Abstract:
A support structure ( 10 ) suitable for use as a support structure to an offshore platform ( 12 ), the support structure ( 10 ) comprising a main support strut ( 30 ) having a lower end and anchorable, in use, to the seabed ( 16 ) and an upper portion arranged, in use, to extend above sea level to a height substantially equal to, or greater than, that of the platform ( 12 ), the support strut ( 30 ) comprising a guide rail ( 62 ) extending upwardly from a level above the sea level ( 18 ) to the top of the support strut ( 30 ) for cooperating with a raising framework ( 90 ) slideably mountable to the guide rail ( 62 ), and further comprising drive means ( 98 ) cooperating between the raising framework ( 90 ) and the guide rail ( 62 ) for elevating the raising framework ( 90 ) relative to the support rail ( 62 ), the support structure ( 10 ) being characterized by: the support strut ( 30 ) and raising framework ( 90 ) each comprising tracks ( 70, 100 ) arranged to substantially align end-to-end when the raising framework ( 90 ) is elevated to the top of the support strut ( 30 ), the tracks ( 70, 100 ), when so aligned, forming a substantially continuous track for laterally transferring a payload ( 42 ) from the raising framework ( 90 ) to the top of the strut ( 30 ).

Description:
BACKGROUND 
     This invention relates to access support, and in particular, but without limitation, to access support suitable for use in conjunction with both existing and new offshore installations. 
     An offshore installation (otherwise known as a platform) can either be manned or unmanned but, in both instances, routine maintenance needs to be carried out from time to time. In order to achieve this, it is necessary to man the platform and lift equipment &amp; supplies onto the platform and this can be accomplished, in most cases, by a crane that is already mounted on the platform. However, in the case where the platform is unmanned, crane usage is infrequent, it becomes degraded over time, due to corrosion, fatigue and exposure to the elements and obsolescence issues cause failures. As such, before any maintenance and/or repair work can take place on the platform, it is often necessary to re-commission the crane prior to work commencing. 
     In order to achieve this, it is therefore necessary to air-lift crewmembers onto the platform using a helicopter, which is a hazardous activity. In addition, flying a crew onto the platform is very expensive. 
     In situations where the crew are required to maintain and/or upgrade a normally unmanned platform, it is also necessary to provide life-support on the platform in case of an emergency. For example, if the weather and/or the conditions are such that the crew cannot be evacuated by sea or air, it is necessary for them to be able to live safely on-board the platform, even if only for a short period of time. Whilst the platform may be supported whilst the crew are on the platform by a standby support vessel and/or a lifeboat system, it is generally not possible to leave crewmembers on an otherwise unmanned platform for extended periods of time. 
     With the passage of time, the platform degrades further, eventually leading to a situation where the required maintenance and remedial work exceeds the capabilities of helicopter intervention. 
     One known solution to this problem is to lift onto the platform temporary living accommodation units, which comprises sleeping quarters, messing facilities, first aid, and office space, etc., but this requires the use of an operational crane and if the crane is not serviceable, then alternative solutions need to be found. In addition, locating these temporary living accommodation units on a producing platform can result in unacceptable risks to personnel. These risks can only be mitigated by shutting down production and this will result in a major loss of revenue. This invention aims to locate the temporary living accommodation away from any hydrocarbon production areas and can incorporate blast and fire protection, which can significantly reduce the risk to personnel. 
     It is known, in such circumstances, to use a so-called “jack up”, which is a rig that can be floated out to the platform and located close by, whereupon legs can be extended downwardly from the jack-up until they rest on the seabed. Further jacking thereby raises the jack-up above the water level providing a temporary structure adjacent the main platform, which can be used for providing the necessary life-support services, equipment and storage space that, is needed for the operators on-board the platform. However, a jack-up is extremely expensive to use and therefore a need exists for an alternative type of support structure, in particular for an offshore platform. 
     It is also known to provide an accommodation support vessel (ASV) adjacent the main platform with a bridge connector, which can be anchored to the platform. However, these ASVs also involve the use of a jack-up for crane installation (albeit for a shorter period of time than if the jack-up were to be used, itself, as the support structure) and this, of course, introduces additional cost and complexity to the procedure. 
     A further consideration is that all of the above solutions rely on helicopter access and this significantly increases the risk to personnel when compared with marine access solutions. However, to date, these marine access solutions have been unable to provide a method of safely docking and remaining on station. 
