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CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to offshore mineral drilling and production platforms of the spar type and, more particularly, is concerned with apparatus for supporting drilling and production risers from a gimbaled table supported above the top of the spar hull wherein the table is compliantly constrained, but allowed limited rotational movement with respect to the spar hull. 
     2. Description of the Prior Art 
     Drilling and production operations for the exploration and production of offshore minerals require a floating platform that is as stable as possible against environmental forces, even in severe weather conditions. Among the six degrees of freedom of a floating platform, the most troublesome to drilling and production operations are the pitch, heave, and roll motions. 
     Present spar type floating platforms typically have drilling and production risers that are supported by means of buoyancy cans attached to each of the individual risers. As the water depth in which a platform will be used increases, the diameter and length of the buoyancy cans must be increased to support the in-water weight of the risers and their contents. Larger diameter buoyancy cans require larger spar center well sizes, which in turn increases the spar hull diameter. Increasing the spar hull diameter and size in turn increases the hydrodynamic environmental loads acting on the spar. A larger size mooring system is then required to withstand the increased environmental loads. The total riser buoyancy can system for deep water spar platforms can become very long and heavy, significantly increasing the fabrication and installation costs. 
     With present spar platforms having a buoyancy can riser support system, as the spar hull displaces laterally in response to environmental loads, the risers undergo a considerable amount of downward motion, or pull-down, with respect to the spar hull. This amount of riser pull-down increases as the water depth and riser length increases, and requires longer jumper hoses, large clear vertical heights between the top of the hull and the drilling deck, and expensive, large stroke keel joints. 
     Consequently, a need exists for improved apparatus for supporting drilling and production risers from a spar type floating platform. Preferably, such an improved apparatus will eliminate the need for riser buoyancy cans. It will preferably also reduce the amount of riser pull-down relative to the spar hull as the spar pitches and displaces in response to environmental forces. Such an improved riser support apparatus will also preferably reduce the amount of fixed ballast required, reduce the need for, or length of, riser jumper hoses, and reduce the size and diameter of the spar hull. It will also preferably be less expensive to build, install, and maintain than individual riser buoyancy can systems in present use. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a riser support and tensioning apparatus and method that satisfies the aforementioned needs. According to one aspect of the invention, for a spar type floating platform having risers passing vertically through the center well of a spar hull, the spar hull having a top surface, apparatus is provided for supporting the risers from the spar hull. The apparatus comprises a table disposed above the spar hull top surface and a plurality of non-linear springs associated with the table and the spar hull for permitting rotational movement between the table and the spar hull. The apparatus also comprises means for attaching the upper ends of the risers to the table. 
     According to another aspect of the invention, for a spar type floating platform having risers passing vertically through the center well of a spar hull, the spar hull having a top surface, apparatus is provided for supporting the risers from the spar hull. The apparatus comprises a table disposed above the spar hull top surface. The table comprises a grid having openings therethrough. The risers pass through respective openings in the table grid. For each riser, at least one riser tensioning hydraulic cylinder is provided, having one end attached to the riser and the opposite end attached to the table, such that the tension in and length of the riser may be adjusted by operation of the riser tensioning hydraulic cylinder. A plurality of elastomeric load pads are disposed between the table and the spar hull for permitting rotational movement therebetween. Larger capacity load pads are located near the center of the table for supporting the majority of the riser tension, and smaller capacity load pads are located near the perimeter of the table for controlling the rotational stiffness of the spar hull. 
     According to a still farther aspect of the invention, for a spar type floating platform having risers passing vertically through the center well of a spar hull, the spar hull having a top surface, apparatus is provided for supporting the risers from the spar hull. The apparatus comprises a table disposed above the spar hull top surface. The table comprises a grid having openings therethrough. The risers pass through respective openings in the table grid. For each riser, at least one riser tensioning hydraulic cylinder is provided, having one end attached to the riser and the opposite end attached to the table, such that the tension in and length of the riser may be adjusted by operation of the riser tensioning hydraulic cylinder. A plurality of table supporting hydraulic cylinders is disposed between the table and the spar hull for permitting rotational movement therebetween. Each table supporting hydraulic cylinder has a first end pivotally attached to the table and a second end pivotally attached to the spar hull. At least one lateral support shaft has an upper end pivotally attached to the table and a lower end. For each lateral support shaft, at least one guide is attached to the spar hull for slidably receiving the lower end of the lateral support shaft. 
