Patent Publication Number: US-2022228688-A1

Title: Swivel stack for transfer of fluid across a rotary interface and method for manufacturing such a swivel stack

Description:
This application is the U.S. national phase of International Application No. PCT/EP2020/061354 filed Apr. 23, 2020 which designated the U.S. and claims priority to EP Patent Application No. 19170914.6 filed Apr. 24, 2019, the entire contents of each of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a swivel stack for transfer of fluid across a rotary interface around a rotation axis between an incoming flow-line and an outgoing flow-line. 
     Moreover, the invention relates to a turret mooring system comprising such a swivel stack. Also, the invention relates to a floating offshore construction provided with such a swivel stack and to a method for manufacturing such a swivel stack 
     Description of the Related Art 
     Toroidal fluid swivels are known in the art for transfer of high-pressure fluids across a rotary interface between an incoming fluid line and an outgoing product piping. Applications for such a swivel include for example offshore oil and gas explorations where high-pressure flows of oil and/or gas are transferred from a (deep-sea) offshore well to a floating vessel such as a Floating Production Storage and Offloading (FPSO) vessel. Typically, such a floating vessel is equipped with a turret mooring system that can couple a mooring buoy or a “mooring structure” and that holds one or more riser lines from the well, to product piping ducts on the vessel. Since the turret mooring system should allow some rotation between the vessel and the buoy, the swivel is likewise adapted to provide rotation between the incoming fluid line and the product piping. 
     In swivel designs, the rotary interface is provided with seals to prevent leakage. Within the rotary interface the seals are subjected to high pressure differences between the high-pressure fluids running through the swivel and ambient. 
     Also, the seals are subjected to mechanical wear due to rotation of the interface. In the prior art, swivels (swivel stacks) thus face several issues affecting their performance and reliability; the main issue being the practical inability to change-out the dynamic seals in-situ, due to the relatively large size of the swivel parts. 
     Another major issue is the difficulty to meet seal design criteria, especially when seal diameter, temperature range, or fluid pressure increases. It is an object of the invention to overcome or mitigate the disadvantages of the prior art. 
     SUMMARY OF THE INVENTION 
     The object is achieved by a swivel stack as disclosed and claimed. The structure of the swivel stack allows a relatively uncomplicated manner of assembly or disassembly, which also enhances the procedure to change-out any seals in the swivel stack. 
     Further, the invention relates to a turret mooring system equipped with a swivel stack as defined above, to a floating offshore construction provided with a swivel stack as defined above and to a method for manufacturing a swivel stack as defined above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be explained in more detail below with reference to drawings in which illustrative embodiments thereof are shown. The drawings are intended exclusively for illustrative purposes and not as a restriction of the inventive concept. The scope of the invention is defined in the appended claims. 
         FIG. 1  shows a cross-section in perspective view of a swivel stack according to an embodiment of the invention; 
         FIG. 2  shows a detailed cross-section of the rotary interface of a swivel stack according to an embodiment of the invention; 
         FIG. 3  shows a detailed cross-section of the rotary interface of a swivel stack according to an embodiment of the invention; 
         FIGS. 4A and 4B  shows detailed cross-sections of a stab-in pipe for use in a swivel stack according to an embodiment of the invention; 
         FIG. 5  shows a perspective view of a swivel stack according to an embodiment of the invention; 
         FIG. 6  shows a perspective view of a portion of a manifold block in accordance with an embodiment of the invention, and 
         FIG. 7  shows schematically a floating offshore construction equipped with a swivel stack in accordance with an embodiment of the invention. 
     
    
    
     In the following description of embodiments, items indicated by an identical reference sign refer to the same or a similar item. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a cross-section in perspective view of a swivel stack according to an embodiment of the invention. 
     A rotary interface  72  of a swivel stack  10  in accordance with the invention is constructed from a plurality of annular elements comprising a group of inner annular elements and an outer annular element that are all centered around a common rotation axis A. 
     In this rotary interface, the outer annular element is configured for rotation around the rotation axis A relative to the group of inner annular elements. 
     The rotary interface comprises a group of inner annular elements  12 ,  14 ,  16  and an outer annular element  18 . The group of inner annular elements comprises a lower annular element  12 , a central annular element  14  and an upper annular element  16 . 
