GB2473733A

GB2473733A - A method of forming and installing a J-tube system

Info

Publication number: GB2473733A
Application number: GB1015546A
Authority: GB
Inventors: Matthew Bristow
Original assignee: Individual
Current assignee: Individual
Priority date: 2009-09-19
Filing date: 2010-09-17
Publication date: 2011-03-23
Anticipated expiration: 2030-09-17
Also published as: EP2539981A2; GB201015546D0; WO2011033267A3; WO2011033267A2; GB0916484D0; GB2473733B

Abstract

A method of forming a J-tube system is provided in which the J-tube is formed from a tubing arrangement comprising a vertical section (1) of pipe with a curved section (2) at the bottom and with a vertical extension piece (3) extending below the heel of the J-tube. During installation, the upper vertical section (1) is secured in position by grouting, i.e. by filling the space between the outside of the upper section (1) and the inside of a top support or supports (4) with grout.

Description

A METHOD OF FORMING AND INSTALLING A J-TUBE SYSTEM

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.

0 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 I 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 Jong 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 details of construction and a more detailed list of comments on the advantages of this method of fixing a J-tube over other methods are given below: Advantages in terms of better fatigue characteristics imparted to main structure, with particular regard to the monopile type wind turbine foundations, are as follows:-a) A consequence of the improved method of fixing a J-tube is that no attachments and/or holes are required to the lower portion of the main structure, i.e. in the vicinity of seabed and for some distance above. This is particularly advantageous for wind turbine monopile type foundations where fatigue is a main design driver. As a consequence, the improved J-tube fixing system allows the monopile to be designed for minimum plate thickness, and hence reduced overall foundation costs.

b) Most offshore wind turbine foundations comprise a driven monopile with a grouted transition piece. Typically the top of the monopile ends just above MSL, with the transition piece grouted over the top of the monopile by a distance of typically 7 to 10 metres. Though the top of the monopile is typically located above MSL, there are no fixed rules in this respect (for example, in deeper water the grouted joint could be located lower down). Typically all substantial fixings for boat landings, J-tubes, and other appurtenances are preferably located on the transition piece. Generally, it is preferred that there are no welded attachments to the monopile as these significantly reduce the fatigue life of the structure, clash with the piling gates during installation, and/or are vulnerable to shear-off during pile driving.

c) A consequence of the above is that conventional J-tubes either i) have to be left unsupported for some distance from the bottom of the transition piece to the seabed, ii) the monopile needs to be designed for reduced fatigue life associated with welded attachments, iii) mechanical attachments to clamp the bottom of the J-tube to the main structure need to be incorporated, or iv) internal J-tubes need to be provided (the latter requiring holes just above the seabed in the main structure). The advantage of the improved J-tube system is that it allows the bottom of the J-tube to be supported directly at the seabed without any deleterious effects on the fatigue performance of the main structure. In addition the improved J-tube system is quick and simple to install.

d) Where J-tubes would otherwise be left unsupported below the bottom of the transition piece, they will need to be designed as cantilevers. Where this cantilever distance is significant (i.e. in all but shallowest water depths) the size of the J-tubes is likely to become impractical (both due to environmental loading and cable pulling forces during installation). The advantage of the improved J-tube system is that it allows the bottom of the J-tube to be supported directly at the seabed with the J-tube being designed as a simply supported or fixed end beam.

e) For the shallower water sites, the improved J-tube fixing system could still be used, but with only the top support(s) (to the transition piece) being utilised. In this case the lower section(s) 3 and pile installation aspect would not be required. The lower portion of the J-tube would thus need to be designed as a cantilever. However, advantage can still be taken of being able to lower the J-tube system to the correct level, etc, by utilising the top supports only to support the whole J-tube assembly.

f) Where welded attachments would otherwise be required, these tend to significantly increase the plate thickness required to maintain adequate fatigue life and hence significantly increase the weight of the main structure (monopile). This is because welded attachments inevitably have a higher stress concentration factor (SCF) than those associated with plain fabricated pipe (the effective difference in SCF can be as high as 1.33 or more). Hence a major advantage of the improved J-tube fixing system is that it allows the weight and cost of the overall foundation to be minimised by allowing by the fatigue performance of the main structure to be fully optimised and plate thicknesses reduced to a minimum.

g) Where internal J-tubes would otherwise be required, these tend to significantly increase the plate thickness required to maintain adequate fatigue life and hence significantly increase the weight of the main structure (monopile). This is because holes, whether elliptical or circular, inevitably have a higher stress concentration factor (SCF) than those associated with plain fabricated pipe (the effective difference in SCF again can be as high as 1.33 or more).

