Patent Publication Number: US-2023143998-A1

Title: Walk-to-work system and method thereof

Description:
TECHNICAL FIELD 
     The present invention relates to a walk-to-work system for a waterborne structure such as a vessel, in particular a walk-to-work system for accessing offshore infrastructure such as a bottom-fixed offshore wind turbine. 
     BACKGROUND AND PRIOR ART 
     Walk-to-work are used on vessels to transport goods/equipment and personnel between the vessel and offshore wind turbines such as bottom-fixed wind turbines. 
     Traditional walk-to-work use a combination of movement compensated offshore gangways and elevator shaft, where the elevator shaft has a fixed height and forms an integrated part of the gangway. Alternatively, the gangway may be mounted on a separate pedestal with access to a separate elevator shaft having a fixed height. The elevator is used to move people and goods from various decks on the vessel to a relevant height of the gangway. The gangway is used to support goods and personnel as they move between the vessel and a relevant height of the wind turbine. 
     As many of the offshore bottom-fixed wind turbines are in areas with large variations between low tide and high tide, the gangway, and in particular the elevator having a fixed height, must be designed for the lowest astronomical tide. This is to ensure that both the gangway and the elevator allow access to a service platform of the wind turbine when the height difference between the sea level and the service platform is at its highest value. Traditionally, this has been solved by simply ensuring that the fixed elevator shaft is sufficiently high. 
     Examples of prior art solutions using walk-to-work for allowing transfer of personnel and good between a vessel and an offshore wind turbine may be found disclosed in the following patent publications: 
     U.S. Pat. No. 4,473,916(A) discloses a transfer apparatus provided with a lift to enable access between a water borne vessel and a wind turbine tower. The lift is mountable to the vessel and comprises a platform and a shaft assembly. The platform is driveable along the lift shaft assembly and the lift comprises a motion compensation arrangement arranged to compensate for movement of the vessel relative to the structure. The lift of U.S. Pat. No. 4,473,916(a) has a fixed height and is integrated to the platform and motion compensation arrangement. 
     CN208683061(U) discloses a tower ladder system for offshore service vessel. The tower ladder system comprises a main frame, a wave compensation boarding device, wave compensation crane device, and an elevator device. The wave compensation boarding gangway device and the wave compensation crane device are mounted outside the main frame and connected to the main frame. The elevator device has a fixed height and is integrated to the main frame. 
     WO2012069835(A1) discloses a lift to enable access between a waterborne vessel and a structure. The lift is mountable to the vessel and comprises a platform and a lift shaft assembly, where the platform is driveable along the lift shaft assembly. 
     The lift further comprises a motion compensation arrangement arranged to compensate for movement of the vessel relative to the structure. The lift/lift shaft of WO2012069835(A1) has a fixed height and is integrated to the platform. 
     When the vessel operates against a wind turbine during high tide, the use of such traditional designed walk-to-work, i.e. with an elevator shaft having a fixed height, can lead to undesired shutdowns of the wind turbine since there is a risk of collision between the wind turbine blades and the highest point of the vessel. The highest point will in most cases be the top point of the elevator having a fixed height. 
     Hence, common for all the above mentioned prior art solutions is a need for a walk-to-work system allowing safe and cost-efficient operations towards different types of wind turbines during all kinds of foreseeable tide conditions, while keeping the frequency of undesired wind turbine shutdowns low. It is thus an object of the present invention to provide a walk-to-work system that allows transfer of personnel and/or goods under any kind of foreseeable tides and/or with low or no risk of collision with the blade of the turbine. 
     SUMMARY OF THE INVENTION 
     The present invention is set forth and characterized in the main claims, while the dependent claims describe other characteristics of the invention. 
     In a first aspect, the invention concerns a walk-to-work system suitable for allowing personnel and/or equipment to move between a first floating marine structure, such as a vessel or other type of marine structure, and a second marine structure, such as a bottom-fixed offshore wind turbine, in particular bottom-fixed wind turbine that have varying height between the lowest point of the wind turbine blade swept area and the sea surface with varying tides, or other type of marine structure. 
     The walk-to-work system comprises a gangway system comprising a height adjustable elongated pedestal and a gangway. The height adjustable elongated pedestal has a first end mountable onto a deck of the vessel. For example, the pedestal may be mounted on an open deck and/or a closed deck. Additionally, the pedestal may also be integrated to a superstructure of the vessel/marine structure. The height adjustable elongated pedestal comprises a first pedestal part and a second pedestal part height adjustably coupled to the first pedestal part, i.e. the pedestal can be adjusted to various heights. 
     A gangway is rotationally coupled to the height adjustable elongated pedestal at a height H g  from the first elongated pedestal end such that the gangway is radially extending at a length L g  from a center axis of the height adjustable elongated pedestal. As used herein, “radially extending” means extending in a direction being perpendicular or at least substantially perpendicular to a longitudinal centre axis of the height adjustable elongated pedestal. Thus, the gangway is rotationally mounted on the upper (second) end of the pedestal, such that when the pedestal is height adjusted, the height of the gangway is consequently also adjusted. 
     The walk-to-work system further comprises an elevator system positioned with a radial offset adjacent to but in a distance from the gangway system, i.e. the elevator system is structurally separated from the gangway and arranged near the gangway such that when an elevator car of the elevator system is elevated to the height of the gangway, the gangway is accessible from the elevator car. 
     The elevator system comprises a height adjustable elongated elevator, a drive system, an elevator car, and a lifting device. 
     The height adjustable elongated elevator has a first elevator end mountable onto the decks of the vessel. The height adjustable elevator shaft comprises a static elevator part and a displaceable elevator part height adjustably coupled to the static elevator part for enabling adjusting the elevator to various heights. 
     The drive system is configured to displace the displaceable elevator part relative to the static elevator part along a longitudinal centre axis of the height adjustable elongated elevator. The drive system may be of any type enabling relative movement between the static elevator part and the displaceable elevator part. For example, the drive system may be a rack and pinion drive or may involve a motorized winch and corresponding set of cables and pulleys, or hydraulic cylinders, or electric actuators. 
     The elevator car is movably connected to the height adjustable elevator, wherein the elevator car is configured to be elevated up to the same height as the gangway for allowing access between the elevator system and the gangway system. 
     The lifting device is configured to move the elevator car of the height adjustable elevator. For example, the lifting device may be a motorized winch with corresponding cables, another alternative may involve a rack and pinion lifting device. 
