Abstract:
Keel joint assemblies are described that permit a degree of rotational movement of a riser within the keel of a floating vessel and greatly reduce the amount of stress and strain that is placed upon the riser, as well. Keel joint assemblies described provide a limiting joint between the riser and the keel opening that permits some angular rotation of the riser with respect to the floating vessel. Additionally, the limiting joint permits the riser to move upwardly and downwardly within the keel opening, but centralizes the riser with respect to the keel opening so that the riser cannot move horizontally with respect to the keel opening. In described embodiments, the limiting joint is provided by a single annular joint that allows that riser to move angularly with respect to the can. In some embodiments, the keel joint assembly incorporates a cylindrical stiffening can that radially surrounds a portion of the riser and is disposed within the keel opening. In these embodiments, a flexible joint is provided between the can and the riser. Supports or guides may be used to retain the can within the keel opening.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority of provisional patent application serial No. 60/308,365 filed Jul. 27, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to methods and devices for providing a stress-relieving joint between a riser and the keel of a floating platform. 
     2. Description of the Related Art 
     Deep water floating platforms use risers to communicate production fluid from the sea floor to the floating production platform. Floating platforms have a portion that lies below the surface of the sea. For stability of the platform, it is desired that there be a very deep draft. The spar, for example, is a popular style of floating platform that has an elongated, cylindrical hull portion which, when deployed, extends downwardly a significant distance into the sea. The lowest portion of the submerged hull is referred to as the keel. Currents in the sea tend to move the floating platform laterally across the sea surface. Despite the presence of anchorages, the platform imparts bending stresses to the riser during lateral movement. Localized, or point, stresses are particularly problematic for risers. 
     One known joint arrangement for use with risers and floating vessels is described in U.S. Pat. No. 5,683,205 issued to Halkyard. Halkyard describes an arrangement wherein a joint means is positioned within a keel opening in the floating vessel to reduce the amount of stress upon a pipe passing through the keel opening. The joint means consists of a radially enlarged sleeve member with an elastomeric annulus at either end that is in contact with both the sleeve member and the pipe. Halkyard&#39;s intent is to reduce stress upon the pipe that is imposed by lateral movement of the floating vessel upon the sea. In order to reduce stress, Halkyard contacts the pipe at two points with an elastomeric annulus, which is described as providing a resilient, somewhat yieldable connection. Unfortunately, Halkyard&#39;s arrangement is problematic since it permits almost no angular movement of the pipe within the sleeve member. While point stresses upon the pipe are reduced, they are still significant. Further, the pipe is required to bend within the confines of the sleeve. This bending, together with the induced point stresses at either end of the sleeve, place significant strain on the pipe. 
     The present invention addresses the problems in the prior art. 
     SUMMARY OF THE INVENTION 
     Keel joint assemblies are described that permit a degree of rotational movement of a riser within the keel of a floating vessel. The assemblies of the present invention greatly reduce the amount of stress and strain that is placed upon the riser, as well. The present invention describes keel joint assemblies that provide a limiting joint between the riser and the keel opening that permits some angular rotation of the riser with respect to the floating vessel. Additionally, the limiting joint permits the riser to move upwardly and downwardly within the keel opening, but centralizes the riser with respect to the keel opening so that the riser cannot move horizontally with respect to the keel opening. 
     In described embodiments, the limiting joint is provided by a single annular joint that allows that riser to move angularly with respect to the can. In some embodiments, the keel joint assembly incorporates a cylindrical stiffening can that radially surrounds a portion of the riser and is disposed within the keel opening. In these embodiments, a flexible joint is provided between the can and the riser. Supports or guides may be used to retain the can within the keel opening. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates an exemplary riser extending upwardly from the sea floor and through a spar-type floating platform. 
     FIG. 2 is a schematic side, cross-sectional view of a first exemplary keel joint assembly constructed in accordance with the present invention. 
     FIG. 3 is a schematic side, cross-sectional view of a second exemplary keel joint assembly constructed in accordance with the present invention. 
     FIG. 4 is a schematic side, cross-sectional view of a third exemplary keel joint assembly constructed in accordance with the present invention. 
     FIG. 5 is a schematic side, cross-sectional view of a fourth exemplary keel joint constructed in accordance with the present invention. 
     FIG. 6 is a schematic side, cross-sectional view of a fifth exemplary keel joint assembly constructed in accordance with the present invention. 
