hole, the lowest top supports 4 may need to be "opened" to allow the curved section 2 to slide past. A typical size for the top supports 4 is 100 mm. larger than the diameter of the J-tube, with the welded attachment to the transition piece 5a also being of tube or pipe, and typically being of the same diameter as the J-tube or slightly larger. The length of the welded attachment to the transition piece 5a is governed by the offset dimension required of the J-tube from the monopole 5b at the seabed 8. Typically this is 0.75 to 2 metres or more from the centre line of the top supports 4 to the face of the monopole 5b.

After installation of the transition piece 5a onto the top of the monopole 5b is completed, installation of the J-tube can commence. Initially, the whole J-tube assembly, i.e. the upper section 1 , the curved section 2 and the lower section 3, is lowered to the seabed 8. Then the J-tube is driven, vibrated, pushed, socketed or otherwise inserted into the seabed 8. Application of the necessary forces to insert the lower section 3 into the seabed 8 is ideally applied at the top of the upper section 1 using conventional and readily available piling equipment, i.e. on-shore type equipment.

Vertical installation of the J-tube assembly ceases when the toe of the lower section 3 reaches the desired embedment depth and/or when the level of the curved section is at the desired level for cable entry, etc. The embedment of the lower section or sections into the seabed 8 provides the horizontal and vertical support required for the bottom of the J-tube assembly throughout its service life.

The actual embedment depth of the lower section 3 will depend on the environmental loading, the size of the J-tube and the ground conditions, etc. and on whether the support for the bottom of the J-tube is intended to be a "pinned" or "fully fixed" feature.

If a scour hole 6 has been allowed to develop adjacent to the monopile 5b, the minimum embedment of the lower section 3 will be governed by the depth of the scour hole 5, whilst the level of the curved section 2 will be governed by the level of the surrounding seabed 8 or a cable trench, etc.

The J-tube is preferably secured in position by grouting, i.e. by filling the space between the outside of the upper section 1 and the inside of the top support(s) 4 with grout. In this way, the grouted connection provides the horizontal and vertical support required by the top of the J-tube assembly throughout its service life. The use of grouting has the advantage that this can be carried out remotely and does not require the use of divers. In addition, the grout provides a convenient way of sealing the inside of the connection from the effects of the environment.

The J-tube can alternatively be secured in position by incorporating a slip layer, membrane or sleeve on the outside of the J- tube or on the inside of the top support(s). This will give the J-tube the ability to slide with respect to the top support(s). This can be advantageous both permanently and temporarily. In this case, the grouting of the gap between the J-tube/slip layer and the top support 4 can still be used as a convenient way of securing the support. In this way, the grouted connection still provides horizontal support but only limited or no vertical support for the top of the J-tube. Alternatively, the top support(s) 4 can be left "dry" such that the J- tube is free to slide within the top support(s). Horizontal support of the J-tube is then provided by the J-tube and the top support(s) 4 being designed as a suitably close fit. Additionally, the top support(s) 4 can be designed as a "spring" support or supports, for either or both of the horizontal and vertical displacements.

The bottom of the space that contains the grout 7 (and the top of the space if required) is sealed to prevent grout loss, or is a sufficiently close fit to keep grout loss to a minimum. Of particular note is the fact that the whole J-tube installation, i.e. both installation of the lower section 3 into the seabed 8 and grouting of the top support(s) 4 need not require the use of divers.

Additional supports may be required to support the top of the upper section 1 , especially if extension pieces are added after initial installation. These are not shown in the drawings. However, as these additional supports are likely to be located well above the wave zone with much better access, these additional supports may be formed either as the top supports shown in the drawings or by conventional clamps, etc.

Draw wires and cables, etc. are installed in the normal way through the upper section 1 and the curved section 2. The lower end of the curved section 2 can terminate in the normal manner, as would be used for conventional J-tubes, e.g. a bell-mouth, cable entry adapters, horizontal extension pieces, or a flexible section of hose, etc. The support of the lower vertical tube 3 in the seabed 8 can be used with other methods of supporting the top of the J-tube assembly. Similarly, the grouted top support can be used without necessarily the need to install the lower section into the seabed.

Though the drawings relate to a typical wind turbine foundation, the invention can be applied to any offshore structure, e.g. a monopile, a lattice structure, or a concrete gravity structure, etc. However, it is envisaged that the invention will be most applicable to the large diameter monopile type foundations associated with offshore wind turbines.

Particular notes regarding the occurrence of scour holes in the seabed and the improved J-tube system, with particular regard to monopile type wind turbine foundations, are as follows:

a) The improved J-tube system can be used where a scour hole has been allowed to develop around the main structure. Scour holes can be accommodated with the improved J-tube system by making the lower section(s) 3 longer than otherwise would be required, and by extending or adding extension pieces to the end of the curved section 2.

b) The lower section(s) 3 will need to be longer than the design embedment depth by the maximum anticipated depth of the scour hole, whilst the curved section 2 will need to be lengthened by the necessary amount to span the maximum anticipated width of the scour hole.