Again a major advantage of the improved Jtube fixing system is that it allows the weight and cost of the overall foundation to be minimised by allowing for the fatigue performance of the main structure to be fully optimised and plate thicknesses reduced to a minimum.

h) Where internal J-tubes would otherwise be required, there are often additional problems associated with the J-tubes being potentially difficult to install. Installation of internal J-tubes can be more costly, more time-consuming and relatively risky operations in terms of potential delays on site, etc. In addition, internal J-tubes are often difficult to satisfactorily seal and often require the use of divers. A major advantage of the improved J-tube fixing system is that it is quick and simple to install, relatively risk-free, and generally does not require the use of divers.

i) Where mechanical attachments to clamp the bottom of the J-tube to the main structure would otherwise be required, these tend to be cumbersome, costly, unproven in operation, time-consuming to install, and relatively risky operations that require use of divers, etc. Again a major advantage of the improved J-tube fixing system is that it is quick and simple to install, relatively risk-free, and generally does not require use of divers.

j) Though the bottom of the improved J-tube system has no direct attachment to the main structure, the top of the J-tube system does.

The top support(s) 4 that support the upper section 1 to the transition piece are anticipated to be attached to the main structure using conventional means, i.e. welded attachments. However, the deleterious effects on the fatigue performance of the main structure are far less noticeable here because the forces and moments in the structure are far lower higher up the structure. Indeed as welded attachments are inevitably required in this region for the boat landings and other appurtenances, there is likely to be no net significant effect on plate thickness of the main structure.

Furthermore, it is anticipated that the welded attachments will be located within the grouted joint that connects the transition piece to the monopile. This gives the opportunity of locating the welded attachments with zones of over-strength, again such that there is likely to be no net significant effect on plate thickness.

k) The ability of the J-tube to be designed as a simply supported or fixed end beam gives greater flexibility in determining the location of the grouted joint that connects the transition piece to the monopile.

Short transition pieces are often preferable as this means that the top of the monopile remains visible above the water level after pile driving, in addition, short transition pieces often produce a significant overall saving in the weight of the main structure. This is because a grouted joint when located higher up the structure needs to be designed for much lower forces and moments. The use of the improved J-tube system maximises the opportunity of using short transition pieces.

Advantages of being able to accommodate significant installation tolerances, with particular regard to the monopile type wind turbine foundations, are as follows:-a) Prior to installation and grouting of the upper section 1 within the top support(s) 4, the J-tube is free to move up and down. This is particularly useful during transportation but also during installation as this means that the improved J-tube system can accommodate significant installation tolerances both with respect to variations in seabed levels but also to variations in the monopile embedment depths. The option of being able to install the J-tube to the desired level means that there is much greater flexibility and tolerance to the final levels of the monopile and transition piece.

b) With monopile type wind turbine foundations, it is anticipated that the top support(s) 4 will be fabricated and assembled as part of the transition piece 5a. At the fabrication stage or prior to transportation, the J-tubes may be installed within the top support(s) (but not permanently fixed). The J-tubes may be lifted as high as possible within the top support(s) in order to ease storage and facilitate transportation, etc. Alternatively, the J-tubes can be installed on site prior to final installation. Once on site, the whole transition piece assembly is grouted to the top of the monopile, whereupon final installation of the J-tubes can begin.

c) Often piling tolerances, variations in seabed levels, development of scour holes, laying of scour protection, variation in depth of cable trenches, and presence of pile heave, all mean that the height of the top of the transition piece from the seabed may be different from that originally intended. In addition, monopiles are typically driven, and to a lesser extent rock-socketed, into the seabed. Variations in ground conditions, presence of bedrock, and/or unpredictable or difficult ground conditions also mean that the embedment depth of the pile may be different to that originally expected. Consequently, the full embedment depth of the monopile may not be practicable and the tolerance required on the embedment depth may be quite significant. In all these cases, the overall height of the J-tube system will be not be fixed but will need to be variable. The J-tube system of the present invention, when required, can accommodate significant installation tolerances of up to ± 5 metres or more, probably far greater than ever required. In fact, the system could be designed to accommodate installation tolerances of ± 10 metres or more.

d) The degree of tolerance required on the embedment depth/J-tube length and the number of turbine locations that are effected will be highly site dependent. On some sites with predictable ground conditions, the variation in seabed level may be less than 0.3 metres whereas, at other sites with difficult ground conditions, a highly mobile seabed and/or poor geotechnical or geophysical information, the ability to accommodate tolerances of up to 5 metres or more on the embedment depth/J-tube length may be beneficial at virtually all the turbine locations.

e) The option of being able to install the J-tube to the desired level also means that the grouted joint that connects the transition piece to the monopile does not need to accommodate the installation tolerances of the monopile, unless specifically desired. This means that the design of the grouted joint can be optimised for a specific fixed length without the need for accommodation of tolerances.

