Abstract:
The invention is a bearing unit and bearing system for supporting a large rotatable element, such as a mooring turret. The bearing unit includes a hydrostatic suspension system which enables the bearing unit to accommodate fabrication tolerances and also enables the bearing unit to conform to relative movements between the ship and the turret, thereby providing a compliant bearing system. The system includes multiple bearing units of the invention which serve as thrust and/or radial bearings for supporting the turret. By manifolding a plurality of bearing units together in a fluidly-isolated group, the pressure applied to the bearing units in that group is self-equalizing so that all the bearing units act in unison to equally support the load, while also allowing some degree of self-alignment and tilting of the load. As a result, the bearing system emulates a self-aligning bearing system and is able to compensate for axial and angular misalignment. The system allows for monitoring of each bearing unit, automatic lubrication of the bearing surfaces, and in situ replacement of bearing liners should wear or damage occur while the system is in operation.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to offshore vessel mooring systems that include a turret rotatably mounted within an opening or well within a vessel and connectable to a seabed mooring. More particularly, the invention relates to a method and apparatus for rotatably supporting a mooring turret within a vessel hull. 
     2. Description of the Prior Art 
     In recent years, the offshore oil and gas drilling industry has gravitated away from fixed platforms and toward floating storage and production vessels. Under this arrangement, a ship, such as a retired tanker, is moored to a mooring buoy, spider, or similar device connected to the seabed at the location of an undersea well. A riser is connected from the undersea well to the ship for delivering the oil or gas product. In this manner, the ship receives the oil or gas product from the undersea well and acts as a temporary storage facility for the product. 
     It is desirable in open or unprotected waters to moor the ship to the mooring buoy in such a manner that the ship is free to rotate or swivel about the mooring in a practice known as weathervaning. By this method, the ship is free to move in accordance with the prevailing currents and winds, while still remaining moored to the seabed. This freedom to swivel is commonly accomplished by mounting a cylindrical mooring turret vertically within the ship in such a manner that the turret is able to rotate or swivel about a vertical axis relative to the ship. The turret is commonly moored by one or more mooring lines know as catenaries which extend to the seabed. A mooring buoy, spider, or other connection joint or platform may be used to interface between the catenaries and the bottom of the turret. In addition, one or more oil production risers extend from a wellhead on the seabed into the turret, and the output from the risers is fed into the tanks in the ship for temporary storage. 
     To enable rotation of the turret relative to the ship, the turret is supported within the turret enclosure by a bearing system. These bearing systems usually include at least one thrust or axial bearing system for supporting axial loads, and at least one radial bearing system for supporting radial loads. Under one conventional arrangement, a thrust bearing system and a first radial bearing system are located near the upper end of the turret, such as on the forecastle of the ship, and a second radial bearing system is located near the bottom of the turret within the turret well. However, it is also known in the art to eliminate the lower radial bearing system to reduce maintenance and alignment problems with the turret, but such an arrangement greatly increases the load and wear on the upper bearing systems. Accordingly, such single-radial-bearing arrangements require an upper bearing system that is durable and compliant. 
     Also, in the case of smaller ships, turrets having rigid bearing systems have been used successfully to enable the turret to rotate relative to the ship. However, in the case of large turrets, and particularly in heavy seas conditions whereby heaving of the ship may cause vessel hull deflections and substantial loads between the turret and the hull, there is a need for some bearing compliance between the turret and the vessel. Compliant bearing systems used in the past for forming an interface between the turret and the ship include spherical self-aligning bearings, compliant plane bearing systems, and crane-wheel-type bearing systems mounted on springs or rubber pads. However, there is a continuing need for improvement over the conventional turret support systems to achieve a less complex, more efficient, and more reliable support system that maintains compliancy between the turret and the ship. 
     SUMMARY OF THE INVENTION 
     Under one aspect, the present invention sets forth a novel bearing pad unit for use in the turret support system of the invention. The bearing unit includes a hydrostatic suspension system which enables the bearing unit to accommodate turret fabrication tolerances and also enables the bearing unit to conform to relative movements between the ship and the turret, thereby providing a compliant bearing system. The bearing unit includes one or more bearing plates supported by a hydrostatic load element. The turret includes a stainless steel liner or race which runs directly against the bearing plates of a plurality bearing units. One or more grease ports are provided in each bearing plate to enable the periodic application of lubricant to the interface between the bearing plates and the stainless steel bearing liner of the turret. 