     Each of inner annular elements  12 ,  14 ,  16  has a cylindrical inner surface at an inner radius RI 1 . The lower and upper annular elements  12 ,  16  each have a cylindrical outer surface at a first outer radius RO 1 . The central annular element  14  has a cylindrical outer surface at a second outer radius RO 2 . The second outer radius RO 2  is smaller than the first outer radius RO 1 . 
     When the inner, central and outer annular elements are stacked on each other, with the central annular element  14  positioned between the lower annular element  12  and the upper annular element  16 , a circular trench is present between facing surfaces  12   a ,  16   a  of the lower annular element and the upper annular element, which surfaces are substantially perpendicular to the rotating axis A. 
     The outer annular element  18  has a cylindrical inner surface at a second inner radius RI 2  and a cylindrical outer surface at a third outer radius RO 3 , and is also centered around the rotation axis A. The second inner radius RI 2  of the outer annular element  18  is somewhat larger than the second outer radius RO 2  of the central inner annular element  14  in such a way there is a radial gap G (see  FIG. 2  and  FIG. 3 ) of about 1 mm between the central inner annular element and the outer annular element. The outer annular element  18  is arranged in the circular trench between the lower and the upper inner annular elements  12 ,  16  such that the cylindrical inner surface  22  of the outer annular element abuts the outer cylindrical surface  20  of the central inner annular element. Along the circumference of the cylindrical inner surface of the outer annular element a recess  24  is provided such that a toroidal chamber  24   a  is formed between the cylindrical outer surface  20  of the central inner annular element and the cylindrical inner surface  22  of the outer annular element  18 . 
     In the outer annular element  18 , a conduit  26  in radial direction is provided between the recess  24  and the outer circumference to provide an outlet (or inlet) of the toroidal chamber  24   a.    
     The upward surface  12   a  of the lower annular element  12  and the downward surface  16   a  of the upper annular element  16  that each extend outward beyond the cylindrical outer surface  20  of the central annular element  14  at the second outer radius RO 2 , are provided with a first stepped surface. The outer annular element  18  has upward and downward surfaces  18   a ,  18   b  with a second stepped surface that is complementary to the first stepped surface. 
     Between the downward facing surface  16   a  of the upper annular element  16  and the upward facing surface  18   b  of the outer annular element  18  and between the upward facing surface  12   a  of the lower annular element  12  and the downward facing surface  18   a  of the outer annular element  18 , annular cavities  28 ,  30 ,  32 ,  34  are formed in which face seal type seal rings are arranged (not shown here). The arrangement of the annular cavities and the seal rings will be described in more detail with reference to  FIG. 2 . 
     Referring to  FIG. 1 , the swivel stack can comprise a plurality of rotary interfaces. To simplify stacking, the lower and upper annular elements  12 ,  16  are designed with a same shape of their upper and lower surfaces  12   a ,  16   a  (perpendicular to the rotation axis A). In this manner an upper annular element  16  of a first rotary interface can be used as a lower annular element  12  of a second rotary interface stacked on top of the first rotary interface. 
     In between each pair of stacked outer annular elements a coupling ring or drive ring  36  is placed. The coupling rings are configured to couple the outer annular element pairwise for joined rotation. One of coupling rings is fitted with a pair of lugs (not shown) designed to receive a pair of drive arms  38  for coupling to an external structure (not shown) on the floating structure (not shown). 
     The swivel stack  10  comprises a base annular element  40  on which a lower support annular element  42  is arranged. The lower support annular element  42  has an upward facing surface  42   a  that has an identical shape as an upward facing surface  12   a  of the lower annular element  12  as described above. The lower support annular element  42  has a stepped upward facing surface on which an outer annular element  18  can be arranged in a manner that annular cavities are present in between the annular elements  42 ,  18 . 
     The base annular element  40  is provided on its circumference with inlet ports  44  (and/or outlet ports) in radial direction that are each configured to be coupled with an incoming flow-line  46  or outgoing flow-line, respectively. Further, each inlet or outlet port  44  is coupled within the base annular element  40  with a conduit  48  extending upward and parallel to the rotation axis A. 
