Abstract:
A high voltage swivel including an inner and an outer body, each body having at least a first and a second ring-shaped conductor element, the conductor elements including two spaced-apart end faces and a curved contact surface, the elements being mutually interconnected at opposing end faces via insulating members at axially spaced-apart positions, the inner and outer bodies being coaxial around a longitudinal axis, the contact surface of each inner conductor element being adjacent to and in electrical contact with an opposed contact surface of a corresponding outer conductor element, the inner and outer bodies being rotatable relative to one another around the longitudinal axis, each conductor element of the inner and outer body connected to a voltage line extending to an input terminal or an output terminal, the insulator members interconnecting the conductor elements situated at a mutual distance in the circumferential direction of the conductor element, the interconnected conductor elements of the inner and the outer body each being supported by a respective support member to form integral units such that the inner and outer bodies are adapted to be mutually decoupled or attached by relative displacement of the integral units in the direction of the longitudinal axis.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a high voltage swivel comprising an inner and an outer body, each body having at least a first and a second ring-shaped conductor element, the conductor elements comprising two spaced-apart end faces and a curved contact surface, the elements being mutually interconnected at opposing end faces via insulating members at axially spaced-apart positions, the inner and outer bodies being coaxial around a longitudinal axis, the contact surface of each inner conductor element being adjacent to and in electrical contact with an opposed contact surface of a corresponding outer conductor element, the inner and outer bodies being rotatable relative to one another around the longitudinal axis, wherein each conductor element of the inner and outer body is connected to a voltage line extending to an input terminal or an output terminal. 
       BACKGROUND OF THE INVENTION 
       [0002]    Such a high voltage swivel is known from U.S. Pat. No. 7,137,822 in the name of the applicant. The known swivel is a high voltage swivel for offshore applications, for instance for distributing electrical power, that is generated on a weathervaning Floating Production, Storage and Offloading vessel (FPSO)—which FPSO is anchored to the sea bed via a turret—to a sub sea power cable. Geostationary hydrocarbon risers extend upwards from a well head to a power plant on the vessel, in which the hydrocarbons are converted into electrical energy. The electrical connection of the rotating vessel to the stationary sub sea power cable leading to shore is achieved by the high voltage swivel in which the stator is connected, via the geostationary swivel part on the vessel, to the sub sea power cable and the rotor is connected to the power plant on the vessel. 
         [0003]    The conductors of the inner and outer bodies of the known high voltage swivel are embedded in solid annular insulator rings which fully surround the conductors, apart from their contact areas. This results in a very good electrical insulation and the use of a solid insulator in stead of air or a dielectric oil allows a compact design and operation at relatively high voltages. The conductors comprise concentric rings each having an annular metal contact surface via which the inner and outer conductors make full contact, such that the mechanical forces and electrodynamic forces as well as the currents are distributed evenly over the full circumference. 
         [0004]    The known swivel has the disadvantage that there is a risk of short circuits after the system has been in operational use for a while and the conductors start to show some wear. When debris originating from wear get in the narrow space between the conductors and the insulating rings, short circuits can be created, causing the swivel to malfunction. Upon wear of the spring elements at the contact surfaces of the annular conductors, the solid insulator rings and conductors of the swivel need to be dismantled in order to obtain access to the electrodes. 
         [0005]    It therefore is an object of the invention to reduce the risk of malfunctioning of the swivel due to debris. It is a further object of the invention to reduce the amount of wear of the conductors and the amount of debris, while obtaining a good electrical contact. It is again an object of the invention to provide a swivel of reduced weight, in which the inner and outer annular conductors are accurately aligned and can take up large mechanical and electrodynamic loads, especially under offshore conditions. The swivel should allow easy handling during assembly and disassembly for inspection or replacement purposes. 
       SUMMARY OF THE INVENTION 
       [0006]    Hereto a high voltage swivel according to the invention is characterised in that the insulator members interconnecting the conductor elements are situated at a mutual distance in the circumferential direction of the conductor element, the interconnected conductor elements of the inner and the outer body each being supported by a respective support member to form integral units such that the inner and outer bodies are adapted to be mutually decoupled or attached by relative displacement of the integral units in the direction of the longitudinal axis 
         [0007]    The open area between the conductors in the axial direction, and the open area between adjacent insulator members in the circumferential direction of the conductor elements result in an open cage-like construction of the inner and outer conductor stacks. Insulating oil can freely flow through the open stacked conductor configuration which prevents occasional debris originating from wear to become trapped, as such debris can be easily removed from the open area without causing short circuits by bridging the space between the conductors. By constructing an open inner and an outer stack of at least two spaced-apart annular conductors, the conductors can be mechanically aligned in a stable manner, and can be easily assembled or disassembled for inspection or repair. The open conductor stacks according to the invention provide a stable and light-weight swivel construction which can withstand large electrodynamic forces and which is especially suitable to remain mechanically aligned under offshore conditions. 
