Patent Publication Number: US-6990917-B2

Title: Large diameter mooring turret with compliant deck and frame

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This non-provisional application is based on Provisional Application Ser. No. 60/344,104 filed Dec. 28, 2001, the priority date of which is claimed for this application. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to mooring systems for offshore vessels and Floating Production Units (“FPUs”) such as Floating Storage and Offloading vessels (“FSOs”), Floating Production Storage and Offloading vessels (“FPSOs”), Floating Storage Drilling Production and Drilling Units (“FPDSOs”) and in particular to turret mooring arrangements, or systems, where a turret is rotatably supported on the vessel and where the turret is fixed to the sea bed by anchor legs so that the vessel can weathervane about the turret. 
   2. Description of the Prior Art 
   Turret mooring systems have been used for some time for FPUs and especially with FPSOs. FPSOs are production platforms typically constructed by reconfiguring existing tanker hulls. FPSOs are the most useful of FPUs in terms of water depth and sea conditions due to their variation in moorings and ship shape configurations. FPSOs are either spread moored (anchored directly to the seafloor and unable to completely weathervane and rotate around a center point of mooring), or they are attached to the seafloor via an internal or external rotatable turret that is moored to the seafloor for 360° weathervaning capability of the vessel.  FIG. 1  is an illustration of a prior art turret moored FPSOV with the turret connected to the sea floor by groups of anchor legs L and risers R running from the sea floor to the turret for rotatable coupling to vessel pipes which run to storage holds. 
   FPSOs compete with other kinds of floating production units such as semi-submersibles, spars, and tension leg platforms. These other systems generally do not have large product storage capacity like FPSOs, but they do have the advantage of easily handling a large number of risers (the flexible pipes and control umbilicals connected between the production unit and subsea wellheads). Large numbers of risers are required for subsea oil fields when it is not desirable to use subsea manifolding connecting several wells together. The number of risers can be from the twenties to ninety or even more. The spread moored FPSO has the advantage of large product storage capacity and also has the space capability for large numbers of risers. One main disadvantage of the spread moored FPSO is the reduced availability for tandem offloading due to occasional bad weather conditions preventing a safe approach of the shuttle tanker to connect to the FPSO. In many locations the rough weather direction changes and can also cause undesirable rolling motions of the vessel that are problematic to the process equipment and to the crew. The competitiveness of all of the above floating production units depends on their advantages and disadvantages. 
   As mentioned above, the present invention is directed to a turret mooring arrangement, and in particular to a rotatably mounted turret of large diameter for the purpose of accommodating a large number of risers and for providing other advantages resulting from a large diameter geostationary turret. Such advantages are summarized below. 
   Prior turret mooring arrangements are known in the art that include turrets of small to moderate diameter where the problems associated with vessel hull deflections are considered. A moonpool (a cylindrical tube extending from top to bottom through a vessel hull) is required to contain and usually support the turret bearing and turret shaft. Flexure of the vessel hull due to sea conditions can cause undesirable structural deflections in the moonpool at the foundations for the turret bearings. This effect can be substantial and detrimental for large moonpool diameters, and unless steps are taken to mitigate such effects, the turret bearings will suffer from high concentrated loads. 
   Prior turret designers have sought to minimize turret diameters due to requirements of roller bearing assemblies requiring flat machined surfaces not exceeding a predetermined diameter. In such arrangement, designers have sought to isolate the flat bearing races with various elastic elements and apparatus in an effort to accommodate hull deflections. Other designers have attempted to provide bearing wheel and rail arrangements for vessel-turret designs. A few of the prior art attempts to solve the problem of vessel hull deflection as it affects bearing operation is presented below. 
   Norwegian Patent No. 165,285 shows a structural suspension that attempts to provide a satisfactory load distribution around a bearing wheel track that may not be flat. Independent radial arms are disclosed to which vertical and radial load rollers are attached. The radial arms attach to a circular ring that twists to add to the flexibility of the bending beam deflection of the arms. This concept is limited in load carrying capacity and limited to relatively small turret diameters. 
