GB2495717A - Gangway for an offshore structure - Google Patents

Abstract

A gangway 30 for accessing an offshore structure, such as a wind turbine, from water level comprises an upper end for connecting to a deck level 28 of the offshore structure; a lower end for providing access to the gangway from a vessel at water level and walkways 32, 34 that extend from the upper end to the lower end in zig-zag fashion with the centre of gravity of the walkways horizontally offset from the upper end; wherein the walkways are arranged to rotate relative to one another such that the gangway may be moved between an extended configuration and a retracted configuration. The gangway further comprises a coupling 42 connected to the lower end for connection to a generally vertical element 10 of the offshore structure to permit vertical movement of the lower end relative to the offshore structure but prevent any horizontal movement. A method of manufacturing the gangway is also claimed.

Description

GANGWAY FOR AN OFFSHORE STRUCTURE

The present invention relates to a gangway for an offshore structure and to a method for installing such a gangway. The offshore structure may for example be a foundation for a wind turbine.

As a consequence of increased energy demands and a desire to generate a larger amount of so called renewable energy an offshore wind industry has developed.

An offshore wind farm avoids many of the restrictions placed on land-based wind farms by planning regulations and suchlike. It also makes transport and installation of large parts to the wind farm site considerably easier, since it is not necessary to design components to be transportable by road, which can restrict the size and weight of individual components. Many designs of offshore wind turbine have been installed and more are in development.

Offshore structures such as offshore wind turbines require a means to permit access to parts of the structure, for example access for personnel carrying out servicing or maintenance work. An offshore wind turbine will typically be accessed infrequently when it is in used, and it will spend significant amounts of time running autonomously between times when personnel need to gain access to the structure.

Viewed from a first aspect, the present invention provides a gangway for accessing an offshore structure from water level, the gangway comprising: an upper end for connecting to a deck level of the offshore structure; a lower end for providing access to the gangway from a vessel at water level; and walkways that, in use, extend from the upper end to the lower end in zig-zag fashion with the centre of gravity of the walkways horizontally offset from the upper end; wherein the walkways are arranged to rotate relative to one another such that the gangway is articulated and may be moved between an extended configuration and a retracted configuration; and wherein the gangway further comprises a coupling connected to the lower end and arranged for connection to a generally vertical element of the offshore structure in order to permit generally vertical movement of the lower end relative to the offshore structure and in order to transfer a horizontal force from the lower end of the gangway to the offshore structure such that, in use, a horizontal force arising from a moment created by the offset centre of gravity of the walkways is supported by the offshore structure.

This arrangement provides a retractable gangway that can be stowed in a retracted configuration during periods when no access to the structure is necessary, and easily deployed to an extended configuration in which the lower end is lowered toward the water level and can be accessed by a vessel at water level. The zig-zag arrangement of the walkways can easily be folded up into a small space. This is a particular advantage for offshore wind turbines since as noted above these will have extended periods of unmanned operation. With the gangway retracted it is difficult for unauthorised persons to access the offshore structure. Also, the retracted gangway is protected from the sea and can avoid direct contact with waves except in severe weatherunder normal weather conditions. A permanently extended gangway would be constantly in contact with the sea and so would be at risk of corrosion and fouling.

The term walkway is intended to encompass any structure for passage of people, such as a path, stairway, ramp or the like. Preferably, the walkways are each of a similar length. In a preferred embodiment the walkways extend along a side of the offshore structure, diagonally across a side surface thereof.

