Abstract:
A fluid swivel including a stationary inner housing assembly and a rotatable outer housing operatively connected to the inner housing assembly. The outer housing includes an annular passage with upper and lower surfaces. Upper and lower seals are positioned between the inner housing assembly and the outer housing in seal recesses. A passage in the inner housing assembly provides fluid of pressure to the annular passage. The fluid exerts a first force axially inward on outer housing outer surfaces, and a larger second force axially outward on the annular passage&#39;s upper and lower surfaces. The second force is greater than the first force so upper and lower ring portions deflect outward such that the elastic axial deflections of the inner housing assembly match the axial deflections of the outer housing, thereby causing the axial clearances between the components at the upper and lower seals to remain almost the same.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of co-pending U.S. patent application Ser. No. 14/178,106, filed Feb. 11, 2014. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    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. 
         [0004]    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. 
         [0005]    2. Description of the Related Art 
         [0006]    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. 
         [0007]    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. 
         [0008]    Separate swivel assemblies are stacked on top of each other with a swivel base fixed to a stationary frame anchored to the sea floor. 
         [0009]    Prior high pressure fluid swivels have provided an inner housing and an outer housing which is rotatably supported on the inner housing by a bearing so that the outer housing is free to rotate about the inner housing. An annular conduit chamber or passage 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 co-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. 
         [0010]    When high pressure is present in the inlet and passes through the annular passage and out the outlet, the pressure in the passage 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. 
         [0011]    As the pressure of flowing fluid increases, the separation between the facing surfaces in which the seals are placed increases. Such separation can be large enough, due to the high fluid pressures, so as to prevent leak-free operation of the swivel at the high pressures by seal extrusion failure. 
         [0012]    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 extrusion gap control. 
         [0013]    The prior art discloses swivels that use exterior pressure sources to apply balancing or “barrier” 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 passage in combination with active pressure compensation. 
         [0014]    U.S. Pat. No. 4,555,118 to Salinger discloses, at  FIG. 4 , a free floating anti-extrusion ring placed above and below an annular passage between inner and outer rings. The free floating anti-extrusion 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 annular 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. 
         [0015]    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 radial 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. 
         [0016]    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. 
         [0017]    A primary aspect 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 recesses formed between inner and outer housings. 
         [0018]    Another aspect 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. 
         [0019]    Another aspect 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. 
         [0020]    Another aspect 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. 
         [0021]    Another aspect of the invention is to provide dynamic upper and lower seal recesses in the outer housing with face seals placed therein where the seal recesses are arranged concentrically outward from the axial center line of the fluid swivel, with the shape of an annular passage in the outer housing compensating for the tendency of clearances behind the seals to open wider axially as high pressure acts inside the swivel. 
         [0022]    Another aspect 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. 
         [0023]    Another aspect of the invention is to size or proportion the dimensions of the structural components of the swivel such that the elastic axial deflections of the tightly assembled together inner housing and seal plate will match the axial deflections of the outer housing&#39;s upper and lower ring portions, thereby causing the axial clearances between the parts at the upper and lower seals to remain almost the same. 
       SUMMARY OF THE INVENTION 
       [0024]    A preferred embodiment of the present invention is a rotatable high pressure sealed joint for a fluid swivel having an outer housing and an inner housing assembly comprising an inner housing and a seal plate. The outer housing includes a radially inward-facing annular passage internally pressurized with high pressure fluid. The radially inward-facing annular passage defines upper and lower ring portions of the outer housing. Two or more dynamic seals are disposed above the upper ring portion and two or more dynamic seals are disposed below the lower ring portion of the outer housing. Dynamic seals are contained by seal grooves or recesses in the outer housing and are in compressed contact with the opposing flat annular surfaces on the seal plate and inner housing. The pressurized internal surfaces of the outer housing annular passage are designed and arranged with respect to the pressurized outer surface area of the upper ring portion and pressurized outer surface area of the lower ring portion so that the upper and lower ring portions elastically deflect upward and downward, respectively, with increasing pressure to compensate for the axially outward elastic deflection of the tightly assembled together inner housing and seal plate. The upward and downward deflection of the outer housing ring portions is the result of having more axial pressure force urging the rings to expand vertically outward than the inward compression pressure force on the smaller radial width of the pressure area between the seal inside diameter and inside diameter of the inner housing. The desired minimum axial net resulting force acting on the outer housing is established by the selection of effective seal diameters and radial and axial dimensions of the annular passage. 
         [0025]    In other embodiments, the seal recesses and their opposite flat annular sealing surfaces can be arranged in other combinations with the same sealing function result such as locating some or all of the seal recesses on the inner housing and some or all on the seal plate. The mating annular sealing surface for each seal is then located on the opposite housing or seal plate. 
         [0026]    The rotatable fluid swivel is further characterized by the outer housing having a radial flow outlet with an internal diameter in communication with the annular passage. The outer housing annular passage is characterized by an internal height and a width. The internal height of the annular passage is sized to be from 50% to 60% of the internal diameter of the radial flow outlet. The width of the annular passage is sized to be about twice the internal height of the annular passage. So sized and arranged, the height of the radial pressure area, bounded by the upper and lower dynamic seals on the outer housing, minimizes the radial pressure force thereby reducing the required outside diameter and weight of the outer housing. Likewise the inward radial pressure force acting on the inner housing is reduced which beneficially reduces the compressive stresses and elastic deflections of the inner housing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    The 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: 
           [0028]      FIG. 1  is a cross section of an illustrative swivel stack assembly with two swivels, each according to a preferred embodiment of the invention, stacked on a swivel 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; 
           [0029]      FIG. 2  is a top view of the swivel stack assembly of  FIG. 1  with section line  1 - 1  indicating the cross-sectional view of the swivel stack assembly of  FIG. 1 ; 
           [0030]      FIG. 3  is a perspective view in cross-section of a swivel according to an embodiment of the invention; 
           [0031]      FIG. 4  is an enlarged sectional view showing stair stepped dynamic face seals between inner and outer housings with an annular passage in the outer housing, with arrows showing forces on the swivel components caused by pressure in the swivel; and 
           [0032]      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 
       [0033]    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, 2 
               