     In a preferred example embodiment of the walk-to-work system, the static elevator part and the displaceable elevator part are shafts being telescopically connected to each other. 
     In another preferred example, the first pedestal part and the second pedestal part are telescopically connected to each other. 
     In another preferred example embodiment, the gangway is rotationally coupled to the second pedestal part, and preferably also fully circumventing the second pedestal having an average diameter Dg and is preferably circular. For example, the gangway may be rotationally coupled to the second pedestal part by a motorized swivel/slewing machinery mounted between the gangway and the second pedestal part. The swivel may be remotely controlled and/or controlled by a control system positioned on the gangway. 
     In another preferred example embodiment, the gangway system further comprises a bridge connected to an outer radial position of the gangway, for example the outermost radial position of the gangway. 
     In another preferred example embodiment, the bridge is pivotally connected to the gangway with a rotational axis oriented perpendicular to the height direction of the pedestal. 
     In another preferred example embodiment, the bridge is configured such that it is length adjustable. 
     In another preferred example embodiment, the length adjustable bridge comprises two bridge parts telescopically connected to each other, thereby achieving the desired length adjustment through relative displacement. 
     In another preferred example embodiment, the walk-to-work system further comprises a motion compensation control system allowing motion compensation of the bridge relative to the gangway. Note that the motion compensation control system may also be configured to perform motion compensation control for the height adjustment of the elongated pedestal and/or the elongated elevator and the rotary mechanism/swivel/slewing machinery allowing rotational movement of the gangway. 
     In another preferred example embodiment, the gangway system further comprises an access platform. Preferably the access platform is arranged adjacent to an outer radial position of the gangway. The term adjacent is defined as a position which allow safe movement of personnel/equipment between the access platform and the gangway. 
     In another preferred example embodiment, the access platform is supported on the elongated pedestal via a support structure, wherein the access platform is arranged near or at the same height Hg of the gangway. The support structure preferably comprises a collar at least partially circumventing the second pedestal below the gangway and a radially extending framework extending from the collar and to an under side of a base of the access platform. The collar may be rotationally mounted onto the second pedestal or fixed. 
     In another preferred example embodiment, the gangway system is configured such that the gangway is allowed to rotate independently of the access platform, see for example the above-mentioned configuration with the motorized swivel/slewing machinery. 
     In another preferred example embodiment, a safety fence is arranged on the access platform, the safety fence being movably connected to a safety barrier of the gangway via a set of tracks such as railings, thereby allowing safe movement of personnel and/or equipment between the access platform and the gangway regardless of the angle of rotation of the gangway. Both the safety barrier and the safety fence have preferably a height that prevents personnel/goods/equipment indented onto the gangway to fall out in allowable weather conditions. 
     In another preferred example embodiment, the gangway system further comprises an access platform and wherein a landing door is vertically mounted to the access platform, preferably at one side of the access platform distal from the gangway. The landing door are preferably parallel and aligned with the elevator car door when the elevator car is elevated to the height of the access platform. 
     In another preferred example embodiment, the landing door is configured to interlock against the elevator car. It should be noted that a relative deflection between the elevator car and the gangway is pre-determined so that it is safe to move between the elevator car and the gangway. Thus, the gangway and the elevator car, when elevated to the height of the gangway are separated by an offset allowing safe movement of personnel and equipment there between. 
     In another preferred example embodiment, the displaceable elevator part is movably connected to the static elevator part via guide rails. Preferably one set of guide rails is mounted on the static elevator part and another of the sets of guide rails is mounted on the displaceable elevator part, wherein the elevator car is movably connected to the guide rails via wheels rotationally mounted on the elevator car such that the elevator is always in contact with at least one set of guide rails. 
     In another preferred example embodiment, a vessel comprises a hull, a deck, a superstructure arranged on the deck and the walk-to-work system, wherein the walk-to-work system is coupled to the deck, for example an open deck and/or a closed deck of the vessel. 
     In another preferred example embodiment, the height adjustable elongated elevator is configured such that its minimum height is lower than a highest point of the vessel&#39;s superstructure. 
     In another preferred example embodiment, the height adjustable elongated pedestal is at least partially integrated to the superstructure of the vessel. 
     In another preferred example embodiment, the vessel comprises a plurality of decks and wherein the static elevator part is fixed to one or more of the plurality of decks of the vessel. 
     In a second aspect, the invention concerns a method for transferring personnel and/or equipment between a first floating marine structure comprising a walk-to-work system according to the invention as defined in the appended claims, and a second marine structure comprising a service platform, the method comprising the steps of:
         A. stabilizing the first floating marine structure relative to the second marine structure,   B. bringing the gangway system in contact with, or in near contact with, for example adjacent to, the service platform by adjusting the height adjustable elongated pedestal to a height that allows access of personnel and/or equipment between the gangway of the height adjustable elongated pedestal and the service platform of the second marine structure,   C. maintaining the contact, or the near contact, by use of a motion compensation control system compensating relative movement between the first marine structure and the second marine structure,
           wherein the motion compensation control system is operationally coupled to at least:
               the pivot point between the gangway and the bridge to ensure motion compensation in the direction around a rotational axis perpendicular to the height direction,   the mechanism/device controlling the height of the pedestal,   the mechanism/device controlling the height of the elevator and   the mechanism/device such as the swivel/slewing machinery controlling the rotation of the gangway around the pedestal,   
               
           D. adjusting the height adjustable elongated elevator to a height that allows an elevation of the elevator car up to the same height as the gangway,   E. elevating the elevator car with personnel and/or equipment from an initial position on the elongated elevator to the same height as the gangway, and   F. transferring personnel and/or equipment between the first floating marine structure via the elevator system and the gangway system to the service platform of the second marine structure.       