   U.S. Pat. No. 5,052,322 to Poldervaart illustrates a bearing fixed to a rigid ring that does not follow deformations of the hull of the ship. A cylindrical tube supporting the rigid ring tends to flex with the vessel hull while the bearing and turret remain relatively isolated from hull deflection. The benefits of this design diminish as the moonpool (or turret insert tube) diameter and hull deflections increase. 
   U.S. Pat. No. 5,515,804 to Pollack shows internal and external turret bearing arrangements with a generally rigid upper mount including a resiliently deflectable support structure that includes a plurality of elastomeric shear pads. These arrangements are also difficult and expensive to scale up to large diameters due to the proportionally increasing size and shear motion capacity of the shear pads. 
   U.S. Pat. No. 5,359,957 to Askestad illustrates radial bearing arms connected to a substructure in the turret which provide individual suspension and can absorb unevenness and deformations in the bearing. Rollers attached to the ends of the radial arms support the turret load. This design is also limited in load carrying capacity by the difficulty of attaching large numbers of rollers for high load capacity. 
   U.S. Pat. No. 5,517,937 to Lunde shows a turret arrangement for accommodating many risers in which the riser tubes are arranged at an angle to minimize the bearing diameter to about eight meters or less while the bottom diameter of the turret is made large in diameter to accommodate the necessary spacing of the risers below the turret. Minimizing the bearing diameter is one way of mitigating the effects of the previously mentioned deflections, but construction complexity and other disadvantages such as limited equipment space inside the turret result from this arrangement. As the numbers of risers increase, their weight eventually overcomes the available capacity of the smaller bearing diameters. 
   U.S. Pat. No. 5,860,382 to Hobdy illustrates a turret with radial bearing rollers arranged with spring assemblies that allow for unevenness of the radial wheel rail and maintain roller contact with their rail. This arrangement of turret and bearing is suitable for risers numbering thirty to forty, but may not be practical for a much larger quantity of risers. The limitation of larger turrets of this design is the low flexibility of the tube-shaped turret structure. The turret is vertically shear-stiff, and the wheel and rail system must therefore be designed for significantly increased loading per wheel to accommodate the out-of-flat condition of the vertically loaded wheel rails. 
   U.S. Pat. No. 6,164,233 to Pollack describes bearing devices that include hydraulic cylinders and pistons to support vertical loads that are arranged to accommodate vessel hull deformations. 
   U.S. Pat. No. 6,263,822 to Fontenot shows elastomeric pads arranged radially and vertically around the main bearing which rotatably supports a mooring turret. This arrangement for shear and compression of elastomeric pads serves to compensate for hull deflection at the main bearing. The elastomeric pads accommodate vertical and radial deflections of the hull. This design is also expensive and may be difficult to scale upward to a large size. 
   U.S. Pat. No. 6,269,762 to Commandeur illustrates a bogie wheel bearing support structure mounted on top of a moonpool tube that extends above the connection to the vessel hull to isolate the bearing structure from the hull deflections. Commandeur also shows elastically deformable elements (rubber filler) beneath the bogie wheels to help even out the load on the wheels. The very tall moonpool tube also serves to isolate radial hull deflections from the bearing assemblies. 
   The advantages of this invention will be more apparent by comparison to prior art turrets. 
     FIG. 2  shows a prior art large turret capable of supporting  43  risers that was supplied for a Petrobras Field Development offshore of Brazil. The illustration is of the turret parts loaded on a barge B for transport from the fabrication yard. The complex arrangement of the lower turret T can be seen in which the turret structure and all of the riser guide tubes are tapered toward the top end in an effort to reduce the upper bearing diameter. The turret structure is of rigid construction. 
     FIG. 3  is a drawing of a prior art turret supplied by SOFEC, Inc. for an offshore oil field in the South China Sea. The turret  200  has a cylindrical tube structure that is relatively rigid in bending and shear. The upper bearing structure and the turret are rigid in the radial and vertical directions. A spring suspension system supporting the upper bearing  202  in combination with a heavily reinforced bearing support  204  structure allows structural deflections of the vessel at the turret insert tube (moonpool) without overloading the bearing. The bearing is a three-row roller bearing mounted in a manner similar to the apparatus of U.S. Pat. No. 5,356,321 to Boatman. This turret arrangement is typical of many in the single point mooring industry utilizing a combination of a lower bearing  208  near the vessel keel with an upper bearing  202  located near the main deck of the vessel. 