The coupling at the lower end bears horizontal forces arising from the weight of the walkways and as a result the mounting of the gangway to the offshore structure is made easier. It is not necessary to align the centre of gravity of the structure vertically with a mounting point or mounting points for carrying the weight of the structure. The gangway can be easily mounted in an offset fashion, and as a result it can be mounted to be supported by a single vertical element such as a column or pile of the offshore structure, with the walkways extending to one side of the vertical element. The gangway does not need vertical structural support from two spaced apart vertical elements and it does not need to be provided with vertical support from multiple spaced apart winches or hanging points. A particularly preferred arrangement is for the gangway to be mounted from a leg at a corner of an offshore structure, extending along a leeward side of the structure. Since the gangway can be mounted on an existing leg it requires no modification of the offshore structure for installation, It can easily be sheltered iii the lee of the structure without the need for a particular orientation of other parts of the structure, since it can be connected to any leg or other vertical element of the structure. Typically the offshore structure might be a jacket type structure with four legs, one at each corner of the structure.

The gangway may comprise a connector for connecting the upper end of the gangway to the offshore structure and this connector should preferably also be capable of carrying a horizontal load. With this arrangement the vertical load arising from the weight of the walkways can be supported at any point along the horizontal extent of the walkways. The vertical load may possibly be supported at the connection of the upper end and by the coupling at the lower end. However, preferably the vertical load is supported by a winch connected to a lower walkway by a cable. The term cable is used herein as a general term to include ropes, chains, cables and other cable type elements for supporting a load in tension. In a preferred embodiment the winch is connected to the lower end of the gangway, which may be an end of a lower walkway.

With a winch supporting a lower walkway of the gangway the gangway can be moved between its retracted and extended configurations by action of the winch. To extend the gangway the winch lowers the cable until the lower end of the gangway is at the desired level for the vessel at water level. The lower end of the gangway may then be rested on the deck of the vessel and permitted to move up and down with wave movement. In this way the gangway provides an access point which does not move relative to the vessel. Preferably, a constant tension winch is used. This acts to raise an lower the gangway in a controlled fashion and to damp erratic movement of the gangway that might occur as a consequence of erratic movement of the vessel.

In a preferred embodiment the coupling connected to the lower end of the gangway is arranged to permit only generally vertical movement of the lower end, when installed on the vertical element in use. Thus, the lower end should be prevented from moving horizontally and should be allow to slide vertically. Hence, the coupling may be arranged to resist loads in all horizontal directions. The coupling is preferably arranged for connection to a vertical element that takes the form of a column or pile of the offshore structure, such as a leg of an offshore platform or similar. The coupling may be arranged to fully or partially encircle the vertical element, for example it may be formed as a U-shape or as a collar about the vertical element. This means that the coupling can easily pass horizontal loads to the vertical element and can also restrain the lower end so that only vertical movement should occur.

Preferably, the coupling includes guide wheels for rolling on the surface of the vertical element. This reduces friction and provides a smooth movement of the coupling in the vertical direction whilst permitting effective transfer of horizontal loads from the gangway to the vertical element.

The gangway may incorporate a bumper for contact with a vessel. The bumper is preferably mounted at the lower end of the gangway and preferably arranged to move with the lower end of the gangway. Hence, the bumper may move vertically as the coupler permits vertical movement. However, the bumper may be prevented from moving horizontally. As a result the bumper can provide a fixed point relative to the vessel and to the lower end of the gangway, which moves up and down with the movement of the vessel. The vessel can be moored to the bumper or can press up against the bumper using a small thrust. The bumper may include a resilient body, such as a rubber body or an inflatable body. The bumper may be for receiving a prow of the vessel and hence may include a V-shaped notch or similar geometry.

The walkways preferably include: an upper walkway with a first end at the upper end of the structure and a second end spaced away from the first end diagonally downwards from the first end; and a lower walkway with a first end at a lower part of the structure and a second end spaced away from the first end diagonally upwards from the first end. There may be additional walkways in between the upper and lower walkways, for example forming an S or sideways W configuration. When four or more walkways are present it may be necessary to have additional couplings for transferring horizontal load to the offshore structure, since otherwise an unrestrained mechanism may be formed.

Alternatively, or in addition, a pantograph mechanism may be used to restrict movement of the walkways.

There may be an intermediate platform or landing connecting adjacent walkways.

