Abstract:
A turret mooring system for a vessel which includes a moon pool comprises a turret which is positioned in the moon pool, a buoy and anchor chain assembly for anchoring the turret to the sea floor, a bearing assembly for rotatably connecting the turret to the vessel, and a bearing support assembly which comprises an upper ring member to which the bearing assembly is connected, a lower ring member which is connected to the vessel in the moon pool, and a number of elongated support beams which are connected between the upper and lower ring members. In addition, the lower ring member is connected to the vessel near the vertical center of the vessel in order to minimize the deflections of the bearing assembly due to hogging and sagging of the vessel.

Description:
This application is based on U.S. Provisional Patent Application No. 60/465,092, which was filed on Apr. 23, 2003. 
   BACKGROUND OF THE INVENTION 
   The present invention is directed to a turret mooring system which comprises a turret that is rotatably supported in the moon pool of a vessel by at least an upper bearing assembly. More particularly, the invention is directed to a bearing support assembly which isolates the upper bearing assembly from deflections of the vessel. 
   Turret mooring systems are commonly used to anchor a vessel to the sea floor. Internal turret mooring systems typically comprise a turret which is rotatably supported in a moon pool that is formed in the hull of the vessel between the bow and the stern. The turret is often supported by both an upper bearing assembly which is connected between the upper end of the turret and the top of the moon pool and a lower bearing assembly which is connected between the lower end of the turret and the bottom of the moon pool. The upper bearing assembly typically comprises a combination axial and radial bearing which is capable of transmitting both the vertical and horizontal mooring loads from the turret to the vessel. The lower bearing assembly usually comprises a radial bearing which transmits only the horizontal mooring loads from the turret to the vessel. 
   A vessel in the open sea is often affected by waves and swells which can cause the vessel to hog and sag. As the vessel hogs and sags, the normally circular cross section of the moon pool deflects into alternating oval configurations. Since the vessel reacts somewhat like a beam in bending, these deflections are greatest at the top of the moon pool, which is normally where the upper bearing assembly is connected. In addition, turret mooring systems are typically designed to provide a clearance between the lower bearing assembly and the turret. As a result, the horizontal mooring loads will cause the turret to pivot about a horizontal axis located near the upper bearing assembly. Unless means are provided to isolate the upper bearing assembly from the deflections of the moon pool and the pivoting of the turret, these displacements may damage the upper bearing assembly and interfere with the operation of the turret mooring system. 
   In certain prior art turret mooring systems, bearing support assemblies comprising elastomeric or steel springs have been employed to isolate the upper bearing assembly from the deflections of the moon pool and to allow the turret to pivot about a horizontal axis. However, these bearing support assemblies contain many parts, require frequent maintenance and are expensive. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, these and other disadvantages in the prior art are overcome by providing a bearing support assembly for a turret mooring system which comprises a turret that is rotatably supported in a moon pool of a vessel by an upper bearing assembly which is secured to an upper portion of the turret. The bearing support assembly comprises an upper ring which is connected to the upper bearing assembly, a lower ring which is connected to the moon pool, and a number of support beams which extend between the upper and lower rings. Furthermore, the lower ring is located near the vertical center of the moon pool where the deflections due to hogging and sagging of the vessel are minimal. In addition, the support beams are preferably designed to deflect slightly and thereby allow the turret to pivot about a horizontal axis located near the upper bearing assembly. 
   Since the lower ring of the upper bearing support assembly is located near the vertical center of the vessel, only minimal deflections of the vessel will be transmitted to the upper bearing assembly through the support beams. In addition, because the support beams will deflect and allow the turret to pivot, the pivoting of the turret will not adversely affect the upper bearing assembly. Thus, the upper bearing support assembly of the present invention provides an effective means to isolate the upper bearing assembly from both the deflections of the vessel and the pivoting of the turret. 
   These and other objects and advantages of the present invention will be made apparent from the following detailed description, with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross sectional view of a turret mooring system which comprises the upper bearing support assembly of the present invention; and 
       FIG. 2  is an enlarged cross sectional view of the upper bearing support assembly shown in  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , the upper bearing support assembly of the present invention, which is indicated generally by reference number  10 , is shown installed in an exemplary turret mooring system  12  for a vessel  14 . The turret mooring system  12  comprises a turret  16  which is mounted in a moon pool  18  that is formed in the hull  20  of the vessel  14 . The turret  16  may be secured to a disconnectable buoy  22  which in turn is anchored to the sea floor using a number of mooring chains  24 . In use, the buoy  22  is hoisted from a submerged position to the turret  16  using a pull-in rope  26  that is attached to a hoist  28 , and the buoy is connected to the turret with a conventional structural collet connector  30 . 
   The turret  16  is rotatably supported in the moon pool  18  by an upper bearing assembly  32  and, when the buoy  22  is attached to the turret, a lower bearing assembly  34 . In this manner, the vessel  14  is allowed to weathervane around the turret  16  while still being firmly anchored to the sea floor. The upper bearing assembly  32  ideally comprises a combination thrust and radial bearing, such as a conventional three-row roller bearing, which supports the weight of the turret  16  and the mooring chains  24  and accommodates the horizontal and vertical mooring loads acting between the turret and the hull  20 . The lower bearing assembly  34 , in contrast, preferably comprises a radial bearing which accommodates only the horizontal loads acting between the buoy  22  and the hull  20 . If desired, the turret mooring system  12  may also include a number of bumper pads  36 , which may be comprised of, for example, rubber or polyethylene, to cushion the lower portion of the turret  16  from the hull  20  prior to connecting the buoy  22  to the turret. 