     The invention therefore aims to provide a solution to one or more of the above problems and/or to provide an improved and/or alternative support structure for use when working on, or servicing, an offshore platform. 
     The invention may also provide a solution, which reduces the risk to personnel whilst addressing one or more of the above problems: the combination of remote temporary living accommodation and marine access may result in a major reduction in the risk to personnel and may facilitate further safety improvements. 
     Various aspects of the invention are set forth in the appendent claims. 
     SUMMARY 
     According to a first aspect of the invention, there is provided a support structure suitable for use as an extension structure to an offshore platform (new or existing), the extension structure comprising a main support strut having a lower end and anchorable, in use, to the seabed or platform and an upper portion arranged, in use, to extend above sea level to a height substantially equal to, or greater than, that of the platform, the support strut comprising a guide rail extending upwardly from a level above the sea level to the top of the support strut for cooperating with a framework mountable to the guide rail, and further comprising drive means cooperating between the framework and the guide rail for elevating the framework relative to the support rail. 
     A second aspect of the invention provides a support structure suitable for use as a support structure to an offshore platform, the support structure comprising a main support strut having a lower end and anchorable, in use, to the seabed and an upper portion arranged, in use, to extend above sea level to a height substantially equal to, or greater than, that of the platform, the support strut ( 30 ) comprising a guide rail extending upwardly from a level above the sea level to the top of the support strut for cooperating with a raising framework slideably mountable to the guide rail, and further comprising drive means cooperating between the raising framework and the guide rail for elevating the raising framework relative to the support rail, the support structure being characterised by: the support strut and raising framework each comprising tracks arranged to substantially align end-to-end when the raising framework is elevated to the top of the support strut, the tracks, when so aligned, forming a substantially continuous track for laterally transferring a payload from the raising framework to the top of the strut. 
     Suitably, the framework can be used to elevate items relative to, or to the top of, the support strut. Suitably, the invention provides a system whereby the support strut can be anchored to the seabed adjacent to a platform and maintained in a fixed relationship thereto, and which enables equipment to be hoisted onto the support strut after installation. Advantageously, this means that the support strut itself, in one embodiment, can be floated to the site of the platform and anchored in position separately from any associated equipment, which can later be affixed to the support strut. Such a configuration may greatly facilitate the initial installation procedure. 
     Thereafter, the invention enables various items of equipment to be hoisted up, and optionally, mounted to the support strut after the support strut has been installed. This conveniently provides a solution to the problem having to use a jack-up to install and/or commission the support structure prior to work commencing. 
     Suitably, the main support strut floats so that it can be floated and/or towed out to the platform by a barge or other support vessel. One or more flotation collars may initially be provided on the support strut to enable it to be up-ended during the installation process. By suitably locating the flotation collar relative to the support strut, the combined centre of buoyancy of the collar(s) and strut can be aligned with the centre of gravity of the strut to enable the strut to be floated in a controlled manner. When the strut is in position, the position of the flotation collar(s) can be adjusted to tilt the support strut into a vertical orientation to allow it to sink vertically to engage the seabed. 
     The support strut suitably comprises an anchorage at its lower end, such as a suction pile or screw pile that enables the lower end of the support strut to be positively engaged with the sea bed. Additionally or alternatively, anchorages may be provided, connected to the support strut by guy wires that serve to stabilise the attitude and/or position of the support strut. 
     Once in position, the support strut can be affixed permanently, temporarily or semi-permanently to the platform, for example, using a connecting frame that can be welded, bolted, riveted etc. to the platform and the support strut. Thereafter, any guy wires can be kept in position, or discarded, as required. 
     Suitably, the sliding framework is adapted to receive a crane, which can be mountable thereto in one embodiment, for example, on tracks. By such a configuration, it may be possible to hoist a crane to the top of the support strut using the framework and guide rail assembly such that when the support frame reaches the top of the support strut, it is then possible to transfer the crane laterally from the framework to the top of the support strut. By such a configuration, it may be possible to install the support strut in the first instance and then to offer-up a framework-mounted crane to support strut via a support vessel, such as a barge boat. 