         [0008]    As the inner and outer conductor stacks each form an integral unit, handling upon installation or replacement is facilitated. The conductors of the inner and outer bodies can be aligned easily by accurate mutual alignment of the units along the longitudinal axis. Disassembling the inner and outer bodies is relatively easy as they can be separated by pulling the units apart in the axial direction. 
         [0009]    From U.S. Pat. No. 4,252,388 a high voltage swivel is known, where the inner body comprises a stack of conductors, dielectric support spacers and washer-like dielectric barriers. The conductors of the outer body are provided by a single or a low number of carbon brushes, each contacting a conductive ring of the inner body at a single position along its circumference. This causes considerable wear and hence contamination from the resulting debris. Moreover, it gives a single, or a low number of, narrow current paths from inner to outer element, which poses limitations on the maximum voltage to be transmitted by the swivel. Furthermore, the mechanical stiffness of the known carbon support brushes is relatively low such that electrical contact is not always optimal and the maximum current passable through the carbon brushes is limited. Upon assembly and disassembly, the outer conductors all need to be individually installed and replaced, which complicates handling of the known swivel parts. 
         [0010]    In one embodiment, the support member comprises a transverse flange extending substantially parallel to the ring-shaped conductor elements. The flanges carrying the conductor stacks can connected to or form a part of a housing containing the conductor elements, and support these elements such that the outer body can rotate relative to the inner body, around the longitudinal axis. 
         [0011]    The inner body may comprise along the longitudinal axis a core element carrying at a lower side the transverse flange, a lower conductor element of the inner body being with a lower end face connected to the transverse flange via spaced-apart insulating members. 
         [0012]    In order to interconnect the spaced-apart conductor elements of the inner body, the elements may comprise at least one radially inwardly projecting conductor part attached to an inner axial conductor that extends inwardly of the ring-shaped conductor elements in an axial direction to an end part that is situated above or below the topmost or lowermost conductor element of the inner body, which end part is attached to a connector terminal. In this way, the inner axial conductors extend inwardly of the ring shaped conductor elements to an output or input terminal without interfering with the relative rotational movement of the inner and outer ring-shaped conductor elements around the longitudinal axis. 
         [0013]    In a similar manner, each conductor element of the outer body may comprise at least one radially outwardly projecting conductor part attached to an outer axial conductor that extends outwardly from the ring-shaped conductor elements in an axial direction to an end part above or below the topmost or lowermost conductor element of the outer body, which end part is attached to a connector terminal. Preferably the connector terminals of the inner and outer body are situated at opposite end parts of the axial conductors, such that sufficient space is available to accommodate the connectors at the end of the power cables that connect to the swivel. 
         [0014]    The connector terminals of the inner and/or of the outer body may be axially directed and can be attached along a circular contour on a radial flange. The power cables connected to the stator and rotors part of the swivel in this embodiment extend, at least in the vicinity of the swivel, in the axial direction. 
         [0015]    Alternatively, the connector terminals at the end parts of the axial conductors of the inner and/or of the outer body may be radially directed and attached along a contour of a ring-shaped support. In this way, the power cables near the rotor or stator part of the swivel may be oriented in a radial direction. 
         [0016]    In one embodiment, radial flange of the inner or outer body is connected to a lower cylindrical housing part, the ring-shaped support of the other body being attached via a rotatable bearing to the lower housing part and to a cover to form a liquid-tight enclosure around the conductor members. Inside the enclosure, a dielectric fluid, such as oil, is accommodated. In a preferred embodiment, the inner and outer bodies are attached to a top cover and to a bottom cover, respectively, which covers are interconnected via an inner cylindrical wall, one of the covers being rotatable relative to the cylindrical wall around the vertical axis, an outer cylindrical wall surrounding the inner wall, which inner wall is provided with apertures and seals that are adapted to open the apertures when a predetermined pressure on the seal is exceeded. In this way, an extra containment compartment is formed for the dielectric fluid by the space between the first and second cylindrical walls, which compartment is only accessible after the seals—for instance formed by burst discs—have been ruptured by a sudden pressure increase, which may be caused by a short-circuit and a sudden increase in pressure due to vaporisation of the dielectric fluid. In this way, the effects of an internal short circuit due to pressure build-up, such as release of explosive gas or projection of hot oil, can be avoided. The internal dimensions of the swivel having an outer containment wall according to the invention, can remain relatively small, as the adverse effects of an internal short circuit are strongly reduced. 