     In each bearing unit, the hydrostatic load element supports the bearing plate or plates and allows minor realignments of the bearing plates to be made while the bearing plates are under load. The hydrostatic load element includes a bearing pad block upon which the bearing plate or plates are mounted. A cylindrical pedestal engages with a cylindrical cavity located in the bearing block for supporting the bearing block. A pressurized hydraulic fluid is disposed within the cylindrical cavity between the pedestal and the bearing block so that the block is hydrostatically supported. A primary fluid seal and a secondary fluid seal are included at the interface between the pedestal and the bearing unit to prevent leakage of the hydraulic fluid. The primary seal is the main load-bearing seal, and is essentially static in service. The secondary seal is included as a backup should the primary seal fail. Also included in the interface between the pedestal and the bearing block is an annular ring bearing which transmits side loads from the block to the pedestal so as to prevent damage to the seals and to prevent direct contact between the block and the pedestal. In addition, if hydraulic pressure is lost in a bearing unit, the bearing block will be supported by a polymer cushion located on top of the pedestal. The cushion protects the pedestal and the block from high contact stresses by preventing direct metal-to-metal contact between the block and the top of the pedestal if hydraulic pressure is lost. 
     Pressurized hydraulic fluid may be pumped into the cylindrical cavity to support the bearing block and to put the bearing plates in contact with the turret bearing race surface. A bleed line is included in the bearing block to enable air in the cylindrical cavity to escape when fluid is pumped into the cylindrical cavity. A fluid supply line runs through the pedestal body and the cushion so that the fluid supply line outlet opening is located on the upper end of the pedestal. The fluid supply line is connectable to the pressurized hydraulic fluid circuit, and a plurality of bearing units may be manifolded together by being placed in isolated fluid communication with each other for equalizing the pressure on each bearing unit, thereby providing a self-adjusting feature among a plurality of bearing units. 
     Accordingly, under an additional aspect, the invention is directed to a system for supporting a turret within a turret well or enclosure. The system includes multiple bearing pad units which serve as thrust and/or radial bearings for supporting the turret. The bearing contact elements are supported hydrostatically so as to compensate for deformations due to fabrication tolerances and vessel hull deflections under load. As a result, the bearing system emulates self-aligning bearings and is able to compensate for axial and angular misalignment. The system allows for monitoring of each bearing unit, automatic lubrication of the bearing surfaces, and in situ replacement of bearing liners should wear or damage occur while the system is in operation. 
     Under another aspect, the invention sets forth a novel method and apparatus for mounting and operating bearing units for supporting a turret within a turret well in a ship&#39;s hull. Under one embodiment, the thrust and radial bearings are mounted in an equally-spaced manner about the perimeter of the turret bearing surface. The thrust bearing units are all manifolded together so that hydraulic fluid is able to flow between the individual thrust bearing units, but the fluid system is otherwise isolated. Similarly, the radial bearing units are manifolded to other radial bearing units, but otherwise isolated from the fluid circuit so that fluid is able to flow between the radial bearing units, but not to the rest of the fluid circuit. By manifolding a plurality of bearing units together, the pressure applied by the bearing units is self-equalizing so that all the bearing units act in unison to equally support the load, while also allowing some degree of self-alignment and tilting of the load. 
     In addition, according to another embodiment, the bearing units are mounted in two or more distinct groups, and preferably three groups, with each group being centered 120 degrees apart from adjacent groups of bearing units. The bearing units in each group are manifolded together, so as to act as a single bearing support, but are not manifolded to either of the other two groups of bearing units. This results in the three distinct groups of bearing units behaving as three single bearing pads, thereby providing a self-aligning compliant support, but allowing no tilting of the load. The arrangement of this second embodiment is particularly advantageous in the case of large diameter turrets of, for example, 10 meters diameter and larger. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and additional objects, features, and advantages of the present invention will become apparent to those of skill in the art from a consideration of the following detailed description of the preferred embodiments of the invention, taken in conjunction with the accompanying drawings. 