This provides a turning place so that personnel using the walkways can go around the corners of the zig-zag. Adjacent walkways may be connected to one another and/or to the intermediate plafform by hinged connectors, thereby permitting the required rotational movement.

With an even number of walkways and the walkways being of similar lengths the lower end of the gangway is preferably generally beneath the upper end, i.e. it may be located in about the same horizontal position and vertically below the upper end when the gangway is in use. With this arrangement the connection for the upper end may be at a first, upper, part of the vertical element of the offshore structure, with the coupling for the lower end joining to the same vertical element at a lower part thereof. It is preferred to have an even number of walkways in this fashion since the gangway can then be supported about a single vertical element of the offshore structure, which may be a column or pile of the offshore structure. Further, with this arrangement when a winch supports the weight of the walkways it may be located at the upper part of the same vertical element.

When there is an odd number of walkways and the walkways are of similar lengths the lower end would be spaced apart horizontally from the upper end of the gangway. This is less preferred, since it requires the lower end to be supported by a vertical element of the offshore structure that is horizontally spaced from the part of the offshore structure that connects to the upper end of the gangway. However some advantages of the gangway can still be realised with this type of arrangement.

In a preferred embodiment the second end of the upper walkway and the second end of the lower walkway are adjacent one another and arranged to permit transit of personnel between the upper and lower walkways. With this arrangement there may be just two walkways in a sideways V configuration and hence the first end of the lower walkway may be at the lower end of the gangway.

In preferred embodiments the walkways comprise stairs. Thus the walkways may be flights of stairs, or the walkways may include a stairway with adjacent ramp. The use of stairs enables the extended position to the walkways to put the walkways at a steeper angle whilst still enabling personnel to mount the walkways without difficulty or risk of slipping.

The gangway may incorporate an access platform at the lower end of the gangway. This may be a horizontal platform extending from the lower walkway. The platform may extend away from the offshore structure in a direction perpendicular to the horizontal extend of the walkways. This provides a space between the structure and the vessel. The access platform may be arranged to land on a deck of a vessel at water level. It may include steps for landing on the deck of the vessel.

In a preferred embodiment the gangway is connected at its upper end to a transition piece for mounting on an offshore foundation, such as a jacket type foundation.

The transition piece may be for supporting a tower for a wind turbine. The transition piece is preferably pre-fabricated onshore and may have a central structure for supporting a tower or the like and outer connecting parts for coupling to vertical elements of the foundation. The central structure and outer connecting parts may be joined by a platform structure and/or by beams or similar. The upper end of the gangway may be connected to a deck structure formed on the transition piece. In one preferred arrangement the transitton piece is integrated with a tower such as a wind turbine tower.

The invention thus extends to a transition piece incorporating a gangway as described above. It is preferred for the transition piece to include connecting parts with rotational symmetry for connection to rotationally symmetrical elements of an offshore foundation structure, for example a jacket structure. With this arrangement the transition piece can be oriented with the foundation structure upon installation in order to ensure that the gangway is located leeward of the structure, There is no requirement for a particular orientation of the foundation structure, since the rotational symmetry means that the transition piece can be rotated to give the correct orientation. Preferably the rotational symmetry is at least threefold. For example, the foundation may comprise four main vertical elements arranged symmetrically with tour mounting points for the transition piece, which would comprise four symmetrically arranged connecting points.

The invention also extends to an offshore structure including a gangway as described above, for example a gangway prefabricated with a transition piece as described above.

The gangway has been developed with a focus on the offshore wind industry and hence in a preferred embodiment the gangway is an access way for an offshore wind turbine. The invention extends to an offshore wind turbine structure incorporating the described gangway. However, the gangway may also be advantageously utilised in other offshore sectors such as oil and gas.