               
                 10A 
                 Upper swivel 
                 1, 3, 4, 5 
               
               
                 10B 
                 Lower swivel 
                 1 
               
               
                 15A 
                 Upper swivel inner housing assembly 
                 1, 4 
               
               
                 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 
               
               
                 18A 
                 Upper ring portion 
                 1, 4, 5 
               
               
                 19A 
                 Lower ring portion 
                 1, 4, 5 
               
               
                 20A 
                 Upper swivel outer housing 
                 1, 3, 4, 5 
               
               
                 20B 
                 Lower swivel outer housing 
                 1 
               
               
                 21A 
                 Upper swivel upper axial bearing 
                 1, 3, 4 
               
               
                 21B 
                 Lower swivel upper axial bearing 
                 1 
               
               
                 22A 
                 Upper swivel lower axial bearing 
                 1, 3, 4 
               
               
                 22B 
                 Lower swivel lower axial bearing 
                 1 
               
               
                 23A 
                 Upper swivel upper radial bearing 
                 1, 3, 4 
               
               
                 23B 
                 Lower swivel upper radial bearing 
                 1 
               
               
                 24A 
                 Upper swivel lower radial bearing 
                 1, 3, 4 
               
               
                 24B 
                 Lower swivel lower radial bearing 
                 1 
               
               
                 26A 
                 Upper inner housing passage 
                 1 
               
               
                 26B 
                 Lower inner housing passage 
                 1 
               
               
                 30A 
                 Upper swivel annular passage 
                 1, 3, 4 
               
               
                  30A D   
                 Upper swivel annular passage internal 
                 1 
               
               
                   
                 diameter 
               
               
                  30A H   
                 Upper swivel annular passage internal 
                 1 
               
               
                   
                 height 
               
               
                     30A W   
                 Upper swivel annular passage radial width 
                 1 
               