     FIG. 1 illustrates a plan view of a first embodiment of a bearing unit of the invention. 
     FlG  2   a  illustrates an elevation view of the bearing unit of FIG. 1, with a sectional view taken along line  2   a — 2   a  of FIG.  1 . 
     FlG.  2   b  illustrates a second embodiment of the bearing unit of FIG. 2 a.    
     FIG. 3 a  illustrates an elevation view of a pedestal of the invention. 
     FIG. 3 b  illustrates a top view of the pedestal of FIG. 3 a.    
     FIG. 3 c  illustrates a cross section view taken along line  3   c — 3   c  in FIG. 3 b.    
     FIG. 3 d  illustrates a cross section view taken along line  3   d — 3   d  in FIG. 3 b.    
     FIG. 4 a  illustrates a first embodiment of a bearing plate for use with the bearing unit of the invention illustrated in FIGS. 1 and 2 a.    
     FIG. 4 b  illustrates a cross section view taken along line  4   b — 4   b  in FIG. 4 a.    
     FIG. 4 c  illustrates a cross section view taken along line  4   c — 4   c  in FIG. 4 a.    
     FlG.  5   a  illustrates a second embodiment of a bearing plate for use with the bearing unit of the invention illustrated in FIG. 2 b.    
     FIG. 5 b  illustrates a cross section view taken along line  5   b — 5   b  in FIG. 5 a.    
     FIG. 5 c  illustrates a cross section view taken along line  5   c — 5   c  in FIG. 5 a.    
     FIG. 6 illustrates a partial cross sectional elevation view of a radial bearing unit of the invention. 
     FIG. 7 a  illustrates an elevation view of a turret supported by a first embodiment of an arrangement of the bearing system of the invention. 
     FIG. 7 b  illustrates a view taken along line  7   b — 7   b  in FIG. 7 a.    
     FIG. 8 illustrates a plan view of a second embodiment of an arrangement of the bearing system of the invention. 
     FIG. 9 illustrates a hydraulic fluid circuit for use with the bearing system of the invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention sets forth a bearing system for use in supporting a large rotatable element, such as for supporting a turret within a turret well enclosure of a ship, or the like. The system includes a plurality of bearing pad units for supporting the turret. In FIGS. 1 and 2 a  there is illustrated a first preferred embodiment of a bearing pad unit  10  of the invention. In its broadest aspect, bearing unit  10  includes an outer member  12  movable relative to an inner member  14  for hydraulically supporting at least one bearing element  16  in contact with the large rotatable element (not shown in FIGS. 1 and 2 a  ). Thus, the preferred embodiment of bearing unit  10  includes a bearing block  20  as part of outer member  12  having a bearing plate  22  as bearing element  16  mounted on an upper bearing-element-support surface  24  of block  20 . Bearing block  20  includes a cylindrical cavity  26  for moveable engagement with inner member  14 , which is in the form of a cylindrical pedestal  28  in the preferred embodiment. Thus, bearing block  20  is axially moveable relative to pedestal  28  along the major axis of pedestal  28 . By the introduction of hydraulic fluid into cylindrical cavity  26 , bearing block  20  can be hydraulically supported on pedestal  28  so that bearing unit  10  is able to act essentially as a hydrostatic load element. However, as will be described in greater detail below, the fluid in bearing units  10  is not entirely static, since fluid is able to flow between two or more fluidly-connected bearing units  10  to enable bearing units  10  to adjust for load variations. 
     As illustrated in FIG. 2 a , a block collar  30  is connected to the lower portion of bearing block  20 . Block collar  30  includes an annular collar shoulder  32  which projects inward toward pedestal  28 , and which will engage with the lower edge of an outwardly-projecting annular pedestal shoulder  34  on pedestal  28 , as also illustrated in FIGS. 3 a - 3   b . The engagement of collar shoulder  32  with pedestal shoulder  34  limits the upward movement of block  20  relative to pedestal  28  when pressurized fluid is introduced into cavity  26 . Thereby, collar  30  is able to retain block  20  on pedestal  28 . However, it is desirable that bearing plate  22  engage with a surface to be supported prior to the contact of collar shoulder  32  with pedestal shoulder  34 . Collar  30  is secured to bearing block  20  by collar machine screws  36 , or other suitable means. Collar  30 , pedestal  28 , block  20 , and the other structural components of the invention may be constructed from stainless steel, carbon steel, cast iron, or any other suitable materials or combinations thereof, taking into account the loads to be supported and the corrosiveness of the environment of use. Furthermore, a dust seal  38  may be included in a cutout  40  located on the inner periphery of collar shoulder  32  for preventing contamination of the fluid seals and cavity  26 . 