   In accordance with the present invention, the upper bearing assembly  32  is supported on the bearing support assembly  10 . Referring to  FIG. 2 , the bearing support assembly  10  comprises an annular upper ring  38  to which the upper bearing assembly  32  is connected, an annular lower ring  40  which is connected to the hull  20 , and a number of elongated support beams  42  which are each secured between the upper and lower rings. In a preferred embodiment of the invention, the bearing support assembly  10  is a pre-fabricated metal structure, that is, the upper ring  38 , the lower ring  40  and the support beams  42  are all constructed of a suitable metal, such as carbon steel, and secured together such as by welding prior to being installed in the moon pool  18 . 
   In the illustrated embodiment of the invention, the upper ring  38  is secured to the upper portion of the turret  16  and the support beams  42  are sufficiently long to allow the lower ring  40  to be secured to the hull  20  near the vertical center of the vessel  14 . At this position, the deflections of the lower ring  40  caused by hogging and sagging of the vessel  14  will be significantly reduced compared with the deflections the lower ring would experience if it were secured to the hull  20  closer to the upper ring  38 . As a result, the upper ring  38  will also experience reduced deflections, and the upper bearing assembly  32  will therefore remain generally circular. Thus, it may be seen that the bearing support assembly tends to isolate the upper bearing assembly  32  from the distortions of the moon pool  18  which are caused by hogging and sagging of the vessel  14 . 
   The length, number and cross sectional configuration of the support beams  42  will depend on the size of the turret  16  and the anticipated vertical mooring loads that will be transmitted through the upper bearing assembly  32 . In addition, the length and cross sectional configuration of the support beams  42  are selected so that they will deflect slightly to allow the turret  16  to pivot about a horizontal axis located near the upper bearing assembly  32  when the turret is subjected to horizontal mooring loads. In an exemplary embodiment of the invention, for example, the support beams  42  may each be constructed of twelve-inch diameter carbon steel tubing and have a length of approximately twenty feet, and a total of twenty four such support beams may be evenly distributed around the bearing support assembly  10 . 
   Referring still to  FIG. 2 , the upper ring  38  preferably comprises a circular base  44  which is attached to the top of a cylindrical hoop  46 . In addition, the upper end of each support beam  42  is ideally attached to an upper rib  48  which optimally is connected to both the base  44  and the hoop  46 . The upper ring  38  may also comprise a stiffener ring  50 , which is preferably attached to the bottom of the hoop  46  and each upper rib  48 , to facilitate the assembly of and provide additional stiffness to the upper ring. 
   If as shown in  FIG. 2  the upper bearing assembly  32  comprises a three-row roller bearing, a lower or outer race  52  of the bearing assembly may be bolted to the base  44  and an upper or inner race  54  of the bearing assembly may be attached to a conventional upper bearing ring  56  which is connected to or formed integrally with the upper end of the turret  16 . Thus, the vertical and horizontal mooring loads which act on the turret  16  will be transmitted through the upper bearing assembly  32  to the upper ring  38 , from the upper ring to the support beams  42 , and from the support beams to the lower ring  40 . 
   The lower ring  40  preferably comprises a circular reaction ring  58  and a number of lower ribs  60 , each of which is secured to the reaction ring and to the lower end of a corresponding support beam  42 . The reaction ring  58  is supported on a conventional moon pool support ring  62  which is attached to the moon pool  18  near the vertical center of the vessel  14 . In addition, the bearing support assembly  10  ideally includes a number of radial and axial jack screws  64  and  66 , respectively, which are operatively engaged between the hull  20  and/or the support ring  62  on the one hand and the reaction ring  58  on the other hand, to provide a means for aligning the axis of rotation of the upper bearing assembly  32  with the axis of rotation of the lower bearing assembly  34 . Once the axes of rotation are properly aligned, the interface between the support ring  62  and the reaction ring  58  may be filled with grout and these two components secured together with, for example, a number of bolts  68  to prevent the separation of the bearing support assembly  10  from the moon pool  18  due to uplift of the turret  16 . 
   In operation of the bearing support assembly  10 , the vertical and horizontal mooring loads acting on the turret  16  are transmitted through the upper bearing assembly  32  to the upper ring  38 . From the upper ring  38 , the mooring loads are transmitted through the support beams  42  to the lower ring  40 , and from the lower ring  40  to the hull  20  of the vessel  14 . In addition, due to the clearance between the turret  16  and the lower bearing assembly  34 , the horizontal mooring loads will cause the turret to pivot about a horizontal axis located near the upper bearing assembly  32 . The resulting bending force will be transmitted through the upper bearing assembly  32  to the support beams  42 , which will deflect slightly to allow the turret  16  to pivot. 
   Since the vessel  14  reacts somewhat like a beam in bending, the deflections of the normally circular shape of the moon pool  18  caused by hogging and sagging of the vessel  14  will be minimized near the vertical center of the hull  20 . Thus, by mounting the lower ring  40  near the vertical center of the hull  20 , the deflections of the lower ring due to hogging and sagging of the vessel  14  will also be minimized. As a result, the upper ring  38  and, thus, the upper bearing assembly  32  will remain relatively circular during operation. In addition, due to the particular construction of the support beams  42 , they will deflect slightly to accommodate the pivoting of the turret  16  caused by the horizontal mooring loads and thereby prevent the upper bearing assembly  32  from becoming damaged. Therefore, the bearing support assembly  10  effectively isolates the upper bearing assembly  32  from both the deflections of the vessel  14  and the pivoting of the turret  16 . 
   It should be recognized that, while the present invention has been described in relation to the preferred embodiments thereof, those skilled in the art may develop a wide variation of structural and operational details without departing from the principles of the invention. Therefore, the present application should be construed to cover all equivalents falling within the true scope and spirit of the invention.