f) Where internal J-tubes would otherwise be required, the holes where the J-tubes pass through the monopile are generally located just above seabed level. These are fixed at the fabrication stage with respect to the embedment depth of the monopile and anticipated seabed level. Therefore this system offers very little tolerance on variation in levels, and could be particularly problematic in variable ground conditions or variations in seabed levels. The improved J-tube system overcomes these problems.

g) Where internal J-tubes would otherwise be required, it is often necessary to centre a plate on the holes to avoid the circumferential welds between the plates passing through or being located too close to the holes, etc. Consequently odd plate heights are often required either between the top of the monopile, and/or bottom of the monopile. With the improved J-tube system, there is no need to centre a plate around the position of the holes, such that odd plate heights are avoided and fabrication costs of the main structure reduced h) Having the ability to accommodate significant tolerances on the embedment depth of the monopile can also help reduce the extent and cost of geotechnical investigations. These are usually carried out some time before the construction phase. Though a detailed geotechnical investigation is always recommended, it is not always cost-effective to carry-out a borehole at every turbine location. The improved J-tube system permits a more flexible approach by allowing the embedment depth of the monopile to be partly determined by the actual conditions encountered on site during installation. This is particularly applicable where ground conditions are highly variable and even when drill-drive techniques may be envisaged. Thus less reliance can be placed on having a very detailed geotechnical investigation. Indeed, the improved J-tube system and its ability to accommodate varying embedment depths may even help compensate for those sites at which the geotechnical investigation is limited or poor.

i) The option of the J-tubes being able to accommodate significant tolerances on the embedment depth of the monopile has been stated above. The advantage of this can be illustrated by way of some examples. For example, this could be particularly important where the monopile has reached refusal during pile driving, e.g. 2 metres or so from the original intended embedment depth. If the ground conditions are better than expected, for example, the reduced embedment depth may well be satisfactory and there is no need to employ very expensive drill-drive techniques that would otherwise be required to get the monopile to the original intended embedment depth. With internal J-tubes and the position of the J-tube holes being fixed, there would be no option but to implement the very expensive drill-drive techniques to get the J-tube holes to the correct position. With the improved J-tube system the revised embedment depth can be easily accommodated without any additional costs.

j) The option of the J-tubes being able to accommodate significant tolerances on the embedment depth of the monopile can also be used where the ground conditions are much stronger than expected and it is simply not necessary to drive, or drill-drive, to the original intended embedment depth. Though this may affect the level of the top of the transition piece, small tolerances may be accommodated by the grouted joint and/or by accepting a higher hub-height.

Alternatively the top of the monopile can be cut-off and removed.

k) Similarly the option of the J-tubes being able to accommodate significant tolerances on the embedment depth of the monopile is also potentially beneficial where the ground conditions are much softer than expected. In this case the monopile can be re-driven to a greater embedment depth with the improved J-tube system being able to accommodate the difference in embedment depth. Though this may affect the level of the top of the transition piece, small tolerances may be accommodated by the grouted joint and/or by accepting a lower hub-height. Alternatively a stub section may be required at the top of the transition piece, but this would be far preferable than trying to adding extension pieces to the monopile on site or re-driving a new monopile.

I) Where tolerances have been accommodated by the J-tube system, the level of the top of the J-tube will vary accordingly. Small tolerances can be accepted by the cable system and/or larger tolerances can be incorporated by use of adding or removing extension pieces. Extension pieces can be welded, cut-off, or utilise bolted flanges as required. The ends of the J-tubes are often terminated with a bell-mouth or connect to the remainder of the cable protection system.

Other advantages of the improved J-tube system, with particular regard to the 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. With monopile type foundations this may often be allowed to occur as a temporary condition between the monopile being installed and the full scour protection being laid. Alternatively on some sites the scour hole may be left as permanent feature, depending on the nature and depth of the ground conditions, etc. Due to the nature of scour, the exact depth and width of scour holes, whether temporary or permanent, are often variable and unpredictable.

b) 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. 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. In both cases, the improved J-tube system can accommodate any variation in size of the scour hole, provided the latter is less than the maximum anticipated size.

c) The improved J-tube system can accommodate scour holes as the J-tube is designed as a simply supported beam or fixed end beam spanning from the seabed (or bottom of the scour hole) to the lowest of the top supports above. Similarly the curved section and any extension pieces can be designed to span horizontally across the width of the scour hole.

d) With the improved J-tube system, the cable pulling forces are more efficiently resisted. As the lower section 3 is attached to the curved section 2, any cable pulling forces exerted around the curved section are more or less transferred directly to the seabed by the lower section(s). As the J-tube is designed as a beam rather than a cantilever, any eccentricity of the cable force, for example due to the presence of a scour hole, is also more efficiently transferred to the ground and/or top support(s).

e) The improved J-tube system is quick and simple to install. There are principally only two operations required; piling installation stage and grouting stage. Both require minimal set-up time and installation time. Both the piling installation stage and grouting stage can expect to be completed within half a day at most, and most likely within hours. Hence total time on site can be reduced to a minimum. This is especially important for offshore structures where the day-rate cost of installation vessels can be substantial.