     Each of the inner annular elements  12 ,  14 ,  16  is provided with at least one through-hole  50  for transport of (hydrocarbon) fluids parallel to the rotation axis A. In each inner annular element  12 ,  14 ,  16  the at least one through-hole  50  is positioned at a location corresponding with the location of the conduit  48  in the base annular element  40 . 
     On the top of the swivel stack  10 , a closing annular element  52  can optionally be arranged. The closing annular element  52  can also function as a support or (fluid) connector for an other swivel stack located above. 
     Within the swivel stack  10 , the inner annular elements  12 ,  14 ,  16  are joined between the base annular element  40  and the closing annular element  52  by a plurality of bolted rods  54  extending through holes arranged on a pitch diameter DP of the annular elements. A more detailed illustration will be given below with reference to  FIG. 2 . 
       FIG. 2  shows a detailed cross-section of the rotary interface of a swivel stack according to an embodiment of the invention. 
     The first and second stepped surfaces have a layout in which four annular cavities  28 ,  30 ,  32 ,  34  are centered around the rotation axis A. The first and second stepped surfaces have a rectangular cross-section and are provided between the central outer annular element  18  and the lower and upper inner annular elements  12 ,  16 . Two of the annular cavities  28 ,  30  are arranged at a first interface A 1  between the facing surfaces  16   a ,  18   b  of the upper annular element  16  and the outer annular element  18 . The other two annular cavities  32 ,  34  are arranged at a second interface A 2  between the facing surfaces  12   a ,  18   a  of the lower annular element  12  and the outer annular element  18 . In each of the annular cavities  28 ,  30 ,  32 ,  34  a face seal type seal ring  56  is installed. 
     In  FIG. 2 , a portion of an arrangement of the lower annular element  12 , the central annular element  14 , the upper annular element  16  in combination with the outer annular element  18  is shown. Also, a portion of a bolted rod  54  is shown, extending through the lower, central and upper annular elements. 
     Above and below the toroidal chamber  24   a , the annular cavities  28 ,  30 ,  32 ,  34  are arranged at a first and second interface A 1 , A 2  between the outer annular element  18  and the upper annular element  16 , and the central annular element  14  and the lower annular element  12 , respectively. 
     At both first and second interfaces A 1 , A 2  an hydraulic area H, i.e., a radial area H where the annular cavities and corresponding seal rings are located, is kept minimal. That is, each of the annular cavities has a radial width W and each seal ring has a corresponding width when placed in the associated annular cavity. Between the two annular cavities  28 ,  30 ;  32 ,  34  in each of the first interface A 1  and the second interface A 2  a non-zero interspace X is arranged. Thus the radial area H has a width equal to the width W of the two annular cavities  28 ,  30 ; 32 ,  34  plus the interspace X in the same interface A 1 ; A 2 . 
     In addition, to keep the hydraulic area H minimal, in each interface one of the two annular cavities  28 ;  32  is arranged directly adjacent to the cylindrical outer end surface  15  of the central annular element  14 . 
     A radial width B of the toroidal chamber  24   a  is equal to or smaller than a height C of the toroidal chamber  24   a . Additionally, the radial width B of the toroidal chamber is smaller than the width of the hydraulic area H (i.e., the width W of the two annular cavities combined  28 ,  30 ; 32 ,  34  plus the interspace X in the same interface A 1 ; A 2 ). At the same time, the inner diameter of the radially inner seal ring is substantially equal to the inner diameter of the toroidal chamber. When the toroidal chamber is under operational pressure, these measures have the effect that the vertical force F 2  on the seals in the first and second interfaces A 1 ; A 2  is smaller than the vertical force F 1  on the upper and lower radial walls of the toroidal chamber  24   a.    
     In addition, in each of the upward and downward facing surfaces  18   a ,  18   b  of the outer annular element  18 , a bushing  58  (plain bearings) is positioned outside the hydraulic area H, thus at larger radius than the radius of the seal rings  56 . 
     In an embodiment, the bolted rods  54  are positioned at a distance to the second outer radius RO 2  so as to leave 5 to 10 mm gap between the bolt hole and an O-ring seal groove  60  holding an O-ring seal ensuring tightness between the central annular element  14  and the lower/upper inner annular element  12 ;  16 . To achieve this, the centre of each rod is located towards the outer radius RO 1  and away from the inner radius RI 1  in the central annular element. 