     FIG. 7 is a schematic side, cross-sectional view of a sixth exemplary keel joint assembly constructed in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 generally illustrates a subsea wellhead  10  that has been installed into the sea floor  12 . A riser  14  is connected to the wellhead  10  and extends upwardly through the waterline  16  to a floating platform  18 . The riser  14  is used to transmit production fluids or as a drilling conduit from the wellhead  10  to production facilities (not shown) on the floating platform  18 . The riser  14  is used to provide a closed conduit from the wellhead  10  to the floating platform  18 . The floating platform  18  shown is a spar-type floating vessel that carries production equipment (not shown) on an upper deck  20 . The hull  22  of the platform  18  is a cylinder having flotation chambers within and a central, vertically-oriented passage  24  through which the riser  14  is disposed. It is noted that the configuration for a passage used in floating platforms varies from platform to platform. Sometimes the passage is lined by a cylindrical wall that extends substantially the entire length of the hull. In other platforms, the passage is partially lined by such a wall, and in still other platforms, there is essentially no lining for the passage. The keel  26  is located at the lower end of the hull  22 . A keel joint, indicated generally at  28 , is used to permit axial upward and downward motion as well as angular deflection of the riser  14  with respect to the keel  26 . It is desired that the keel joint  28  be constructed to preclude localized bending stresses in the riser  14  that could damage it, resulting in structural failure of the riser  14 . 
     Referring to FIG. 2, there is shown a first, and currently most preferred, exemplary keel joint arrangement  30  that can be used as the keel joint  28  to support the riser  14 . The keel joint arrangement  30  includes a stiff cylindrical can  32  that radially surrounds a portion of the riser  14 . The can  32  is retained within and disposed away from the walls of the keel opening or passage  24  by supports or guides  34  that are securely affixed with the hull  22 . While there are only two upper and two lower supports  34  shown in FIG. 2, it should be understood that there are actually more such supports  34 , perhaps four or more upper and four or more lower supports  34  and that the supports are located to surround the circumference of the riser  14 . The supports  34  have rounded, non-puncturing ends  36  to contact the outer wall of the can  32 . It is noted that the supports  34  are not affixed to the can  32 , thereby permitting the can  32  to move upwardly and downwardly within the passage  24 . The keel joint arrangement  30  maybe thought of an “open can” arrangement since the can  32  is affixed to the riser  14  by a stress joint (straight or tapered)  38  proximate the lower end of the can  32  while the upper end  40  of the can  32  is not secured to or maintained in contact with the riser  14 . The exemplary stress joint  38  illustrated consists of a pair of radially enlarged collars  42  that surround the riser  14  and are affixed to the inner radial surface of the can  32 . The collars  42  are shown to be fashioned of metal. However, the collars  42  may also be fashioned of a suitable elastomeric material. The collars  42  may be substantially rigid so as to permit a small amount of angular movement of the riser  14  with respect to the can  32 . Alternatively, the collars  42  may be relatively flexible to permit additional angular movement. 
     In operation, the riser  14  can move angularly to a degree within the can  32  under bending stresses. Illustrative directions of such relative angular movement are shown in FIG. 2 by arrows  33  about rotation point  35 . During such angular movement, the outer walls of the riser  14  are moved closer to or further away from the inner walls of the keel opening  24 . The stress joint  38  forms a fulcrum. The can  32  is stiff enough that it transfers stresses directly from the stress joint  38  to the supports  34 , thereby preventing any significant stresses from being seen by the upper portion of the riser  14 . Generally, this arrangement allows the upper portion of the riser  14  to have a smaller cross section than the stress joint  38 . 
     FIG. 3 illustrates an alternative embodiment for a keel joint arrangement  50  that is useful as a keel joint  28 . In the keel joint arrangement  50 , a heavy walled wear sleeve  52  radially surrounds a portion of the riser  14 . The wear sleeve  52  may or may not be secured to the riser  14  in a fixed relation, such as by the use of welding or retaining rings such as are known in the art. A central portion of the wear sleeve  52  has an external annular ring  54  that extends radially outwardly and forms the portion of the sleeve  52  having the largest exterior diameter. The ring  54  presents an outer radial surface that is vertically curved in a convex manner. The outer radial surface of the ring  54  may also be frustoconical in shape. Below the annular ring  54  is a lower inwardly tapered portion  56 . Above the ring  54  is an upper inwardly tapered portion  58 . A partially-lined passage, designated as  24 ′, in the hull  22  of the floating vessel  18  has an open upper end  60  that is outwardly flared for installation purposes. The flare of the upper end assists in guiding the sleeve  52  and ring  54  into place when lowering the riser  14  through the hull  22 . The lower end of the passage  24  has an annular recess  62  that is sized and shaped for the annular ring  54  to reside within. The recess  62  presents an inner surface that is vertically curved in a concave manner so that the outer convex surface of the annular ring  54  can be matingly engaged. If the outer radial surface of the ring  54  is frustoconical in shape, however, the inner surface of the recess  62  will be made complimentary to that frustoconical shape. 