         [0017]    In a preferred embodiment of a high voltage swivel, spring plates are fixed to the conductor element at the contact surface of the inner or outer conductor element, arranged side by side, a length direction of the spring plates extending in the circumferential direction of the conductor element. Placing the spring plates, which have a louver like construction, with their length direction in the circumferential direction of the swivel, an equal resistance in both rotational directions is achieved. This results in even force distribution and reduced wear of the spring plates, while good conductive contact is maintained between the inner and outer ring shaped conductor elements at all times. 
         [0018]    For improving the ease of construction, the spring plates can be situated on the contact surface of the inner conductor, which is easily accessible. The spring plates may have a length that is smaller than 0.1 preferably smaller than 0.05 times a circumferential length of the conductor contact surface, such that at least 10 sets, preferably at least 20 sets of substantially parallel spring plates can be placed at the contact surface for optimising the electrical conductive contact between inner and outer conductors. 
         [0019]    In a further embodiment, the spring plates may be formed in a mounting frame, one of the conductors having at a contact surface a coupling member for engaging with the mounting frame, the mounting frames covering at least a part of the contact surface of the conductor element. The mounting frames with the conductors can be manufactured separately with high accuracy and can be easily mounted on the contact surface of the inner or outer conductor ring. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    Some embodiments of a high voltage swivel according to the invention will by way of non-limiting example be explained in detail with reference to the accompanying drawing. In the drawing: 
           [0021]      FIG. 1  shows a cross-sectional view of a first embodiment of a high voltage swivel according to the invention, 
           [0022]      FIG. 2  shows a detail of the conductor elements of  FIG. 1  on an enlarged scale, 
           [0023]      FIG. 3  shows a cross-sectional view of an inner conductor stack of a second embodiment of a high voltage swivel according to the invention, 
           [0024]      FIG. 4  shows a perspective view of the inner conductor stack of  FIG. 3 , 
           [0025]      FIG. 5  shows a cross-sectional view of an outer conductor stack of the second embodiment of a high voltage swivel according to the invention for cooperation with the inner conductor stack of  FIGS. 3 and 4 , 
           [0026]      FIG. 6  shows a perspective view of the outer conductor stack of  FIG. 5 , 
           [0027]      FIG. 7  shows a perspective view of a high voltage swivel, with the inner and outer conductor stacks of  FIG. 3-6  in an assembled configuration, 
           [0028]      FIG. 8  shows a cross-sectional view of the high voltage swivel of  FIG. 7 , 
           [0029]      FIG. 9  shows a perspective view of the high voltage swivel outer housing, 
           [0030]      FIG. 10  shows a schematic top plan view of the swivel of  FIG. 8  showing the grouped arrangement of the swivel connector terminals, 
           [0031]      FIG. 11  shows a perspective view of a third embodiment of a high voltage swivel with all connector terminals in an axially oriented configuration, 
           [0032]      FIG. 12  shows an embodiment of a high voltage swivel having an inner housing and an outer containment housing, 
           [0033]      FIG. 13  shows an inner ring-shaped conductor element comprising a number of spring plates mounted in mounting frames, attached to the contact surface, 
           [0034]      FIG. 14  shows a detail of the mounting plate carrying louver-type spring plates according to  FIG. 13 , 
           [0035]      FIG. 15  shows a top view of the inner conductor element, the spring plates and the outer conductor element, and 
           [0036]      FIG. 16  schematically shows an offshore system with a floating power plant comprising a high voltage swivel of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0037]      FIG. 1  shows a high voltage swivel  1  comprising a stationary outer body and a rotatable inner body. The outer and inner bodies are formed by inner and outer conductor stacks  2 ,  3  that are co-axially aligned around longitudinal axis  4  and are enclosed in a housing  5 . The outer and inner stacks  2 ,  3  are each comprised of four ring-shaped conductor elements  7 , 7 ′, 8 ,  8 ′, 9 , 9 ′ and,  10 , 10 ′, one conductor element for each phase and one for connection to ground voltage level. The conductors elements  7 - 10  and  7 ′- 10 ′ are provided with aligned holes through which an insulating connecting rod  12 ,  13  is guided. Around the connecting rods  12 ,  13 , insulating spacers  15 ,  15 ′, 16 ,  16 ′,  17 ,  17 ′, 18   18 ′,  19 ,  19 ′ are provided, which abut against end faces  45 , 45 ′, 46 , 46 ′ (see  FIG. 2 ) of pairs of adjacent conductor elements  7 , 8 ;  7 ′, 8 ′;  8 , 9 ;  8 ′, 9 ′ and  9 , 10 ;  9 ′, 10 ′ and maintain a predetermined axial distance between these adjacent conductor elements. The lower spacer  15  of the inner conductor stack  3  is connected to a lower support member  22 , and the upper spacer  19  is adjacent an upper support member  23 . The connecting rod  13  clamps the insulating spacers  15 - 19  and the ring-shaped conductors  7 - 10  between the upper and lower support members  22 , 23  such that the support members, the connecting rods, the insulating spacers and the ring-shaped conductors form an integral unit. The same holds for the outer conductor stack  2  wherein the ring-shaped conductors  7 ′- 10 ′ are interconnected via the outer connecting rod  12 , and the insulating spacers  15 ′- 19 ′ that are clamped between upper support member  25  and lower support member  26 . 