     In a preferred example embodiment, the first marine structure is a vessel preferably a vessel as described above. 
     In another preferred example embodiment, the second marine structure is an offshore wind turbine, such a bottom fixed offshore turbine. 
     In another preferred example embodiment, step B further comprises bringing the gangway system in contact with, or near contact with, the service platform by use of a length adjustable bridge, wherein one end of the length adjustable bridge is pivotally connected to an outer radial position of the gangway. Hence, the contact, or near contact, is achieved by pivoting the bridge until the bridge&#39;s other end is in the desired height position relative to the service platform. 
     In another preferred example embodiment, step C further comprises
         compensating relative movement between the first floating marine structure and the second marine structure   by rotating the gangway relative to the height adjustable elongated pedestal, and/or   by adjusting the height of height adjustable elongated pedestal, and/or   if the gangway system comprises a length adjustable bridge, by adjusting the length of the length adjustable bridge.       

     In another preferred example embodiment, the gangway system further comprises an access platform supported on the elongated pedestal such that the access platform is arranged adjacent to an outer radial position of the gangway and wherein step D further comprises adjusting the height of the height adjustable elevator to allow elevation of the elevator car up to the same height H g  as the gangway adjacent to the access platform. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    illustrates schematically a walk-to-work system according to a first example embodiment of the invention mounted in the transversal direction of a waterborne vessel and operating against an offshore wind turbine. 
         FIG.  2    illustrates schematically the walk-to-work system of  FIG.  1    in a lower and upper position, where  FIG.  2 A  shows a side view of the walk-to-work system in a lower (detailed illustration) and upper position (contour illustration) and  FIG.  2 B  shows a perspective view of the walk-to-work system in a lower (detailed illustration) and upper position (contour illustration). 
         FIG.  3    illustrates in perspective details of a height adjustable elevator forming part of the inventive walk-to-work system, wherein  FIG.  3 A  shows the telescopic elevator in an upper position and  FIG.  3 B  shows the telescopic elevator in a lower position. 
         FIG.  4    illustrates in profile details of the height adjustable elevator of  FIGS.  3 A and  3 B , where  FIG.  4 A  show the sides of the height adjustable elevator in a lower (detailed illustration) and upper position (contour illustration), while  FIGS.  4 B and  4 C  show the height adjustable elevator in a lower (detailed illustration) and upper position (contour illustration), from the front and from the back, respectively. 
         FIG.  5    illustrates a perspective and top view of the walk-to-work system according to the first example embodiment of the invention, where  FIG.  5 A  shows the walk-to-work system with an access platform having handrails and  FIG.  5 B  shows a top view of the access platform and a gangway of the gangway system rotating independently from the access platform. 
         FIG.  6    illustrates schematically a walk-to-work system according to a second example embodiment of the invention mounted in the transversal direction of the vessel and operating against an offshore wind turbine, where  FIG.  6 A  shows the height adjustable elevator of the walk-to-work system in a low elevated position and  FIG.  6 B  shows the walk-to-work system in a high elevated position. 
         FIG.  7    illustrates schematically a walk-to-work system according to a third example embodiment of the invention mounted in the transversal direction of the vessel and operating against an offshore wind turbine, where  FIG.  7 A  shows the height adjustable elevator of the walk-to-work system in a low elevated position and  FIG.  7 B  shows the height adjustable elevator in a high elevated position. 
         FIG.  8    illustrates schematically a walk-to-work system according to a fourth example embodiment of the invention mounted in a transversal direction of the vessel and operating against an offshore wind turbine, where  FIG.  8 A  shows the height adjustable elevator of the walk-to-work system in a high elevated position and  FIG.  8 B  shows the height adjustable elevator in a low elevated position. 
         FIG.  9    illustrates schematically a walk-to-work system according to a fifth example embodiment of the invention mounted in the transversal direction of a vessel and operating against an offshore wind turbine, where  FIG.  9 A  shows the height adjustable elevator of the walk-to-work system in an high elevated position and  FIG.  9 B  shows the height adjustable elevator in a low elevated position. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following, specific example embodiments of the invention will be described in more detail with reference to the drawings. However, the invention is not limited to the example embodiments and illustrations contained herein. It is specifically intended that the invention includes modified forms of the embodiments, including portions of the embodiments and combinations of elements of different embodiments. It should be appreciated that in the development of any actual implementation, as in any engineering or design project, specific decisions must be made to achieve the developer&#39;s specific goals, such as compliance with system and/or business-related constraints. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication and manufacture for the skilled person having the benefit of this disclosure. 
       FIG.  1    shows a first example embodiment of a walk-to-work system  1  in accordance with the invention. 
     In  FIG.  1    the walk-to-work system  1  of a first example embodiment of the invention is mounted in a transversal direction of a vessel  300  and allows access between the vessel  300  and a relevant position, for example a service platform  410 , of an offshore structure such as an offshore wind turbine installation  400 . Thus, when the walk-to-work system  1  is mounted in a transversal direction of the vessel  300  the direction of the access passage between the vessel  300  and a relevant position of the offshore structure is primarily perpendicular or oblique to the vessel longitudinal direction. The walk-to-work system  1  may alternatively be mounted in a different direction on the vessel  300 , for example in a longitudinal direction of the vessel, or other directions. 
     The walk-to-work system  1  comprises a gangway system  200  and an elevator system  100  positioned structurally independently from the gangway system  200 . 
     The gangway system  200  is used to support personnel and/or equipment as they move from the vessel  300  to a particular height of the wind turbine  400 , for example at the height of a service platform  410  mounted to the wind turbine tower  420 . 
     The elevator system  100  allows personnel and/or equipment to move from one or more decks, such as opened deck  310  and closed deck  320  from the vessel  300  to the desired height of the gangway system  200 , such as the height of a gangway  210  of the gangway system  200 . 
     Having accessed the wind turbine  400 , the personnel may carry out tasks such as maintenance, reparation, servicing, or other type of tasks on the wind turbine  400 . 
