Abstract:
A hull-to-caisson interface connection assembly accommodates high tension loads between the hull and a caisson of a Spar-type platform, while facilitating angular flexing motion and constraining lateral and axial movement. The interface connection assembly includes an annular bearing shoulder at the top end of caisson, and a laminated bearing assembly mounted in the bottom of the hull and defining a passage through which the upper portion of the caisson passes, so that the caisson shoulder seats against the upper end of the bearing assembly. The bearing assembly includes a laminated structure of alternating steel and elastomeric flex elements, bonded to each other to flex together as a unit, rather than sliding relative to each other. The laminated structure of the bearing assembly supports the vertical tensile loads applied by the weight of the caisson on the hull, while also accommodating the angular loads applied between the caisson and the hull.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     Not Applicable  
       FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not Applicable  
       BACKGROUND OF THE INVENTION  
       [0003]     This invention relates to floating offshore storage, drilling or production platforms and more particularly, platforms of the Spar type. More specifically, the invention relates to an improved interface connection assembly between the Spar hull and a caisson extending downwardly therefrom.  
         [0004]     Floating platforms are typically employed in water depths of about 500 ft. (approximately 152 m) and greater, and are held in position over a seabed well site by mooring lines anchored to the sea floor, or by motorized thrusters located on the sides of the platform, or both. Although floating platforms are more complex to operate because of their movement in response to wind and water conditions, they are capable of operating in substantially greater water depths than are fixed platforms, and are also more mobile, and hence, easier to move to other well sites. There are several different types of known floating platforms, including so-called “drill ships,” tension-leg platforms (“TLPs”), “semi-submersibles,” and “Spar” platforms.  
         [0005]     Spar-type platforms comprise an elongate, substantially cylindrical (or multi-cylindrical) buoyant hull that supports one or more decks above the surface of the water when floating in an upright operating position, in which an upper portion of the platform hull extends above the waterline and a lower portion is submerged below it. Because part of the platform hull is above the water, it is subject to forces of wind, waves, and water currents which cause a constant movement of the platform. Generally, a Spar-type floating platform is designed to be installed for a 25 to 30 year service life.  
         [0006]     Despite their relative success, Spar-type platforms include some aspects that need improvement to accommodate various applications and/or a larger spectrum of environmental conditions. For example, in typical Spar designs and configurations, at least one tubular caisson extends downwardly from the bottom of the Spar hull. The caisson(s) may extend some distance below the Spar, or to the sea floor. Often the inside of the caisson will include a plurality of tubulars and/or control bundles. Because the caisson extends below the floating spar hull, there is typically relative, flexing motion at the interface connection area between the Spar hull and the caisson. Currently existing hull-to-caisson interface connections do not accommodate very high tensile loads, the bending/flexing relative motion between the spar hull and the caisson for the life of the installation, and the very demanding fatigue life. Since the wind, waves, and water currents are always present in various intensities, the hull/caisson interface connection must be designed to accommodate the very demanding fatigue life due to the high loads and constant relative motion.  
         [0007]     It would therefore be desirable for an improved Spar hull-to-caisson interface connection that accommodates high tension loads between the spar hull and the caisson while still facilitating angular, flexing motion (rotation) while also simultaneously constraining lateral movement and vertical axial movement. The result would be highly-improved fatigue characteristics of the interface between the Spar hull and the caisson.  
       SUMMARY OF THE INVENTION  
       [0008]     In accordance with the present invention, a Spar hull-to-caisson interface connection assembly is provided that accommodates high tension loads between the hull and caisson of a Spar-type platform, while facilitating angular, flexing motion (rotation) and simultaneously constraining lateral movement and vertical axial movement. Broadly, the interface connection assembly comprises an annular bearing shoulder provided at the top end of caisson, and a laminated bearing assembly mounted in the bottom end of the hull and defining a passage through which the upper portion of the caisson passes, so that the caisson shoulder seats against the upper end of the bearing assembly. The bearing assembly comprises a laminated structure of alternating steel and elastomer flex elements that are bonded to each other so that they flex together as a unit, rather than sliding relative to each other. The laminated structure of the bearing assembly supports the vertical tensile loads applied by the weight of the caisson on the hull, while also allowing a smooth and efficient accommodation of the angular and rotational loads applied between the caisson and the hull, so as to reduce fatigue at the hull/caisson interface.  