         [0038]    Each of the inner conductor elements  7 - 10  comprises a conductor part  30  forming a radial extension, which conductor part supports an inner axial conductor  31  that is provided with an insulating sheath  32 . The axial conductor  31  extends within the inner ring-shaped conductor elements  7 - 10  and has an end part  34  situated above the topmost conductor, the closed contour of elements  19 , 19 ′, which end part  34  is attached to a radially oriented connector terminal  33 . The outer conductor elements  7 ′- 10 ′ comprise a radially oriented outer conductor part  35  that is connected to outer axial conductor  36  having a lower end  37  situated below the lowermost ring-shaped conductor elements  10 , 10 ′. The outer axial conductor  36  extends with a lower end  37  below the lowermost ring-shaped conductor element  10 ′ and is attached to a connector terminal  38  which is radially oriented. For each inner ring-shaped conductor element  7 - 10  and for each outer ring-shaped conductor element  7 ′- 10 ′ two inner and two outer axial conductors  32 , 36  as well as two connector terminals  33 , 38  are provided for an even distribution of input and output currents. 
         [0039]    The housing  5  of the high voltage swivel provides a fluid-tight containment of dielectric oil. An outer housing part  40 , supporting the outer conductor stack  2  is connected via an axial-radial bearing  42  to an inner housing part  43  supporting the inner conductor stack  3 . 
         [0040]    After disconnecting the bearing  42  and detaching the axial conductors  32 ,  36  from their respective connector terminals  33 ,  38 , the outer conductor stack  2  can be lifted in the direction of the longitudinal axis  4 , to disconnect the conductor stacks  2 , 3  for maintenance or repair. The open space between the conductor elements  7 - 10 ; 7 ′- 10 ′ and between adjacent insulating spacers  15 , 15 ′; 16 , 16 ′; 17 , 17 ′ and  18 , 18 ′ that are situated at the same axial positions, leaves free access for dielectric oil circulation and prevents debris from being trapped between pairs of adjacent ring-shaped conductor elements  7 , 7 ′- 10 , 10 ′ and hence avoids short-circuits from being formed. The free transport of debris by the oil by natural convection ensures that debris are not trapped at a fixed position such that chances of a short circuit being caused by these debris is strongly reduced. Also heat, generated upon current transfer between the inner and the outer conductor stacks, is transported by convection to the surrounding oil and by the oil to the metal housing  5 . Heat transported to the housing  5  will be transferred to the ambient air by convection. The oil inside the housing  5  is not actively circulated. 
         [0041]    The height H 1  of the swivel  1  may lie between 0.7 m and 2.0 m, for instance about 1.5 m. The axial distance H 2  between adjacent ring-shaped conductor elements can lie between 6 cm and 25 cm, for instance 15 cm. A thickness H 3  of the ring shaped conductor elements  7 , 7 ′- 10 , 10 ′ can range from 3 cm to 10 cm, for instance 5 cm. The width W of the conductor elements may lie between 10 cm and 20 cm, for instance 15 cm. The outer diameter D 1  of the swivel is for instance between 1.5 m and 2.5 m, for instance 2 m, whereas the outside diameter D 2  of the outer conductor stack  2  can be between 1 m and 2 m, for instance 1.3 m 
         [0042]      FIG. 2  shows an enlarged detail of two adjacent conductor elements  7 , 7 ′. The insulating spacers  19 , 19 ′;  18 , 18 ′ abut against upper end faces  45 ,  45 ′ and lower end faces  46 ,  46 ′ of the conductor elements  7 , 7 ′. Each conductor element  7 , 7 ′ has a curved contact surface  48 , 48 ′ which contact surfaces are placed in opposed sliding relationship to transfer currents from one conductor element to the other. 