     FIG. 4  is a drawing of a prior art turret designed and supplied by SOFEC, Inc. for an oil field offshore of Brazil. An upper bearing system, located near the main deck of the vessel, includes a radial wheel/rail bearing  210  and an axial wheel/rail bearing  212  to provide rotational support between the turret and the vessel. No lower radial bearing was provided. A wheel and rail bearing system was provided for the vertical load to withstand large loads, because hull deflections concentrate the load onto only a fraction of the wheels. The vertical load rollers were designed with sufficient excess capacity per roller to carry the total load on only a portion of the total number of rollers. Radial wheels mounted on springs that spread the load over many radial wheels accommodates the radial deflections. The turret is stiff in both the radial and vertical directions. 
   For small diameter turrets, an axial roller bearing assembly can be provided between the turret and the vessel. Such roller bearing assemblies require that the bearing races be flat, machined surfaces. Such races have in the prior art often been isolated from ovaling due to vessel sagging and hogging by various elastic arrangements between a lower bearing race and the vessel. As the diameter of the turret becomes very large, roller bearing assemblies are not feasible due to the inability to machine flat surfaces for the very large diameter. Wheel-rail assemblies can be installed between the turret and the vessel, as described above, but for very large turrets carrying a very large number of risers, the forces on certain wheels due to the sagging or hogging of the vessel can become so large as to make it impractical to provide a very large turret for accepting a very large number of risers. The above very large number of risers connotes a number of from 40 to 120 risers. 
   Summing up, the problems for designing a very large turret (VLT) in the past have been either of vertically and radially stiff construction combined with various expensive devices to isolate the bearing, or they are limited in their range of diameter and load carrying capacity. The problems associated with a relatively inflexible structure limits the economic benefits of a large diameter turret, requires larger bearing capacities, and tends to reduce the wear life of the bearings. 
   3. Identification of Objects of the Invention 
   A primary object of the present invention is to provide an economical turret arrangement that has inherent structural flexibility, thereby making practical a large diameter main bearing that supports a very large turret. 
   Another object of the present invention is to provide an economical large diameter turret mooring arrangement for an FPSO that will accommodate a very large number of risers (either flexible non-metallic pipe or rigid steel pipe flow lines) where the large number of risers greatly exceeds those presently known in the art. 
   Another object of the present invention is to provide a practical turret configuration of sufficient size that allows a weathervaning vessel to be used as a floating production unit (FPU) with at least as many risers as can be connected to a non-weathervaning FPU such as a spread moored ship-shaped vessel or a semi-submersible vessel. 
   Another object of the present invention is to provide a wheel and rail bearing arrangement for a very large turret (VLT) frame configuration that has sufficient flexibility so that vessel hogging and sagging deflection causes a maximum load per wheel to increase not more than preferably about 50 percent greater than would occur with the rails in a perfectly flat plane, and not exceeding 150 percent greater than would occur with the rails in a flat plane. 
   Another object of the present invention is to provide a turret with a flexible structural frame configuration that allows a sliding-type lower bearing of a diameter greater than 12 meters diameter to be used near the vessel keel elevation in combination with an upper bearing greater than about 14 meters diameter located near the vessel main deck. 
   Another object of the present invention is to provide a turret with a flexible structural frame configuration with elastomeric bumper pads attached to the lower turret near the vessel keel elevation in combination with an upper bearing greater than about 14 meters diameter located near the vessel main deck. 
   Another object of the present invention is to provide a turret with a flexible structural frame configuration that allows the optional installation of protective riser tubes between the chain table and the main deck without appreciably increasing the stiffness of the turret frame. 
   SUMMARY OF THE INVENTION 
   The objects identified above, as well as other features and advantages of the invention are provided by a turret configuration in which the turret includes an upper section, a lower section, and a coupling structure such as at least three vertical columns or riser tubes alone for coupling the upper and lower sections together. The turret mooring arrangement is rotatably supported on a vessel that floats at the surface of the sea and that can weathervane about the turret. The lower section of the turret is anchored by at least three mooring lines that extend to the sea floor for anchoring the turret in a substantially geostationary position. 
