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FIELD OF THE INVENTION 
     This invention relates to a swivel device for installation at a well for subsea production of oil or gas and adapted to be connected to a production vessel at the sea surface, comprising a main or fluid swivel having at least two fluid passages and an electric/hydraulic auxiliary swivel for signal communication and power transfer. 
     BACKGROUND OF THE INVENTION 
     When developing marginal offshore production fields it is important to keep the costs low. A substantial cost factor with known forms of development, is due to subsea pipelines and cables between christmas trees at the wells and product receiver sites, for example a platform or a floating production vessel. Typically there may here be the question of distances like 2 km. In this connection it is to be noted that modern drilling technology makes it possible to produce by means of a reduced number of christmas trees, because the boreholes or wells can have several branches. This will make it possible to develop subsea fields in some instances by employing only one subsea christmas tree. 
     International patent application PC/NO96/00201 dated Jul. 8, 1996 relates to a system for offshore production of hydrocarbons by employing a moored production vessel or ship. The present invention can be regarded as a further development thereof and is based on the idea that the production vessel can by lying directly above or in the immediate vicinity of the christmas tree at the seabed and that the well fluid flows are transferred directly between the christmas tree and the vessel without relying on pipelines or cables on the seabed. 
     Advantageously the solution to be described in the following specification can employ a particular and preferably simplified form of christmas tree, being more or less integrated into an arrangement according to the invention as will appear from the following description. 
     Thus, in a device as stated in the introduction above, the novel and specific features according to the invention primarily comprise therein that the fluid swivel is provided with a rotatable swivel housing at the top of a stationary christmas tree, preferably comprising a small number of valves, such as a production master valve and an annulus master valve, and that said passages are through-running vertically in a central core member of the fluid swivel in order to make possible well intervention from the upper side of the fluid swivel. 
     On the basis of this fundamental solution, which in actual practice can be embodied in various forms, it is possible to obtain a number of advantages, of which in particular the following are mentioned: 
     The production vessel employed does not need any modifications worth mentioning, in relation to common ship designs, and therefore will be relatively inexpensive, 
     the same production vessel can be used for installing and possibly retrieving the swivel device, or also for well maintenance, which contributes to reduced operational costs, 
     very small marginal subsea fields can be profitable by using this novel arrangement, so that the degree of extraction can be increased for the fields, 
     the equipment employed can be re-used by moving it from field to field. 
     SUMMARY OF THE INVENTION 
     The present invention is a swivel device for installation at a well for subsea production of oil or gas and adapted to be connected to a production vessel at the sea surface comprising a main fluid swivel having at least two passages and an auxiliary swivel. The swivel device is provided with a rotatable swivel housing on top of a stationary christmas tree and that the passages are through-running vertically in a central core member of the fluid swivel, in order to make possible well intervention from the upper side of the fluid swivel. Further, at the lower part of the christmas tree, there is provided a force transferring housing being rotatably journalled about a common vertical axis on a base structure. The swivel device further comprises a mooring yoke having two legs, the free ends of which are rotatably connected to either side of the force transferring housing, and the opposite ends of which are adapted to be attached to the end of a mooring line. Between the force transferring housing and the swivel housing, there is provided a mechanical coupling for causing the swivel housing to perform the same rotary movements as the force transferring housing under the influence of the yoke. 
     Further, the mechanical coupling between the force transferring housing and the swivel housing comprises a rod element extending in parallel to the common vertical axis at the side of the christmas tree and the swivel housing. 
     The swivel device of the present invention may further comprise a retrievable top member incorporating the auxiliary swivel. The top member at its lower part is provided with a connector adapted to cooperate with the core member of the fluid swivel. 
     The top member may further contain a control module for the christmas tree and may be provided with a handling head for installation and retrieval. 
     The swivel device of the present invention may further comprise a bridging arm extended outwards and downwards from the top member and provided with a connector plate at its lower end. The connector plate is adapted to cooperate with a complementary connector part associated with a laterally projecting connecting member on the swivel housing. 
     A further embodiment of the instant invention provides the swivel housing with laterally directed connecting members for at least two risers being oriented substantially in the same plane as a connecting member for an umbilical. The connecting members are directed substantially outwards centrally between the yoke legs. 