     The drive means cooperating between the framework and the guide rail for elevating the framework relative to the support rail can be provided in any number of ways. In a first embodiment of the invention, the drive means comprises a pulley system, which is suitably motor-driven, which enables items of equipment to be hoisted up the support strut by pulling on a pulley cable connected at one end to the framework and at the other end to a driving motor. In alternative embodiments of the invention, the guide rail comprises a toothed section forming a rack and the drive assembly comprises a gear adapted to engage the rack of the guide rail such that the framework can be driven directly up the guide rail by the cooperation of the motor-driven drive gear cooperating with the rack of the guide rail. 
     Suitably, means is provided for preventing the inadvertent and/or unintentional lowering of the framework relative to the guide rail. This can be provided in any one of a number of ways including the provision of a fall-arrest device associated with a hoisting cable (in the case of a pulley hoist system), or, in a preferred embodiment, by the use of a pawl cooperating between the framework and the rack of the guide rail, which is able to ratchet freely up the guide rail, but which engages with the rack when the motor drive assembly is stopped and/or disengaged. In a yet further possible embodiment of the invention, the fall-arrest device comprises a plurality of catches located on the guide rail that sequentially engage with the framework as it is elevated and which are arranged to inhibit and/or prevent inadvertent downward movement of the framework relative to the guide rail. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of the invention shall now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a perspective view of a support structure in accordance with the invention installed adjacent a platform; 
         FIG. 2  is a schematic plan view of a connecting framework located between the support structure and the platform; 
         FIG. 3  is a schematic perspective view of an embodiment of a support strut; 
         FIG. 4  is a perspective view showing the detail of the guide rails shown in  FIG. 3 ; 
         FIG. 5  is a plan view showing the detail of the guide rails and raising framework of  FIG. 4 ; 
         FIG. 6  is a schematic side view showing the operation of the raising framework of the support structure; 
         FIGS. 7 to 16  are a sequence showing the installation and assembly of the support structure; 
         FIG. 17  is a sequence showing how the support structure of an embodiment of the invention can be used to hoist a support vessel out of the water; 
         FIGS. 18 and 19  are schematic side views showing a fall-arrest arrangement for use in conjunction with the raising framework described herein; 
         FIG. 20  is a perspective view of an embodiment of the raising framework of the invention; 
         FIG. 21  is a perspective view of an embodiment of a boat lift suspended from the raising framework of  FIG. 20 ; 
         FIG. 22  is a perspective view of an embodiment of a support structure used in conjunction with an existing offshore platform; 
         FIG. 23  is a perspective view of an embodiment of a support structure used in conjunction with a new build offshore platform; 
         FIG. 24  is a simplified view of the support strut of the structure; 
         FIG. 25  illustrates how an alternative embodiment of the raising framework can be connected to the support strut of  FIG. 24 ; and 
         FIG. 26  is a perspective, schematic view, of a cradle for a boat hoist useable in conjunction with the invention. 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG. 1 , a support structure  10  is located adjacent an offshore platform  12 . The platform  12  comprises a support framework  14  anchored to the seabed  16  for supporting, above sea level  18  a weather deck  20 , a cellar deck  22  and a spider deck  24 . The platform usually comprises equipment, a crane and wellheads, which are not shown for clarity in the drawings. 
     The support structure  10  is located next to the platform  12  and comprises a main support strut  30  that is anchored to the seabed  16  using a suitable attachment, which, in the illustrated embodiment, is a suction pile  32 , although other anchorages may be used depending on the type of seabed (rock, sand, silt, gravel, etc.). An upper part of the support strut  30  is connected to the platform  12  by connecting steelwork (not visible in  FIG. 1 ). The support structure  10  additionally comprises a deck  106  upon which are located modular accommodation units  36  and lifeboats  38 . The top of the support strut  30  flares outwardly to form an integrally-formed pedestal  40 , which provides a base for a crane  42 . As such, the support structure  10  is located beside the platform  12  and provides a crane  42  for hoisting equipment onto the deck  106  of the support structure  10  and onto a deck  20  of the platform  12 , as required. 
     The support structure  10  is thus comprised of a number of components, these being: interface steelwork (for connecting the support structure to the platform); a main support strut and suction pile; a crane pedestal; a crane, accommodation and installation system; and a power system for independently powering the support structure, for example, a diesel generator. 