   The arrangement utilizes a known bearing system, that is, a wheel and rail system that can be manufactured economically in sizes larger than 14 meters diameter. The phrase, “very large turret” (VLT), as used herein, refers to turrets requiring moonpool diameters larger than about 14 meters and up to about 35 meters. The moonpool diameter is limited only by the available width of the vessel into which the moonpool (turret insert tube) is fitted. The turret frame is configured in a way that provides sufficient flexibility to allow the turret main deck to conform to the vessel deck flexure shape as the vessel bends in the so-called “hogging and sagging” conditions. The bending flexure of the vessel hull causes the bottom or lower supporting surfaces on the vessel on which the wheels or rollers are supported to elastically flex and not remain in a flat plane. The load carrying frame members of the turret flex in concert with the vessel hull due to turret loads and thereby spread the loads to turret mounted upper rails for the wheels more uniformly than is possible with a stiff turret frame. 
   The upper section of the turret includes an axial/radial bearing assembly. This assembly permits the vessel to weathervane about the turret while resisting loadings caused by weather conditions, including sea conditions, causing the vessel to heave, pitch, roll, and yaw in the sea. The bearing assembly uses the commercially available Amclyde type flanged wheel and rail construction that can be manufactured economically for rail sizes larger than 12 meters diameter. The bearing foundation or support structure attached to the vessel hull bends and flexes with the vessel hull. The main deck of the turret is capable of flexing under the vertical load of the turret weight, mooring legs, and the weight of the risers and due to its flexible design follows the flex of the vessel. Certain geometric ratios such as main deck thickness to diameter; main deck thickness to depth of vessel hull; and column diameter to column length are required to be within certain ranges to provide the required flexibility without causing detrimental large stresses in the frame members. 
   The lower section of the turret includes a chain table to which mooring legs are attached, a structural coupling arrangement such as vertical columns which connect the chain table to the main deck, and riser tubes which protectively enclose the risers between the chain table and the main deck. An alternative embodiment of the invention places elastomeric bumper units at the outside diameter of the chain table to occasionally react against the inside of the moonpool. 
   Existing tanker vessels in the (Very Large Crude Carrier) VLCC class are available in the industry for FPSO conversion. The hull width of the VLCCs range from 50 meters to as much as 70 meters beam width. These vessels, with moonpool diameters of up to about 30 to 35 meters, can accept turrets that are practical according to the invention and that are large enough to accommodate between forty and one hundred twenty risers arranged in not more than two concentric rows at the bottom of the turret. 
   This invention, as defined below by the claims, makes possible a Very Large Turret (VLT) for a very large crude carrier (VLCC) converted into a weathervaning FPSO vessel. A weathervaning vessel is advantageous as compared to a spread moored vessel, because it provides safer shuttle tanker mooring for tandem offloading and more up-time for offloading. A VLT, i.e., one capable of handling between forty and one hundred twenty risers, has many advantages. All such advantages result from the large bearing diameter in combination with a bearing foundation and bearing arrangement which rotatably couples the vessel to a relatively flexible turret (as compared to prior turrets) capable of conforming to hogging and sagging deflections of the vessel hull. Advantages of the VLT according to the invention are summarized below. 
   1. The increased riser capacity allows a deep water field operator to no longer be required to use subsea manifolding because of space limitations on the turret. This feature provides maximum flexibility for field layout. 
   2. The VLT economically provides sufficient space for oversized riser tubes that allow maximum flexibility in riser location at the turret. 
   3. The increased space on the turret for manifold modules allows utilization of conventional valves rather than higher cost compact values. 
   4. The manifold module can be large enough for choke valves in all production and test situations. This feature allows all production and test swivels to be of lower pressure rating for higher reliability. 
   5. The manifold space can be large enough for a high pressure gas manifold to split the gas flow to a reinjection header and to a gas sales riser. 
   6. The space on the turret is sufficient for large pig launcher/receivers for instrumented pigs. 