               
                 32A 
                 Upper swivel radial flow outlet 
                 1, 2, 3 
               
               
                 40     
                 First upper dynamic seal 
                 3, 4, 5 
               
               
                 40A 
                 First upper seal recess 
                 4 
               
               
                 41     
                 First lower dynamic seal 
                 3, 4, 5 
               
               
                 41A 
                 First lower seal recess 
                 4 
               
               
                 42     
                 Second upper dynamic seal 
                 3, 4, 5 
               
               
                 42A 
                 Second upper seal recess 
                 4 
               
               
                 43     
                 Second lower dynamic seal 
                 3, 4, 5 
               
               
                 43A 
                 Second lower seal recess 
                 4 
               
               
                 44     
                 Third upper dynamic seal 
                 3, 4, 5 
               
               
                 44A 
                 Third upper seal recess 
                 4 
               
               
                 45     
                 Third lower dynamic seal 
                 3, 4, 5 
               
               
                 45A 
                 Third lower seal recess 
                 4 
               
               
                 46     
                 Fourth upper dynamic seal 
                 3, 4, 5 
               
               
                 46A 
                 Fourth upper seal recess 
                 4 
               
               
                 47     
                 Fourth lower dynamic seal 
                 3, 4, 5 
               
               
                 47A 
                 Fourth lower seal recess 
                 4 
               
               
                 52     
                 Swivel spacer 
                 1 
               
               
                 52A 
                 Upper clearance gap 
                 4, 5 
               
               
                 52B 
                 Lower clearance gap 
                 4, 5 
               
               
                 54     
                 Base manifold 
                 1 
               
               
                 56     
                 Manifold connection pipes 
                 1 
               
               
                 58A 
                 Upper swivel manifold passage 
                 1 
               
               
                 58B 
                 Lower swivel manifold passage 
                 1 
               
               
                 60A 
                 Upper pipe 
                 1, 2 
               
               
                 60B 
                 Lower pipe 
                 1, 2 
               
               
                 61     
                 First upper annular surface 
                 4 
               
               
                 62     
                 First lower annular surface 
                 4 
               
               
                 63     
                 Second upper annular surface 
                 4 
               
               
                 64     
                 Second lower annular surface 
                 4 
               
               
                 70     
                 Threaded bolts 
                 1 
               
               
                 72     
                 Threaded bolts 
                 1 
               
               
                 98     
                 First static seal 
                 4, 5 
               
               
                 99     
                 Second static seal 
                 4, 5 
               
               
                 100      
                 Swivel base 
                 1 
               
               
                 105      
                 Clearance gap 
                 4 
               
               
                 120      
                 Torque reaction arm 
                 2 
               
               
                 200      
                 Swivel stack 
                 1, 2 
               
               
                 A 1   
                 Lateral component of the circumferential 
                 4 
               
               
                   
                 sealed area of the outer housing 
               
               
                 D P   
                 Internal diameter of radial flow outlet 
                 1 
               
               
                 F H   
                 Horizontal force acting on outer housing 
                 4 
               
               
                 F v1   
                 Inward vertical force acting on outer 
                 4 
               
               
                   
                 housing 
               
               
                 F v2   
                 Outward vertical force acting on outer 
                 4 
               
               
                   
                 housing 
               
               
                 H eff   
                 Effective height 
                 5 
               
               
                 P 
                 Pressure in annular passage 
                 4, 5 
               
               
                   
               
             
          
         
       
     
