Abstract:
A fluid swivel ( 10 A) that includes a stationary annular structure ( 16 A,  17 A) defining upper and lower annular seal slots ( 61, 62 ), and a rotatable outer housing ( 20 A) operatively connected to the stationary annular structure ( 16 A,  17 A), the outer housing defining a radial groove ( 30 ) that has upper and lower surfaces. Upper and lower seals ( 40, 41 ) are positioned between the stationary annular structure ( 16   a,    17   a ) and the rotatable outer housing ( 20   a ) and disposed in the upper and lower seal slots ( 61, 62 ). An inner fluid manifold ( 26 A) is disposed in the stationary annular structure ( 16 A,  17 A) and arranged to provide fluid of pressure P to the radial groove ( 30 ), the fluid exerting a first force (Fv 1 ) longitudinally inwardly on the outer surface of the outer housing ( 20 A), and a second force (Fv 2 ) longitudinally outwardly on the upper and lower surfaces of the radial groove ( 30 ), the second force (Fv 2 ) having a greater magnitude than the first force (Fv 1 ) so the outer housing ( 20 A) deflects outwardly, pushing the upper and lower seals ( 40, 41 ) into the upper and lower seal slots ( 61, 62 ).

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates in general to swivel equipment for transferring fluids. In particular, the invention relates to a fluid swivel joint for a swivel stack assembly adapted for transferring fluids between tankers, storage vessels and the like and one or more conduits beneath the ocean surface. The fluid of the swivel may be product such as hydrocarbons to be transferred from the seabed to a vessel or may be water or gas to be transferred from the vessel to the seabed for well stimulation. 
         [0003]    Still more particularly, the invention relates to a sealing arrangement for a fluid swivel joint which uses the mechanical design of the joint with the pressure of the fluid flowing through the joint to substantially prevent seal glands, and extrusion gaps in which dynamic seals are placed, from enlarging as a function of high pressure of the fluid commonly encountered on offshore loading terminals for oil and gas tankers. 
         [0004]    2. Description of the Prior Art 
         [0005]    The offshore search for oil and gas has greatly expanded in recent years and progressed into deep rough waters such as the North Sea. To facilitate production of oil and gas from remotely located offshore fields, complex mooring systems for offshore loading terminals, which serve as centralized production sites for the entire field, have been developed. Flexible fluid lines, called risers, extend from a subsea location to the mooring site to permit the transfer of fluids between a moored vessel and a subsea location. For example, certain fluid lines may be used to convey oil and gas into the floating vessel while other fluid lines may be used to inject liquids or gases back from the vessel into subsea wells for purpose of control, well stimulation, or storage. 
         [0006]    Floating vessels can be moored to a single point mooring system, which permits the vessel to weathervane and rotate 360° about a single mooring point. To permit the vessel to rotate and move freely without causing twisting or entanglement of the various risers to which the vessel is attached, it is necessary to provide a swivel mechanism to connect the fluid lines to the mooring site. Furthermore, since a plurality of risers are involved, it is necessary that swivels be stacked in order to have the capability of accommodating multiple fluid lines or risers. 
         [0007]    Separate swivel assemblies are stacked on top of each other with a swivel stack base fixed to a stationary frame anchored to the sea floor. 
         [0008]    Prior high pressure product swivels have provided an inner housing and an outer housing which is rotatively supported on the inner housing by a bearing so that the outer housing is free to rotate about the inner housing. A toroidally shaped conduit chamber is formed between the two housings when the two housings are placed in registration with each other. An inlet from the inner housing communicates with the chamber, and an outlet in the outer housing communicates with the chamber. Upper and lower dynamic seals in the form of face seals or radial seals are placed in grooves or gaps between axially opposed or radially opposed surfaces of the inner and outer housings to prevent fluid from leaking past the two facing surfaces while the high pressure fluid is present in the chamber. 
         [0009]    When high pressure is present in the inlet and passes through the toroidal chamber and out the outlet, the pressure in the chamber acts to separate the inner housing and the outer housing from each other. In other words, the inner housing is forced to contract radially inward as a consequence of the force generated by the fluid pressure acting on an effective area between the two dynamic seals; the outer housing is forced to expand radially outward by the force of the fluid pressure acting on an effective area between the upper and lower dynamic seals. Separation occurs between the facing surfaces as a result of high fluid pressure in the chamber. High pressure as used herein is meant to be at the level of 2,000 psi and above. 