Viewed from a second aspect, the invention provides a method of manufacture of a gangway for accessing an offshore structure from water level, the method comprising: providing an upper end for connecting to a deck level of the offshore structure; providing a lower end for access to the gangway from a vessel at water level; providing walkways that, in use, extend from the upper end to the lower end in zig-zag fashion with the centre of gravity of the walkways horizontally offset from the upper end; connecting the walkways together to enable relative rotation of adjacent walkways such that the gangway is articulated and may be moved between an extended configuration and a retracted configuration; and providing a coupling connected to the lower end of the gangway and arranged for connection to a generally vertical element of the offshore structure in order to permit generally vertical movement of the lower end relative to the offshore structure and in order to transfer a horizontal force from the lower end to the offshore structure such that, in use, a horizontal force arising from a moment created by the offset centre of gravity of the walkways is supported by the offshore structure.

The method may include providing features of the gangway as disGussed above in relation to the first aspect. The method of manufacture of the gangway may include prefabrication of the gangway onshore, with or without a transition piece as described above.

The method of manufacture may include offshore installation of the gangway and/or of the gangway and transition piece. The offshore installation steps may include mounting the gangway to an offshore foundation, preferably via a step of installation of a transition piece prefabricated with the gangway. The method may further include connecting the lower end of the gangway to a vertical element of the offshore foundation using the coupling.

Whilst sea level and the sea-bed are referred to herein it will be understood that the structures described herein may also be used in any suitable body of water, such as seas, oceans, estuaries, inland lakes and reservoirs. Hence, any reference to sea level should be understood to mean a datum water level for the desired location of the structure in any body of water. Similarly, any reference to the sea-bed should be understood to refer to the bottom of the body of water.

As used herein the terms top and bottom and other terms relating to relative vertical location, such as upper and lower are intended to reference the normal installed orientation of the described structures. Thus, for example, the top of a pile is the part that is uppermost when it is installed, and the bottom of a pile is the opposite end of the pile.

Naturally the parts of the structure may be manufactured and transported in other orientations.

Certain preferred embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings in which: Figure 1 is a partially exploded isometric view of a jacket and wind turbine, with some ancillary parts omitted for clarity; Figure 2 is a side elevation of the jacket; Figure 3 is a plan view of the jacket with the transition piece omitted; Figures 4 to 10 show the installation of the jacket on the sea-bed; Figure 11 is an illustration of the detail of the connection between piles within the legs of the jacket, with exaggerated diameter; Figure 12 is an elevation of a part of the installed jacket with gangway with the gangway retracted; Figure 13 is a similar elevation to Figure 12, with the gangway deployed; and Figure 14 shows the detail of the connection between the lower portion of the gangway and a leg of the jacket.

The preferred embodiment is described in the context of a jacket for a wind turbine, as shown in Figure 1. The wind turbine and tower can be of generally conventional construction and hence are not described in any further detail. The tower is mounted on a transition piece 2 that is installed atop the jacket 4. The jacket 4 comprises three jacket modules 6 secured to the sea-bed by piles 8. Figure 1 shows the extent of the piles beneath the jacket into the sea-bed. In this preferred embodiment the jacket modules 6 are 12 m in height and width, with a generally cubic shape. The piles 8 may extend by around 30 m into the sea-bed, the actual depth being decided depending on soil conditions and on the expected loading on the wind turbine. The jacket 4 is sized so as to allow for one of the three modules to protrude above the reference water level.

Figures 2 and 3 show the jacket 4 in greater detail. Each jacket module 6 is a cube shape with four hollow legs 10 along the vertical edges, which are joined on each vertical outer surface of the cube by a truss structure formed by two cross-beams 12 in an X shape. The preferred embodiment shown in the drawings does not include any other cross beams. However, if required for structural strength then horizontal cross-beams can be added above and/or below the X-shaped beams 12. As can be seen clearly in the plan view of Figure 3 the jacket modules 6 do not include any beams across the middle of the structure either inside the cube shape or at the horizontal outer surfaces of the cube shape. The preferred embodiment uses tubular hollow beams. In the context of jacket designed to support an offshore wind turbine the hollow beams for this 12 m cube have a diameter of 1.2 m for the hollow legs 10 and a diameter of 0.6 m for the cross-beams 12.