1) There are few development costs required with the improved J-tube system. Operations utilise well established technologies, e.g. conventional fabrication methods, pile installation and grouting technologies. Designs can be developed with confidence with knowledge that potential for things to go wrong on site is reduced to a minimum.

g) The improved J-tube system is relatively risk-free in terms of the potential for things to go wrong on site, or the potential for delays to develop on site. Operations are mostly low tech and based on well established technologies. In addition, the improved J-tube system is relatively non-weather dependent, i.e. installation can be carried out with all weather conditions commensurate with that of the installation vessel. The above are important where a number of installations need to be completed within a tight schedule.

h) The use of divers is generally not required with the improved J-tube system. The use of divers is generally not preferred due to safety issues, the additional resources that are required, dependence on the weather, cost, and time constraints, etc. 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) In service the improved J-tube system is supported at the top by the top support(s) 4 and at the bottom by the seabed 8. The J-tube is designed to resist the environmental loading and cable pulling forces as a simply supported beam, propped cantilever, or fixed end beam. In this respect the top grouted connection to the top support(s) 4 is regarded as being fully fixed' whilst the bottom connection to the seabed can be designed as either pinned' or fully fixed'.

b) A fixed end beam design is generally more economical than a simply supported design, i.e. a smaller J-tube diameter is required commensurate with the minimum diameter required for cable installation. In addition, a fixed end beam has up to two redundancies, i.e. J-tubes can be designed with inclusion of plastic hinges near the ends (cables inside can accommodate some deformation).

C) The top support(s) 4 can either be a single relatively long support or a plurality of much shorter supports. A single support would preferably be a minimum of 3 or 10 diameters or greater in length in order to be able to resist all the necessary forces and moments, but also to provide a degree of accuracy in vertical alignment when lowering the J-tubes to the seabed and installing the J-tubes into the seabed. A pair of much shorter top supports separated by a specified vertical distance would also be able to resist the necessary forces and provide the required degree of alignment. Each top support, if part of a pair, would typically be a minimum of 1 to 3 diameters in length.

d) 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.

For monopile type wind turbine foundations, this is likely to be within the lower proportion of the transition piece, i.e. within the grouted joint that connects the transition piece to the monopile. Typically the top of grouted joint is located just above MSL, but there are no fixed rules in this respect. Typically, therefore, the main span of the J-tubes will be similar to the distance from the bottom of the transition piece to the seabed.

e) It is also possible, if not preferable, to double-up the function of the boat landings to act as the top support(s), if required. Typically, the main tubes forming the boat landings are circa 350 to 400 mm diameter; thus J-tubes of diameter 275 to 325 mm diameter would fit very nicely within main tubes of boat landings with a nominal gap left for grouting. Alternatively, the size of the main boat landing tubes could be optimised to suit the preferred size of the J-tubes, or vice versa. 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, other than the J-tubes themselves, thus leading to significant savings in secondary steel and overall foundation costs. In addition, it is possible to design the boat landing as a composite structure (two tubes separated by grout) in order to further reduce the thickness of the members, etc. Typically, there are two main boat landing tubes and two J-tubes required per turbine location. Where only one J-tube is required, one main tube of the boat landing can remain unused. Where three or more J-tubes are required, dummy boat landing tubes can be added as required.

f) 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.

Conversely, the level of the lowest of the top supports can also be raised, particularly for the shallowest water depths, with or without a scour hole, in order to maintain adequate clearance between the bottom of the lower section 3 and the seabed 8 when the transition piece 5a is installed.

g) 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.

However, in the case of a conical monopile where the difference in diameters is relatively modest, it is anticipated that the upper and lower sections 1 and 3 will remain vertical, with different length attachments for the top support(s) 4. Longer attachments are required above the conical section in order to maintain verticality and to achieve adequate clearance away from the monopile at seabed level. Longer attachments have the advantage anyway that they generally help reduce the induced bending moments and forces.

h) 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. The inclusion of an offset would tend to limit the choice of installation technique, generally favouring push or socketed installations. The offset would need to be located such that it does not clash with the top support(s) 4. In addition, shear keys within the top supports 4 or radial fins within the seabed embedment may be required to prevent rotation due to eccentric loading.