     According to another aspect of the invention, for a spar type floating platform having risers passing vertically through the center well of a spar hull, the spar hull having a top surface, apparatus is provided for supporting the risers from the spar hull. The apparatus comprises a table disposed above the spar hull top surface. The table comprises a grid having openings therethrough. The risers pass through respective openings in the table grid. For each riser, at least one riser tensioning hydraulic cylinder is provided, having one end attached to the riser and the opposite end attached to the table, such that the tension in and length of the riser may be adjusted by operation of the riser tensioning hydraulic cylinder. A plurality of pedestals is provided, each pedestal having a lower end attached to the spar hull and an upper end higher than the table for hanging the table therefrom. For each pedestal, at least one non-linear spring is associated with the table, the pedestal, and the spar hull for permitting rotational movement between the table and the spar hull. 
     According to still another aspect of the invention, for a spar type floating platform having risers passing vertically through the center well of a spar hull, apparatus is provided for suspending and tensioning a riser from a surface associated with the spar hull, and for permitting limited rotational movement between the riser and the surface. The apparatus comprises a hydraulic cylinder having one end attached to the riser and the other end attached to the surface. The tension in the riser may be adjusted by operation of the hydraulic cylinder. Means is provided for permitting rotational movement between the riser and the surface. 
     According to still another aspect of the invention, a method is provided for supporting a riser at a floating spar hull, the spar hull having a top surface. The method comprises the step of connecting a table to the spar hull, wherein the table has a limited range of rotational movement with respect to the spar hull top surface in response to environmental forces acting on the spar hull. The method further comprises the steps of suspending the riser from the table and of tensioning the riser. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     For a more complete understanding of the invention, and the advantages thereof, reference is now mad to the following detailed description of the invention taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic, side elevation view in cross-section of a spar type floating platform having a riser support apparatus of the present invention. 
     FIG. 2 is a plan view of the riser support table of the present invention. 
     FIG. 3 is a side, cross-sectional view of an apparatus of the present invention for supporting and tensioning the risers. 
     FIG. 4 illustrates an alternative, ball-in-socket device that may be used in the apparatus of FIG.  3 . 
     FIG. 5 is a schematic, side elevation view in cross-section of the upper portion of the spar hull and an embodiment of the riser support apparatus of the invention utilizing elastomeric load pads. 
     FIG. 6 is a schematic, side elevation view in cross-section of the upper portion of the spar hull illustrating an alternative embodiment of the invention utilizing table supporting hydraulic cylinders. 
     FIG. 7 is a schematic, side elevation view in cross-section of the upper portion of the spar hull illustrating an alternative embodiment of the invention wherein the riser support table is hanging from pedestals attached to the spar hull. 
     FIG. 8 illustrates an embodiment of the invention utilizing both elastomeric load pads and table supporting hydraulic cylinders. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, and more particularly to FIG. 1, there is schematically shown a side elevation view of a spar type floating platform, generally designated  10 , employing a riser support apparatus of the present invention. Spar platform  10  includes spar hull  12  having buoyancy tanks  14  at its upper end. Production risers  16  and drilling riser  18  extend from wells (not shown) on the sea floor  20  up through keel joint  22  at the lower end of spar hull  12 . The risers  16  and  18  extend up through the center well  24  of spar hull  12  and are tied at their upper ends to riser support apparatus  26 . Riser support apparatus  26  includes riser support table  28 , which is compliantly supported above top surface  30  of spar hull  12  by non-linear springs  32 . Trees  34  are attached to the upper ends of risers  16  and  18 . Spar hull  12  floats at and extends slightly above water surface  36 . 
     Referring now to FIG. 2, there is shown a plan view of riser support table  28 . Table  28  is made up of beams  38  interconnected to form a grid. Production risers  16  and drilling riser  18  pass through respective openings  40  of the grid of table  28 . 
     FIG. 3 illustrates an apparatus of the present invention for supporting and tensioning risers  16  and  18  from riser support table  28 . As seen in FIG. 3, riser support bracket  42  is clamped or welded to riser  16  above table  28 . Riser tensioning hydraulic cylinders  44  located below riser support bracket  42  have pistons  46  attached to riser support bracket  42 . The bottoms of hydraulic cylinders  44  are attached to table  28  by elastomeric flex units  48 . Elastomeric flex units  48  permit relative rotation between hydraulic cylinders  44  and table  28 , and thus between riser  16  and table  28 . Some degree of rotation between risers  16  and  18  and table  28  is necessary because risers  16  and  18  will tend to remain parallel to the axis of spar hull  12 , or tilt with spar hull  12 , as table  28  rotates relative to spar hull  12 . Elastomeric flex units include rigid portions  50  and flexible portions  52  between rigid portions  50 . Rigid portions  50  are preferably made of steel, and flexible portions  52  are preferably made of an elastomeric material. 