The lowermost jacket module 6 includes mud mats to support the jacket module on the sea-bed. The mud mats may be adapted according to soil conditions in a conventional fashion. The jacket 4 also includes J-tubes (not shown) for passage of cables and the like from the sea-bed to the top of the jacket 4. The J-tubes (not shown) are mounted to the base of legs 10 of the lowermost jacket module 6 and can be rotated.

Each J-tube includes an angled section with a lower horizontal part that receives a cable running generally horizontally along the floor, a bend that turns the cable through a right angle, and an upper vertical part so that the cable exits the angled section running vertically upward toward the top of the jacket. Unlike a conventional J-tube the J-tubes in this preferred embodiment do not include a vertical section of tubing running from the angled section to the top of the jacket. Instead, the cabling is passed from the vertical exit portion of the angled section to the top of the structure without being enclosed in a tube. Rings mounted on legs 10 of the modules 6 keep the cables in place during wave and tidal water movements.

After installation of the jacket 4 the J-tube closest to the incoming cabling is rotated in order to direct the lower horizontal part of the angled section in the direction of the cabling. The cabling can then be installed through the J-tube and up to the top of the structure via the rings.

The transition piece 2 comprises a central circular structure for supporting a turbine tower and four triangular beams 14 extending outwardly away from the central structure and tapering toward outer connecting parts at the end of each beam 14, which connect to the tops of piles extending upwardly from the uppermost jacket module 6. The beams 14 have a box beam construction.

Further details of the structure of the jacket 4 will be apparent from the following discussion of installation of the jacket 4, with reference to Figures 4 to 10. The three jacket modules 6, transition piece 2 and piles 8 are conveyed to the installation site by barge 16 and the first, lowermost, jacket module 6a is lowered to the desired position on the sea-bed by crane. Figure 4 shows a barge 16 with second and third jacket modules 6b, 6b and the transition piece 2 on deck, and the first jacket module 6a located on the sea-bed. Four foundation piles Ba are inserted into the hollow legs 10 of the first jacket module 6a and as shown in Figure 5 these are driven through the legs 10 and into the sea-bed. The pile driving is done partly in air and partly with the hammer submerged, As noted above, the extent of the foundation piles Ba into the sea-bed might be about 30 m.

The upper end of the foundation pile Ba is left protruding from the tops of the legs 10 of the first jacket module 6a.

For this example using 12 m high jacket modules 6 the exposed length of the pile is at least 4 m. Two of the foundation piles 8a are exposed by 4 m. The other two piles are left with a greater exposed height in order to provide guide piles to facilitate simple alignment of the next jacket module. In this preferred embodiment a first guide pile extends 5.5 m above the tops of the legs 10 and a second guide pile extends 4.5 m above the tops of the legs.

As an alternative to driving the foundation piles Ba into the sea-bed with a hammer the piles could be installed in pre-drilled holes and secured in place by grout or the like.

When the foundation piles Ba are in place the second jacket module 6b is lowered onto the exposed upper ends of the foundation piles Ba. This step is shown in Figure 6.

The second jacket module 6b is first aligned with the first guide pile, which is the foundation pile that extends out of the lowermost jacket module 6a by the greatest height.

The second jacket module is landed on this first guide pile, and this provides a first fixed point. Then, when the first guide pile is engaged with the second jacket module 6b, then the jacket module can be rotate to align with the second guide pile. Landing the second jacket module on the second guide pile fixes the jacket module rigidly in place, preventing further rotation. When it is slid down the first and second guide piles it should be correctly aligned with the remaining two foundation piles 8a, allowing it to be easily located on all four piles.