       FIG. 2  schematically illustrates the interface steelwork  50  that connects the main support strut  30  to a platform  12  platform at the spider deck  24 , cellar deck  22  and/or weather deck  20  levels: the simplest arrangement being a connection at the spider deck  24  level with access then being via the platform access system from the spider deck  24  to the cellar deck  22 . However, connections at cellar deck  22  and weather deck  20  levels would provide multiple access and egress routes and align with the use of a crane  42  supported from the support strut  30 . The interaction between the platform  12  and the support strut  30  also needs to be considered due to each supporting the other, however, for installations with weight, load and/or pile load constraints the support strut  30  and suction pile  32  can be sized to alleviate these constraints. 
     The interface steelwork  50  comprises a part-circular profile  52  that seats against, and which can be welded to the support strut  30 , along with at least two splayed connectors  54  that extend between the part-circular profile  52  and the spider deck framework  24 . Additional bracing may be provided to add rigidity to the connecting steelwork, where necessary. 
     The interface steelwork  50  fulfils four functions: as installation aid for the support strut  30  and suction pile  32 ; support for the crane installation system (described below); support for the support strut  30  and suction pile  32 ; and support for the access walkways to and from the platform. 
     Suitably, the interface steelwork  50  will be pre-fabricated and installed on the platform using bolted connectors, which require a minimum of preparation work on the platform  12 . The interface steelwork  50  also incorporates a locating device and clamps to fix the support strut  30  before it is rotated to a vertical orientation. As such, the interface steelwork provides a pivot point and securement for the support strut  30  as it is tilted to a vertical orientation. Once the suction pile  32  is set at the correct depth, the clamps (not shown) can be closed and secured. 
     The support strut  30  and suction pile  32  are designed in accordance with the site-specific requirements: the main considerations being the locations of the access and egress levels; the crane requirement; platform support and environmental considerations. In addition, the support strut  30  is designed to float so that it can be towed to site, which can reduce installation costs significantly. 
     In situations where a crane  42  is specified, the support strut  30  and suction pile  32  design needs to be modified to accommodate both the crane installation loads and the crane operating loads. 
     Because most crane pedestals are typically of a larger diameter than what is required for the support strut, a pedestal is provided at the top of the support strut, as shown in  FIG. 3 . In  FIG. 3 , the support structure  10  comprises a hollow, tubular support strut  30  with a suction pile  32  at its base—the hollow tubular construction allowing it to float, when in a horizontal orientation, but submersible, when desired, for example, by partially flooding it with sea water. The support strut&#39;s  30  upper end is fitted with a crane pedestal  40 , which is of a larger diameter at its upper edge than that of the support strut. A flared portion  60  is provided to transmit the loads into the support strut  30  and the overall height of the support structure  10  can be adjusted off-site by appropriately sizing the support strut and by sliding the pedestal  40  relative thereto prior to welding into position. 
     The support structure  10  additionally comprises a pair of guide rails  62  that extend from a point level with the upper edge  64  of the pedestal  40  to a point above sea level  18 . The guide rails  62  are rigidly connected to the support strut  30 , at intervals, by connectors  66 , which, in practice, would comprise triangulation elements (not shown for clarity) to form a rigid connection between the two. 
     The upper surface  68  of the pedestal comprises a pair of parallel tracks  70  that project beyond the edge  64  of the pedestal and overlie the upper ends of the guide rails. 
     The guide rails  62  are shown in greater detail in  FIGS. 4 to 6  of the drawings. In  FIG. 4  it can be seen that each guide rail  62  comprises a central bar portion  72  having a smooth front surface that faces away from the support strut  30 , and a smooth rear surface  76  that faces towards the support strut  30 , in use. The front  74  and rear  76  surfaces provide rolling surfaces for respective, vertically spaced-apart sets of rollers  78 ,  80  to which a raising framework (not shown) is connected. The sides of each guide rail  62  are provided with toothed formations  82 , which engage with the teeth of driving gears (not shown in  FIG. 4 ) to enable the raising framework to be driven along the guide rails, either upwardly or downwardly, as required. 
       FIG. 5  is a plan view of the guide rails  62  described above. In  FIG. 5 , the raising framework  90  comprises a pair of arms  92  that extends behind the guide rails  62  at a relatively elevated position to support a set of rollers  80  that bear against the rear rolling surface  76  of the guide rails  62 . The raising framework additionally comprises a cross bar  94  at a relatively lower position to which another set of rollers  78  are connected, which bear against front rolling surface  74  of the guide rails  62 . The toothed portions  82  of the guide rails  62  project sideward from the guide rails  62  and are engaged by motor-driven gears  96  to raise or lower the raising framework  90 . 