     Bearing unit  10  includes two fluid seals for increased reliability. A primary fluid seal  42  is located at a peripheral annular undercut  44  on pedestal  14 , immediately adjacent to a lip  46  on the upper end  48  of pedestal  28 . Thus, primary fluid seal  42  is retained between lip  46  and an undercut shoulder  50  formed by undercut  44 . Primary fluid seal  42  is preferably a circular polymer seal having a generally V-shaped cross section, and may further include a securing O-ring  52  for added assurance. Primary seal  42  bears the full hydraulic load when bearing unit  10  is under pressure. A secondary fluid seal  54  is located in a peripheral annular recess  56  in block  20  at the interface between block  20  and block collar  30 . Secondary fluid seal  54  may be of the same type and material as primary fluid seal  42 , but of a slightly larger diameter. Secondary fluid seal  54  provides retention of any fluid leakage past primary fluid seal  42 , and thereby contributes to the reliability of bearing unit  10 . 
     Immediately below primary fluid seal  42  there is located a radially-acting ring bearing  58 . Ring bearing  58  is located on the opposite side of pedestal shoulder  34  from block collar  30 , and is constructed as a circular ring of bearing bronze, nickel-bronze alloy, or other relatively lubricious high-bearing-strength material. Ring bearing  58  is of a slightly greater diameter than pedestal shoulder  34 , and absorbs and transmits lateral forces imposed on bearing block  20 , thereby protecting primary fluid seal  42  and secondary fluid seal  54  from excessive wear due to side loading. Thus, side loads imposed on bearing plate  22  due to friction, or the like, are transmitted by ring bearing  58  to pedestal  28 . Ring bearing  58  also prevents direct metal-to-metal contact between pedestal  28  and bearing block  20 , while the relative lubricity of ring bearing  58  allows low friction axial movement of bearing block  20  relative to pedestal  28  even during side loading. In addition, block  20  and block collar  30  include lubrication ports  59  for enabling lubrication of the interface between block  20  and pedestal  28 . Furthermore, it should be noted that other materials may be substituted for bronze for forming ring bearing  58 , including synthetic materials. One preferred alternative material is a synthetic polymer tape of sold under the brand name Thoratape™, available from Thordon Bearings, Inc. of Canada, which may be wrapped around pedestal  28  below primary seal  42  to serve as ring bearing  58  in place of the bronze ring. 
     Bearing plate  22  is retained on bearing block  20  by recessed machine screws  60 , as illustrated in FIG.  1 . Furthermore, a circular projection  62  is centrally located on upper surface  24  of block  20  for engaging with a circular recess  64  which is centrally located in the underside of bearing plate  22 . This arrangement acts to transfer lateral forces from bearing plate  22  to block  20 , rather than having to rely solely on the shear strength of machine screws  60 . As also illustrated in FIGS. 4 a - 4   c , bearing plate  22  is preferably a rectangular bronze plate having a synthetic lining of low friction TRAXL bonded to its surface. TRAXL is a brand name used by Thordon Bearings, Inc. of Canada, and is a synthetic bearing lining typically applied to a bronze or stainless steel backing. Of course, the invention is not limited to a particular material or lining for the bearing plates, and any suitable material may be used for forming the bearing plates of the invention. Lubrication ports  66  are provided in bearing plate  22  to enable the periodic application of lubricant to the surface of the plate through lubrication channels  68 . Application of lubricant such as grease may be accomplished manually or automatically using known systems. 