     In this manner, the clamping force on each of the inner annular elements  12 ,  14 ,  16  is mainly acting on the O-ring seal groove  60  to limit opening and risk of extrusion of the O-ring seal out of the O-ring seal groove  60  when associated flow lines and toroidal chamber in the swivel stack  10  are under high operating pressure. This technique translates the attempt to minimize the prying effect on the bolted rods (see below). 
     It is recognized that a pressure induced end cap force is acting on the hydraulic area H—from the inner diameter of the O-ring seal groove  60  to an outer diameter of the outer annular cavity groove (holding the face seal). This force tends to open both the interface between the central annular element and lower annular element and the interface between the central annular element and upper annular element. The pressure induced end cap force is amplified by the ratio of the distance of the hydraulic area H to the inner radius RI 1  to the distance of the bolt  54  to the same inner radius RI 1  (prying effect): obviously the larger the distance of the bolt  54  to inner radius RI 1 , the smaller the tensile load on the bolt  54 . This property is vital for this type of design. 
     According to an embodiment, the first and second stepped surfaces have a layout in which at least two annular cavities  28 ,  30 ,  32 ,  34  are centered around the rotation axis A with half of the at least two of the annular cavities located at a first interface between the downward facing surface of the upper annular element and the upward facing surface of the outer annular element and the other half of the at least two annular cavities located at a second interface between the upward facing surface of the lower annular element and the downward facing surface of the outer annular element, respectively. 
       FIG. 3  shows a detailed cross-section of the rotary interface of a swivel stack according to an embodiment of the invention. 
     The rotary interface shown in  FIG. 3  is largely identical to the rotary interface of  FIG. 2 . Features with identical reference sign as in  FIG. 2  will not be described here. 
     In the interface shown in  FIG. 3 , additional recesses  62  are provided in the outer annular element  18  on either side of the toroidal chamber  24   a  to fit two additional seal rings  64 , piston orientated. The additional seal rings  64  may be referred to as isolation seals, providing further isolation of the toroidal chamber  24   a  in the circumferential direction of the rotary interface. 
       FIGS. 4 a  and 4 b    show details of a stab-in tube  66  for use in a swivel stack  10  according to an embodiment of the invention. 
     In the through-holes  50  extending through the inner annular elements  12 ,  14 ,  16  of the swivel stack  10  a stab-in tube  66  can be placed to provide a conduit that runs between the base annular element  40  and the central annular element  18  associated with the respective through-hole connected to radial conduit  26  of the central annular element. 
     The stab-in tube  66  has closed end caps  68  and is provided with openings  70   a ,  70   b  in its side wall at the level of the radial conduit  44  of the base annular element  40  and the level of the radial conduit  26  in the central annular element  18 , respectively. 
     In this manner a flow path for the fluid flowing through the stab-in tube is obtained that is leak-proof. 
       FIG. 4A  shows a cross-section of the stab-in tube  66 .  FIG. 4B  shows a cross-section of a swivel stack with a stab-in tube  66  mounted in the through-holes  50  of the inner annular elements  12 ,  14 ,  16 . 
       FIG. 5  shows a perspective view of a swivel stack according to an embodiment of the invention. 
     The swivel stack  10  depicted here, comprises the base annular element  40 , a number of rotary interfaces  72 ,  74 ,  76 ,  78  and a closing annular element  52 . 
     Further, the swivel stack  10  comprises a pair of coupling arms  30  attached to one of the coupling rings  36 . The coupling arms are configured to be coupled to a vessel (not shown) in which the swivel stack is mounted. The coupling arms provide a fixed orientation of the rotary interfaces with respect to the vessel. 
     On the outer cylindrical end surface of each rotary interface additional probing ports  80  can be present, which provide access to leak ports (not shown) in the hydraulic area H. 
       FIG. 6  shows a perspective view of a manifold block in accordance with an embodiment of the invention. 
     When the swivel stack  10  is arranged on a turret mooring system on a floating object such as a vessel, the ports  44  for incoming/outgoing fluid on the base annular element  40  of the swivel stack can be coupled to riser lines  82  attached to a manifold structure (earthbound part) within the turret of the turret mooring system. 