         [0009]     A more complete understanding of the invention can be obtained by reference to the following detailed description of the preferred embodiments thereof in connection with the attached drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is an elevational view of a Spar-type platform, of a type that may incorporate a hull/caisson interface connection in accordance with the present invention;  
         [0011]      FIG. 2  is an elevational view of the portion of  FIG. 1  enclosed within the dashed outline  2  in  FIG. 1 , partially broken away to show a Spar hull/caisson interface connection assembly in accordance with a first preferred embodiment if the present invention;  
         [0012]      FIG. 3  is a cross-sectional view taken along line  3 - 3  of  FIG. 2 ;  
         [0013]      FIG. 4  is a longitudinal cross-section view of a hull/caisson interface connection assembly according to the first preferred embodiment of the present invention;  
         [0014]      FIG. 5  is a cross-sectional view taken along line  5 - 5  of  FIG. 4 ;  
         [0015]      FIG. 6  is top perspective view of an elastomeric element of the hull/caisson interface connection assembly of  FIG. 4 ;  
         [0016]      FIG. 7  is a semi-schematic cross-sectional view of the hull/caisson interface connection assembly of  FIG. 4 , showing the relative flexing motion between the hull and the caisson facilitated by the present invention;  
         [0017]      FIG. 8  is a longitudinal cross-section view of a hull/caisson interface connection assembly according to a second preferred embodiment of the present invention; and  
         [0018]      FIG. 9  is a top plan view of the hull/caisson interface connection of  FIG. 8   
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     Referring first to  FIGS. 1-3 , an elevational view of a Spar-type offshore oil and gas drilling and production platform  10  is illustrated in  FIG. 1 , in which the platform  10  is shown floating upright in a deep body of water. The platform  10  comprises a hull  12 , a substantial portion of which is submerged below the surface  14  of the body of water. The hull  12  extends above the surface  14  of the water to support a deck  16  that carries drilling and production equipment, crew living quarters, etc., as is conventional. Fixed or variable ballast elements (not illustrated) may be disposed within the submerged portion of the hull  12  to lower the center of gravity of the platform  10  substantially below its center of buoyancy, thereby enhancing the stability of the platform  10  by increasing its natural period above the period of waves in the body of water. The hull  12  advantageously includes one or more helical strakes  18  that extend radially outwardly from the hull  12 , and that are arranged so as to define at least one generally helical band extending around the periphery of the submerged portion of the hull  12 . The purpose of the helical strake or strakes  18  is to prevent or minimize vortex-induced vibrations, as is well-known in the art.  
         [0020]     The platform  10  further comprises at least one cylindrical caisson  20  that extends vertically down from the hull  12 . The caisson  20  is coupled to the hull  12  by means of an interface bearing assembly  22  ( FIG. 2 ), in accordance with the present invention. The bearing assembly  22 , which will be described in detail below, is seated in an annular receptacle  24  mounted in the open bottom end of the hull  12 , as shown in  FIG. 2 . The bearing assembly  22  defines a central passage  23  (see  FIGS. 4 and 5 ) through which the upper portion of the caisson  20  passes as it enters the bottom of the hull  12  through a central opening  25  in the receptacle  24  (see  FIG. 4 ), so that the bearing assembly  22  is disposed coaxially around the upper portion of the caisson  20 . As will be discussed below, the diameter of the passage  23  increases from the upper end of the bearing assembly  22  to the lower end thereof. The caisson  20  may advantageously include a flared rim  26  around its upper end. The weight of the caisson  20  applies a vertical tensile load (indicated by arrow “A” in  FIG. 1 ) on the hull  12 , while wind, waves, and currents apply angular flexing loads (indicated by arrow “B” in  FIG. 1 ) between the hull  12  and the caisson  20 .  
         [0021]     Drilling, production, and export from the platform  10  all require vertical conduits through the water column between the seabed  28  and the platform  10 . These conduits are typically provided by a plurality of risers  30  that extend from the seabed  28 , upwardly through the caisson  20  and the hull  12  to the deck  16 . In a typical Spar-type platform, the risers  30  may be secured to the deck  16  by tensioning devices (not shown), or they may be supported on the deck and descend therefrom in a modified catenary shape to the seabed  28 . Other means, well-known in the art, may be employed to support individual risers, especially those of a smaller diameter.  