   7. Space on the turret is provided for storing quantities of chemical injection fluids and pumps. This feature reduces the number of high pressure fluid paths in the swivel stack for the chemicals. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The objects, advantages, and features of the invention will become more apparent by reference to the drawings that are appended hereto and wherein like numerals indicate like parts and wherein illustrative embodiments are shown, of which: 
       FIG. 1  is an illustration of a prior art FPSO vessel floating on the sea with anchor legs connected between a seafloor and a rotatably mounted turret on the vessel, with numerous flowlines on the seafloor coupled to and flexible risers supported from the turret in the floating vessel; and 
       FIGS. 2 ,  3  and  4  depict prior art turrets and turret moored vessels as described above; 
       FIG. 5  illustrates an embodiment of the invention in a transverse cross sectional view of a turret in a vessel, and shows an upper bearing located near the main deck of the vessel, but does not include a lower bearing near the vessel keel; 
       FIG. 6A  illustrates greatly exaggerated flexure of the turret frame of  FIG. 5  when acted upon by horizontal forces on the chain table; 
       FIG. 6B  illustrates greatly exaggerated flexure of the turret frame of  FIG. 5  when acted upon by vertical forces on the chain table; 
       FIG. 7A  is a sketch of a turret moored vessel showing a greatly exaggerated example of the vessel in a “sagging” condition with a flexible turret main deck conforming to the shape of the vessel sag; 
       FIG. 7B  is an exaggerated sketch of the turret of  FIG. 7A  with a flexible main deck; 
       FIG. 7C  is a perspective view of a large diameter turret with a flexible turret having a main deck capable of conforming to a sagging or a hogging shape of a supporting surface subject to vessel sag or hog; 
       FIG. 8  is a sketch which illustrates wheel loads distributed around a circular roller track (rail), with a linear load variation that occurs when an upper rail of the turret and a bottom rail of the vessel are both in a perfectly flat condition and an external load “Fv” is acting vertically at an eccentric location “e” from the center of the bearing; 
       FIG. 9  illustrates wheel loads distributed around a circular roller track (rail) that has been deformed by bending deflection of the vessel hull, and shows that the upper rail attached to a stiff turret does not conform to the shape of the roller track on the vessel hull; 
       FIG. 10  illustrates wheel loads distributed around circular track rails that have been deflected by vessel hull bending and shows that where the turret structure above the upper rail is sufficiently flexible, the upper rail conforms to the out-of-plane shape of the lower rail; 
       FIG. 11  illustrates the basic geometry of the flexible turret frame of a preferred embodiment of the invention and defines certain dimensions used as parameters for turret design; 
       FIG. 11A  is a graph of the parameter δ/D 1  as a function of a ratio A 1 /D 1  with an indication of acceptable ranges of those parameters to achieve a sufficiently flexible turret in order to meet specified characteristics; 
       FIG. 12  is an enlarged view of the upper turret structure of the arrangement illustrated in  FIG. 5 , showing the bearing system and related apparatus; 
       FIG. 13  is an enlarged cross section view of the arrangement illustrated in  FIG. 5  showing the area at one side of the turret at the bearing and riser supports; 
       FIG. 13A  illustrates a preferred embodiment wherein the riser tube hangs from the main deck of the turret frame; 
       FIGS. 13B ,  13 C,  13 D and  13 E illustrate the construction assembly of riser tubes for a preferred embodiment of this invention; 
       FIG. 14  shows an embodiment of the invention with a top plan view of the turret winch deck of the arrangement illustrated in  FIG. 5 ; 
       FIG. 15 , is a plan view of the turret main deck of the arrangement illustrated in  FIG. 5  where manifold piping and equipment are omitted from the view for clarity; 
       FIG. 16  shows a plan view of the chain table of the arrangement illustrated in  FIG. 5 ; 
       FIG. 17  shows an alternative embodiment of the invention with a transverse cross section view of a turret and vessel and illustrating a flexible frame structure turret supported by an upper bearing at the main deck of the vessel, and by a lower bearing near the vessel keel; 
       FIG. 18  shows another embodiment of the invention in a transverse cross section view of a turret and vessel, and illustrates a turret having a flexible frame structure which is rotatably supported by an upper bearing at the main deck of the vessel, and an elastomeric bumper pad near the vessel keel; 
       FIG. 19  shows another embodiment of the invention and shows in an elevation view riser tubes also serving as the structural members connecting the chain table to the turret main deck; and 
       FIG. 20  shows another embodiment of the invention with a transverse section view of a turret where the turret main deck is connected to the chain table by a small diameter tube. 
   