I) 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. In this way, advantage can still be taken of being able to lower the J-tube system to the correct level, etc. but by utilising an alternative upper support arrangement. It is anticipated, however, that the grouted top support of the improved J-tube system would be preferred because of its greater speed of installation and, in particular, on no reliance on the use of divers.

j) 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. It is anticipated, however, that the lower top supports 4 would still be grouted (i.e. where access is difficult and below the water level) and would then be adequate to support the whole J-tube assembly safely in at least a temporary condition until all supports are secured.

k) 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. The J-tubes may be lifted as high as possible within the top supports 4 in order to ease storage and facilitate transportation. Alternatively, the J-tubes can be installed within the top supports 4 on site prior to final installation.

This can be achieved by, for example, inserting the upper section 1 into the top support 4 from below.

I) 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. These are generally composed of two semi-circular components, hinged on one side and bolted closed on the other side. The advantage of this is that this may speed up or simplify the insertion of the J-tube assembly into the top supports 4 before the transition piece 5a is lifted and installed onto the top of the monopile 5b. These may, however, be locked closed before J-tube installation begins. In addition, this may also provide a back-up facility whereby the J-tube can be removed if damaged during installation, or if the embedment depth is insufficient.

m)The lower section 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 before the transition piece 5a is lifted and installed onto the top of the monopile 5b. The benefits of the latter may be ease of transportation and/or the provision of adequate clearance during manoeuvring of the transition piece 5a.

In this case, the lower section or sections can be welded or bolted into their final position whilst on the installation vessel. Alternatively, the curved section 2 can be welded or bolted to the upper section 1.

n) The top support(s) and J-tubes are preferably manufactured from structural steel, but other materials could also be used. The curved section 2 of the J-tube can be fabricated, hot forged, or formed from proprietary bolted sections, etc as for conventional J-tubes.

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. The internal diameters of the upper section 1 and of the curved section 2, and the radius of the curved section 2, are determined to suit cable installation requirements.

o) 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 3 need not be directly in line with the upper sectioni, though a smaller rather than larger offset is preferential to minimise secondary bending moments during piling operations. Indeed, the lower section 3 does not even have to be the same diameter (or even of the same cross-section) as the upper section 1. Furthermore, the lower section 3 can in fact be two or more tubes (or other section shapes) if required, e.g. one on each side and lapped with the upper section 1. In addition, it is preferable that the inner space of the lower section 3 is not continuous with the rest of the tubes in order to avoid snagging of the cables and prevent ingress of soil during piling operations.

p) Where a fabricated junction is envisaged, the lower section 3 can be simply cut-to-shape, lapped and welded to the top of the curved section 2 and to the bottom of the upper section 1. Alternatively many other fabrication methods are possible. A cast steel junction has the advantage that the junction can be strengthened as necessary to resist the installation forces. A typical cast steel junction is anticipated to weigh less than approximately 250 kg. A proprietary section of pipe may be an off-the-shelf product with or without integral bolted flanges, e.g. spun pressure pipe. If other materials are used, the manufacturing method will vary accordingly.

q) Where a scour hole is allowed to develop around the main structure prior to J-tube installation, the lower section(s) 3 will need to be longer than the design embedment depth by the anticipated maximum depth of the scour hole. In addition, horizontal extension pieces to the end of the curved section 2 may also be required to span the anticipated maximum width of the scour hole.

r) The distance between the lowest top support 4 and the seabed 8 should be greater than the length of the lower section 3 pIus the height of the curved section 2. For shallow water depths and locations with a deep scour hole, this may become impractical to achieve and the top supports 4 will then need to be "opened" to allow the curved section 2 to pass, this requiring the use of divers.

Conventional "openable" hinged and/or bolted mechanical clamps can be used in this instance. These are generally composed of two semi-circular components, hinged on one side and bolted closed on the other side. It is anticipated that only the lowest top support 4 will need to be opened to allow the J-tube to drop. However, in this situation, it is envisaged that at least two remaining top supports 4 will remain closed or unopenable so that the stability of the J-tube assembly is maintained during installation.

s) The use of openable top supports 4 would be considered as a non-standard or less preferred option, and would generally only be required for the shallowest water depths and then only when combined with a deep scour hole. However, the degree of use of openable top supports 4 can be reduced by having the lowest top support 4 as high up the main structure 5 as possible and the avoidance of large embedment depths associated with deep scour holes. Once the J-tube assembly has been lowered, 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. Furthermore, it may be possible to close and grout the one openable top support 4 at a later date when conditions are better suited to the use of divers. In this temporary condition, especially before any significant scour hole has developed, adequate support of the J-tube should be provided by the remaining top supports 4.

t) Earthing cables can be connected between the J-tube system and the concentric support tubes, or main structure, in order to provide any necessary electrical continuity.