         [0022]     Turning now to  FIG. 4 , the interface connection assembly in accordance with a first preferred embodiment of the present invention includes an annular shoulder  32  extending radially outward from the top end of the caisson  20 . The bearing assembly  22  coaxially surrounds the upper portion of the caisson  20  just below the shoulder  32 . The uppermost part of the bearing assembly  22  is a steel top bearing ring  34 , disposed so that the bottom surface of the shoulder  32  rests on top of the top bearing ring  34 . The lowermost part of the bearing assembly  22  is a steel bottom support ring  36  that is seated in the receptacle  24  in the bottom end of the hull  12 , as described above. Arranged between the top bearing ring  34  and the bottom support ring  36  of the bearing assembly  22  is a plurality of flex elements, comprising a plurality of steel flex rings  38  alternating with, and separated by, elastomeric flex rings  40 . Both the steel flex rings  38  and the elastomeric flex rings  40  are formed in a “dished” or arcuate configuration, defined by a common radius of curvature, as are the bottom surface of the top bearing ring  34  and the top surface of the bottom support ring  36 . The steel rings  38 , the elastomeric rings  40 , the top bearing ring  34 , and the bottom support ring  38  are bonded together by well-known techniques. The flex elements  38 ,  40  are enclosed within an elastomeric outer sleeve  41 , preferably about 1-2 cm in thickness, that is fixed between the top bearing ring  34  and the bottom support ring  36 .  
         [0023]     The configuration of one of the steel flex rings  38  is illustrated in  FIG. 5 , which also shows the interior of the caisson  20  and the risers  30  passing through it. The configuration of one of the elastomeric flex rings  40  is shown in  FIG. 6 . The elastomeric flex rings  40  are preferably made of a nitrile copolymer of butadiene and acrylonitrile, marketed under the tradename “BUNA-N.” Other suitable elastomeric materials include copolymers of tetrafluoroethylene marketed by DuPont Performance Elastomers under the trademark “VITON®”, and by Seals Eastern, Inc. under the trademark “AFLAS®.” Other suitable elastomeric materials will suggest themselves to those of ordinary skill in the pertinent arts. The particular elastomer selected will depend on the environmental conditions to be encountered and the physical characteristics desired in the bearing assembly  22 . Thus, the elastomeric material may be selected for its specific physical characteristics, such as hardness and shear modulus. Likewise, the physical dimensions of the flex elements  38 ,  40  will be selected depending on the specific application and environment.  
         [0024]     Although the flex elements  38 ,  40  are shown, in this first exemplary embodiment, as being continuous annular elements, they may be configured as a plurality of discrete cylindrical flex element stacks disposed in an annular arrangement, as discussed below in connection with the embodiment shown in FIGS.  8  an  9 .  
         [0025]     From  FIG. 4  it can be seen that the inside diameter of each successive flex element  38 ,  40  (going from the upper end of the bearing assembly  22  to the lower end thereof) is slightly greater than the inside diameter of the flex element  38  or  40  immediately above it, thereby resulting in the diameter of the central passage  23  of the bearing assembly  22  increasing from the top of the bearing assembly  22  to the bottom thereof. This tapered configuration of the central passage  23  facilitates the relative angular motion between the caisson  20  and the receptacle, which is affixed to the hull  12 , and which supports the bearing assembly  22 , as mentioned above.  
         [0026]     In a preferred embodiment of the invention, there are preferably about ten to about thirty each of the steel rings  38  and the elastomeric rings  40 , vulcanized and bonded together (and to the upper bearing ring  34  and the lower support ring  36 ) by any suitable means known in the art to form a laminated structure in which the flex elements  38 ,  40  flex together, instead of sliding, in response to angular motions of the caisson  20  relative to the hull. By thus flexing, instead of sliding, the flex elements  38 ,  40  avoid the so-called “slip-stick” effect, in which relative motion between the caisson  20  and the hull  12  would only occur only when the static friction forces between adjacent flex elements are overcome by the angular flexing and bending loads to which the caisson  18  and the hull  12  are subject. This “slip-stick” effect would thus cause a “jerking” action, inducing erratic bending moments in the caisson, with a resultant reduction in the fatigue life of the hull/caisson interface. By eliminating this “slip-stick” effect, the bearing assembly  22 , with its laminated flex elements  38 ,  40 , starts to flex with any flex-inducing load applied to the caisson  20  and/or the hull  12 , instead of requiring a load that exceeds the friction forces between unbonded flex elements. Thus, the laminated (bonded) flex element arrangement in the bearing assembly  22  substantially eliminates erratic bending moments.  