         [0010]    As the pressure of flowing product increases, the separation between the facing surfaces in which the seals are placed increases. Such separation can be large enough, due to the high product pressures, so as to prevent leak-free operation of the product swivel at the high pressures by seal extrusion failure. 
         [0011]    Swivel component deformation has been the subject of much effort by prior developers. The prior art has considered the idea of adding more material to the swivel components so that deformation as a function of pressure—especially high pressure in the 5,000 to 10,000 psi range—will resist deflection. With high pressures, however, the swivel components, i.e., the inner and outer housings, become so large and heavy that they are disadvantageous from weight, cost, handling, and size standpoints, and without necessarily achieving the desired gap control. 
         [0012]    The prior art discloses swivels that use exterior pressure sources to apply balancing or “buffer” fluid pressure at the dynamic seal interface. Examples of such “active” pressure compensation for dynamic seal gap control are shown in U.S. Pat. No. 4,602,806 to Saliger; U.S. Pat. No. 4,669,758 to Feller et al., U.S. Pat. No. 5,411,298 to Pollack; U.S. Pat. No. 6,053,787 to Erstad et al., and U.S. Pat. No. 4,662,657 to Harvey et al. All of these patents disclose separate anti-extrusion rings above and below the annular fluid manifold in combination with active pressure compensation. 
         [0013]    U.S. Pat. No. 4,555,118 to Salinger discloses, at  FIG. 4 , a free floating anti-extrusion ring placed above and below a toroidal passage between inner and outer rings. The free floating antiextrusion ring is initially displaced (i.e., at zero pressure) from the inner joint ring by a small seal extrusion gap. In operation, the internal pressure of the pressurized fluid in the toroidal passage is transmitted to the outer side of the anti-extrusion ring such that the pressure differential across the seal presses the anti-extrusion ring against the outer surface of the inner ring. In other words, the seal extrusion gap width varies as a function of internal pressure. Metal to metal contact of the anti-extrusion ring with the annular surface of the inner ring can cause friction and scoring problems during operation. 
         [0014]    U.S. Pat. No. 4,819,966 to Gibb, at  FIGS. 2 ,  3 , and  4 , shows an annular ring having an annular groove which registers with the inlet of an inner housing. An annular chamber is formed outwardly in the annular ring such that upper and lower lips are created in the annular ring that face the exterior surface of the inner housing. The lips carry dynamic seals and are forced into sealing engagement about the cylindrical surface of the inner housing above and below the inlet when pressure is in the chamber. A constant seal gap is maintained as a function of pressure by proper shaping of the chamber and the ring and the lip. A lubricating system may also be provided for injecting a controlled fluid. 
         [0015]    U.S. Pat. No. 6,450,546 to Montgomery and Roy shows a sealed fluid joint for a fluid swivel in which a pressure balanced middle housing ring is mounted between an inner housing and outer housing ring. Pressure balance is achieved by providing an inner annulus chamber or cavity between the inner housing and middle housing ring and an outer annulus chamber or cavity between the middle and outer housing ring. Holes or passages through the middle housing ring fluidly connect the inner and outer chambers. Dynamic seals are placed in seal glands between the inner housing and middle housing ring. Static seals are placed in seal glands between the middle and outer housing rings. The arrangement transfers component deformation due to product fluid pressure from the dynamic seal interface to the static seal interface by exposing fluid product pressure to a smaller effective area at the dynamic seals on the inner side of the middle housing ring than an effective area at the static seals on the outer side of the middle housing ring. The counter forces generated by the product fluid pressure over two different effective areas on the middle housing ring deforms the middle housing ring radially in a predetermined direction and amount as a function of increasing pressure. Control of radial deformation of the middle housing ring is passive, because it depends on a geometrical arrangement of dynamic and static seals on both sides of the middle housing ring and is proportional to the product fluid pressure. 
       3. Identification of Objects of the Invention 
       [0016]    A primary object of the invention is to provide a fluid swivel arrangement that is capable of flowing high pressure product through it while minimizing product leaking past dynamic seal grooves formed between inner and outer housings. 
         [0017]    Another object of the invention is to provide a fluid swivel arrangement for a predetermined high pressure rating, path diameter, and arrangement and shape of the components that minimizes the swivel outside diameter, height, and weight. 