     The elevator system  100  comprises a height adjustable elevator  110 , an elevator car  130  movably connected to the height adjustable elevator  100 . As shown on  FIG.  1  to  6   , the height adjustable elevator  110  may be a telescopic elevator shaft or other type of height adjustable elevators as shown on  FIG.  7  to  9   . 
       FIG.  2    shows the walk-to-work system  1  of the first example embodiment in two different elevated positions, a retracted/low elevated position shown in continuous back lines, and an extended/high elevated position shown in continuous black lines illustrating only the main contours of the walk-to-work system  1 . 
     The elevator car  130  is configured to be elevated to the same height as the gangway  210  for allowing access between the telescopic elevator shaft  110  and the gangway system  200 . The elevator system  100  further comprises a lifting device  150  configured to move the elevator car  130  along the telescopic elevator shaft  110 . 
       FIG.  3    shows detailed views of the height adjustable elevator  110  of the first example embodiment shown in  FIG.  1    when the height adjustable elevator  110  is a telescopic elevator shaft. The telescopic elevator shaft  110 , which form part of the inventive walk-to-work system  1 , may be a conventional personnel and goods telescopic elevator shaft for offshore use. 
     The height adjustable elevator  110  comprises a static elevator part  111  that is elongated and mounted onto a deck of the vessel  300  such as an open deck  310  or closed decks  320 , a displaceable elevator part  112  that is elongated and height adjustable relative to the static elevator part  111 , and an elevator car  130 . Thus, the elevator  110  may be elevated to a high position as illustrated in  FIG.  3 A  or to a low position as shown in  FIG.  3 B . 
       FIG.  4    provides further details side ( FIG.  4 A ) and front ( FIGS.  4 B and  4 C ) views of the telescopic elevator shaft  110  of the first example embodiment when the elevator is mounted onto the decks  310 ,  320  of the vessel  300 , wherein the retracted/low elevated position is illustrated in continuous black lines and the extended position is illustrated in black dotted lines. Note, that the illustrations of the extended/high positions only show the main contours of telescopic elevator shaft  110 . 
     The static elevator part  111  is a vertical structure having a pre-determined height. Note that, the term ‘vertical’ should not be interpreted in a strict mathematical sense, but as substantially vertical, i.e. where deviations from vertical axes are possible, such as deviation between +/−5 degrees. As shown in  FIG.  3 A  and  FIG.  3 B , the static elevator part  111  may have a rectangular shape. A first elevator end  113  of the height adjustable elevator  110  is securely mounted to the vessel  300 , while a second elevator end  114  is located vertically above the first elevator end  113 . In principle, the static elevator part  111  may take any forms. 
     The static elevator part  111  is mounted onto the vessel  300 , for example to an open deck  310  as shown in  FIG.  1   . The static elevator part  111  may be freestanding on the open deck  310  of the vessel  300 , as shown in  FIG.  1   . Alternatively, or in addition, the static elevator part  111  can be completely or partially integrated to a superstructure (not shown) of an open deck  310  of the vessel  300 . Alternatively, or in addition, the static elevator part  111  may also be integrated to one or more closed decks  320  of the vessel  300 , as shown in  FIG.  2   . 
     An advantageous effect of the first example embodiment of having the static elevator part  111  integrated to one or more closed decks  320  and/or into the vessel superstructure, is that it reduces, or eliminate, the elevator&#39;s  100  footprint on the open deck  310 , thereby leaving more available deck space to the vessel&#39;s  300  superstructure. 
     The static elevator part  111  has a fixed height Hsp that is pre-determined, typically Hsp is about 10 to 25 meters. In general, it should not have a height exceeding the highest point of the vessel superstructure, to minimize the probability that it becomes the reason for any turbine shutdown. A width Wsp of the static elevator part  111  is pre-determined based on space needed for the height adjustable elevator  110  due to the size of the elevator car  130 , and the required cross section to carry structural loading. Typically, the footprint of the height adjustable elevator  110  is minimized to ensure maximize available free deck space on vessel. 
     The static elevator part  111  is preferably made of materials that allows lifting of heavy loads in relevant offshore wind turbine operations. For example, the static part  110  can be made of any type of high strength structural metallic material, preferably weldable structural steel, typically with high strength or extra high strength steel, casted and forged parts may also be used where suitable. 
     In the particular configuration of the first example embodiment as best illustrated in  FIG.  3   , the displaceable elevator part  112  is a substantially vertical frame or mast that is movably connected to the static elevator part  111  via sets of double guide rails  140   a , 140   b , where one  140   a  of the sets of double guide rails is mounted along the static elevator part  111  and another  140   b  of the sets of double guide rail is vertically mounted on the displaceable elevator part  112 . Other means may also be used for movably connecting the displaceable elevator part  112  to the static elevator part  111 . 
     Note that vertical is hereinafter referred to as the direction perpendicular to the sea at rest. 
     Further, a drive system  160  is in  FIGS.  3  and  4    shown connected to the displaceable elevator part  112 . The drive system  160  is used to adjust the height of the displaceable part  112  relative to the static elevator part  111 . In  FIGS.  3  and  4   , the drive system  160  is a rack and pinion drive system. However, it may be of any type enabling relative movements between the static elevator part  111  and the displaceable elevator part  120 . Other examples may involve a motorized winch and corresponding set of cables and pulleys, or hydraulic cylinders, or electric actuators. 
     As best illustrated in  FIG.  3    and  FIG.  4   , an elevator car  130  may be movably connected to the sets of double guide rails  140   a , 140   b  via for example sets of double wheels  170   a , 170   b  mounted to the elevator. The elevator car  130  is configured such that, when the displaceable elevator part  112  is extended relative to static elevator part  111 , the elevator car  130  is always in contact with at least one set of guide rails  140   a ,  140   b.    
     To allow vertical movement of the elevator car  130  along the set of double guide rails  140   a , 140   b , the elevator car  130  is in the first example embodiment operationally connected to a lifting device  150 . In  FIGS.  3  and  4    the lifting device is shown as a motorized winch with corresponding cables  151  and pulley  152 . Another alternative may involve a rack and pinion lifting device. 