     Under a second embodiment, as illustrated in FIG. 2 b , a pair of smaller bearing plates  70  may be located on upper surface  24  of block  20  instead of single bearing plate  22 . As also illustrated in FIGS. 5 a - 5   c , bearing plates  70  include a downwardly projecting key member  72  which is used to secure bearing plates  70  to upper surface  24  of block  20 . This key member  72  fits within a key slot  74  formed on upper block surface  24  and enables bearing plates  70  to be removed from bearing unit  10  for repair or replacement without necessitating access to the upper or bearing surface  75  of bearing plates  70 . Accordingly, bearing plates  70  may be removed from a bearing unit  10  during use of adjacent bearing units  10 , without requiring dismantling of the entire bearing unit  10 . In addition, key members  72  also serve the same shear-transferring purpose as circular projection  62  and circular recess  64  in the first embodiment, and, accordingly, circular projection  62  and circular recess  64  are not required for the second embodiment. As with the first embodiment  22  of the bearing plate, bearing plates  70  may include lubrication ports  66  and channels  68 , and are constructed of similar materials. In addition, lubrication ports  66  may be formed on both sides of plate  70  so that plate  70  may be interchangeably used on either end of block  20 . 
     Referring back to FIGS. 1 and 2 a , Pedestal  28  may be secured to a suitable support surface (not shown) by using a two-piece clamp plate  76 . Clamp plate  76  annularly engages an annular groove  78  formed in the lower end of pedestal  28 . Thus, clamp plate  76  encircles pedestal  28  in a collar-like manner for securely retaining pedestal  28 . Clamp plate  76  may then be bolted or otherwise secured to the surface such as with bolts  80 . In addition, clamp plate  76  includes a brace assembly  82  which projects upward adjacent to block  20 . Brace assembly  82  is positioned so as to prevent rotation of the generally rectangular block  20 . This serves to keep bearing plates  22 ,  70 , properly oriented with respect to the bearing race of the element being supported (not shown). 
     While the foregoing embodiments of the invention are primarily intended for use in supporting an axial load, the bearing unit of the invention may also be used as a radial bearing. Thus, in a third embodiment, as illustrated in FIG. 6, a radial bearing unit  11  includes swivelable bearing plates  84  mounted on upper surface  24  of block  20 . Radial bearing unit  11  is essentially the same as bearing unit  10 , with the exception of the arrangement of bearing plates  84 . Swivel bearing plates  84  are able to pivot about a pivot axis  86 , which enables bearing plates  84  to conform to a cylindrical (curved) bearing race (not shown) rather than a flat bearing race. This enables a plurality of radial bearing units  11  to be arranged circumferentially around the cylindrical periphery of a large rotatable element for supporting radial loads imposed on and by the rotatable element. Swivel bearing plates  84  also include lubrication ports and channels, as with the bearing plates  22 ,  70  of the first two embodiments, and may be similarly constructed. 
     As illustrated in FIGS. 3 a - 3   c , pedestal  28  includes a main fluid port  88  for connection to a source of pressurized hydraulic fluid (not shown in FIGS. 3 a - 3   c ). Fluid port  88  runs in the direction of the primary axis of pedestal  28 , and has an opening in a cylindrical depression  90  formed on the upper surface of pedestal  28 . As illustrated in FIGS. 2 a  and  6 , a bleed port  92  is provided in block  12  for enabling air in cylindrical cavity  26  to exit when cavity  26  is being filled with hydraulic fluid. Accordingly, pressurized hydraulic fluid may be pumped into cylindrical cavity  26  through main fluid port  88 , thereby displacing air in cylindrical cavity  26  through bleed port  92 . In addition, pedestal  28  may include a cushion  94  located in depression  90  on top of pedestal  28 . Cushion  94  may be formed of a suitable synthetic material compatible with hydraulic fluid, such as Thorflex™, a material sold by Thordon Bearings, Inc. of Canada. Cushion  94  serves to protect pedestal  28  and block  20  from high contact stresses by preventing direct metal to metal contact between block  20  and the top  48  of pedestal  28  if hydraulic pressure is lost. Cushion  94  is mounted in depression  90  using machine screws (not shown) and screw holes  96 , as illustrated in FIGS. 2 b  and  2   e . In addition, cushion  94  includes a through-hole (not shown) which aligns with main fluid port  88  for permitting fluid to pass from main fluid port  88  into cavity  26 . 