         [0018]    Another object of the invention is to provide shapes and arrangements of inner and outer housings with seals between them that minimize relative internal deflections so as to assume proper function and long life of the seals. 
         [0019]    Another object of the invention is to provide a fluid swivel arrangement with inner and outer housings arranged so that internal areas subjected to high fluid pressure are minimized, thereby allowing the parts to be reduced in size and weight. 
         [0020]    Another object of the invention is to provide dynamic upper and lower seal grooves in the outer housing with face seals placed therein where the seal grooves are oriented radially outwardly from the center line of the fluid swivel, with the shape of a radial groove cavity in the outer housing compensating for the tendency of clearances behind the seals to open wider as pressure acts inside the swivel. 
         [0021]    Another object of the invention is to match the bending stiffness of the inner housing with the bending stiffness of the outer housing such that elastic matching occurs with the inner and outer housings expanding about the same amount in the same direction when high pressure fluid is carried by the swivel, so that the inner and outer housings effectively move together with increasing pressure and the clearance between the parts remains almost the same. 
         [0022]    Another object of the invention is to provide upper and lower dynamic seals where the upper dynamic seals are characterized by a slightly larger diameter than the opposing lower dynamic seals, thereby providing a positive downward force to prevent the outer housing from floating upwardly, thereby preventing excessive vertical force of the seals against the surfaces of the outer housing. 
       SUMMARY OF THE INVENTION 
       [0023]    The objects identified above, as well as other advantages and features of the invention, are incorporated in a sealed fluid joint for a rotatable fluid swivel having inner and outer housings with a fluid pressure carrying radial annular groove cavity in the annular outer housing that faces upper and lower radial annular seal grooves, with face seals disposed therein which seal against surfaces of the inner housing. The areas in the radial annular groove cavity are designed and arranged with respect to the areas of the upper and lower radial annular seal grooves so that the outer housing deflects outwardly with increasing pressure to compensate for inner deflections of said face seals inwardly. 
         [0024]    The rotatable fluid swivel is further characterized by an inner fluid manifold in the inner housing that communicates with the radial annular groove cavity in the annular outer housing. The inner fluid manifold has a diameter D in , while the annular grove cavity is characterized by an internal height and a width. 
         [0025]    The internal height of the annular groove cavity is sized to be from 50% to 60% of the internal diameter of the inner fluid manifold. The width of the annular groove cavity is sized to be about twice the internal height of the annular groove cavity. So sized and arranged, the height of pressure area acting on the primary seal in the lateral annular seal groove is such that the diameter and weight of the outer housing is minimized, while also reducing the compressive load and stresses in the inner housing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    The objects, advantages, and features of the invention will become more apparent by reference to the drawings which are appended hereto, wherein like reference numbers indicate like parts, and wherein an illustrative embodiment of the invention is shown, of which: 
           [0027]      FIG. 1  is a cross section of an illustrative swivel stack assembly with two swivels, each according to the invention, stacked on a swivel stack base with one inlet of the base fluidly coupled to an outlet of one swivel and a second inlet of the base fluidly coupled to an outlet of a second swivel; 
           [0028]      FIG. 2  is a top view of the swivel stack assembly of  FIG. 1  with section line  1 - 1  indicating the cross section view of the top swivel of the assembly of  FIG. 1 ; 
           [0029]      FIG. 3  is an enlarged sectional view of a swivel according to the invention; 
           [0030]      FIG. 4  is an enlarged sectional view showing stair stepped dynamic face seals between inner and outer housings with a radial groove in the outer housing, with arrows showing forces on the swivel parts caused by pressure in the swivel; and 
           [0031]      FIG. 5  is another diagrammatic illustration of  FIG. 4  showing effective pressure height area in the swivel according to the invention. 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0032]    The aspects, features, and advantages of the invention mentioned above are described in more detail by reference to the drawings wherein like reference numerals represent like elements. The following table provides a list of reference numerals used throughout the specification and the features that they represent: 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
               