         [0043]    The inner and outer conductor stacks  2 , 3  constitute a stable and robust structure in which mechanical forces and electro dynamic forces, as well as the currents are distributed evenly over the full circumference of the rings-shaped conductor elements. At the same time, full rotational freedom is provided between the rotating swivel part (e.g. the outer stack  2 ) that is fixed to the floating offshore structure, which structure may comprise an offshore weathervaning power generating unit, such as a wind turbine unit, an FPSO and the like, and the geostationary swivel part (e.g. the inner conductor stack  3 ) that may be connected to a sub-sea power cable. 
         [0044]      FIG. 3  shows an embodiment wherein the inner conductor stack forms an integral unit  50  having four interconnected ring-shaped conductor elements  51 , 52 , 53  and  54 . The conductor elements  51 - 54  are interconnected by the insulating spacers  55 ,  66 ,  67  that are distributed along the circumference of the conductor elements. The lower conductor element  54  is supported via lowermost insulating spacers  55  onto a transverse flange  56  near a lower end  58 ′ of a central support member  57 . Two inner axial conductors  58 ,  59  extend upwardly, along the inside of the ring-shaped conductor elements  51 - 54  from the lower conductor element  54  to connector terminals  60 ,  61 . The same applies for each conductor element  51  and  53 , such that in total six connector terminals are provided for the three conductor elements  51 ,  53  and  54 . The conductor element  52  defines a ground voltage level, and is coupled to a single ground connector terminal  62  (see  FIG. 4 ). 
         [0045]    As can be clearly seen from  FIG. 4 , the inner axial conductors  58 , 59  are connected to the conductor element  54  via the radially inwardly projecting conductor part  63 . The conductor parts  64 ,  65  of the conductor elements  51  and  53  are also indicated in  FIG. 4 . Near an upper end  70  the connector terminals  60 ,  61 ,  62  are mounted in a cylindrical support  72 . The cylindrical support  72  forms part of the outer housing and is at its upper edge connected to a top wall  73 . The top wall  73  is also connected to an upper end of the central support member  57  to form a rigid integral unit  50 . Along a lower edge of the cylindrical support  72 , a bearing  75  is attached for rotatably connecting to a lower housing part of the swivel. 
         [0046]      FIG. 5  shows the outer conductor stack wherein an integral unit  80  is formed by four outer ring-shaped conductor elements  51 ′, 52 ′, 53 ′ and  54 ′. The conductor elements  51 ′- 54 ′ are interconnected via the insulating spacers  81 , 82 , 83 . Adjacent spacers  81 , 82  are placed at a relatively large mutual distance S of for instance 40 cm such that a largely open configuration of the stacked conductor elements  51 ′- 54 ′ is obtained. As can be seen from  FIG. 6 , the outer conductor elements  51 ′- 54 ′ are coupled to outer axial conductors  85 , 86 , 87 , 88 , wherein conductor  88  is shown without the outer insulating sheath. The conductors  85 - 88  are coupled to outer conductor parts  89 , 90  which project from the conductor elements  51 ′- 54 ′ in a radial direction. Near the lower end  92 , the outer axial conductors  85 - 88  are coupled to connector terminals  93 , 94  extending in an axial direction and being mounted in a lower ring-shaped support  95 . The lower ring-shaped support  95  is with its edge connected to a lower cylindrical wall part  96  of the outer housing  101 . 
         [0047]      FIG. 7  shows a perspective view of the integral units  50  and  80  of the inner and outer conductor stacks in the assembled configuration, coaxially aligned around the longitudinal centre line  100 , whereas  FIG. 8  shows the assembled integral units  50 ,  80  in cross-sectional view. The outer housing  101  has only schematically been indicated in  FIG. 8 . 
         [0048]      FIG. 9  shows the outer housing  101  of the swivel providing a fluid tight containment of the insulating oil that surrounds the inner and outer conductor stacks with the cylindrical support  72  rotatably attached to lower cylindrical wall part  96  via the bearing  75 . The housing  101  is closed by the top wall  73  and by the lower ring-shaped support  95 . The connector terminals  60 , 62  project through the housing wall in a liquid-tight manner. 