Particular notes regarding details of fabrication of the improved J- tube system, with particular emphasis on monopile type wind turbine foundations, are given below:- a) It is anticipated that the top support(s) is or are to be attached to the main structure using conventional means, i.e. welded attachments. In addition, t is also possible to double-up the function of the boat landings to act as the top support(s), if required. The advantage of utilising the main tubes of the boat landings as the concentric support tubes is that little or no additional steel work is required.

b) The lowest of the top supports, whether part of the boat landing system or not, can be extended downwards in order to reduce the span of the J-tubes as required. In this way, the size of the members can be optimised to produce the most effective structure. c) It is anticipated that the upper and lower sections 1 and 3 will be substantially vertical. However, on some types of main structure, e.g. lattice type foundations with sloping leg geometry or where the monopiles incorporate a conical section, the J-tube may be inclined. d) The upper part of the J-tube may also incorporate an offset or dogleg, though this is normally less preferred in view of the relatively modest differences in diameters that are usually involved.

e) If necessary, the grouted top supports of the improved J-tube system can be substituted by conventional mechanical clamps or other types of support (that would also permit sliding of the J-tube assembly). In this case, only the lower section or sections 3 and the pile installation aspect of the improved J-tube fixing system would be required.

f) The highest top supports 4, i.e. those positioned some distance above the wave zone, may have good access if they are sited near a platform. In this case, the grouting may not be required and conventional clamps could be used instead, particularly if vertical extension pieces are to be added at a later date. g) The top supports 4 may be fabricated and assembled as part of the transition piece 5a. At the fabrication stage, or prior to transportation, the J-tubes may be assembled within the top supports 4, but not permanently fixed. Alternatively, the J-tubes can be installed within the top supports 4 on site prior to final installation. h) The lower section(s) 3 can be fixed permanently to the rest of the J- tube prior to transport to site or after insertion of the J-tube assembly into the top support(s) 4. Alternatively, the curved section 2 can be welded or bolted to the upper section 1.

i) The top support(s) and J-tubes are preferably manufactured from structural steel, but other materials could also be used. Horizontal extension pieces, bell-mouths, or flexible sections, etc can be added to the end of the curved section 2 as required to suit each particular cable installation.

j) The junction between the lower section 3, the curved section 2 and the bottom of the upper section 1 can be fabricated, cast steel, or proprietary section of pipe. The lower section(s) 3 need not be directly in line with the upper section 1. Where a fabricated junction is envisaged, the lower section(s) 3 can be simply cut-to-shape, lapped and welded to the top of the curved section 2 or to the bottom of the upper section 1.

k) The top supports 4, though designed to function as a closed tube, can nevertheless be fabricated as conventional "openable" hinged and/or bolted mechanical clamps, perhaps to facilitate transportation or lowering of the J-tube assembly to the seabed. In this case, it is anticipated that the supports 4 will be permanently locked closed before the grouting operations begin. The grouting operations will then be carried out in the normal way. Particular notes with regard to the piling operation stage of the improved J-tube system, with particular emphasis on monopile type wind turbine foundations, are given below: - a) Driving installation methods utilise an impact hammer and are usually the most widely available. Driving methods may not be suitable where large piling forces occur or large inertia forces are introduced into the J-tube assembly. However, if the pile resistance is sufficiently low and/or the use of static weights is sufficiently large, the driving forces may be quite nominal and the lower section(s) 3 may literally be "tapped" into position, rather than "driven", such that large inertial forces are avoided.

b) Vibration installation methods utilise a vibro-driver and are particularly suitable for granular soils. The piling forces are generally not as large as with driven methods.

c) Hydraulic pushing of piles utilise a hydraulic press and piles installed in this way are known as 'push piles' or 'jacked piles'. This method is also known as the press-in method or silent piling. The reactive resistance necessary to push the piles into the ground is created from temporary weights or anchoring to an adjacent structure. Hydraulic pushing is particularly advantageous with the improved J- tube system as it creates little or no inertial forces in the system, is nearly vibration-free, and creates little noise.

d) For hydraulic pushing, the reactive force can be applied to the top of the J-tube assembly directly, or indirectly by the use of a "follower", e.g. a short length of tube that fits on top of the J-tube assembly. In the latter case, it is therefore also possible to incorporate a narrow section of sheet pile onto the "follower" for the hydraulic press to grip onto. e) The support of the bottom of the J-tube within the seabed can be significantly improved, if required, by the use of radial fins welded to the embedded part of the lower section(s) 3. Radial fins will also help to prevent rotation of the J-tube system.

f) Where very strong soils are encountered, water jetting techniques can be used or, where stronger rock is encountered, holes can be predrilled or drill-drive rock-socket techniques employed.

Particular notes regard to the design aspects of the supports of the improved J-tube system, with particular emphasis on monopile type wind foundations are given below:- a) Flexibility can be incorporated into the design of the J-tube assembly in order to help accommodate differential movements and any relative movement between itself and the monopile without inducing any excessive forces or displacements,

b) Lateral forces and displacements in the system can be controlled by the embedment depth of the lower section(s) into the seabed. The vertical forces and displacements in the J-tube assembly and the induced bending moments in the top support(s) can be controlled by the number, length of the top support(s) to the monopile and by the embedment depth of the lower section(s) into the seabed.