     After risers  16  and  18  are installed on table  28 , hydraulic cylinders  44  may be operated to adjust the tension and lengths of the risers to provide the correct fixed ballast to the spar hull from the riser weight, and to compensate for temperature changes in the risers caused by the produced fluid and the temperature of the surrounding risers. 
     FIG. 4 illustrates an alternative device to elastomeric flex units  48  for permitting relative rotation between hydraulic cylinders  44  and table  28 . In this embodiment, a segment of a ball  54  is attached to the bottom of hydraulic cylinder  44 , and a mating cup  56  is attached to table  28 . Spherically shaped surface  58  of cup  56  slidingly engages the spherical surface of ball segment  54 , and permits relative rotation between hydraulic cylinder  44  and table  28 , and thus between riser  16  and table  28 . 
     FIG. 5 illustrates a first embodiment of a riser support apparatus of the present invention. In this embodiment, elastomeric load pads  58  and  60  function as non-linear springs  32  for compliantly supporting table  28  above top surface  30  of spar hull  12 , as described with reference to FIG.  1 . Elastomeric load pads  58  and  60  are sized to be strong enough to support the tension in all of the risers  16  and  18  and with a spring rate that keeps the heave period of the spar platform and the riser support system larger than the dominant wave period. Elastomeric load pads  58  and  60  are placed laterally around table  28  in such a manner as to allow table  28  to rotate to a limited degree relative to spar hull top surface  30  as spar hull  12  pitches in response to environmental forces. This relative rotation is necessary to prevent large axial tension and compression fluctuations in risers  16  near the outer perimeter of table  28 . Risers  16  are axially secured at their upper ends to table  28 , and at their lower ends to the sea floor. Therefore, if table  28  were rigidly fixed in its position above spar hull top surface  30  without any means for relative rotation therebetween, a tilt of spar hull  12  from its normally vertical position would induce large compressive loads in the risers  16  on the side of spar hull  12  tilted down. This large compressive load would overstress and eventually buckle these risers. Similarly, the risers  16  on the opposite side of spar hull  12  would experience large tensile loads. The large variations in axial tension and compression in risers  16  would result in unacceptable fatigue damage to risers  16  over the lifetime of the installation. The relative rotation between table  28  and spar hull  12  permitted by elastomeric load pads  58  and  60  allows the upper ends of risers  16  to “float” with respect to upper surface  30  of spar hull  12 , and thus prevents large axial tension and compression fluctuations in risers  16  resulting from environmentally induced pitching of spar hull  12 . 
     As seen most clearly in FIG. 2, large capacity elastomeric load pads  58  are located near the center of table  28  for supporting a large portion of the riser tension. Smaller capacity elastomeric load pads  60  are located near the perimeter of table  28  for controlling the rotational stiffness of table  28  with respect to spar hull  12 . The combined axial stiffness of all the risers  16  and  18  installed on the spar platform varies in direct proportion to the number of risers installed. When fewer risers are installed, their combined axial stiffness is reduced proportionately. Therefore, the vertical stiffness of the riser support apparatus does not normally require adjustment as risers  16  and  18  are added to, or removed from, table  28 . Furthermore, regardless of the number of risers installed on table  28 , the heave period of the spar platform and riser support system will be greater than the dominant wave period if the appropriate spring rate is chosen for elastomeric load pads  58  and  60 . 
     As additional risers are suspended from table  28 , the rotational stiffness of the riser support system may be increased by inserting additional smaller capacity elastomeric load pads  60  around the perimeter of table  28 . Alternatively, variable stiffness elastomeric load pads may be used for load pads  60 . These commercially available load pads have an interior, sealed air chamber that can be pressurized or depressurized as needed to adjust their stiffness. 
     FIG. 6 illustrates an alternative embodiment of a riser support apparatus of the present invention. In this embodiment, table supporting hydraulic cylinders  62  and  63  function as non-linear springs  32  for compliantly supporting table  28  above top surface  30  of spar hull  12  as described with reference to FIG.  1 . Large capacity hydraulic cylinders  62  are located near the center of table  28  for supporting a large portion of the riser tension. Smaller capacity hydraulic cylinders  63  are located near the perimeter of table  28  for controlling the rotational stiffness of table  28  with respect to spar hull  12 . In order to permit table  28  to rotate about both horizontal axes with respect to spar hull  12 , the upper ends of hydraulic cylinders  62  and  63  are pivotally attached to table  28 , and the lower ends are pivotally attached to spar hull  12 . 