     The displaceable elevator part  112  has a height Htp and a width Wtp that are pre-determined. The height Htp is defined from the height Hsp and the maximum required access height for the particular vessel in question, typically Htp is about 4 m to 20 m. This will however vary from vessel to vessel based on the actual design parameters for the actual wind turbine fields it is intended to operate. The sum of H SP  and H TP  should give sufficient height to operate the elevator system  100  when gangway system  200  is in upper position. The width Wtp of the displaceable elevator part  112  is pre-determined based on the size of the elevator car  130 , and the required cross section to carry structural loading. Typically, the Wtp is minimized to avoid oversizing of static elevator part  111  width Wsp. 
     The displaceable elevator part  112  may preferably be made of a material that can resists to lift heavy loads and is suitable for offshore use. For example, the displaceable elevator part  112  can be made of any type of high strength structural metallic material, preferably weldable structural steel, typically with high strength or extra high strength steel. Casted and forged parts may also be used where suitable. 
     An advantageous aspect of the walk-to-work system  1  lies in the elevator system  100  being height adjustable. Reducing the height of the elevator system  100  is advantageous when operating the walk-to-work system  1  against the wind turbine  400  during high tide conditions, while increasing the height of the elevator system  100  is advantageous during low tide conditions. 
     Particularly, the height adjustable elevator  110  highest point, thereby also the highest point of the elevator system  100 , can advantageously be height adjusted to a minimum height H Emin , wherein H Emin  represents the vertical height between the sea surface, for example at highest astronomical tide conditions, and the highest point of the height adjustable elevator  110 . H Emin  achieved by the invention is advantageously smaller than that achieved by traditional design from the prior art in similar highest tide conditions, due to the fact the elevator  110  is height adjustable whereas in the prior art the elevator has a fixed height. 
     Since in most cases the highest point of the height adjustable elevator  110  is also the highest point of the vessel  300 , allowing a retraction of the height adjustable elevator to the minimum height H Emin  that is smaller than the height achieved in the prior art solutions is an advantage in that it reduces the risk of collision with wind turbine blades in the blade swept area A BSA , in particular at highest tide conditions. As a consequence, this particular elevator system  100  contributes to reduce the risk that the wind turbine must be shut down when the vessel  300  operates against the wind turbine. Consequently, it also avoids costs caused by loss in energy production. 
     Another advantage of the elevator system  100  of the invention is that the elevator car  130  can be elevated at any height along the height adjustable elevator  110 . Thus, the elevator car  130  is not limited to be elevated to specific floor levels at specific heights. Thereby, the elevator car  130  can be elevated at any height of the gangway  200  to provide access directly to the gangway  210 , or via an access platform  240  if such a platform is used as further described below. This advantageously increases the operational flexibility of the walk-to-work system  1  compare to prior art traditional design. 
     To allow successful operation of such an elevator system  100 , the gangway system  200  is herein made structurally independent of the elevator system  100 . This independent configuration effectively reduces, or even removes, limitations of the elevator system  100  design related to the presence of the gangway system  200 . For example, a height adjustable such as a telescopic elevator  110  is technically challenging, and even impossible, in a traditional walk-to-work system  1  where the elevator system is a structural integral part of the gangway system  200 . 
       FIG.  5    shows a detailed perspective ( FIG.  5 A ) and top ( FIG.  5 B ) view of the gangway system  200  and elevator system  100  of the first example embodiment. The arrows shown in  FIG.  5 A  illustrated the possible movement of the main elements of the walk-to-walk system  1 . 
     As mentioned above, the gangway system  200  is mounted on the vessel  300  and is structurally independent from the telescopic elevator  100 . The gangway system  200  of the first example embodiment may be mounted as a freestanding unit on the open deck  310  and/or as an integrated unit to one or more closed decks  320 . 
     The gangway system  200  comprises a height adjustable elongated pedestal  220  and a gangway  210 . The gangway system  200  may be a conventional offshore gangway system. The gangway system  200  may in the first example embodiment further comprise a bridge  230  that may be length adjustable (see arrow on  FIG.  5 A ), such as a telescopic bridge as shown on  FIGS.  1 , 2 , and  5 - 9   . 
     The adjustable elongated pedestal  220  of the first example embodiment has a first pedestal part  220   a , shown as an outer pedestal part on  FIGS.  1 ,  2 ,  5  and  6   , and a second pedestal part  220   b , shown as an inner pedestal part on  FIGS.  1 , 2 , 5  and  6   . The second pedestal part  220   b  is typically coupled to the first pedestal part  220   a , thereby allowing the second pedestal part  220   b  to move vertically relative to the first pedestal  220   a  (see arrow in  FIG.  5   ). One end of the pedestal  220  is fixed to the vessel  300 , for example by welding. 
     The gangway  210  in the first example embodiment is preferably coupled to the second pedestal part  220   b , where the coupling is preferably such that the gangway  210  may rotate around a vertical axis (see arrow  FIG.  5 A ), for example via a motorized swivel/slewing machinery.  FIG.  5    shows an advantageous example where the gangway  210  is partly circumventing the inner pedestal part  220   b.    
     If the gangway  210  is rotatable, the bridge  230  the first example embodiment may be made rotatable as the bridge is connected to the gangway  210 . A rotation of the gangway  210  would thus result in a corresponding rotation of the bridge  230 . 
     Alternatively, the bridge  230  may be rotationally coupled to the second pedestal part  220   b  such that independent rotations of the gangway  210  and the bridge  230  is achieved. 
     In yet an alternative configuration the first example embodiment, the gangway system  200  may comprise an access platform  240  as depicted  FIGS.  2 ,  5  and  6   . The gangway system  200  is configured such that the access platform  240  is arranged on or the near the same vertical level as the gangway  210  but does not rotate with the gangway  210 . The access platform  240  and gangway  210  are at the same vertical height and attached to the pedestal  220 . Further, the access platform  240  is arranged adjacent to the gangway  210  as best illustrated in  FIG.  5 B , thereby allowing personnel and/or equipment to safely move between the gangway  210  and access platforms  240 . 
     To allow the gangway  210  in this example embodiment to rotate independently of the access platform  240 , the latter may be coupled to the pedestal  220 , preferably to the inner pedestal part  220   b , using a support structure  250 . With particular reference to  FIG.  5   , such a support structure  250  may comprise a collar  251  at least partly circumventing the inner pedestal part  220   b  below the gangway  210  and a radially extending framework extending from the collar  251  to an underside of a base of the access platform  240 . The collar  251  may be either rotationally mounted onto the inner pedestal part  220   b  or fixed. 
     The access platform  240  in this example embodiment may have a rectangular shape as best shown in  FIG.  5 B . But any shapes allowing the above-mentioned transfer of personnel and/or equipment are feasible. 