     In operation, the keel joint arrangement  50  helps to prevent damage to the riser  14  from bending stresses. The wear sleeve  52  is located at the keel  26  where the primary bending stresses are imparted to the riser  14  and, therefore, is designed to absorb most of those stresses and prevent them from being imparted directly to the riser  14 . The interface of the ring  54  and the recess  62  provides a fulcrum wherein the riser  14  can move angularly with respect to the hull  22 . In addition, the elongated upper tapered portion  58  will tend to bear against the length of the passage  24 ′, thereby reducing or eliminating localized, or point, stresses. 
     Referring now to FIG. 4, there is shown a keel joint arrangement  70 , which is a second alternative embodiment that is useful as the keel joint  28 . The keel joint arrangement  70  employs centralizer assemblies  72  that are secured within the passage  24  of the hull  22 . Preferably, the centralizer assemblies  72  are spaced angularly about the circumference of the passage  24 . In a preferred embodiment, the centralizers  72  comprise hydraulically actuated piston-type assemblies, the piston arrangement being illustrated schematically by two  72   a ,  72   b . In practice, the two arms  72   a ,  72   b  would be nested one within the other in a piston fashion and would be selectively moveably with respect to one another. In an alternative embodiment, the centralizer assemblies  72  comprise hinged assemblies wherein the two arms  72   a ,  72   b  are hingedly affixed to one another at hinge point  72   c . Actuation of the centralizer assembly in this case would move the arm  72   a  angularly with respect to the arm  72   b  about the hinge point  72   c , thereby permitting the arm  72   a  to be selectively moved into and out of engagement with the riser  14 . The centralizers  72  are energized via hydraulic lines (not shown) to urge the riser toward the radial center of the passage  24  to resist contact between the riser  14  and the passage  24 . The centralizers  72  have rounded, non-puncturing tips  74  that bear upon the riser  14 . Preferably, the non-puncturing tips comprise either wear pads or rollers for engagement of the riser  14 . It is noted that the piston-type centralizer assemblies  72  may be actuated mechanically rather than hydraulically. Also, the centralizer assemblies&#39; attachments to the passage  24  may be softened, such as through use of springs or rubber, in such a way as to decrease bending stresses by yielding to riser deflection. In a further alternative embodiment, the centralizers  72  will comprise members that have a hinged attachment to the passage  24 . 
     FIG. 5 depicts a third alternative embodiment for the keel joint  28 . Keel joint assembly  90  includes a riser collar  92  that surrounds a portion of the riser  14  proximate the keel  26 . The collar  92  is not affixed to the riser  14  but instead permits sliding movement of the riser  14  upwardly and downwardly through the collar  92 . The collar  92  is generally cylindrical but includes a bulbous central portion  94  and two tapered end portions  96 ,  98 . A guide sleeve  100  radially surrounds the collar  92  and features an interior rounded profile  102  that is shaped and sized to receive the bulbous portion  94  of the collar  92 . An exterior landing profile  104  is located at the lower end of the guide sleeve and is shaped and sized to form a complementary fit with a landing profile  106  formed into the keel  26 . The passage  24 ′ is constructed identically to the passage  24 ′ described earlier in that it has an open upper end with an outward flare. 
     To assemble the keel joint arrangement  90 , the collar  92  and guide sleeve  100  are assembled onto the riser  14 . Then the riser  14  is run through the passage  24 ′ and the landing profile  104  of the guide sleeve  100  is seated into the matching profile  106  in the keel  26 . In operation, the riser  14  can slide upwardly and downwardly within the collar  92  as necessary to compensate for movement of the floating platform  18 . Rotation of the platform  18  with respect to the riser  14  is permitted between the riser  14  and the collar  92  as well as between the collar  92  and the guide sleeve  100 . Angular movement of the riser  14  with respect to the platform  18  is accommodated by rotation of the bulbous portion  94  within the rounded profile  102  of the guide sleeve  100 . Alternatively, a rubberized flex joint of a type known in the art (not shown) might be used to accommodate angular rotation. 