     Air-over-oil accumulators  64  are hydraulically connected to smaller capacity hydraulic cylinders  63  for providing them with an adjustable spring rate. For a stiff spring rate, a relatively small amount of air should be maintained in accumulators  64 . The use of hydraulic cylinders  63  with air-over-oil accumulators  64  provides greater operational flexibility than the riser support apparatus of FIG.  5 . Both the tension force and the stiffness of hydraulic cylinders  63  can easily be adjusted over time by simply increasing or decreasing the air pressure in accumulators  64 . 
     Because table supporting hydraulic cylinders  62  and  63  operate in compression and are hinged at their opposite ends, table  28  must be laterally supported with hydraulic cylinders  62  and  63  in their upright position to prevent table  28  and hydraulic cylinders  62  and  63  from folding down flat against upper surface  30  of spar hull  12 . Lateral support shafts  66  provide the required lateral stability to the riser support apparatus of FIG.  6 . The upper ends of lateral 
     support shafts  66  are pivotally attached to table  28  so as to permit relative rotation between table  28  and spar hull  12 . The lower ends of shafts  66  are loosely fitted within guides  68  attached to spar hull  12 . Lateral support shafts  66  slide axially within guides  66  as table  28  tilts with respect to upper surface  30  of spar hull  12  in response to environmental loads. For a spar hull  12  having a center well  24  of square cross-sectional shape, four lateral support shafts  66  are preferably used, one being located near each of the four corners of center well  24 . 
     FIG. 7 illustrates another alternative embodiment of a riser support apparatus of the present invention. In this embodiment, table  28  is partially supported from the bottom only by elastomeric load cells  58  located near the center of table  28 . To provide additional vertical support and the necessary lateral stability, table  28  is hung from pedestals  70 . The lower ends of pedestals  70  are rigidly attached to spar hull  12 , and their upper ends are higher than table  28  so that table  28  may be hung therefrom. Table supporting hydraulic cylinders  63  are used to provide limited rotational movement to table  28 . With this arrangement, table  28  is naturally stable because it is suspended from an upper support structure. 
     FIG. 7 illustrates two ways in which table  28  may be hung from pedestals  70  by hydraulic cylinders  63 . The first way is illustrated at the right end of table  28 . Here, hydraulic cylinder  63  has an upper end pivotally connected to the top of pedestal  70  and a lower end pivotally connected to table  28 , so that hydraulic cylinder  63  directly supports table  28  from pedestal  70 . Air-over-oil accumulator  64  is placed on table  28  near, and is hydraulically connected to, hydraulic cylinder  63  to provide it an adjustable spring rate as described above with reference to hydraulic cylinders  63  in FIG.  6 . 
     The second way in which table  28  may be hung from pedestals  70  is illustrated at the left end of table  28 . Here, pulley  72  is pivotally mounted near the top of pedestal  70 . Cable  74  passes over the top of pulley  72  and has one end attached to table  28  and the opposite end attached to the upper end of hydraulic cylinder  63 . The lower end of hydraulic cylinder  63  is attached to spar hull  12  so that the tension in cable  74  is borne by hydraulic cylinder  63 . Air-over-oil accumulator  64  is placed on spar hull  12  near, and hydraulically connected to, hydraulic cylinder  63  as described above. Although not illustrated, hydraulic cylinder  63  could instead be mounted on table  28  and connected to the opposite or right end of cable  74 . In that case, the left end of cable  74  opposite hydraulic cylinder  63  would be connected directly to spar hull  12 . 
     FIG. 8 illustrates a combination of some of the above described alternative embodiments of the riser support apparatus of this invention. Such a combination of features may provide the most desirable system in terms of operational flexibility. Large, rather stiff elastomeric load pads  58  placed under and near the center of table  28  support the majority of the tension in risers  16  and  18 . Four lateral support shafts  66  pivotally attached to table  28  and located near the corners of center well  24  of spar hull  12  provide the needed lateral stability to table  28 . Smaller capacity table supporting hydraulic cylinders  63  located under and near the perimeter of table  28  provide the proper rotation stiffness. Depending on the direction of rotation of table  28 , hydraulic cylinders  63  could act in either compression or tension. The tension and sniffiness of hydraulic cylinders  63  can be adjusted by adjusting the air pressure in accumulators  64  to keep the overall rotational stiffness of table  28  at the desired level over time as wells are drilled and additional production risers  16  are installed. 
     A coupled computer aided design analysis was performed to compare a number of variable design parameters of a spar floating platform having a riser support system of the present invention with those of a traditional spar platform having risers individually supported by buoyancy cans. The analysis was based on the following fixed design parameters for both types of spar platforms: 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 Design Basis 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Water depth: 
                 4500 feet 
               