     A landing door or the like  260  may in this example embodiment be vertically mounted to one side of the access platform  240  distal to the gangway  210 . The elevator door may be mounted along part of or the entire width of the access platform  240 . 
     To meet intentional safety standard, the landing door  260  in this example embodiment is preferably interlocked against the elevator car door  131  to prevent opening of the doors to the access platform  240  when the landing door  260  and the elevator car door  131  are not vertically aligned according to the applicable elevator safety standard. Typically, a misalignment of more than 20 cm will prevent opening of the doors  260 , 131 , but this may vary depending on the applicable standard. Emergency operation of doors in any position is possible by use of special tools, such as a key. 
     As shown in  FIG.  5 B , the landing door  260  in this example embodiment shall be parallel and aligned with a door  131  of the elevator car  130  when the latter has been elevated to the height of the access platform  240 , as also shown in  FIG.  1    and  FIG.  2   . This allows personnel and/or equipment to safely access the gangway  210  via the access platform  240 . 
     The access platform  240  in this example embodiment may further comprise a safety fence  270  to ensure the safety movement of personnel and/or equipment between the elevator car  130  and the gangway  210 . As best illustrated on  FIG.  5 B , the safety fences  270  may be movably connected via the set of tracks to a safety barrier  280  of the gangway  210 , such that a surface area covered by the gangway  210  and access platform  240  is always safely secured for personnel and/equipment to move onto regardless of the rotational position of the gangway  210  relative to the access platform  240 . 
     Operations of the gangway system  200  and elevator system  100  may in this example embodiment be controlled separately or together by a control system (not shown), for example they may be controlled by separate or combined Programmable Logic Controllers (PLC) or Industrial Computers (IPC). 
     During normal operation, the elevator car  130  of height adjustable elevator  110  may be interlocked to the gangway  210  via interlocking means such a mechanical interlock, alternatively the elevator is interlocked to the access platform  240  if such a platform is used. This allows a floor of the elevator car  130  and a floor of the gangway  210  to always be at the same elevation above the vessel deck  300  when these are interlocked to each other. 
     Height adjustment and alignment of the displaceable elevator part  112  and elevator car  130  relative to the gangway  210  may in this example embodiment be achieved by using position encoders arranged on the gangway  210  (or the access platform  240 , if used), the displaceable elevator part  112 , and/or in the interlock, for example in the PLC or IPC. Optionally, proximity sensors may be used to verify the exact position of the displaceable elevator part  112  and elevator car  130  relative to the gangway  210  (or relative to the access platform  240  if used). Alternatively, any sensors such as laser, LiDAR, mechanical switches, Optical Machine Vision may also be used to ensure alignment between the height adjustment of the displaceable elevator part  112 , the height adjustment of the elevator car  130  and the height adjustment of the gangway  210  (and access platform  240 , if used). Alternatively, an operator may override the interlock between the elevator car  130  and gangway  210  or access platform  240  and adjust the respective heights independently. 
       FIG.  6    shows a second example embodiment of the invention that is similar to the first embodiment except for the static elevator part  111  and the displaceable elevator part  112 . In the second example embodiment, the static elevator part  111  comprises two first support means  180  stationary mounted at a distance from each other on the open deck  310  of the vessel  300  and two hollow elongated first cylinders  181 . The first cylinders  181  are vertically mounted onto the first support means  180 . 
     Further in this second example embodiment, the displaceable elevator part  112  comprises two elongated second cylinders  182 . Each second cylinder  182  is height adjustably coupled, i.e. telescopically coupled, to one of the first cylinders  181  at an end distal from the first support means  181 . The other end of each second cylinders  182  is connected to a second support  183  means supporting an elongated elevator mast  185 . 
     The elevator car  130  (not shown in  FIG.  6   ) of the second example embodiment is integrated into the housing of elevator shaft  185 . The elevator mast  185  may comprise several floors  184  allowing personal and/or equipment to move between the elevator car  130  and the gangway  210  when a predetermined floor  184  is elevated to and aligned with the height of the gangway  210 . 
     The elevator car  130  (not shown) of the second example embodiment is integrated into the housing elevator mast  185  of the displaceable elevator part  112 . Thus, the elevator car  130  may be elevated within the elevator mast  185  by a lifting device  150 . Alternatively, or in addition, the elevator car may also be elevated by elevating the displaceable elevator part  112  relative to the static elevator part  111 , even when the elevator car  130  is stationary relative to the elevator mast  185 . With this arrangement, the elevator car may be elevated up to the same height as the gangway  210  to allow access between the elevator system  100  and the gangway system  200 . 
     In the second example embodiment, the displaceable elevator part  112  is height adjusted relative the static elevator  111  by means of a drive system  160  (not shown), for example rack and pinion, lifting winches, hydraulic cylinders or the like. 
       FIG.  6 A  shows the height adjustable elevator  110  of the second example embodiment in a retracted/low elevated position, while  FIG.  6 B  shows the height adjustable elevator  110  of the second embodiment in an extended/high elevated position. It should be noted that  FIGS.  6 A and  6 B  do not show the situation when a floor of the elevator car  130  is aligned with the gangway  210 . 
       FIG.  7    shows a third example embodiment of the invention that is similar to the first embodiment except for the static elevator part  111  and the displaceable elevator part  112 . In the third example embodiment, the static elevator part  111  is a channel extending through the plurality of decks  310 , 320  of the vessel  300 , and the displaceable elevator part  112  is an elongated elevator mast  185  that is height adjustably coupled to the channel through the plurality of decks. 
     The elevator car  130  (not shown) of the third example embodiment is integrated into the displaceable elevator part  112 , i.e. the elongated elevator mast. Thus, the elevator car  130  may be elevated within the displaceable elevator part  112  by means of a lifting device  150  (not shown) or the like. Alternatively, or in addition, the elevator car may also be elevated by elevating the displaceable elevator part  112 , i.e. the elevator mast, relative to the static elevator part  111 , even if the elevator car  130  is stationary relative to the displaceable elevator part  112 . With this arrangement, the elevator car  130  may be elevated up to the same height as the gangway  210  to allow access between the elevator system  100  and the gangway system  200 . 