     According to an embodiment, a riser line  82  is equipped with a pipe flange  84 . The pipe flange  84  of the riser line  82  is then coupled to an associated port  44  on the base annular element  40  by a flange  88  of spool piece  86  or connecting tube. The spool piece  86  is equipped with a spool piece flange  89  that is configured for connecting to the pipe flange  84  of the riser line  82 . Spool pieces of different lengths and with different orientations of connectors on the flanges  88 ,  89  can be used for making a fluid connection between a riser line  82  and a port  44  for incoming/outgoing fluid. 
       FIG. 7  shows schematically an example of a floating offshore construction equipped with a swivel stack in accordance with an embodiment of the invention. 
     A floating production unit  1  such as an FPSO vessel, or in general an offshore vessel, is moored at a location at sea near a reservoir R in the seabed. Process equipment  2  on the vessel is shown schematically. 
     The floating production unit  1  is turret moored. In  FIG. 7 , according to an embodiment, the floating production unit is shown as turret moored, by means of a turret mooring system. 
     Turret mooring systems provide a turret mooring structure comprising a turret structure  3  such as a mooring buoy and a support structure mounted on either the outside or the inside of the floating production unit  1 . The turret structure  3  is anchored to the seabed with anchoring lines  5 . Riser lines  82  (and other lines and other equipments  90  such as umbilical lines, gas/water injection lines, electric power lines, valves/shutters, etc.) are extending between the reservoir R under the seabed and the turret structure  3 . The support structure, provided on the floating production unit  1 , has a receptacle for receiving the turret structure  3 , such that rotation of the floating production unit  1  about the turret structure  3  is still possible. In this manner, the floating production unit  1  can weathervane under influence of wind, waves, currents and/or drifting ice and adopt the position of least resistance with regard to the environment, while the riser lines remain at their unrotated position. 
     A swivel stack  10  according to an embodiment of the invention is arranged in the turret mooring system to provide one or more rotary interfaces between the riser lines  82  and process equipment  2  on the floating vessel. 
     According to an embodiment, the inner and outer annular elements are obtained from steel forgings and machined to the desired final shapes. The steel forgings may be based on carbon steel or stainless steel. 
     The invention has been described with reference to some embodiments. The swivel stack shown is described here by way of an example. Configurations with a different number of rotary interfaces can be constructed within the scope of the invention. 
     Obvious modifications and alterations will occur to the person skilled in the art upon reading and understanding the preceding detailed description, which is to be considered in all respects only as illustrative and not restrictive. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. 
     REFERENCES 
     
         
         
           
             Floating production unit  1   
             Processing equipment  2   
             Turret structure  3   
             Anchoring line  5   
             Swivel stack  10   
             Inner annular elements  12 ,  14 ,  16   
             Cylindrical outer end surface  15   
             Upward surface  12   a    
             Downward surface  16   a    
             Outer annular element  18   
             Downward facing surface  18   a    
             Upward facing surface  18   b    
             Outer cylindrical surface  20   
             Cylindrical inner surface  22   
             Recess  24   
             Toroidal chamber  24   a    
             Conduit  26   
             Annular cavities  28 ,  30 ,  32 ,  34   
             Coupling ring or drive ring  36   
             Pair of drive arms  38   
             Base annular element  40   
             Lower support annular element  42   
             Inlet/outlet ports  44   
             Flow-line  46   
             Through-hole  50   
             Closing annular element  52   
             Bolt  54   
             Face seal type seal ring  56   
             Bushing  58   
             O-ring seal groove  60   
             Additional recess  62   
             Isolation seal  64   
             Stab-in tube  66   
             Opening  70   a ,  70   b    
             Rotary interface  72 ,  74 ,  76 ,  78   
             Probing port  80   
             Riser line  82   
             Pipe flange  84   
             Spool piece  86   
             Spool piece flange  88   
             Equipment  90   
             Interface A 1 , A 2   
             Rotating axis A 
             Pitch diameter DP 
             Radial gap G 
             Hydraulic area H 
             Radial width of chamber B 
             Height of chamber C 
             Vertical force F 1 , F 2   
             Reservoir R 
             First outer radius RO 1   
             Second outer radius RO 2   
             Third outer radius RO 3   
             First inner radius RI 1   
             Second inner radius RI 2   
             Radial width W 
             Interspace X