     The swivel device may be characterized such that the wellhead comprises a conductor tube, characterized in that the force transferring housing is rotatably provided on a base structure being connected to the conductor tube for transferring mooring forces directly from the yoke to the conductor tube. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following description the invention will be explained more closely with reference to the drawings, in which: 
     FIG. 1 shows a simplified overview of a total arrangement with a production vessel associated with an installation at the seabed and being moored thereto, and 
     FIG. 2 shows an example of a preferred embodiment of the device according to the invention, as seen in elevation and partially in section. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows an overview of the total arrangement when a production vessel or ship  3  operates in association with a well or a similar installation as generally shown at  20  on the seabed  1 . Risers or hoses  31  and  32  are shown here in their full length up to the ship  3 , where they are connected to a processing unit  3 A being shown purely schematically. Moreover a mooring line  69  is extending up through the sea surface  2  to mooring means at the bow portion of the ship  3 . An intermediate region of the mooring line  69  is provided with a buoyancy element and likewise the risers  31  and  32  have buoyancy bodies at a lower portion for elevating the risers or hoses from the bottom  1 . This general system is described more closely in the above mentioned International patent application. 
     In FIG. 2 there is indicated with hatching those parts and components which take part in the swing or turning movement when a production vessel moves around a subsea installation under the influence of wind and weather, in particular waves. The arrangement is preferably built up with a central and common vertical axis  20 X coinciding with the axis of a wellhead  48 , i.e. the top of the production tubing, when a subsea oil or gas well is concerned. 
     FIG. 2 in relatively much detail primarily shows more or less common structures at the top of an oil or gas well at the seabed  1 . A concrete plate or base  45  forms a foundation on seabed  1  and has a central opening for supporting a self-adjusting ball joint type frame  46  which in turn is connected to a common conductor tube  47 . In a known manner this can extend for several tens of meters down into the structure underneath the seabed  1 . Within conductor tube  47  the actual production tubing is hanging, and ending at the top in the wellhead  48  itself. 
     A base structure  49  serves for supporting the whole arrangement above it. Through strong, supporting structure elements the base structure  49  is directly connected to the conductor tube  47  for transferring mooring forces thereto. Journalled at the base structure  49  there is shown a strong, carousel-like housing  60  being rotatable about the central axis  20 X. Housing  60  is provided with preferably diametrically opposed attachment members  61  in the form of projecting studs for a yoke  63 , the upper end  64  of which is adapted to be connected to one or more mooring lines  69 . It will be seen that yoke  63  has two legs being united at an upper end or cross piece  64 . Yoke  63  can assume various angular positions by swinging about the horizontal axis extending diametrically between the attachment members  61 , whereby the angular range of the yoke movement at least extends upwards to an approximate vertical position, whereas the lowermost angular position in practice is restricted in view of umbilical  33  and/or risers  31  and  32 . 
     During operation with a moored production vessel, this, through the intermediary of line or lines  69  and thereby yoke  63 , will cause rotation of the hatched parts of the arrangement about the axis  20 X. A corresponding rotary movement takes place in the actual swivel device, whereby the central core member  25  is stationary together with valve block  50 , whereas the remaining parts of the swivel including the swivel housing  24  participate in the rotary movement. Thus, swivel housing  24  is journalled at  30  on top of valve block  50 . 
     In order to secure a simultaneous and common rotary movement of housing  60  and swivel housing  24 , there is shown a mechanical guide or coupling in the form of an upright rod element  72  being supported at its lower end by a bracket  71  attached to housing  60 , and further up extending through a sleeve  73 A on a supporting arm  73  projecting laterally from swivel housing  24 . This mechanical coupling has been made sufficiently rigid for swivel housing  24  to perform the same rotary movements as the carousel or housing  60  under the influence of yoke  63 , when this is connected to a surface vessel through mooring line  69 . In addition to the rotary interconnection between housing  60  and the fluid swivel housing  24  as just explained, rod element  72  in cooperation with sleeve  73 A can also have a useful and advantageous function during installation of the swivel device on a wellhead. For this purpose sleeve  73 A has a downward wide cone  73 B for cooperation with the upper end of rod element  72  when landing the fluid swivel and possible christmas tree  50  on wellhead  48 . The christmas tree or block  50  at its lower part has a mounting or connector device  50 B for detachable mounting on wellhead  48 , as previously known per se. 