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) Prior to the piling installation stage beginning, the J-tube is lowered to the seabed, whereupon the J-tube is driven, vibrated, pushed, socketed or otherwise inserted into the seabed. Application of the necessary forces to drive, vibrate, or push the lower vertical section into the seabed would ideally be applied at the top of the upper section 1 using conventional and readily available (i.e. onshore type) pi'ing equipment.

b) Before lowering to the seabed, the J-tube assembly is orientated in the horizontal plane to suit the required cabling layout. The upper and lower sections are generally anticipated to be 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.

c) Vertical installation of the J-tube assembly ceases when the toe of the lower section 3 reaches the desired embedment depth, and/or the height of the curved section 2 reaches the desired level for cable entry, etc. Where a scour hole 6 has been allowed to develop adjacent to the main structure 5, the minimum embedment of the lower section(s) 3 will be governed by the bottom of the scour hole 6, whilst the level of the curved section 2 will be governed by the level of the surrounding seabed or cable trench, etc. d) 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, particularly where a large inertial mass of the curved section and any extension pieces, etc, exists to one side. 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. The piling forces associated with 5 or metres embedment in a wide range of soils are expected to be quite low. In softer soils, the use of static weights may be sufficient in themselves to install the J-tube assembly in position, particularly as sufficient crane capacity and weights may already be available on the installation vessel. Alternatively, if the static weights are sufficiently large and/or the soil resistance is sufficiently low, the lower section(s) 3 may literally be "tapped" into position, rather than "driven", such that large inertial forces are avoided.

e) Vibration installation methods utilise a vibro-driver and are particularly suitable for granular soils. The piling forces are not as large as with driven methods, but whether this is acceptable to the large offset mass of the curved section, etc, would need to be investigated for each particular case. However, the piling forces associated with 5 or 10 metres embedment in loose or medium dense granular material are expected to be quite low.

f) 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 curved section 2 or in any attached extension pieces, etc, is nearly vibration-free, and creates little noise. This type of installation technique works most efficiently with open-ended piles and often the installation equipment is designed to operate in a confined space (advantageous for operating from deck of jack-up barge, etc).

Furthermore, the reactive force required to push the lower vertical tube of the J-tube to a maximum depth of 5 or 10 metres is expected to be comparatively low. Water jetting can be used to aid installation in difficult ground conditions.

g) It is anticipated that the reactive force necessary to install the lower section(s) into most soft to moderately strong soils will be less than tonnes, though the actual force will very much depend on the conditions at each site. Water jetting can be used to reduce the reactive force in the stronger soils if required. An example of a suitable hydraulic press is the Giken UPI5O. This has a reactive force capacity of 150 tonnes and the whole unit weighs less than 10 tonnes. The availability of hydraulic presses is fairly common in Europe and quite widespread in the Far East, where push piles are far more common. Whatever equipment is used, the hydraulic press can be mounted on a simple frame that sits atop the transition piece, or is alternatively mounted off the installation vessel. 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 possible to incorporate a narrow section of sheet pile onto the "follower" for the hydraulic press to grip onto, as many off-the-shelf hydraulic presses are designed to install sheet piles.

Notwithstanding the above, the actual optimum equipment would need to be investigated for each particular case.

h) The actual embedment depth of the lower section(s) 3 will depend on the environmental loading, the size of the J-tube, and the ground conditions, etc, and whether the support for the bottom of the J-tube is to be designed as pinned' or fully fixed'. In addition, where a scour hole 6 is allowed to develop around the main structure 5 prior to J-tube installation, the lower section(s) 3 will need to be longer than the design embedment depth by the anticipated maximum depth of the scour hole. It is anticipated that the embedment depths for a pinned support will typically be less than 5 metres, and the embedment depth for a fully fixed support will be less than 10 metres, though the final depth will be very much site dependent.