     FIGS. 7 a - 7   b  illustrate a first arrangement for mounting and operating a plurality of bearing units  10 ,  11  for supporting a large rotatable element, such as a mooring turret  98  mounted in the hull of a ship  99 . A first set of a plurality of bearing units  10  are arranged in a radially symmetrical, equally spaced pattern for acting as thrust bearings for axially supporting turret  98 . A second set of a plurality of bearing units  11  are symmetrically arranged within a turret well enclosure  100  for acting as radial bearings. Thus, the thrust bearing units  10  are in sliding contact with a first bearing race  102  located on a downward-facing flat surface located near the upper end of turret  98 . A second bearing race  104  having a cylindrical configuration is provided on the outer periphery of the cylindrical surface of turret  98  for engagement with radial bearing units  11 . First and second bearing races  102 ,  104  are preferably formed of stainless steel, although other suitable materials may also be used. It will be apparent that as turret  98  rotates about a vertical axis relative to ship  99 , bearing races  102 ,  104  slide across bearing plates  22 ,  70 ,  84 , while bearing units  10 ,  11  serve to maintain the spatial position of turret  98  relative to ship  99  and turret enclosure  100 , and thereby prevent binding, contact, and the like. 
     In the embodiment illustrated in FIGS. 7 a - 7   b , once thrust bearing units  10  are pressurized, the fluid circuit is isolated from the fluid pumping unit (not shown in FIGS. 7 a - 7   b ) and thrust bearing units  10  are all manifolded together in fluid communication so that hydraulic fluid is able to flow between the individual bearing units  10 , but not back to the rest of the fluid circuit. Thus, the pressure applied by bearing units  10  is self-equalizing so that all bearing units  10  act in unison to equally support the load, while also allowing some degree of self-alignment and tilting of the load. For example, if a greater load is applied to one side of the bearing arrangement, say, due to deflections on turret  98 , the bearing units  10  on the side under greater load will tend to depress under the greater pressure, and the fluid in those bearing units  10  will circulate out of those bearing units  10  and toward the bearing units  10  on the opposite side of turret  98 . As the unequal load is relieved, the pressure applied to each bearing unit  10  will equalize, and, accordingly, the fluid will return to the bearing units  10  that were formerly depressed. This enables the bearing arrangement of FIGS. 7 a - 7   b  to act as a compliant, self-adjusting bearing system. Radial bearing units  11  may be similarly manifolded together in a group so that they also are compliant and self-adjusting. 
     In a second embodiment, as illustrated in FIG. 8, a plurality of thrust bearing units  10  are mounted in three distinct pad groups  110   a ,  110   b , and  110   c , with each pad group being centered 120 degrees apart from adjacent pad groups  110   a ,  110   b , and  110   c . The bearing units  10  in each individual pad group  110   a-c  are manifolded together, so that the distinct pad group acts as a single bearing support, but are not manifolded to the bearing units  10  in either of the other two pad groups  110   a-c . This results in the three groups of bearing units  10  behaving as three single bearing pads, thereby providing a self-aligning support within each pad group, but allowing no tilting of the load (in this case, turret  98 ). The arrangement of this second embodiment is particularly advantageous in the case of large diameter turrets of, for example, 10 meters diameter and larger. 
     FIG. 9 illustrates a portion of an exemplary fluid circuit of the invention that may be used with the bearing arrangement of FIG.  8 . The fluid circuit of the invention includes a number of conventional components, such as a fluid sump, main pump, purge pump, accumulators, and the like, which are well known in the art, and which are illustrated schematically as pump unit  112 . FIG. 9 further illustrates the fluid circuit schematic for pad groups  110   a ,  110   b , and  110   c . Each pad group  110   a-c  is connected to a main fluid line  114  and a purge/flush line  116 . A main line valve  118  and a purge/flush line valve  120  are included for each bearing unit  10 , so that each bearing unit  10  may be isolated, such as in the case of a bearing unit  10  requiring repair, replacement, deactivation due to fluid seal leakage, or the like. A bleed/purge fluid line  122  is also provided, and a bleed valve  124  is provided for each bearing unit  10  to enable bleeding/purging of individual bearing units  10 . In addition, each pad group  110   a-c  includes a main line isolation valve  126 . Isolation valves  126  enable each pad group  110   a-c  to be isolated from the pump unit  112 . However, by positioning main line valves  118  in the open position and purge line valves  120  and bleed valves  124  in the closed position, each bearing unit  10  in a particular pad group  110   a-c  remains in fluid communication with only the other bearing units  10  in that particular pad group  110   a-c , and thus, the bearing units  10  in each pad group  110   a-c  are manifolded to each other, but not to bearing units  10  in other pad groups  110   a-c . The fluid pressure in each pad group  110   a-c  may be monitored by pressure gauges  128 , or the like to determine that each pad group  110   a-c  remains properly pressurized. 
