Abstract:
A docking device for facilitating a transfer between a vessel and an offshore structure is provided. The docking device includes a first docking component mounted on the vessel and a second docking component mounted on the offshore structure. The first and second docking components are connectable such that an elevation of a bow or stern of the vessel is facilitated in the connected state.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority to European Patent Office application No. 12166650.7 EP filed May 3, 2012, the entire content of which is hereby incorporated herein by reference. 
       FIELD OF INVENTION 
       [0002]    The present invention relates to a docking device for facilitating a transfer between a vessel and an offshore structure. 
       BACKGROUND OF INVENTION 
       [0003]    Offshore structures, e. g. monopiles, jacket structures or floating structures are increasingly used in particular with the growing demand for offshore wind farms. For erection, maintenance and repair of such offshore structures a safe, secure and cost effective transfer of personnel and light weight equipment between vessels and offshore structures is required. 
         [0004]    At present the transfer of personnel and equipment is performed by various vessel types and transfer systems. These range from transfer via small fast rescue boats till special vessels equipped with a motion compensated access platform. 
         [0005]    Transfer vessels—with a so called intelligent bow fender—are known to facilitate transfer vessel to push towards bumper guards on e. g. a transition piece of a wind turbine installation detects the bollard push force applied on the fender which again is used to determine a minimum friction between fender and bumper guards on the monopile. The friction facilitates ability to maintain the bow of the vessel in a vertically stand still position. However, in practice a transfer is only possible when the fixed-in-position bumper guards on the transition piece can be accessed exactly. Depending on the direction and force of the wind and/or water current and/or waves it may be very difficult to firstly guide the vessel to the accurate position of the bumper guards, but also to maintain the vessel in correct position afterwards. 
         [0006]    All known docking devices are not designed to operate in wave heights of more than 2 meters, accordingly the access of offshore structures is depending on weather or sea conditions. 
       SUMMARY OF INVENTION 
       [0007]    It is therefore an object of the present invention to provide a docking device which enables a transfer to and from offshore structures even in adverse weather conditions. 
         [0008]    According to the present invention this object is achieved in the above defined docking device in that it comprises a first docking component mounted on said vessel and a second docking component mounted on said offshore structure, whereby the first and second docking components are connectable such that bow or stern of the vessel can be elevated in the connected state. 
         [0009]    The inventive docking device is based on the idea that a safe and secure access and transfer to and from an offshore structure is possible when bow or stern of the vessel can be established in a secure and fixed position in relation to a second docking component mounted on the offshore structure. In the secure and fixed position bow or stern of the vessel is elevated in relation to the sea surface so that wave movement or wave impact do not impair the transfer. 
         [0010]    According to a further development of the inventive docking device it may be envisaged that the offshore structure comprises several second docking components disposed in circumferential direction. Accordingly the docking device is capable of docking in all directions by elevating bow or stern beyond wave impact allowing for a safe and secure transfer of personnel and equipment. 
         [0011]    Preferably the first docking component is formed as a toothed wheel or a pinion and the second docking component is formed as a rack, whereby the first and second docking components act as a rack and pinion arrangement in the connected state. The rack and pinion connection is a strong and stable connection allowing bow or stern of the vessel to be elevated. 
         [0012]    According to the invention the toothed wheel or the pinion of the first docking component can be actuated in order to elevate the vessel. Accordingly the transfer procedure starts with a docking procedure in which the first and second docking components are brought into a connected state followed by an elevation procedure. When the vessel has reached the elevated position a transfer of personnel and equipment from vessel to offshore structure and reverse is possible. 
         [0013]    According to a second, alternative embodiment of the invention the first docking component may comprise a retractable hook and a second docking component is formed as a railing, whereby the hook locks to the railing in the connected state. This embodiment has the advantage that the offshore structure can be accessed in all directions so that docking manoeuvres for facilitating a transfer during adverse weather conditions are possible. During the docking procedure the retractable hook is connected to the railing of the offshore structure, afterwards the hook is retracted so that bow or stern of the vessel is elevated. 
         [0014]    In the inventive docking device the first docking component may further comprise a wire which can be fixed at the railing in the connected state. The wire takes the load from the vessel and unloads the retractable hook. 
         [0015]    A very safe and stable connection between the first docking component and the second docking component can be achieved when the hook comprises a hydraulic lock. 
         [0016]    In the inventive docking device it may be envisaged that a fender is disposed between vessel and offshore structure in order to protect them. 
         [0017]    Further the invention relates to a method for facilitating a transfer between a vessel and an offshore structure with a docking device. 
         [0018]    According to the inventive method a docking device is used comprising a first docking component mounted on said vessel and a second docking component mounted on said offshore structure, whereby the first and second docking components are connected and bow or stern of the vessel are elevated safely beyond impact of actual sea state. 