         [0027]     The upper end of the caisson  20  may advantageously include a stress joint portion  42  where the caisson  20  joins the shoulder  32 . The stress joint portion  42  is formed with a tapered wall thickness that gradually increases as it approaches the shoulder  32  along a radiused juncture  44 . This feature provides an improved distribution of stress within a bending tubular member, such as the caisson  20 .  
         [0028]     The function of the hull/caisson interface connection assembly of the present invention is illustrated in  FIG. 7 . As mentioned above, the interface connection assembly comprises the shoulder  32  at the top end of the caisson  20  and the bearing assembly  22 . As the caisson  20  is subject to an angular flexing load, indicated by the arrow “B”, the caisson  20  pivots relative to a vertical axis  50 , bringing the caisson shoulder  32  to bear against the bearing assembly  22 , which flexes in response to the loads applied thereto by the shoulder, as indicated by the arrow “C”, to accommodate these loads. The tapered internal diameter of the bearing element central passage  23  provides the leeway for the pivoting action of the caisson  22 . Tensile loads along the axis  50 , indicated by the arrow “A” in  FIG. 1 , are likewise absorbed by the bearing assembly  22 .  
         [0029]     A hull/caisson interface connection assembly, in accordance with a second preferred embodiment of the invention, is illustrated in  FIGS. 8 and 9 . In this embodiment, a caisson  60  includes an annular shoulder  62  extending radially from the caisson  60  near the upper end thereof. The shoulder  62  includes a sloped or angled lower surface  64  that is provided with a plurality of upper sockets  66  that are equidistantly spaced in an annular arrangement. The upper end of the caisson  60  passes through a central opening  68  in an annular receptacle  70  mounted in the open bottom end of the hull  12 . The receptacle  70  includes a sloped or angled support surface  72  having a plurality of lower sockets  74 , each of which is circumferentially located so as to correspond with one of the upper sockets  66 . The lower sockets  74  are at a greater radial distance from a caisson central axis  76  than are the upper sockets  66 .  
         [0030]     A bearing assembly, comprising a plurality of substantially cylindrical flex element stacks or blocks  78 , is mounted between the lower surface  64  of the caisson shoulder  62  and the support surface  72  of the receptacle  70 , so as to surround coaxially the upper portion of the caisson  60  that is above the receptacle  70 . Specifically, each of the flex element stacks or blocks  78  has a steel top bearing element  77  that is seated in one of the upper sockets  66 , and a steel bottom support element  79  that is seated in a corresponding one of the lower sockets  74 , so that the stacks or blocks  78  are arranged around the periphery of the caisson  60  and extend radially outward therefrom, much like spokes on a wheel, as best shown in  FIG. 9 .  
         [0031]     As shown in  FIG. 8 , each of the flex element stacks or blocks  78  includes, between the top bearing element  77  and the bottom support element  79 , a laminated structure comprising a plurality of disc--like steel flex elements  80  alternating with a plurality of similarly-shaped elastomeric flex elements  82 . The flex elements  80 ,  82  may advantageously be concave or dish-shaped, when viewed from the top, with a common radius of curvature, thereby resembling small versions of the flex rings  38 ,  40  that are used in the above-described first embodiment, but without a central aperture. The flex elements  80 ,  82  may be made of materials that are the same as, or similar to, the annular flex elements  38 ,  40  of the above-described first embodiment, and they are vulcanized and bonded together by any suitable means, as discussed above in connection with the first embodiment. Each of the flex element stacks or blocks  78  is encased in an elastomeric sleeve  84 , which is similar to the sleeve  41  described above in connection with the first embodiment.  
         [0032]     Each of the flex element stacks or blocks  78  flexes as a unit, as does the above-described arrangement of annular flex element rings  38 ,  40  in the first embodiment. Furthermore, the above-described advantages of the first embodiment are also achieved in this second embodiment. Although eight flex element stacks or blocks  78  are shown in the illustrated embodiment, the number of the stacks or blocks  78 , their specific physical dimensions, and the number of individual flex elements  80 ,  82  forming each stack or block  78 , will vary according to the specific needs and demands of the Spar structure in which they are employed. One advantage of this second embodiment is that the use of multiple flex element stacks or blocks removes constraints on the size of the individual flex elements.  
         [0033]     Although an exemplary embodiment of the invention has been described above by way of example only, it will be understood by those skilled in the field that modifications may be made to the disclosed embodiment without departing from the scope of the invention, which is defined by the claims that follow.