     The elevator mast may comprise several floors  184  allowing personal and/or equipment may to move between the elevator car  130  and the gangway  210  when a predetermined floor is elevated to and aligned with the height of the gangway  210 . 
     In the second example embodiment the displaceable elevator part  112  is height adjusted relative the static elevator  111  by means of a drive system  160  (not shown), for example rack and pinion, lifting winches, hydraulic cylinders or the like. 
       FIG.  7 A  shows the height adjustable elevator  110  of the third example embodiment in a retracted/low elevated position, while  FIG.  7 B  shows the height adjustable elevator  110  of the third embodiment in an extended/high elevated position. 
       FIG.  8    shows a fourth example embodiment of the invention that is similar to the first embodiment except for the static elevator part  111  and the displaceable elevator part  112 . 
     In the fourth example embodiment, the height adjustable elevator  110  is an elongated elevator mast. The static elevator part  111  is the bottom part of the elevator mast  185  mounted onto the deck  310  of the vessel  300 , alternatively or in addition, the static elevator  111  of the fourth example embodiment may be integrated to one or more close decks  320 . 
     Further, the displaceable elevator part  112  of the fourth example embodiment is a top part of said elevator mast  185 , wherein the top part is pivotally coupled, and thereby height adjustable, relative to the bottom part of the elevator mast  185 . The top part may be pivoted by means of a pivoting system, i.e. a drive system  160 . The pivoting system may displace the displaceable elevator part  112  (top part) relative to the static elevator part  111  (bottom part), such that the height of the height adjustable elevator  110  is decreased or pivoted such that the height of the height adjustable elevator  110  is increased.  FIG.  8 A  shows that the top part may pivoted from a high position (shown in dotted lines) on top of the bottom part to a low position (shown in continuous lines) on the side of the bottom part. 
     The elevator car  130  (not shown) of the fourth example embodiment is integrated into the elevator shaft, i.e. within the bottom and top part of the elongated elevator mast  110 , 185 . Thus, the elevator car  130  may be elevated within the height adjustable elevator  110  by means of the lifting device  150  (not shown) or the like combined with means (not shown) for elevating the elevator car between the bottom and top part of the elevator shaft when the top part is in the high position. 
     Means for elevating the elevator car  130  between the bottom and top part may involve a set of guide rails (not shown) or the like. One set of guide rails may be arranged on the bottom part and another set of guide rails is arranged on the top part, such the elevator is always in contact with a set of guide rails when moving between the bottom and top part of the elevator mast  185 , 110 . With this arrangement, the elevator car may be elevated up to the same height as the gangway  210  to allow access between the elevator system  100  and the gangway system  200 . 
     The elevator shaft may comprise several floors  184  allowing personal and/or equipment may to move between the elevator car  130  and the gangway  210  when a predetermined floor is elevated to and aligned with the height of the gangway  210 . 
       FIG.  8 B  shows the height adjustable elevator  110  of the fourth example embodiment in a retracted/low elevated position, while  FIG.  8 A  shows the height adjustable elevator  110  of the fourth embodiment in an extended/high elevated position. 
       FIG.  9    shows a fifth example embodiment of the invention that is similar to the first embodiment except for the static elevator part  111  and the displaceable elevator part  112 . In the fifth example embodiment, the height adjustable elevator comprises a static elevator part  111  and displaceable elevator part  112 , where the static elevator part  111  is an elongated channel mounted onto and extending through the plurality of decks  310 , 320  of the vessel  300  and further protruding vertically from the deck  310  of the vessel  300 . The static elevator part  111  further comprises first support means  180  arranged on the vertical sides of the elongated channel to support the structure of the elongated channel. Further in the fourth embodiment, the displaceable elevator part  112  is a framework structure height adjustably coupled to the static elevator  111 . As shown in  FIG.  8    the displaceable elevator part  112  is telescopically coupled to the static elevator part  111 . 
     A drive system  160  (not shown) of the fourth example embodiment is configured to displace the displaceable elevator part  111 , i.e. the framework structure, relative to the static elevator part  111 , i.e. the elongated channel. In the fourth embodiment this is achieved by connecting the displaceable elevator part  112  to the second elongated pedestal  220   b  of the gangway system  200  by a second support means  183  extending from the second pedestal  220   b  to a position on the displaceable elevator part  111  such that when the height of the second elongated pedestal  220   b  is adjusted the displaceable elevator part  112  is also adjusted as it is physically connected to the second pedestal  220   b.    
     The elevator car  130  is movably connected to the height adjustable elevator  110  and may be elevated along the height adjustable elevator  110 . The elevator car may be movably connected to a sets of guide rails (not shown) via for example sets of double wheels (not shown) rotationally mounted to the elevator. The elevator car  130  may be configured such that, when the displaceable elevator part  112  is extended relative to static elevator part  111 , the elevator car  130  is always in contact with at least one set of guide rails. 
     To allow vertical movement of the elevator car  130  along the height adjustable elevator  110 , the elevator car  130  is connected to a lifting device (not shown). The lifting device may be a motorized winch with corresponding cables and pulley. Another alternative may involve a rack and pinion lifting device. 
     With this arrangement, the elevator car may be elevated up to the same height as the gangway  210 , i.e. it may be elevated along the elongated channel and the framework structure, to allow access between the elevator system  100  and the gangway system  200 . 
       FIG.  9 A  shows the height adjustable elevator  110  of the fifth example embodiment in an extended/high elevated position, while  FIG.  9 B  shows the height adjustable elevator  110  of the fourth example embodiment in a retracted/low elevated position. 
     As in the first embodiment, the elevator system  100  and gangway  200  of the walk-to-work system  1  of the second, third, fourth and fifth example embodiments may be partially integrated to the vessel  300  superstructure (not shown). 
     In all example embodiments, the walk-to-work system  1  is shown arranged in the transversal direction of the vessel  300  as shown in  FIG.  1   , alternatively it may be arranged in the longitudinal direction of the vessel  300  (not shown) or other ways. 
     It is appreciated that certain features of the invention, which, for clarity, have been described above in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which, for brevity, have been described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination. 
     