               
                   
                   
                 Figures where  
               
               
                 Reference  
                   
                 Reference 
               
               
                 Numeral 
                 Feature 
                 Numeral Appears 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 5 
                 Longitudinal axis 
                 1 
               
               
                 10A 
                 Upper swivel 
                 1, 3, 4, 5 
               
               
                 10B 
                 Lower swivel 
                 1 
               
               
                 16A 
                 Upper swivel inner housing 
                 1, 3, 4, 5 
               
               
                 16B 
                 Lower swivel inner housing 
                 1 
               
               
                 17A 
                 Upper swivel seal plate 
                 1, 3, 4, 5 
               
               
                 17B 
                 Lower swivel seal plate 
                 1 
               
               
                 18  
                 Upper ring portion 
                 4 
               
               
                 18A 
                 threaded bolts 
                 1 
               
               
                 19  
                 Lower ring portion 
                 4 
               
               
                 20A 
                 Upper swivel outer housing 
                 1, 3, 4, 5 
               
               
                 20B 
                 Lower swivel outer housing 
                 1 
               
               
                 21A 
                 Upper swivel upper bearing 
                 1, 3, 4 
               
               
                 21B 
                 Lower swivel upper bearing 
                 1 
               
               
                 22A 
                 Upper swivel lower bearing 
                 1, 3, 4 
               
               
                 22B 
                 Lower swivel lower bearing 
                 1 
               
               
                 23A 
                 Upper swivel radial bearing 
                 1, 3, 4 
               
               
                 23B 
                 Lower swivel radial bearing 
                 1 
               
               
                 26A 
                 Upper inner housing inlet 
                 1 
               
               
                 26B 
                 Lower inner housing inlet 
                 1 
               
               
                 30  
                 Radial groove 
                 1, 3, 4 
               
               
                 30A 
                 Radial flow outlet 
                 2 
               
               
                 40  
                 Upper dynamic seal 
                 3, 4, 5 
               
               
                 41  
                 Lower dynamic seal 
                 3, 4, 5 
               
               
                 42  
                 Backup upper dynamic seal 
                 3, 4, 5 
               
               
                 43  
                 Backup lower dynamic seal 
                 3, 4, 5 
               
               
                 44  
                 Upper secondary dynamic seal 
                 3, 4, 5 
               
               
                 45  
                 Lower secondary dynamic seal 
                 3, 4, 5 
               
               
                 46  
                 Backup secondary upper dynamic seal 
                 3, 4, 5 
               
               
                 47  
                 Backup secondary lower dynamic seal 
                 3, 4, 5 
               
               
                 52  
                 Swivel connectors 
                 1 
               
               
                 52A 
                 Upper shoulder 
                 4, 5 
               
               
                 52B 
                 Lower shoulder 
                 4, 5 
               
               
                 54  
                 Base housing 
                 1 
               
               
                 56  
                 Fixed spools 
                 1 
               
               
                 58A 
                 Upper swivel base inlet 
                 1 
               
               
                 58B 
                 Lower swivel base inlet 
                 1 
               
               
                 60A 
                 Upper spool 
                 1, 2 
               
               
                 60B 
                 Lower Spool 
                 1, 2 
               
               
                 61  
                 First upper annular slot 
                 4 
               
               
                 62  
                 First lower annular slot 
                 4 
               
               
                 63  
                 Second upper annular slot 
                 4 
               
               
                 64  
                 Second lower annular slot 
                 4 
               
               
                 70  
                 bolts 
                 1 
               
               
                 98  
                 Primary static seal 
                 4 
               
               
                 99  
                 Secondary static seal 
                 4 
               
               
                 100  
                 Swivel Base 
                 1 
               
               
                 105  
                 Space 
                 4 
               
               
                 120  
                 Pick-up arms 
                 2 
               
               
                 200  
                 Swivel stack 
                 1 
               
               
                 A 1   
                 Lateral component of the circumferential 
                 4 
               
               
                   
                 sealed area of the outer housing 
                   
               
               
                 D P   
                 Internal diameter of upper spool 
                 1 
               
               
                 F H   
                 Horizontal force 
                 4 
               
               
                 F v1   
                 Inward vertical force 
                 4 
               
               
                 F v2   
                 Outward vertical force 
                 4 
               
               
                 H eff   
                 Effective height 
                 5 
               
               
                 P 
                 Pressure in radial groove 
                 4, 5 
               
               
                   
               
             
          
         
       
     