When the second jacket module 6b is in place follower piles 8b are installed in the legs 10 of the second jacket module 6b. The follower piles 8b have a length that is the -10-same as the height of the jacket modules 6, and hence they protrude out of the tops of the legs of the second jacket module 6b by the same length that the foundation piles 8a protrude from the lowermost jacket module 6a. This means that the follower piles 8a will include first and second guide piles with extra height corresponding to the extra height of the guide piles in the foundation piles 8a, The third jacket module 6c and a further set of follower piles 8b are then installed atop the second module 6b in a similar fashion.

When all the jacket modules 6 and piles 8 have been installed grout is pumped into the space between the piles 8 and the jacket legs 10. The grout is pumped upward from the base of the jacket using conventional grout injection techniques and a grouting vessel 17, which is shown schematically in Figure 8. The grout forms the connection between the piles 8 and the jacket legs 10 and ensures that the jacket columns can carry vertical tensile loads.

The exposed pile ends at the top of the partially constructed jacket are cut to make them level with each other, and to provide an exposed length suitable for supporting the transition piece 2, which is installed on the exposed pile ends as shown in Figure 9. The transition piece 2 is pre-fabricated with an access way 18, which is not shown in Figure 9 but is described below with reference to Figures 12, 13 and 14. Before the transition piece 2 is installed it is rotated so that the access way 18 is on the leeward side of the jacket 4. Since the transition piece 2 and the other parts of the jacket 4 have a rotationally symmetrical structure it can be installed in any rotational orientation relative to the jacket modules 6.

With the transition piece 2 in the correct orientation the four connecting parts of the transition piece 2, which take the form of cylindrical sockets with similar dimensions to the hollow legs 10 of the jacket modules 6, are placed on the top ends of the piles 8, and then grout is injected to secure the transition piece 2 in place. The jacket 4 is then complete, as shown in Figure 10. The transition piece 2 may optionally be installed with the turbine tower already in place, the tower and transition piece 2 having been assembled together onshore.

Further details of the inter-module and inter-pile connections will now be described with reference to Figure 11. Figure 11 shows a cross-section of the jacket leg and piles at a join between two modules, with the diameter of the leg and piles exaggerated compared to the overlap length. As explained above, the length of overlap The hollow legs 10 of the jacket modules 6 include internal lugs 20 protruding from the wall of the leg and acting to locate the piles 8 in the centre of the hollow legs 10 and also to maintain a set minimum spacing between the piles 8 and the walls of the hollow legs 10. In addition, the legs 10 include a connecting ring 22, which in this case is at the bottoms of the legs 10 of the upper module 6. The connecting ring 22 has an inner diameter generally larger than the outer diameter of the legs and includes a tapered part that acts to guide the two legs into alignment andto give a tight, wedged fit of one leg onto the other. An 0-ring seal is also included, to ensure that the connecting ring 22 seals the joint between the two legs 10 and contains the grout 23 that is later pumped into the space between the piles 8 and the walls of the hollow legs 10.

The two piles 8 are designed to interlock using a plug and socket type arrangement. The lower pile 8 has a male joint 24 consisting of a truncated cone protruding upward from a flat, with a shoulder about the base of the cone. The shoulder can be used for impacting the hammer when a foundation pile 8a is driven into the sea-bed. The upper pile 8 has a female joint 26 of complementary shape, with a frustoconical volume for receiving the truncated cone of the male joint 24. Advantageously, the cone shape provides a tight fit that prevents undesirable ingress of grout into the join between two piles 8. The foundation pile 8a has the male joint 24 at its upper end. The follower piles 8b are provided with a female joint 26 at one end and a male joint 24 at the other end.

Figures 12 and 13 show further detail of the constructed jacket and tower and ancillary parts that are installed on the transition piece 2. These ancillary parts include a deck area 28 on the transition piece that joins to a gangway 30, and a crane 31. The crane 31 is used to load paris and equipment onto the deck area 28 from a vessel 33 below. The deck area 28 also joins to a stairway up into the tower that provides access into the tower from beneath the base of the tower. Advantageously this avoids the need to cut a doorway into the tower, which means that the tower is not weakened by holes in its structure.