       
         
           
               
             
               
                   
               
               
                 List of reference numerals/letters: 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                  1 
                 Walk-to-work system 
               
               
                 100 
                 Elevator system 
               
               
                 110 
                 Height adjustable elevator/Telescopic elevator 
               
               
                 111 
                 Static elevator part 
               
               
                 112 
                 Displaceable elevator part 
               
               
                 113 
                 First elevator end 
               
               
                 114 
                 Second elevator end 
               
               
                 130 
                 Elevator car 
               
               
                 131 
                 Elevator car door/lift car door 
               
               
                 140a 
                 First set of double guide rails 
               
               
                 140b 
                 Second set of double guide rails 
               
               
                 150 
                 Lifting device 
               
               
                 151 
                 Lifting device cable 
               
               
                 152 
                 Lifting device pulley 
               
               
                 160 
                 Drive system 
               
               
                 170a 
                 First set of double wheels 
               
               
                 170b 
                 Second set of double wheels 
               
               
                 180 
                 First support means 
               
               
                 181 
                 First cylinders 
               
               
                 182 
                 Second cylinders 
               
               
                 183 
                 Second support means 
               
               
                 184 
                 Floors 
               
               
                 185 
                 Elevator mast 
               
               
                 200 
                 Gangway system 
               
               
                 210 
                 Gangway 
               
               
                 220 
                 Height adjustable elongated pedestal 
               
               
                 220a 
                 First/outer elongated pedestal part 
               
               
                 220b 
                 Second/inner elongated pedestal part 
               
               
                 221 
                 First elongated pedestal end 
               
               
                 222 
                 Second elongated pedestal end 
               
               
                 230 
                 Bridge/Telescopic bridge 
               
               
                 231 
                 First bridge part 
               
               
                 232 
                 Second bridge part 
               
               
                 240 
                 Access platform 
               
               
                 250 
                 Support means/Support structure 
               
               
                 251 
                 Collar 
               
               
                 260 
                 Landing doors 
               
               
                 270 
                 Safety fence 
               
               
                 280 
                 Safety barrier 
               
               
                 300 
                 Vessel/waterborne structure 
               
               
                 301 
                 Hull of the vessel 
               
               
                 302 
                 Superstructure of the vessel 
               
               
                 310 
                 Open deck/Deck 5 
               
               
                 320 
                 Closed decks/Deck 1, 2, 3 and 4 
               
               
                 400 
                 Offshore wind turbine/offshore structure 
               
               
                 410 
                 Service platform - of the wind turbine 
               
               
                 420 
                 Tower 
               
               
                 A BSA   
                 Blade swept area 
               
               
                 Dg 
                 Diameter of the gangway 
               
               
                 H g   
                 Height between the first elongated pedestal end and the gangway 
               
               
                 H sp   
                 Height of the static elevator part 
               
               
                 H tp   
                 Height of the displaceable elevator part 
               
               
                 Lg 
                 Length of the gangway&#39;s radius 
               
               
                 Wsp 
                 Width of the static elevator part