         [0033]    The invention is for a novel high pressure swivel where two of the novel swivels are illustrated in  FIG. 1  stacked on top of each other. Two swivels, including upper swivel  10 A and lower swivel  10 B of swivel stack  200  are shown stacked on top of a fixed swivel base  100 . Two swivels are shown for illustrative purposes in  FIG. 1 , but a single swivel could be provided or more inner housing bases may be provided to accommodate several swivels. As shown in  FIG. 1 , an upper swivel inner housing  16 A, and a lower swivel inner housing  16 B are secured by bolts  70  extending from the top of upper swivel  10 A through lower swivel  10 B, through swivel connector  52  and into base housing  54 . 
         [0034]    The swivel base  100  is fixed to a substantially geostationary point (not shown) of an offshore mooring terminal. Fixed spools  56 , arranged and designed to carry high pressure fluids, enter upper and lower swivel base inlets  58 A,  58 B of base housing  54 . Of course other spools can be provided about the circumference of the base housing  54  to provide fluid communication to other swivels in a stack. An upper spool  60 A provides fluid communication between base inlet  58 A and upper inner housing inlet  26 A of upper swivel  10 A. Lower spool  60 B provides fluid communication between base inlet  58 B and lower inner housing inlet  26 B of lower swivel  10 B. Spools  60 A,  60 B are spaced about a longitudinal axis  5  of the stack of swivels. 
         [0035]    Upper swivel  10 A is constructed with an upper swivel inner housing  16 A having an upper swivel seal plate  17 A attached to its top by means of threaded bolts  18 A. Upper swivel outer housing  20 A is rotatively carried on inner housing components  16 A,  17 A by means of upper swivel upper and lower bearings  21 A,  22 A and upper swivel radial bearing  23 A. 
         [0036]    As illustrated in  FIG. 1 , the lower swivel  10 B may be constructed similarly to upper swivel  10 A, with the two swivel assemblies bolted together. Lower swivel  10 B may be constructed much like upper swivel  10 A with lower swivel upper and lower bearings  21 B and  22 B, and lower swivel radial bearing  23 B, providing rotational support of the lower swivel outer housing  20 B to the lower swivel inner housing  16 B and a lower swivel seal plate  17 B. As mentioned above, upper swivel  10 A and lower swivel  10 B are coupled together, and to the base housing  54 , by threaded bolts  70 . Although much of the following disclosure references upper swivel  10 A and its parts, it is to be understood that the features and principles discussed apply equally to the lower swivel  10 B. In addition, a swivel stack according to the present invention may have additional swivels, each sharing features and characteristics in common with those discussed herein. 
         [0037]    In the upper swivel  10 A, a radial flow outlet  30 A in the upper swivel outer housing  20 A is radially aligned with a radial groove  30 , which extends circumferentially about the upper swivel outer housing  20 A. When upper swivel outer housing  20 A turns about upper swivel inner housing members  16 A/ 17 A, radial groove  30  is always in fluid communication with the upper inner housing inlet  26 A, which fluidly connects to spool  60 A and base inlet  58 A. 
         [0038]      FIG. 2  is a top view of the swivel stack of an embodiment of the present invention. Spools  60 A,  60 B positioned 180° degrees from each other. A single radial flow outlet  30 A is illustrated for fluid communication with spool  60 A, but multiple radial outlet passages can be provided about the outer housing. Pick-up arms  120  may be mounted to the top of the swivel stack assembly. 