As noted above, the deck area 28 and gangway 30 are prefabricated on the transition piece 2 and during installation of the transition piece 2, with or without the tower, it is rotated to place the gangway 30 on the leeward side of the jacket. The final stage of the installation process is to connect the gangway 30 to a leg 10 of the upper jacket module 6. Prefabrication of these ancillary parts with the transition piece 2 means that complicated construction and assembly occurs on land rather than at the offshore installation site.

The gangway 30 provides access to the deck area 28 and turbine tower from a vessel. It is an articulated stairway with upper and lower parts in the form of flights of stairs 32, 34 that join the deck area 28 to an access platform 36 at the base of the stairway. The upper flight 34 is connected to the deck area 28 at one end and extends away from the deck area 28 to an intermediate platform 38. At the intermediate platform 38 the direction of the stairway reverses and the intermediate platform 38 connects to the lower flight 34, which runs back toward the deck area 28, ending at the access platform 36, which in this embodiment is located generally beneath the starting point for the upper flight 32, where it joins the deck area 28.

The upper and lower flights 32, 34 are rotatably connected to each other via the intermediate platform 38 and the upper flight 32 is rotatably connected to the deck area 28. The rotating connections allow the gangway 30 to be lowered toward water level in order to place the access platform 36 at deck level for a vessel 33 below. When the gangway 30 is not in use the access platform 36 can be retracted and raised to a stowed configuration with the upper and lower flights 32, 34 folding together. Figure 12 shows the raised/stowed configuration of the gangway 30 and Figure 13 shows the extended/in use configuration.

A constant tension winch 40 on the deck area 28 is attached to the access platform 36 via cables and supports the vertical load that arises from the weight of the gangway 30. The winch 40 is used to raise and lower the gangway 30. When a vessel 33 is present and the gangway 30 is lowered the access platform 36 sits on the deck of the vessel. As the vessel rises and falls due to water movement the constant tension winch 40 will raise and lower the gangway 30 to match the movement of the vessel 33.

As a result the access platform 36 will not move up and down relative to the vessel 33, which wilt make it easier to use the gangway 30.

An important feature of the gangway 30 is the connection of the base of the stairway to the leg 10 of the uppermost jacket module 6. This is shown in more detail in Figure 14, which includes a close up view of the base of the stairway shown in the deployed configuration with a vessel present. A collar 42 around the leg 10 guides the vertical movement of the access platform 36 and prevents it from moving horizontally.

The collar 42 also provides a reaction force to support horizontal loads arising from the weight of the gangway 30. It will be understood that since the stairway extends sideways from its mounting point at the deck area 28 then a moment is created about the mounting point to the deck area 28. The collar 42 provides a horizontal reaction against this moment in order to support the offset gangway 30. The collar 42 includes guide wheels 44 so that it can move smoothly up and down the leg 10. The connection of the upper flight 34 to the deck area 28 also bears a horizontal load, which is equivalent to the horizontal reaction at the coJlar 42, but in the opposite direction.

The collar 42 is connected to the access platform 36 by a shaft 46. Also connected to the collar 42 is a bumper 48. The bumper 48 is designed to receive the prow of the vessel 33 and to hence keep the vessel fixed in place horizontally. The vessel can use a small amount of thrust to keep the prow of the vessel 33 pressed against the bumper 48. The bumper 48 will move up and down with the vessel 33 since, as with the access platform 36, it is supported by the constant tension winch 40.