     In initial operation, bleed valves  124 , main line valves  118 , and isolation valves  126  are opened, while purge line valves  120  remain closed. Pump unit  112  is used to supply pressurized hydraulic fluid to bearing units  10 . Upon bleeding of all air from bearing units  10 , bleed valves  124  are closed. Bearing units  10  are then pressurized to a desired pressure so as to bring bearing plates  22 ,  70  into contact with first bearing race  102  and to thereby support turret  98 . Isolation valves  126  are then closed so that each pad group  110   a-c  is isolated from the other pad groups  110   a-c . However, each bearing unit  10  in a particular pad group  110   a-c  remains in fluid communication with the other bearing units  10  in that particular group  110   a-c . Thus, each pad group  110   a-c  acts as a single bearing unit, while the individual bearing units  10  in the pad group  110   a-c  are able to compensate among themselves for misalignments, irregularities in the bearing race  102 , or the like, by fluid flow between the bearing units  10  in that group. In addition, the number of bearing units  10  in each pad group  110   a-c  do not have to be uniform. For example, pad group  110   a  might consist of eight bearing units while pad groups  110   b  and  110   c  might only consist of six bearing units. This may be advantageous if pad group  110   a  is in line with the major axis of the ship and is subject to greater loads than pad groups  110   b  and  110   c.    
     The radial bearing units  11  may also be arranged in distinct pad groups in the manner described above. In addition, it is not necessary that the pad groups be distinctly spaced from each other. For example, bearing units  10 ,  11  shown in the arrangement of FIGS. 8 a - 8   b  may also be manifolded into pad groups if so desired. Under one such preferred arrangement, the thrust bearing units  10  may all be manifolded together, while the radial bearing units  11  may be manifolded into groups of three or four separate pad groups. Other such manifolding combinations will also be apparent to those skilled in the art, and it is to be understood that the embodiments shown are merely exemplary. 
     Should it be necessary to repair or replace a bearing unit  10 , (or a radial bearing unit  11 ) while the bearing system is in use, main line valve  118  is first closed to isolate the bearing unit  10  to be replaced from the other bearing units  10  in that group. Purge line valve  120  is then opened and purge line  116  is used to remove the fluid from that bearing unit  10 , while not affecting the operation of the remaining bearing units  10 . Following repair or replacement, purge line  116  is used to repressurize the repaired bearing unit  10  and the repaired bearing unit  10  is put back into fluid communication with the other bearing units  10  in its pad group  110   a-c  by closing purge line valve  120  and opening main line valve  118 . In addition, it should be apparent that the schematic for a single pad group, for example, pad group  110   a , represents the operation schematic for the first embodiment described above with reference to FIGS. 7 a - 7   b  in which all the bearing units  10  are manifolded together, and, accordingly, further description of the fluid circuit operation of that embodiment is not believed to be necessary. 
     Thus, the present invention sets forth a novel bearing unit and bearing operation system for use in supporting a large rotatable object. While the best mode of the invention has been set forth in a manner applied to a support system for a turret in an offshore mooring system, it will be apparent to those skilled in the art that other applications for the invention may also be advantageous. In addition, variations in the specific structure of the invention will also be apparent. For example, the positions of the block and the pedestal may be reversed so that the pedestal acts as a ram for supporting the bearing element. Also, other types of bearing elements might be substituted for bearing plates  22 ,  70 ,  84 . For example, rollers might be mounted on top of block  20  for use as the bearing elements for contacting bearing races  102 ,  104 . Other structural variations will also be apparent and are believed to be within the scope of the invention. Accordingly, while the foregoing disclosure sets forth exemplary embodiments of the present invention, it is to be understood that the invention is not limited to the particulars of the foregoing embodiments, but is limited in scope only as set forth in the following claims.