   DESCRIPTION OF THE INVENTION 
   The illustrations of the preferred embodiments of the invention are described by reference to the Figures briefly described above and include reference numbers for the following items: 
   
     
       
         
             
             
           
             
                 
             
           
          
             
               100 
               Flexible frame turret 
             
             
               1 
               Turret main deck 
             
             
               2 
               Column 
             
             
               3 
               Chain table 
             
             
               4 
               Bearing Foundation 
             
             
               5 
               Turret insert tube (Moonpool) 
             
             
               6 
               Pump deck 
             
             
               7 
               Manifolds 
             
             
               8 
               Winch deck 
             
             
               9 
               Winch 
             
             
               10 
               Swivel stack 
             
             
               11 
               Swivel torque tube 
             
             
               12 
               Load wheels 
             
             
               13 
               Radial wheels 
             
             
               14 
               Uplift wheels 
             
             
               15 
               Radial spring assembly 
             
             
               16 
               Riser tube 
             
             
               17 
               Riser bend stiffener 
             
             
               18 
               Riser 
             
             
               19 
               Chain support 
             
             
               20 
               Mooring chain 
             
             
               21 
               Winch line 
             
             
               22 
               Piping 
             
             
               23 
               Safety valves 
             
             
               24 
               Riser support clamp 
             
             
               25 
               Riser tube slip joint 
             
             
               26 
               Rail 
             
             
               27 
               T-Beam 
             
             
               28 
               Horizontal sheave 
             
             
               29 
               Moveable vertical sheave 
             
             
               30 
               Vessel main deck 
             
             
               31 
               Radial beam 
             
             
               32 
               Center ring 
             
             
               33 
               Riser support tube 
             
             
               34 
               Outer ring 
             
             
               35 
               Chemical tank 
             
             
               36 
               Chemical pump unit 
             
             
               37 
               Seal 
             
             
               38 
               Elastomeric bumper pad 
             
             
               39 
               Clearance gap 
             
             
               40 
               Chain installation deck 
             
             
               41 
               Chain hang-off bracket 
             
             
               42 
               Lower bearing 
             
             
               43 
               Vessel hull structure 
             
             
               44 
               Riser tube flange 
             
             
               45 
               Hanging riser tube 
             
             
               46 
               Welding fixture 
             
             
               47 
               Weld joint 
             
             
               48 
               Riser tube collar 
             
             
               49 
               Riser end fitting 
             
             
               50 
               Riser tube hole 
             
             
               51 
               Hull elastic curve 
             
             
               52 
               Main deck elastic curve 
             
             
               53 
               Central column 
             
             
                 