Claims (1)

1. <claim-text>CLAIMS: 1. A gangway for accessing an offshore structure from water level, the gangway comprising: an upper end for connecting to a deck level of the offshore structure; a lower end for providing access to the gangway from a vessel at water level; and walkways that, in use, extend from the upper end to the lower end in zig-zag fashion with the centre of gravity of the walkways horizontally offset from the upper end; wherein the walkways are arranged to rotate relative to one another such that the gangway is articulated and may be moved between an extended configuration and a retracted configuration; and wherein the gangway further comprises a coupling connected to the lower end and arranged for connection to a generally vertical element of the offshore structure in order to permit generally vertical movement of the lower end relative to the offshore structure and in order to transfer a horizontal force from the lower end of the gangway to the offshore structure such that, in use, a horizontal force arising from a moment created by the offset centre of gravity of the walkways is supported by the offshore structure.</claim-text> <claim-text>2. A gangway as claimed in claim 1, comprising a connector for connecting the upper end of the gangway to the offshore structure, the connector being capable of carrying a horizontal load.</claim-text> <claim-text>3. A gangway as claimed in claim 1 or 2, comprising a winch for connection to the lower end of the gangway to support the weight of the walkways.</claim-text> <claim-text>4. A gangway as claimed in claim 3 wherein the winch is a constant tension winch.</claim-text> <claim-text>5. A gangway as claimed in any preceding claim, wherein the coupling connected to the lower end of the gangway is arranged to permit only generally vertical movement of the lower end and to prevent horizontal movement, when installed on the vertical element in use.</claim-text> <claim-text>6. A gangway as claimed in claim 5, wherein the coupling is arranged for connection to a verlical element that takes the form of a column or pile of the offshore structure, and is arranged to fully or partially encircle the vertical element.</claim-text> <claim-text>7. A gangway as claimed in claim 6, wherein the coupling comprises a collar for mounting about the vertical element.</claim-text> <claim-text>8. A gangway as claimed in any preceding claim, wherein the coupling includes guide wheels for rolling on the surface of the vertical element.</claim-text> <claim-text>9. A gangway as claimed in any preceding claim, comprising a bumper for contact with a vessel, the bumper being mounted at the lower end of the gangway and arranged to move vertically with the lower end of the gangway.</claim-text> <claim-text>10. A gangway as claimed in any preceding claim, wherein the walkways include: an upper walkway with a first end at the upper end of the structure and a second end spaced away from the first end diagonally downwards from the first end; and a lower walkway with a first end at a lower part of the structure and a second end spaced away from the first end diagonally upwards from the first end.</claim-text> <claim-text>11. A gangway as claimed in any preceding claim, wherein the lower end of the gangway is generally beneath the upper end.</claim-text> <claim-text>12. A gangway as claimed in any preceding claim, wherein the walkways comprise stairs.</claim-text> <claim-text>13. A gangway as claimed in any preceding claim, comprising an access platform at the lower end of the gangway.</claim-text> <claim-text>14. A transition piece for mounting on an offshore foundation, the transition piece comprising a gangway as claimed in any preceding claim.</claim-text> <claim-text>15. An offshore wind turbine structure comprising a gangway as claimed in any preceding claim, 16. A method of manufacture of a gangway for accessing an offshore structure from water level, the method comprising: providing an upper end for connecting to a deck level of the offshore structure; providing a lower end for access to the gangway from a vessel at water level; providing walkways that, in use, extend from the upper end to the lower end in zig-zag fashion with the centre of gravity of the walkways horizontally offset from the upper end; connecting the walkways together to enable relative rotation of adjacent walkways such that the gangway is articulated and may be moved between an extended configuration and a retracted configuration; and providing a coupling connected to the lower end of the gangway and arranged for connection to a generally vertical element of the offshore structure in order to permit generally vertical movement of the lower end relative to the offshore structure and in order to transfer a horizontal force from the lower end to the offshore structure such that, in use, a horizontal force arising from a moment created by the offset centre of gravity of the walkways is supported by the offshore structure.17. A method as claimed in claim 16 comprising providing features of the gangway as claimed in any of claims ito 13.18. A method as claimed in claim 16 or 17 comprising prefabrication of the gangway onshore with a transition piece for an offshore structure.19. A method as claimed in claim 18 comprising offshore installation of the gangway and transition piece.20. A method as claimed in claim 19, wherein the offshore installation steps include connecting the lower end of the gangway to a vertical element of the offshore foundation using the coupling.</claim-text>