         [0034]    The invention is for a novel high pressure swivel where two of the novel swivels are illustrated in  FIG. 1  vertically secured together. 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 three or more swivels may be stacked together. 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 , or other fastening means such as segmented clamps, extending from the upper swivel  10 A into and through lower swivel  10 B, through swivel spacer  52  and into base manifold  54 . 
         [0035]    The swivel base  100  is fixed to a substantially geostationary point (not shown) of an offshore mooring terminal. Manifold connection pipes  56 , arranged and designed to carry high pressure fluids, are connected to the base manifold  54  and fluidly communicate with manifold passages  58 A,  58 B of base manifold  54 . Of course other connection pipes can be provided about the circumference of the base manifold  54  to provide fluid communication to other swivels in a stack. An upper pipe  60 A provides fluid communication between base manifold passage  58 A and upper inner housing passage  26 A of upper swivel  10 A. Lower pipe  60 B provides fluid communication between base manifold passage  58 B and lower inner housing passage  26 B of lower swivel  10 B. Pipes  60 A,  60 B are spaced about a longitudinal axis  5  of the stack of swivels. 
         [0036]    Upper swivel  10 A is constructed with an upper swivel inner housing assembly  15 A having an inner housing  16 A and a seal plate  17 A attached to the top of the inner housing  16 A by means of threaded bolts  72 . Upper swivel outer housing  20 A is rotatively carried on the inner housing assembly  15 A by means of upper swivel upper and lower axial bearings  21 A and  22 A, and upper swivel upper and lower radial bearings  23 A and  24 A. 
         [0037]    As illustrated in  FIG. 1 , the lower swivel  10 B may be constructed similarly to upper swivel  10 A, with the two swivel assemblies connected together. Lower swivel  10 B may be constructed much like upper swivel  10 A with lower swivel upper and lower axial bearings  21 B and  22 B, and lower swivel upper and lower radial bearings  23 B and  24 B, providing rotational support of the lower swivel outer housing  20 B to the lower swivel inner housing  16 B and 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 manifold  54 , by threaded bolts  70  or other means. 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  200  according to the present invention may have additional swivels, each sharing features and characteristics in common with those discussed herein. 
         [0038]    In the upper swivel  10 A, a radial flow outlet  32 A in the upper swivel outer housing  20 A is radially aligned with an annular passage  30 A, which extends circumferentially about the upper swivel outer housing  20 A. When upper swivel outer housing  20 A turns about upper swivel inner housing assembly  15 A, annular passage  30 A is always in fluid communication with the inner housing passage  26 A of the upper swivel  10 A, which fluidly connects to pipe  60 A and base manifold passage  58 A. Referring to  FIG. 1 , the annular passage  30 A has an outer diameter  30 A D  and an internal height  30 A H  sized to be from 50% to 60% of an internal diameter D P  of the radial flow outlet  32 A. A radial width  30 A W  of the annular passage  30 A, as measured from a radially inward-facing annular surface of the outer housing  20 A, is sized to be about twice the internal height  30 A H  of the annular passage  30 A. 
         [0039]      FIG. 2  is a top view of the swivel stack of an embodiment of the present invention. Pipes  60 A,  60 B are positioned 180° or any chosen angle from each other. A single radial flow outlet  32 A is illustrated for fluid communication with pipe  60 A, but multiple radial flow outlets can be provided about the outer housing. Torque reaction arms  120  may be mounted to the outer housings  20 A and  20 B. Preferably, the torque reaction arms  120  are connected to the weathervaning floating vessel by link arms (not shown) which forces the outer housings  20 A and  20 B to rotate with the floating vessel 
         [0040]      FIG. 3  is a perspective view of the upper swivel  10 A in cross-section 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 axial bearings  21 A and  22 A, as well as radial bearings  23 A and  24 A. Dynamic seal pairs  40 ,  42 ;  41 ,  43 ;  44 ,  46 ;  45 ,  47  provide sealing of the inner and outer housings in response to high pressure fluid in inner housing passage  26 A and annular passage  30 A. 
         [0041]      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 axial bearings  21 A,  22 A, and upper swivel radial bearings  23 A and  24 A. Static seals  98 ,  99  are positioned between inner housing  16 A and seal plate  17 A to prevent high pressure fluid in annular passage  30 A from passing therebetween. Inner housing assembly 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 seal recesses  40 A,  41 A, respectively. A backup second set of upper and lower annular dynamic face seals  42 ,  43  are placed in second upper and lower seal recesses  42 A,  43 A, which are of greater diameter than the first upper and lower seal recesses  40 A,  41 A. The first and second upper seals  40  and  42  sealingly engage a flat first upper annular surface  61  of the seal plate  17 A and the first and second lower seals  41  and  43  sealingly engage a flat first lower annular surface  62  of the inner housing  20 A. 