     Swivel device  50  has a stationary, central core member  25  with axially through bores or passages comprising a production bore  21  and an annulus bore  22 . Both of these communicate downwards with corresponding fluid passages or conduits in valve block  50 , for the purpose, inter alia, of the various valve functions made possible thereby. Specifically it is to be noted in this connection that these fluid bores or passages extend vertically from the top of the central core member  25  of the swivel and preferably with direct prolongation in corresponding passages  56 , 57  as indicated in christmas tree  50 . Thus, well intervention from the upper side of the fluid swivel is made possible. This however, is on condition that a top member  77  of the total swivel device is removed, as will be described more closely below. 
     Moreover, around core member  25  the swivel device  20  has two or more annular fluid passages with associated seals and bearing elements, as generally shown at  27 . These parts of a fluid swivel are previously known per se, for example from Norwegian patent No. 177.780, illustrating an axially separable swivel device, being primarily intended for other uses. 
     Swivel housing  24  is provided with connecting members generally denoted  31 A and  32 A for risers or hoses  31  and  32  adapted to extend upwards through the water to the production vessel. (See FIG.  1 ). Whereas connecting members  31 A and  32 A for fluid transfer are located relatively centrally at swivel  20 , an upper connecting member  33 A for an umbilical  33  is located at an upper part of swivel housing  24 , being adapted to rotate about axis  20 X during the turning movements mentioned. A bearing  30  supports swivel housing  24  so that it can rotate in relation to the christmas tree or block  50 . 
     By means of a connector  20 B a top member represented by its encasing  77  is incorporated in the total swivel device  20  in FIG.  2 . Connector  20 B permits separation of top member from and retrieval thereof from the remaining swivel device, as will appear from the following description. An auxiliary swivel  28  for transferring signals and power, is located in top member  77  and this also accomodates a control module  29  for the christmas tree. A slip ring mechanism  79  serves for the required rotary coupling of the various wire connections between said components in top member  77  and christmas tree  50 , possibly also other units being incorporating in the wellhead. 
     On the other hand there are provided electric and hydraulic connections from top member  77  through a bridging arm  77 B extending from a sidewall of the top member encasing  77  outwards and downwards along the side of connector  20 B, and provided with a connector plate  78  or the like for cooperation with a complementary connector part on a connecting member  33 A for umbilical  33 . Accordingly, this can, through connection  78 , bridge arm  77 B and the interior of top member  77  including slip ring mechanism  79 , communicate with christmas tree  50  and so forth. 
     As indicated with reference numeral  26  at the lower part of top member  77 , this is rotatably journalled on connector  20 B. A synchronous and simultaneous rotary movement of top member  77  under the influence of yoke  63  is secured by means of rod element  72 , entering into a sleeve  75 A connected to the top member encasing  77  through a radial arm  75 . Sleeve  75 A has a downwardly open cone  75 B which like cone member  73 B below, contributes to a safe landing and installation of top member  77  separately on the upper end of core member  25 , at the same time as connection is established at  78 . For such installation or retrieval of top member  77  this is provided with an upper handling head  77 A, which in a manner known per se can be configured and dimensioned for cooperation with suitable tools. It is obvious that these operations by means of handling head  77 A can also depending on the circumstances, comprise the whole swivel device, including the fluid swivel  24 , 27  and the christmas tree or block  50  as well as connector  50 B thereon, for installation at wellhead  48 . 
     The laterally directed connecting members  31 A and  32 A for risers are preferably oriented in the same plane as connecting member  33 A for umbilical  33 , and all connecting members are preferably directed substantially outwards from swivel housing  24  centrally between the yoke legs  63 . It can be practical to provide isolation valves  31 B and  32 B between the connecting members and swivel housing  24 , as will appear from FIG.  2 . These isolation valves can also act as wing valves, which can be of interest in view of the preferred, simple form of christmas tree  50 .

Summary:
A swivel assembly for installation at a well for subsea production of oil or gas and adapted to be connected to a production vessel at the sea surface. The swivel assembly comprises a main fluid swivel having at least two paths and an electric/hydraulic auxiliary swivel for signal communication and power transfer. The fluid swivel is provided with a rotatable swivel housing at the top of a stationary christmas tree, preferably comprising a small number of valves, such as a production master valve and an annulus master valve. The paths are through-running vertically in the central core member of the fluid swivel so as to make possible well intervention from the upper side of the fluid swivel.