         [0039]      FIG. 3  is a perspective view of the upper swivel  10 A illustrating the upper swivel inner housing  16 A and upper swivel seal plate  17 A in registration with the upper swivel outer housing  20 A and rotatively supported thereto by bearings  21 A, and  22 A, as well as lower radial bearing  23 A. An upper radial bearing (not shown) can be provided corresponding to bearing  23 A. Dynamic seal pairs  40 ,  42 ;  41 ,  43 ;  44 ,  46 ;  45 ,  47  provide sealing of the inner and outer housing in response to high pressure fluid in inner fluid manifold  26 A and radial groove  30 . 
         [0040]      FIG. 4  shows an enlarged partial cross section of the upper swivel  10 A, including upper swivel inner housing components  16 A and  17 A, and upper swivel outer housing  20 A rotatively supported by upper swivel upper and lower bearings  21 A,  22 A, and upper swivel radial bearing  23 A. Static seals  98 ,  99  are positioned between inner housing  16 A and seal plate  17 A to prevent high pressure fluid in radial groove  30  from passing therebetween. Components  16 A and  17 A are static; they are bolted together and move together as one unit. A first set of upper and lower annular dynamic face seals  40 ,  41  are placed in first upper and lower annular slots  61 ,  62 . A backup secondary set of upper and lower annular dynamic face seals  42 ,  43  are placed in second upper and lower annular slots  63 ,  64 , which are of greater diameter than the first upper and lower annular slots  61 ,  62 . 
         [0041]      FIG. 4  further illustrates the forces on the upper swivel outer housing  20 A caused by high fluid pressure in radial groove  30 , as well as the space  105  between upper swivel inner components  16 A,  17 A and upper swivel outer housing  20 A.  FIG. 4  exaggerates space  105  to illustrate that high pressure acts behind dynamic seals  40  and  41 , including over upper and lower shoulders  52 A and  52 B, to force dynamic seal  40  into face seal slot  61  and to force dynamic seal  41  into face seal slot  62 . In other words, an inward vertical force F v1  is applied behind dynamic seals  40  and  41  toward the upper swivel outer housing  20 A by virtue of the fluid pressure. The total inward force F v1  is equal to the fluid pressure P times the circumferential sealed area of the outer housing, the lateral component of which is identified as A 1  in  FIG. 4 . 
         [0042]    The radial groove  30  is shaped so as to cause upper and lower ring portions  18 ,  19 , which face opposite to the dynamic seals  40 ,  41 , to deflect outward and spread apart under the force of pressure P in the radial groove  30 . See the arrows labeled “DEFLECTION DIRECTION” of  FIG. 4 . These deflections partially compensate for any detrimental deflections of the upper swivel seal plate  17 A above and the upper swivel inner housing  16 A below. 
         [0043]    The outward deflections of upper and lower ring portions  18 ,  19  are caused by pressure P acting on the surfaces of radial groove  30 . The circumferential area of the surfaces of the groove on which the pressure P acts, or circumferential groove area, is designed to be greater than the circumferential seal area discussed above. The outward vertical force F v2 , which is generated by pressure P, acts to deflect the upper and lower ring portions  18 ,  19  in opposition to inward force F v1 . The force F v2  is greater than F v1 . The net force F v2 −F v1  deflects the upper ring portion  18  outward and upward and the lower ring portion  19  outward and downward. As a result, the clearances of the seal slots  61 ,  63  and  62 ,  64  remain substantially constant with pressure increasing to very high levels. 