         [0042]      FIG. 4  further illustrates the forces on the upper swivel outer housing  20 A caused by high fluid pressure in annular passage  30 A, as well as the clearance gap  105  between upper swivel inner housing assembly  15 A and upper swivel outer housing  20 A.  FIG. 4  exaggerates clearance gap  105  to illustrate that high pressure acts against dynamic seals  40  and  41 , including against surfaces defining upper and lower clearance gaps  52 A and  52 B, to force dynamic seal  40  against first upper annular surface  61  and to force dynamic seal  41  against first lower annular surface  62 . 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 l  in  FIG. 4 . 
         [0043]    The radially inward-facing annular passage  30 A defines upper and lower ring portions  18 A and  19 A of the outer housing  20 A. The annular passage  30 A is shaped so as to cause the upper and lower ring portions  18 A and  19 A, which face opposite to the dynamic seals  40 ,  41 , to deflect outward and spread apart under the force of pressure P in the annular passage  30 A. See the arrows labeled “DEFLECTION DIRECTION” of  FIG. 4 . These deflections partially and sufficiently compensate for any detrimental upward deflection of the upper swivel seal plate  17 A above and lower deflection of the upper swivel inner housing  16 A below. 
         [0044]    The outward deflections of upper and lower ring portions  18 A,  19 A are caused by pressure P acting on the surfaces of annular passage  30 A. The circumferential area of the surfaces of the annular passage  30 A on which the pressure P acts, or circumferential passage area, is designed to be greater than the circumferential seal area A l  discussed above. The outward vertical force F v2 , which is generated by pressure P, acts to elastically deflect the upper and lower ring portions  18 A,  19 A 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 A outward and upward and the lower ring portion  19 A outward and downward. As a result, the metal-to-metal clearances of the seal recesses  40 A,  42 A to annular surface  61 , seal recesses  41 A,  43 A to annular surface  62 , seal recesses  44 A,  46 A to annular surface  63 , and seal recesses  45 A,  47 A to annular surface  64  remain substantially constant with pressure increasing to very high levels. Opposite from the pressurized side of each seal are small (almost too small to be easily seen in the figures) metal-to-metal spaces or gaps between stationary inner housing  16 A, or seal plate  17 A, and rotating outer housing  20 A. This gap is small, in the range of 0.4 to 1.2 millimeters, and it is crucial that these surfaces never close together to touch and cause very expensive damage to the metal parts. Also it is crucial that the gaps do not increase excessively under pressure such that the seals fail by high pressure extrusion into these gaps. 
         [0045]    A horizontal force F H  causes upper swivel outer housing  20 A to deflect outwardly but has little effect on the clearances at annular surfaces  61 ,  63  and  62 ,  64 . 
         [0046]      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 radially outwardly from the upper swivel inner housing components  16 A,  17 A. The effective height H eff  of the pressure area on which pressure P is acting is the height acting around annular passage  30 A of the outer housing and upper and lower clearance gaps  52 A and  52 B. The area is reduced by the placement of seals 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. 
         [0047]    Additional Features 
         [0048]    The invention embodied in the swivel illustrated in  FIGS. 1-5  is characterized by additional features, including: 
         [0049]    (1) The dynamic seals discussed above may include as primary seals the first upper seal  40 , first lower seal  41 , a second upper seal  42 , and second 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 third upper seal  44 , a third lower seal  45 , a fourth upper seal  46 , and fourth lower seal  47  may also be provided. The third and fourth upper seals  44  and  46  are placed in third and fourth upper seal recesses  44 A and  46 A, respectively, which are of greater diameter than the first and second upper seal recesses  40 A and  42 A. The third and fourth upper seals  44  and  46  sealingly engage a flat second upper annular surface  63  of the seal plate  17 A. The third and fourth lower seals  45  and  47  are placed in third and fourth lower seal recesses  45 A and  47 A, respectively, which are of greater diameter than the first and second lower seal recesses  41 A and  43 A. The third and fourth lower seals  45  and  47  sealingly engage a flat second lower annular surface  64  of the inner housing  16 A. 
         [0050]    (2) The static seals of  FIGS. 4 and 5  may include a primary static seal  98  and a secondary static seal  99 . 
         [0051]    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 . 
         [0052]    (3) The annular passage  30 A 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 radial flow outlet  32 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. 
         [0053]    While the invention has been described in detail above with reference to specific embodiments, it will be understood that modifications and alterations in the embodiments disclosed may be made by those practiced in the art without departing from the spirit and scope of the invention. All such modifications and alterations are intended to be covered. In addition, all publications cited herein are indicative of the level of skill in the art and are hereby incorporated by reference in their entirety as if each had been individually incorporated by reference and fully set forth.