     In  FIG. 6  it can be seen how the raising framework  90  cooperates with the guide rails  62  to enable a payload, a crane in the illustrated example, to be hoisted up the support strut  30  using an engine  98  for driving the gears  96 . 
     As can also be seen in  FIG. 6 , the raising framework  90  is adapted to carry a pair of parallel tracks  100  that are arranged to align with the tracks  70  on top of the pedestal  40  of the support structure  10 . By raising the raising framework  90  to the top of the guide rails  62 , the respective tracks  100 ,  70  meet end-to-end, suitably by the provision of complementarily chamfered ends, to enable a payload to be transferred laterally (by rolling it along the tracks  100 ,  70 ) from the raising framework  90  to the top of the pedestal  40 . 
     The installation of the support structure proceeds as shown in the sequence of  FIGS. 7 to 19  of the drawings. 
     In  FIG. 7 , the support strut  30  and suction pile  32  are floated to the platform  12 , with pre-installed connecting steelwork  50  in place, and one or more floatation collars  102  are used to maintain the strut  30  in a horizontal orientation. In  FIG. 8 , the support strut  30  is up-ended, for example by partially flooding it, until it reaches a vertical orientation as shown in  FIG. 9 . At this point, the location of the strut  30  can be adjusted freely, or the strut  30  can be located against pre-installed connection steelwork (not shown) on the platform  12 . The strut  30  can then be sunk by allowing the flotation collars  102  to slide upwardly until the suction pile  32  engages the sea bed  16 . The suction pile can then be evacuated (or the strut otherwise anchored to the sea bed  16 ) to hold it in position and the floatation collars  102  removed. 
     In  FIG. 12 , a support vessel  104  approaches the support structure  10  at low tide such that the raising framework  90  can be offered up beneath the lower ends of the guide rails (not shown for clarity). As the tide rises, as shown in  FIG. 13 , the raising framework  90  engages the ends of the guide rails  62  and can be driven up the support strut  30  with its first payload, in this case, a crane  42 , using the drive gears previously described. When the raising framework  90  reaches the top of the support strut  30 , the crane  42  can be transferred to the top of the support strut on the rails  70 ,  100  previously described, and locked into position. 
     Now that the crane  42  has been installed, it is possible to use the crane  42  to transfer other items from the support vessel  104 , such as a deck  106  and to install it on the support strut  30 . The deck  106  would have to be installed piecewise. Thereafter, living accommodation units  36  and the like can be hoisted, using the crane  42 , onto the deck  106  of the support structure  10  to complete the installation. 
     At this point, as shown in  FIG. 14 , the raising framework  90  can be jettisoned, or it can be left in place to act as a davit system for raising a fast intervention vessel  124  out of the water, as shown in  FIG. 17 . Such a configuration allows the fast intervention vessel  124  to be hoisted safely out of the water so that it no longer moves relative to the support structure  10 , thus facilitating the safe transfer of crew from the vessel  124  to the support structure  10 , and also providing lifeboat or life-support for the support structure  10  should that be necessary. 
     The fast intervention vessel  124  can be hoisted using a set of under-hull slings or by attachment of crane hooks to hard eyes on the deck of the vessel  124 . Once hoisted into position relative to the raising framework, linkages can be used to free the crane for other uses. Alternatively, on a low tide, the vessel  124  can be located below the raising framework  90  and connect thereto by slings or wires, and the raising framework driven up the support strut  30  in the previously described manner to hoist the vessel  124  out of the water. Such an arrangement is shown in  FIG. 17  of the drawings, whereby the vessel  124  can be connected to the raising framework  90  by a set of linkages  108  that connect to hard eyes  110  on the deck of the vessel  124 . 
     The crane  42  can be powered by an internal combustion engine, and fuel tanks therefor can be conveniently located within the interior of the strut  30  or pedestal  40 . 
     A fall-arrest device is also provided for the raising framework  90  to prevent inadvertent falls, for example, in the event of an engine  98  failure. The fall-arrest device can comprise a pawl arrangement that ratchets against the toothed racks  82  of the guide rails  62 , or a supplementary set of catches can be provided, as shown in  FIGS. 18 and 19 . In  FIG. 18 , it can be seen that the guide rails  62  are provided with a series of catches  112  and that the raising framework  90  comprises a pivoting loop  114  that successively hooks over the catches  112  as the raising framework  90  is raised. Lowering of the frame  90  can be effected by dis-engaging the loop. However, in the event of an engine failure, the raising framework  90  drops until the loop  114  engages one of the catches  112  thereby preventing further descent of the raising framework  90 . 