These embedment depths are comparatively shallow and the piling forces are expected to be comparatively low compared to many onshore applications (the latter of which are commonly installed in a wide range of soils up to 20 metres deep, or much more). It is anticipated that the piling forces will be well within the capabilities of most conventional onshore type piling equipment and, therefore, such piling equipment will be of small to moderate size. Hence the total piling installation times are expected to be relatively small.

i) The support for the bottom of the J-tube is designed as a laterally loaded pile, e.g. using the concept of p-y springs, etc. The J-tubes need to resist primarily lateral loads, with the axial loads being comparatively small. Consequently, the embedment depths should rarely be required to be greater than 10 diameters for a pinned support, or 20 diameters for a fully fixed support. In stronger soils, the actual embedment depths may be significantly less.

j) 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. This could be useful where the upper soils are relatively soft and/or the depth of drivable overburden soils is limited. Radial fins will also help to prevent rotation of the J-tube system if, for example, an offset is incorporated in the upper part of the J-tube system.

k) Where very strong soils are encountered, water jetting techniques can be used. Many rocks that are already very weak, e.g. chalk, or are highly weathered, may be no stronger than many soils. In many cases, because of the relatively short embedments required and the fact that the upper layers of many rock strata are likely to be heavily weathered, the simpler piling techniques may well be possible.

Where stronger rock is encountered, the simpler piling methods may not be possible, however, it is anticipated most sites to have at least some depth of overburden soils. Where strong rock is present near the surface, holes can be predrilled or drill-drive rock-socket techniques employed.

I) The lower section(s) 3 and the pile installation aspect of the improved J-tube fixing system can be used whatever the method of supporting the top of the J-tube assembly, ie. whether the top supports are secured by grouting or conventional mechanical clamps or other types of support are used, as discussed above, that do not rely on the use of grouting. In this way, advantage can still be taken of being able to lower the J-tube system to the correct level but utilising an alternative upper support It is anticipated, however, that a grouted top support would be preferred because of its greater speed of installation and, in particular, on no reliance on the use of divers.

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) A significant feature of the present invention is that it has the ability to accommodate large differential movements without forming a hole in the seabed. Unlike a very stiff attachment, the large span/depth ratio of the J-tube (depth being the distance from the seabed to the lowest upper support) means that the force necessary to deflect the bottom of the J-tube is much less than the force req uired to develop a hole in the seabed. To put it another way, as a result of the inherent flexibility of the J-tube, it is capable of accommodating any relative movement between itself and the monopile without inducing any excessive forces or displacements, b) Interaction between the lower section 3 and the main structure (e.g. the monopile), i.e. where transfer of forces occur between the two through the ground, is anticipated to be small This is because the J-tube is likely to be that much smaller, and therefore less stiff, than the main structure. In addition, as the soil between the lower section 3 and the main structure (the monopile) acts like a stiff spring, the transfer of forces is likely to degrade rather than attract additional forces. Whatever the scenario, for each particular case, the effects of interaction can be investigated by considering either separate support springs for both the monopile and the J-tube, or by considering dependant spring supports for the bottom of the J-tube assembly.

c) As the J-tube is likely to be far less stiff than the monopile, it can accommodate significant differentiate movement relative to the monopile and/or surrounding ground without inducing large stresses in the system. In addition, it is likely that the J-tube will deflect to a certain degree with the monopile, such that any induced bending moments will be even lower. Furthermore, a hole or slot is less likely to develop around the bottom of the J-tube as the soil reactions required to form these holes cannot easily be resisted by the bottom of the J-tube acting as a cantilever, i.e. the J-tube system will preferentially deflect to alleviate the load rather than resist the forces. Even if a significant slot or hole forms around the bottom of the J-tube, again the lateral displacements required to close the gap generally do not generate significant bending moments in the J-tube system, etc. The improved J-tube system is therefore very tolerant to whatever possible support conditions are likely to be imposed upon the bottom of the J-tube.

d) In addition to lateral loads, the bottom of the J-tube will also support some vertical loads. These can be due to self-weight, but also due vertical displacement caused by rotation and flexure of the monopile. However, the maximum vertical load is likely to be limited by the axial capacity of the ground, rather than the steelwork. In addition, the embedment depth of the J-tube into the seabed can be varied in order to control the forces. Furthermore, the vertical support offered by the ground does not affect the overall stability of the improved J-tube system, i.e. the vertical support offered by the seabed is a redundant support.

e) The vertical loads in the J-tube system will induce bending moments in the attachments that connect the top supports to the monopile, which will need to be designed accordingly. The vertical displacements and associated vertical forces and induced bending moments can be controlled by the number, length, and stiffness of these attachments to the monopile, etc. f) If necessary the J-tube top support can be designed as a permanent sliding joint (i.e. by using alternative materials to grout or designing support accordingly, as discussed below).

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) It is anticipated that both the upper section 1 of the J-tube and the top support(s) 4 will preferably be constructed from pipe or tubing, with the J-tube fitting inside the top support(s) 4. A close-fit or loose 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. The diameter or size of the hole in the centre of the end plate andlor diaphragm will govern the closeness of the fit.