               
                   
                 Topside weight: 
                 39,000 tons 
               
               
                   
                 Topside VCG above hull top: 
                 80 feet 
               
               
                   
                 Wind sail area: 
                 68,000 square feet 
               
               
                   
                 Wind center of pressure: 
                 150 feet 
               
               
                   
                 Number of wells: 
                 20 
               
               
                   
                 Well pattern: 
                 5 × 5 
               
               
                   
                 Production risers: 
               
               
                   
                 outer casing outer diameter: 
                 13.375 inches 
               
               
                   
                 outer casing thickness: 
                 0.48 inches 
               
               
                   
                 inner casing outer diameter: 
                 10.75 inches 
               
               
                   
                 inner casing thickness: 
                 0.797 inches 
               
               
                   
                 tubing outer diameter: 
                 5.5 inches 
               
               
                   
                 tubing thickness: 
                 0.415 inches 
               
               
                   
                 Outer casing design pressure: 
                 4000 psi 
               
               
                   
                 Inner casing design pressure: 
                 8500 psi 
               
               
                   
                 Tubing design pressure: 
                 8500 psi 
               
               
                   
                 Fluid weights under production: 
               
               
                   
                 Outer casing: 
                 8.55 ppg 
               
               
                   
                 Inner casing: 
                 15.5 ppg 
               
               
                   
                 Tubing: 
                 5.5 ppg 
               
               
                   
                 Riser tree elevation: 
                 55 feet 
               
               
                   
                 Total riser weight at tree elevation: 
                 872 kips 
               
               
                   
                 Riser weight at keel: 
                 736 kips 
               
               
                   
                 Riser wet weight per foot: 
                 191 lb/ft. 
               
               
                   
                 Riser EA/L: 
                 325 kips/ft. 
               
               
                   
                   
               
             
          
         
       
     
     The coupled design analysis resulted in the following design parameters for spar platforms having each type of riser support system: 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                 Traditional spar 
                 Spar with riser 
               
               
                   
                 with riser 
                 support system 
               
               
                   
                 buoyancy cans 
                 of invention 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Spar center well 
                 wet 
                 wet 
               