       FIG. 20  is a perspective view of an embodiment of a raising framework  90 , as described previously with reference to  FIGS. 4 to 6 . 
     From  FIG. 20 , it can be seen that the guide rails  62  comprise a central bar portion  72  having a smooth front surface (not visible) that faces away from the support strut  30 . The guide rails  62  are mounted to a lattice-type support framework  200  that is affixed to the support strut  30 . The lattice-type framework  200  comprises a vertical rail  202  having a smooth rear surface  76  that faces towards the support strut  30 , in use. The front and rear  76  surfaces provide rolling surfaces for respective, vertically spaced-apart sets of rollers  78 ,  80  to which a raising framework (not shown) is connected. The sides of each guide rail  62  are provided with toothed formations  82 , which engage with the teeth of driving gears  96  to enable the raising framework  90  to be driven along the guide rails, either upwardly or downwardly, as required. 
     The raising framework  90  comprises a pair of arms  92  that extend behind the guide rails  202  at a relatively elevated position to support a set of rollers  80  that bear against the rear rolling surface  76  of the guide rails  202 . The raising framework  90  additionally comprises another set of rollers (not visible) which are arranged to bear against front rolling surface of the guide rails  62 . The toothed portions  82  of the guide rails  62  project sideward from the guide rails  62  and are engaged by motor-driven gears  96  to raise or lower the raising framework  90 . 
     The raising framework  90  thus cooperates with the guide rails  62  to enable a payload, such as a support vessel, or crane, to be hoisted up the support strut  30  using set of motors  204  for driving the gears  96 . 
       FIG. 20  additionally shows the raising framework  90  comprising four suspension lines  208 , to which a boat hoist  210 , as described with respect to  FIG. 21  below, can be connected, in use. 
     In  FIG. 21 , the raising framework  90  comprises four suspension lines  208 , which can be manufactured from steel rope, metal tubes or bars, and which hang below the raising framework  90 . The boat hoist  210  comprises a pair of spaced-apart side walls  212 , formed by a metal framework, and a base wall  214  manufactured also from a framework. The dimensions of the boat hoist  210  are selected to accommodate a support vessel  104 , which can be driven into the framework when the boat hoist  210  is lowered to slightly below sea level. 
     The support vessel  104  can thus be located within the boat hoist  210 , and raised out of the water by the raising framework  90 , as previously described (in particular, with reference to  FIG. 17  above). The provision of a dedicated boat hoist  210  is particularly advantageous because it obviates the need for crew members to attach and detach hoist lines  108 , which can be difficult or dangerous in heavy seas. 
     In certain embodiments (not shown), a flexible and/or reticulated support sheet is affixed to the upper edges of the side walls  212  and hangs between them above the base  214  wall of the boat hoist  210 . The provision of a flexible sheet or net enables the support vessel  108  to be retained securely by the boat hoist  210 , i.e. by the sheet conforming to the shape of the underside of the hull. Such a configuration additionally reduces the likelihood of point-loading the hull of the support vessel  108  (for example, where the keel would otherwise engage the base wall struts). 
     Given that the boat hoist  210  is likely to be used in heavy seas, wave suppression means and/or fendering may be provided on the boat hoist  210 , although not shown in the drawings. For example, inflatable tubes may be affixed to the upper edges of the side walls  212  of the boat hoist  210 , thereby cushioning the support vessel  108  from impacts with the side walls  212 , as well as providing shelter from the waves. Further, the inflatable tubes, or booms/pontoons may extend axially away from the boat hoist  210 , and may provide a relatively protected entrance and exit to the boat hoist  210 . 
       FIG. 22  is a more detailed version of  FIG. 1 , albeit with a boat hoist  210  fitted thereto. It will be noted that the embodiment shown in  FIG. 22  comprises an additional modification to the raising framework, which may be used in conjunction with any of the embodiments described herein. The modification is shown in particular with reference to  FIGS. 24 and 25 . 