         [0044]    A horizontal force F H  causes upper swivel outer housing  20 A to deflect outwardly but has little effect on the clearances of the seal slots  61 ,  63  and  62 ,  64 . 
         [0045]      FIG. 5  is the same cross section of a portion of a swivel, but illustrates the shortened radial internal pressure height, or effective height H eff , on which fluid pressure in the swivel is acting. This tends to force upper swivel outer housing  20 A outwardly from the upper swivel inner housing members  16 A,  17 A. The effective height H eff  of the pressure area on which pressure P is acting is the height acting around the groove of the outer housing and upper and lower shoulders  52 A and  52 B. The area may be reduced by the placement of seals  40 ,  41  in a stair-step arrangement, bringing them as close together as practical. A reduction in the pressure area results in a reduction of the compressive load and stresses in the inner housing. Further, the reduced effective height H eff  reduces the radial force applied to the upper swivel outer housing  20 A so that its diameter and weight can be reduced, resulting in a lighter, smaller swivel. 
         [0046]    Additional Features 
         [0047]    The invention embodied in the swivel illustrated in  FIGS. 1-5  is characterized by additional features, including: 
         [0048]    (1) The dynamic seals discussed above may include as primary seals an upper seal  40 , a lower seal  41 , a backup upper seal  42 , and a backup lower seal  43  to seal between upper swivel inner housing components  16 A,  17 A and outer housing  20 A. See  FIGS. 4 and 5 . A secondary seal system, including an upper secondary seal  44 , a lower secondary seal  45 , a backup secondary upper seal  46 , and a backup secondary lower seal  47  may also be provided. 
         [0049]    (2) The static seals of  FIGS. 4 and 5  may include a primary static seal  98  and a secondary static seal  99 . 
         [0050]    Separate oil barrier systems may be provided on the primary dynamic seals  40 ,  42  and  41 ,  43  and on the secondary dynamic seals  44 ,  46 , and  45 ,  47 . 
         [0051]    (3) The upper dynamic seals  40 ,  42  and  44 ,  46 , that radially surround the inner housing components  16 A,  17 A, may be of a slightly larger diameter than the corresponding opposing lower dynamic seals  41 ,  43  and  45 ,  46 . Such larger diameter of the upper dynamic seals as compared to the lower dynamic seals produces a positive downward force that minimizes upper swivel outer housing  20 A from “floating” upward and excessively forcing the seals vertically. 
         [0052]    (4) The radial groove  30  in the upper swivel outer housing  20 A is wide and of short height, as illustrated in  FIG. 4 , with the height being about 50% to 60% of the internal diameter D P  of the inner housing inlet ( 26 A). As described above, the dynamic seals  40 ,  42 ;  41 ,  43 ;  44 ,  46 ;  45 ,  47  are preferably placed as close together as practical in a stair step arrangement. Such geometries contribute to the reduction of pressure height in the area acting on the upper swivel outer housing  20 A. (See, e.g., effective height H eff  in  FIG. 5 ). With a predetermined internal pressure rating, that pressure acts on a smaller pressure area, resulting in a smaller radial force. As a consequence, the diameter and weight of the outer housing is reduced, compared to prior swivels of the same pressure rating. The shorter effective pressure height H eff  area also reduces the compressive load and stresses in the upper swivel inner housing components  16 A and  17 A.