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) It is anticipated that the required grout strength need only be of medium to high strength, e.g. 30 to 50 MPa, in order to transfer all the necessary forces, etc. In addition, it is anticipated that the required grout volumes will be relatively small, especially if the space between the outside of the upper section 1 and the inside of the top support 4 is kept to 25mm or less.

f) Grout pipes to the top support(s) 4 can be pre-installed in a fabrication shop prior to transportation to site. These would normally expect to extend to near the top of the main structure and/or level with the deck of the installation vessel (e.g. a jack-up barge). No additional fabrication or installation need be required on site other than to connect the top of the grout pipes to the grouting equipment.

g) In the case of conventional offshore wind turbines that often incorporate a grouted joint between the transition piece and the monopile, the grouting can be carried out utilising the existing equipment.

h) 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. A permanent sliding support may be preferable in some circumstances, for example, in order to reduce the forces to be resisted by the top supports where they connect to the transition piece. It is anticipated, however, that grouting of some sort will be used in the majority of situations, either as a permanent fixed grouted joint or as a grouted joint incorporating a slip layer, b) The forces to be resisted by the attachments of the top supports to the transition piece will be site dependent and will vary according to the size of the J-tube, the lengths of the attachments, the relative stiffness of the components, the environmental conditions and the ground conditions. In addition, in some cases, a fixed external J-tube can act as an extra support for the overall monopile structure and can consequently attract a significant vertical load. In all these cases, the forces 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 monop lIe 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. However, 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. The slip layer is intended to transfer all lateral loads across the joint whilst allowing 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.

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. The use of an inert elastomeric material 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. This can be advantageous, particularly where the J-tube is too stiff or of a relatively short span, in order to control either the forces within the system or the displacements at the mud-line. 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 W02008151660 and in EP1616377.

The former also specifically relates to a foundation for a wind turbine. Patent Specification W02008151660 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 W02008151660 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.

Claims

Claims:- 1. A method of forming a J-tube system in which the J-tube is formed from an upper section, a curved section at the bottom of the upper section and one or more lower sections extending below the heel of the J-tube.
2. A method as claimed in Claim 1, in which the upper section and the curved section are of tubular form.
3. A method as claimed in Claim 2, in which the inner spaces of the upper section and the curved section are continuous and unobstructed.
4. A method as claimed in Claim 1, in which the lower section or sections is or are generally in line with the upper section.
5. A method as claimed in Claim 1, which includes the use of a lower section of tubular form.
6. A method as claimed in Claim 5, in which the lower section is rigidly connected to the heel of the J-tube.
7. A method of forming a J-tube system substantially as hereinbefore described with reference to the accompanying drawings.
8. A J-tube system formed by the method claimed in any one of the preceding claims.
9. A method of installing a J-tube system as claimed in Claim 8, 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 firmly attached to a main structure.
10. A method as claimed in Claim 9, in which the whole J-tube assembly is driven, vibrated, pushed, socketed or otherwise inserted into the seabed.
11. A method as claimed in Claim 10, in which vertical installation of the J-tube assembly 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.
12. A method as claimed in Claim 9, in which the upper section is enclosed within or restrained by the top support or supports relative to which the J-tube assembly is free to slide up and down.
13. A method as claimed in Claim 12, in which, prior to installation, the J-tube assembly is held in an elevated position above the seabed and is then lowered to the seabed.
14. A method of installing a J-tube system, formed by the method claimed in any one of Claims 1 to 7, 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.
15. A method as claimed in Claim 14, in which the in-fill material is grout.
16. A method as claimed in Claim 14, in which a slip layer (or other suitable element that permits sliding) is incorporated onto the outside of the upper section or onto the inside of the top support or supports.
17. A method as claimed in Claim 15, in which in order to contain the grout, the upper section is enclosed by the top support or supports in a manner that prevents grout loss.
18. A method as claimed in Claim 17, in which containment of the grout loss is achieved by the use of concentric pipes or tubes.
19. A method as claimed in Claim 18, in which the bottom of the space that contains the grout is sealed to prevent loss of grout, or the pipes or tubes are a sufficiently close fit.
20. A method as claimed in any one of Claims 9 to 19, in which draw-wires, cables, risers or the like are installed through the upper section and the curved section.
21. A method of installing a J-tube system, formed by the method claimed in any one of Claims 1 to 7, in which the upper section is secured in position by ensuring that there is a sufficiently tight fit, or a dry joint, between the outside of the upper section and the inside of a top support or supports.
22. A method of installing a J-tube system, formed by the method claimed in any one of Claims 1 to 7, in which the upper section is secured in position by securing it mechanically to a top support or supports.
23. A method as claimed in any one of Claims 9 to 22, in which extension pieces are added to the top of the upper section and/or to the end of the curved section.
24. A method of installing a J-tube system substantially as hereinbefore described with reference to the accompanying drawings.