               
                 Center well size (feet) 
                 75 × 75 
                 50 × 50 
               
               
                 Spar hull diameter (feet) 
                 158 
                 150 
               
               
                 Draft (feet) 
                 650 
                 650 
               
               
                 Hard tank depth (feet) 
                 255 
                 245 
               
               
                 Freeboard (feet) 
                 55 
                 55 
               
               
                 Truss height (feet) 
                 360 
                 380 
               
               
                 Soft tank height (feet) 
                 35 
                 25 
               
               
                 Hull steel weight (tons) 
                 29,937 
                 29,200 
               
               
                 Fixed ballast (tons) 
                 36,668 
                 21,844 
               
               
                 Riser tension supported (tons) 
                 0 
                 14,160 
               
               
                 Variable ballast (tons) 
                 12,347 
                 14,398 
               
               
                 Number of mooring lines 
                 16 
                 16 
               
               
                 Mooring pattern 
                 4 × 4 
                 4 × 4 
               
               
                 Pretension (kips) 
                 650 
                 550 
               
               
                 Fairlead elevation (feet) 
                 255 
                 245 
               
               
                 Upper chain 
               
               
                 diameter (inches) 
                 5.875 
                 5.875 
               
               
                 length (feet) 
                 250 
                 250 
               
               
                 Wire 
               
               
                 diameter (inches) 
                 5.375 
                 5.125 
               
               
                 length (feet) 
                 6000 
                 5500 
               
               
                 Lower chain 
               
               
                 diameter (inches) 
                 5.875 
                 5.875 
               
               
                 length (feet) 
                 200 
                 200 
               
               
                   
               
             
          
         
       
     
     There are several advantages attained by the use of the gimbaled table riser support system of the present invention with a spar type floating platform. First, the magnitude of spar pitch motions are reduced 10 to 25 percent from those of a traditionally designed spar with buoyancy cans. Second, because the gimbaled table supports the risers, the riser weight replaces fixed ballast in the spar hull. Therefore, the amount of fixed ballast required is greatly reduced by approximately 40 percent. Third, the need for buoyancy cans for supporting the risers is eliminated. This also eliminates released buoyancy can concerns and the need for buoyancy can guide structures. Fourth, riser pull-down relative to the spar hull is significantly reduced, which reduces jumper hose requirements. Fifth, a simplified keel joint design may be used. Sixth, the present invention permits easier drilling and production operations and easier access to trees and risers. Seventh, the riser tensioning system becomes more manageable and inspectable. Eighth, riser interference is essentially eliminated. Ninth, the spar hull diameter and center well size may be reduced. This in turn reduces the mooring line size requirement. Tenth, the smaller sea floor riser pattern reduces the amount of lateral offset of the spar platform. Eleventh, slip joint requirements are reduced, and requirements for drilling tensionsers and workover riser tensioning are eliminated. Twelfth, special workover buoyancy requirements are eliminated. Thirteenth, the smaller size center well permits reduced topside dimensions. Fourteenth, tensioning system redundancy is not required for each individual riser. Therefore, the need for an extra buoyancy chamber in each riser is eliminated. Finally, a riser support system of the present invention is less expensive to build, install, and maintain than the individual riser buoyancy can system in present use. 
     The gimbaled table riser support system and method of the present invention, and many of its intended advantages, will be understood from the foregoing description of example embodiments, and it will be apparent that, although the invention and its advantages have been described in detail, various changes, substitutions, and alterations may be made in the manner, procedure, and details thereof without departing from the spirit and scope of the invention, as defined by the appended claims, or sacrificing any of its material advantages, the form hereinbefore described being merely exemplary embodiments thereof.

Summary:
For a spar type floating platform having risers passing vertically through the center well of a spar hull, there is provided apparatus for supporting the risers from a gimbaled table supported above the top of the spar hull. The table flexibly is supported by a plurality of non-linear springs attached to the top of the spar hull. The non-linear springs compliantly constrain the table rotationally so that the table is allowed a limited degree of rotational movement with respect to the spar hull in response to wind and current induced environmental loads. Larger capacity non-linear springs are located near the center of the table for supporting the majority of the riser tension, and smaller capacity non-linear springs are located near the perimeter of the table for controlling the rotational stiffness of the table. The riser support table comprises a grid of interconnected beams having openings therebetween through which the risers pass. The non-linear springs may take the form of elastomeric load pads or hydraulic cylinders, or a combination of both. The upper ends of the risers are supported from the table by riser tensioning hydraulic cylinders that may be individually actuated to adjust the tension in and length of the risers. Elastomeric flex units or ball-in-socket devices are disposed between the riser tensioning hydraulic cylinders and the table to permit rotational movement between the each riser and the table.