             
          
         
       
     
   
     FIG. 1  illustrates a prior art FPSO V floating on the sea with anchor legs L and numerous flexible risers (i.e., flexible marine hoses) R hanging from the turret to the seafloor. Other known variations of riser systems including steel catenary and hybrid steel and flexible riser systems are suitable for the turret of this invention. 
     FIG. 5  is a transverse sectional view of one preferred embodiment of the invention. The flexible frame turret  100  comprises three primary components: turret main deck  1 ; connecting structure such as columns  2 ; and chain table  3 . At least three pillars or columns  2 , but preferably six, connect main deck  1  to chain deck  3  with structural moment connections that transfer the axial forces and moments from chain table  3  to main deck  1 . A single pillar or cylindrical structure could be substituted for pillars or columns  2  between deck  1  and chain table  3 . (See  FIG. 20 ) Mooring chain  20  is the upper section of the mooring leg; it is attached to chain table  3  by a pivoting ratchet type chain support  19 . A radial array including at least three mooring legs, but preferably a total of nine legs in three groups of three legs each, is commonly used where each leg comprises various known combinations of chain, wire rope, synthetic or polyester rope, all connected together with suitable shackles and fittings to a termination point on the seafloor at anchors or piles. Chain installation deck  40  provides access to workers to handle chain during mooring leg installation. The slack end of chain  20  is secured to deck  40  by chain hang-off bracket  41  after winch  9  pulls mooring leg  20  to an appropriate tension. 
   Risers  18  extend from the sea floor beneath the flexible frame turret  100  and extend through chain table  3  and through riser tubes  16  to main deck  1 . A riser bend stiffener  17  restrains each riser  18  horizontally and transfers horizontal forces of the risers to the chain table  3 . The riser tubes  16  protectively enclose each riser  18  from chain table  3  to main deck  1 . 
   When environmental forces cause the vessel to move from its neutral calm water position, vertical and horizontal mooring restoring forces of anchor legs  20  act on chain table  3  and are transferred through pillars or columns  2  (or other suitable structure) to main turret deck  1 , and through three sets of wheels  12 ,  13 ,  14 , into bearing support  4 , as shown below by reference to  FIGS. 12 and 13 . Turret insert tube  5  is a primary load transfer structure attached inside the vessel hull structure  43 . Subsea currents, surface wave motions, and vessel offset motions also cause vertical and horizontal riser forces to act on the turret. Riser forces are significant because of the great number of risers provided. As few as 40 and up to as many as 120 risers  18  are contemplated for use with the preferred embodiment of the invention. Vertical riser forces of risers  18  are transferred upward through each riser tube  16  and are primarily reacted by turret main deck  1 . Horizontal riser forces are transferred horizontally at chain table  3  and are primarily reacted by chain table  3 . 
     FIG. 6A  illustrates the flexible nature of the turret frame  100  with horizontal forces represented by arrow F x  applied to chain table  3 . The cumulative horizontal forces of the risers and anchor legs are represented by a single vector F x . The deflected frame shape of chain table  3 , pillars or columns  2  and turret main deck  1  is exaggerated in the drawing for clarity. Horizontal force “F x ” causes chain table  3  to deflect horizontally a distance “X 1 ” until the internal forces and moments in the frame  100  reach equilibrium. Clearance gap  39  provides sufficient space for horizontal elastic deflections of chain table  3 . The entire turret frame  100  including main deck  1 , pillars or columns  2  (or other suitable connecting structure such as a small diameter tube), and chain table  3 , contribute to the total flexibility. All of the pillars or columns  2  bend elastically while being partially constrained by their direct connection to main deck  1  and chain table  3 . 