     A fourth alternative exemplary embodiment for the keel joint  28  is shown in FIG.  6 . Keel joint assembly  110  incorporates a flexible cage assembly to permit relative movement between the riser  14  and the floating vessel  18 . A flexible cage assembly  112  is formed of an inner riser sleeve  114  and an outer keel sleeve  116 . A central cage  118  adjoins the two sleeves  114 ,  116 . The cage  118  includes an upper ring  120 , a central ring  122 , and a lower ring  124 . There are a series of upper spokes  126  that radiate upwardly and outwardly from the central ring  122  to the upper ring  124 . There are also a series of lower spokes  128  that radiate outwardly and downwardly from the central ring  122  to the lower ring  124 . The upper and lower spokes  126 ,  128  are each arranged in a spaced relation from one another about the circumference of the central ring  122 . The spokes  126 ,  128  are fashioned from a material that is somewhat flexible yet has good strength under both tension and compression. It is currently preferred that the spokes  126 ,  128  are fashioned of a steel alloy, although other suitable materials may be used. The spokes  126 ,  128  are elastically deformable as necessary to allow the riser  14  to move angularly within the passage  24 ′. Angular deflection of the riser  14  results in non-uniform deflection of upper spokes  126  and lower spokes  128 . The upper ring  120  affixes the upper spokes  126  to the outer keel sleeve  116 . The lower ring  124  is not affixed to the outer keel sleeve  116 . 
     The outer keel sleeve  116  is seated within the passage  24 ′ by means of a landing profile  130  that is shaped and sized to be seated within a complimentary seating profile  132  at the lower end of the passage  24 ′. Locking flanges  134  are secured onto the lower side of the keel  26  to secure the outer keel sleeve  116  in place. In a manner known in the art, the locking flanges  134  may be selectively disengaged, or unlocked, and subsequently retrieved by upward movement of the riser  14  with respect to the passage  24 ′, i.e., by pulling upwardly on the riser string. 
     During operation, the cage  118  holds the riser  14  in a semi-rigid manner that permits some flexibility. The riser  14  can move angularly with respect to the hull  22  due to the flexibility of the spokes  126  and  128  of the cage  118 . Loading from movement of the riser  14  is transferred by the upper spokes  126  to the keel sleeve  116  which, in turn transfers the loading to the hull  22 . Because the keel sleeve  116  engages the passage  24 ′ of the hull  22  along substantially its entire length, point loading is avoided. 
     FIG. 7 depicts a fifth alternative embodiment for use as the keel joint  28 . Keel joint arrangement  130  includes an open top can structure, which is shown incorporated into the riser  14  as a sub  132  at is affixed at either end to other riser sections  134 ,  136 . The can sub  132  includes a pair of concentric tubular members. The inner tubular member  138  has the same interior and exterior diameters as a standard riser section. The outer tubular member, or can,  140  is coaxial with the inner tubular member  138  and is affixed to the inner tubular member  138  by a flange adapter, or stress joint,  142  that joins the two pieces together proximate the lower end of the sub  132 . While FIG. 7 shows the flange adapter  142  to be an annular metallic collar that is integrally formed into both the inner and outer tubular members  138 ,  140 , it might also comprise a separate collar or elastomeric member as well as a flexible casing. 
     A cylindrical guide sleeve  144  radially surrounds the open top can sub  132 . The guide sleeve  144  is securely affixed to the outer tubular member  140  by, for example, welding. Supports  146  are used to secure the guide sleeve  144  within the passage  24  of the hull  22 . The supports  146  maintain the guide sleeve  144  a distance away from the wall of the passage  24  so that the guide sleeve  144  is substantially radially centered within the passage  24 . The supports  146  are preferably formed of structural beams. The supports  146  are arranged in two tiers, an upper tier and a lower tier, and each tier surrounds the circumference of the passage  24 . The outer tubular member  140  is stiff enough that it transfers stresses directly from the flange adapter  142  to the guide sleeve  144 . Because the guide sleeve  144  and the outer tubular member  140  are affixed along substantially their entire length, point stresses are avoided. In addition, the supports transmit loads or stresses from the guide sleeve  144  to the passage  24  walls. The length of contact between the outer tubular member  140  and the guide sleeve  144  allows for a longer vertical riser stroke than arrangements wherein there is less contact area, such as the arrangement  30  shown in FIG.  2 . 
     While described in terms of preferred embodiments, those of skill in the art will understand that many modifications and changes may be made while remaining within the scope of the invention.