         [0019]    Further features of the invention are disclosed in the subclaims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The invention and its underlying principle will be better understood when consideration is given to the following detailed description of preferred embodiments. 
           [0021]    In the accompanying drawings: 
           [0022]      FIG. 1  shows a vessel with an inventive docking device; 
           [0023]      FIG. 2  shows the vessel of  FIG. 1  approaching an offshore structure; 
           [0024]      FIG. 3  shows the connection of the vessel to the offshore structure; 
           [0025]      FIG. 4  shows the elevation of the vessel; 
           [0026]      FIG. 5  shows a vessel with an inventive docking device according to a second embodiment of the invention; 
           [0027]      FIG. 6  shows the vessel of  FIG. 5  approaching an offshore structure; 
           [0028]      FIG. 7  shows the connection of the vessel to the offshore structure; 
           [0029]      FIG. 8  shows the vessel in an elevated state; and 
           [0030]      FIG. 9  shows the vessel and the offshore structure of  FIG. 8 , whereby a wire is used. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0031]    In  FIG. 1  a vessel  1  is shown which is equipped with a first docking component in the form of a pinion  3 . Basically the pinion  3  is a toothed wheel which can be actuated, e. g. rotated by a motor. The rotatable pinion  3  is used for establishing a connected state with an offshore structure. 
         [0032]      FIG. 2  shows the vessel  1  approaching an offshore structure  4 . In this embodiment the offshore structure  4  is a monopile being used as a foundation for a wind turbine. However, the docking device can be used for other offshore structures like jackets, gravity foundations, drilling platforms or even semisubmersible structures as well. 
         [0033]    The offshore structure  4  is provided with second docking components  5  which are formed similar to a ladder. Each of said second docking components  5  comprises two parallel struts  6 , which are connected by cross struts  7 . As can be seen in  FIG. 2  several second docking components  5  are provided in circumferential direction of the offshore structure  4 . 
         [0034]      FIG. 3  shows vessel  1  and offshore structure  4  in a connected state. The toothed wheels of pinion  3  are engaged in the clearances of the second docking component  5  at the outer side of the offshore structure  4 . This way stable and secure connection between vessel  1  and offshore structure  4  is established. 
         [0035]      FIG. 4  shows vessel  1  and offshore structure  4  in the connected state. In the connected state the pinion  3  of the first docking component of the vessel  1  is rotated by a motor, consequently the bow  2  of the vessel  1  is elevated in relation to the sea surface  8 . The pinion  3  fixes the bow  2  vertically. First the bow  2  is levelled highest as a result of wave movement. After fixing the bow  2  will be elevated another short distance, typically a few centimetres to ensure bow clearance of vertical wave impact. The procedure implies in theory that only or primarily static vertical forces between docking components shall be accounted for during docking and undocking. In this state the bow  2  of vessel  1  is elevated beyond wave impact. This allows for a safe and secure transfer of personnel and equipment to and from offshore structure  4 . The docking device as shown in  FIG. 1-4  radically expands the weather window in which transfers can be performed compared to conventional docking devices. 
         [0036]      FIG. 5  shows another embodiment of a vessel  9  which is equipped with a first docking component in the form of a retractable hook  10 . The hook  10  is disposed at a retractable boom  11  which is actuated hydraulically. Opening and closing the hook  10  is performed hydraulically as well. 
         [0037]      FIG. 6  shows vessel  9  approaching an offshore structure  12 . When the vessel  9  approaches the offshore structure  12  the hook  10  of the retractable boom  11  is open. 
         [0038]    The offshore structure  12  is provided with a circular railing  13 .  FIG. 7  shows vessel  9  and offshore structure  12  in a connected state. Vessel  9  and boom  11  are steered until hook  10  reaches the railing  13 . As the railing  13  is ring-shaped docking of hook  10  can take place even in adverse weather conditions. Once the vessel  9  has reached the offshore structure  12  the hook  10  is closed which clamps the railing  13 . After a stable connection state has been reached the retractable boom  11  is actuated such that it is levelled so that the bow  2  of vessel  9  is elevated in relation to the sea surface. The vessel  9  is lifted with its bow  2  so that it is no longer impaired by wave movement. 
         [0039]      FIG. 9  shows a similar embodiment whereby an additional wire is used in the connected state. The wire  14  is provided on a roll  15  on the vessel  9 . After the connection between the first docking component, namely hook  10  and boom  11  and the second docking component, namely railing  13 , has been established the wire  14  is in addition fixed at railing  13  so that the wire  14  takes the load from the vessel  9 . In this connected state a transfer of personnel and equipment is possible in both directions. 
         [0040]    All above inventions implies a system operation, which eliminate or significantly reduce dynamic forces to be obtained in the docking system. 
         [0041]    Although the present invention has been described in detail with reference to the preferred embodiment, the present invention is not limited by the disclosed examples from which the skilled person is able to derive other variations without departing from the scope of the invention.