Particular notes with regard to sealing of the end of the top support(s) and, in particular, of the use of concentric support tubes and grouting procedures, with particular emphasis on monopile type wind turbine foundations, are given below:- a) The fit of the J-tubes within the top supports can be made by the provision of simple end plates or internal diaphragms at the ends of the concentric top support(s). The end plates and/or diaphragms will also act as centralisers.

b) A close-fit between the upper section 1 and the top support(s) 4 may aid more accurate vertical alignment of the J-tube assembly during this piling stage. Alternatively, simple guides can be added to aid piling.

c) If the upper section 1 is a loose fit within the top support(s) 4, then rubber seals or similar can be utilised. If the upper section 1 is a close enough fit within the top support(s) 4 then rubber seals or similar may not be required.

d) The grout 7 can be introduced into the space between the outside of the upper section 1 and the inside of the top support(s) 4 by filling by gravity from the top of the top support(s) 4 or, preferably, by pressure grouting.

e) Grout pipes to the top support(s) 4 can be pre-installed in a fabrication shop prior to transportation to site.

f) Alternative materials can be used instead of the grout if required.

Grout is generally preferred because it is economical to use and can readily be pumped. However, any suitable inert material with minimal structural properties may be used to fill the gap between the outside of the J-tube and the inside of the top support(s).

Particular notes with regard to the provision of permanent sliding support to the top support(s), with particular emphasis on monopile type wind turbine foundations, are given below: - a) During the installation phase, the upper section 1 is free to slide within the top support(s) 4 until the joint is permanently grouted. However, this joint can also be designed as a permanent sliding joint. This can be achieved by, for example, the provision of a slip layer or a dry joint or by the use of an elastomeric filler material. Grouting will still be required to seal the gap between the slip layer and either the outside of the J-tube or the inside of the top support(s) 4, i.e. grouting can be used either for a permanent fixed grouted joint or for a grouted joint incorporating a slip layer,

b) The forces to be resisted by the attachments of the top supports to the transition piece can largely be alleviated by the incorporation of a permanent sliding top support. Furthermore, and crucially for some foundation designs, a permanent sliding top support will also accommodate slippage of either straight-sided or conical grouted connections (used to connect the transition piece and monopile together) or where settlement of the monopile is anticipated,

c) A slip layer can be applied to the outside of the upper section 1 or to the inside of the top support(s) 4. It is anticipated that the slip layer will be added to the J-tubes during the fabrication stage or prior to lowering of the J-tubes to the seabed 8. The slip layer is intended to transfer all lateral loads across the joint whilst allowing some vertical or sliding movement of the J-tube,

d) The slip layer can be formed, with various levels of sophistication, from layers of building paper, layers of HDPE, layers of PTFE, coatings of mastic or bitumen/asphalt, heat-shrinkable sleeves of HDPE or PEX, sheets of rubber, cork or other elastomers or combinations thereof. In addition, several proprietary or composite products are available, including various slip-membranes, compounds and low friction slip- sleeve pipe wrap, etc.

e) If the joint is left "dry", rubber or elastomeric gaskets may be used to transfer the lateral forces. Alternatively, a simple close-fit between the upper section 1 and internal diaphragms of the top support(s) 4 may be sufficient.

f) A sliding support can also be achieved by filling the gap between the outside of the J-tube and the inside of the top support(s) with an elastomeric material rather than grout. This also has the advantage that it can also provide corrosion protection for the hidden components. Two-part elastomeric materials are available in pourable and pumpable forms, and

g) If required, the top supports, in particular the lowest top supports, can be designed as "spring" supports. A spring support may be formed by the provision of an elastomeric or compressible collar, blocks or filling material. A spring support effectively increases the span of the lower part of the J-tube, thereby increasing the ability of the J-tube system to accommodate any relative movement between itself and the monopile.

Comparison with the prior art a) Other currently accepted ways of forming a J-tube system for an offshore facility are disclosed in WO2008151660 and in EP1616377. The former also specifically relates to a foundation for a wind turbine. Patent Specification WO2008151660 discloses a two-part tubular arrangement that is hinged in the middle, thereby enabling one part of the J-tube to be lowered to the sea floor. Patent Specification WO2008151660 does not specifically deal with fixing of the J-tube system to the main structure, and the method of lowering the J-tube to the sea floor is quite different.

b) Patent Specification EP1616377 relates to a mechanically complex telescopic arrangement which again does not specifically deal with fixing of the J-tube system to the main structure, and the method of lowering the J-tube to the sea floor is quite different.

The improved J-tube system improves on previous methods in that it can potentially lead to reduced overall foundation costs. The savings in overall foundation costs can be attributed to both reduced fabrication costs and reduced installation costs. Fabrication costs can be reduced, not only with the J-tube system itself being efficient, but indirectly by imparting better fatigue characteristics to the main structure. The latter is particularly important to wind turbine foundations, or similar structures, where fatigue is a main design driver. In addition the improved J-tube fixing system can tolerate much larger variations in seabed levels and/or embedment depths of the main structure.