         [0049]      FIG. 10  shows a top plan view of a swivel  110  with outer conductor stack  111  and inner conductor stack  112 . The connector terminals are arranged in two groups  113 ,  114  of three connector terminals each, each connector terminal in a group being attached to a conductor element with a different phase. The power cables  115 ,  116  attached to respective connecter terminals in a group of connector terminals  113 ,  114  are intertwined such that losses in the power cables  115 ,  116  due to the electrical fields generated by the currents in these cables are reduced. The connector terminal  117  for connecting to ground voltage is placed at a circumferential distance from the groups  113 ,  114 . 
         [0050]      FIG. 11  shows an embodiment wherein the connector terminals at the top end of the swivel are mounted in a horizontal support flange  120 , such that both upper connector terminals  121 , and lower connector terminals  122  extend in an axial direction. 
         [0051]      FIG. 12  shows a swivel symmetrical around longitudinal axis  123 , having an outer housing around the stacked conductors  124 , with a cylindrical wall  126  a bottom wall  125  and a top wall  132 . Connector terminals  130 ,  131  are attached to the top wall  132  and bottom wall  125 . A second wall  127  with bottom  133  and top  129  is placed around the inner wall  126 . No oil is present in the space defined between the walls  126  and  127 . In the wall  126 , a number of apertures  134  are provided that are closed off by a burst disc  128  which breaks away upon a pressure increase in the space defined by the wall  126 . When, due to a short circuit, the pressure rises sharply in the space within inner wall  126 , the burst disc  128  will break away from the aperture  134  such that oil can flow into the space between the inner wall  126  and the outer wall  127 . Hereby the risk of projection of hot oil from the swivel containment and ejection of explosive gases from the swivel is sharply reduced and operational safety of the swivel is increased. 
         [0052]      FIG. 13  shows an inner ring-shaped conductor element  140  which at its contact surface is provided with a number of mounting frames  150 ,  151 , each mounting frame carrying six spring plates  152 , 153  in a louver like configuration, as can be seen from  FIG. 14 . The spring plates compensate for the mechanical tolerances between the rigid inner and outer ring-shaped conductor elements and provide a secure conducting contact between the conductor elements at all times. The spring plates extend generally with their length direction L in the circumferential direction of the conductor elements, such that upon rotation an even force distribution for rotations in either direction is obtained, reduced wear of the spring plates occurs and a good conductive contact is established with the outer conductor element  141 . 
         [0053]    As can be seen from  FIG. 15 , the mounting frames  150 ,  151  are at their rear side provided with grooves, into which projections  154 ,  155  on the peripheral surface of the conductor element  140  fit. In the embodiment that is shown, the conductor member  140  is covered along its complete circumference by the mounting frames  150 ,  15  which can be easily replaced in case of damage to the spring plates. It is however also possible to cover only part of the circumference of the conductor element with spring plates, by placing the mounting frames  150 ,  151  at a relatively large mutual distance. 
         [0054]      FIG. 16  shows an offshore system comprising a Floating Production, Storage and Offloading vessel (FPSO)  260  which is anchored to the sea bed  261  via a turret  262 , at the bottom of which anchor lines  263  and  264  are attached. The vessel  260  can weathervane around the turret  262 , which is geostationary. A product riser  265  extends from a sub sea hydrocarbon well to a product swivel (not shown) on the FPSO  260  and from the product swivel via a duct  65 ′ to production and/or processing equipment on the FPSO. In a power generation unit  266 , gas produced from the well is converted into electricity which is supplied to a swivel  267  according to the present invention. The power lead  268  extending from the power generation unit  266  is attached to conductors on the outer element of the swivel which is stationary relative to the vessel  260 . The power lead  269 , extending to the sea bed is connected to the electrical conductors of the inner element of the swivel  267  which is fixedly attached to the turret  262 . The power lead  269  may extend to an unmanned platform  270  attached to the sea bed via product riser  270 ′, such as a gas riser, or may extend to an on-shore power grid  71 , or may be connected to heating elements  275 ,  276  of a substantially horizontal hydrocarbon transfer duct  277  between two floating structures  272 ,  273 . 
         [0055]    It should be noted that in stead of with an FPSO  260 , the swivel according to the present invention can also be used with other offshore power generating constructions such as weathervaning wind turbines.