     As can be seen in  FIGS. 24 and 25 , which are simplified views for clarity, the support strut  300  comprises four racks  220  in a cruciform arrangement thus dividing it into quadrants separated by the respective racks  220 . The raising framework  90 , as shown in  FIG. 25 , comprises four machine housings  234  which locate around the exterior of the support strut  300 , and which support the gears and motors which engage with the racks  220 . The raising framework  90  can be driven up or down the support strut  300  by motor- or engine-driven drive gears or wheels located within the machine housings  234 , which cooperate between the support strut  300  and raising framework  90  to drive it up or down, as required. 
     The raising framework  90  is connected to the support strut  300 , as previously described. The raising framework can likewise be used to hoist a support vessel (not shown) out of the water, a crane (not shown) to the top of the support strut  300 , or other equipment and components, as previously described. 
     Notably, because the raising framework  90  of the embodiment shown in  FIG. 25  surrounds the support strut  300 , the upper struts can form the transfer rails for a payload, which can be slid sideways atop the support strut  300  when raised, and left there when the raising framework  90  lowers again. 
     It will be appreciated from the foregoing that although the invention is particularly suited to servicing operations for existing platforms, e.g. crane replacement etc., it is equally applicable to new installations, and an example of a new build offshore platform  400  and support structure  10  is shown in  FIG. 23  of the drawings. The new build platform  400  comprises a main supporting monopole or support strut/structure  402  to which a main deck  404  is mounted. The operational equipment of the platform  400 , e.g. a drilling rig (not shown) is located on the main deck  404 . 
     A pair of additional support struts are provided: a first support strut  300 , as described herein, which comprises a raising framework  90 , crane  42 , deck area  38  for temporary living accommodation etc.; and a second strut  410 , which supports an additional deck  406 . The additional deck  406  usefully provides an alternative location for certain items, e.g. equipment to support drilling operations etc., and also enables the support structure  10  of the invention to be truly “independent” of the main platform. The support struts  300 ,  410 ,  402  comprise foundations  424  at their lower ends, enabling them to be driven into a sea bed with relative ease. Further, because the support struts  300 ,  410 ,  402  are interconnected by cross-struts  408 , a tripod-type structure is formed, which is inherently more stable than a mono-pile, and can be driven vertically into the sea bed by differentially varying the pressure in the foundations  424  during the driving process (as described in greater detail in UK Patent Application No: GB 12407991.7). 
     Referring now to  FIG. 26  of the drawings, a cradle  248  for the boat hoist  210  previously described comprises a steel support frame comprising two spaced-apart side walls  212  manufactured from steel sections  250  welded to form a rigid, triangulated structure. The side walls  212  are interconnected at their lower edges by a base wall  214  comprising steel sections  252  welded to, and spanning, the lower edges of the side walls. 
     The cradle  248  additionally comprises, extending outwardly at an angle, from each of its corners, a retractable boom  256 . Each boom  256  comprises a pair of spaced-apart, horizontal metal tubes that terminate at their free ends, with a float  258 . The floats  258  serve to stabilise the cradle  248  when floating in the water, or when lightly supported by the suspension lines (not shown). The angling of the booms  256  provides a tapered entrance and exit for the cradle  248  facilitating the entry and exit of a support vessel (not shown). 
     A flexible skin (e.g. of sheet plastics, or canvas), or a sheet metal skin  254 , is provided on the exterior of the side walls  212  and booms  256 , and optionally, below the base wall  214  of the cradle  248 , to buffet the waves and to create a relatively calm “harbour” within the confines of the cradle  248 . 
     Attached to the cradle  248 , spanning the side walls, are a set of flexible slings  260 , which engage with the contoured underside of the support vessel (not shown) as it is raised out of the water. As previously discussed, the slings could be replaced by a net or a flexible sheet to more evenly distribute the transference of the weight of the support vessel (not shown) to the cradle  248 , thereby stabilising it and reducing the likelihood of hull punctures. 
     The invention is not restricted to the details of the foregoing embodiments, which are merely an example of an embodiment of the invention. For example, the foregoing description has focussed on the use of the support structure an alternative to an accommodation type jack-up. However, there are other types of jack-up, such as a drilling jack-up, to which the concept of the invention may offer an alternative solution. Specifically, the crane of the invention could be used to lift a drilling rig onto a platform that was designed to support such weight. This could be a particularly attractive concept to those energy companies looking to drill for shale gas offshore and utilise, where possible, their existing infrastructure.