     FIG. 6B  illustrates the flexible nature of the turret frame when downward vertical loads “F z ” and “F r ” act on chain table  3 . The cumulative downward force of the anchor legs is represented by the vector F z . The cumulative vector “F z ” is applied to chain table  3  through the connection of mooring chain  20  and chain support  19 . (See  FIG. 5 ). Force “F z ” does not necessarily act at the geometric center of the turret, a condition that causes non-symmetrical deflection of the frame that is not illustrated. Force “F z ” is transferred from chain table  3  through pillars or columns  2  to turret main deck  1 . Force vector “Fr” represents the downward force exerted by each riser  18  through riser tube  16  onto main deck  1 . Each riser force vector “Fr” may have a different numerical value resulting in a non-uniform distribution of load onto main deck  1 . The deflected frame shape resulting from “F z ” and “Fr” is exaggerated in  FIG. 6B  for clarity. Pillars or columns  2  bend elastically in a different curve from that illustrated in  FIG. 6A . 
     FIG. 7A  is a greatly exaggerated sketch of a vessel hull in which an internal turret  100  of the invention is installed and rotatably supported within a moonpool of the hull. The  FIG. 7A  sketch shows the vessel bent into a so-called “sagging” condition such that a hull elastic curve  51  is characterized by a radius of curvature R 1 , which is many times greater than C 1 , the distance from elastic curve  51  to the vessel main deck  30 . From elementary beam theory it is known that the elastic curve passes through the horizontal neutral axis of each cross section of a beam in bending, in this case, the vessel hull. 
     FIGS. 7B ,  12 , and  13  illustrate wheels  12  mounted between upper and lower rails  26 U,  26 B. The lower rail  26 B is mounted on the turret insert tube  5  or “moonpool” of the vessel. The upper rail  26 U is mounted on a surface of the turret main deck and is positioned concentrically with bottom rail  26 B.  FIG. 7B  shows the turret  100  with an exaggerated sketch illustrating the flex of the turret  100 . As shown in  FIGS. 7A and 7B , the arrow R 2  represents a radius of curvature of the turret main deck elastic curve  52  of turret main deck  1 , while the arrow R 3  represents a radius of curvature of the bottom surface of the turret main deck  1 . Since the radii R 1 , R 2 , and R 3  are very large, and the radii of curvature depicted in  FIG. 7A  all are much greater than the distance C 1 , then a common radius of curvature can be assumed for R 1 , R 2 , and R 3 . That is,
 R 1 &gt;&gt;C 1 , and R 1 ≈R 2 ≈R 3 =R. 
   The hull bending stress σ h  can be represented approximately as: 
         σ   h     =     E   ⁡     (       C   1     R     )           
           where   ⁢           ⁢   E   ⁢           ⁢   is   ⁢           ⁢   Youngs   ⁢           ⁢   Modulus     =     Stress   Strain       ,     for   ⁢           ⁢   the   ⁢           ⁢   structural   ⁢           ⁢   material     ,       
 
   and predicts the elongation or compression of an object as long as the stress is less than the yield strength of the material. 
   In  FIG. 7B , the thickness h 3  of the turret main deck  1  results in a distance C 2  from the top plate of main deck  1  to elastic curve  52 . The radius of curvature R 2 , which can be represented by a common radius R as indicated above, results in a turret main deck bending stress σ t , 
         σ   t     =         E   ⁡     (       C   2     R     )       ⁢           ⁢   and   ⁢           ⁢     σ   t       =         σ   h     ⁡     (       C   2       C   1       )       .           
 
   It can be seen that the hull bending stress σ h  is greater than turret main deck bending stress σ t  due to hogging and sagging. 
   The bottom rail  26 B elastically deflects approximately in the shape of vessel main deck  30 . See  FIG. 13  for an enlarged view of rail  26 B. The turret  100  of a preferred embodiment this invention, as illustrated in  FIG. 7B , is designed to have a flexibility so that main deck  1  conforms to follows the bend of the vessel when the vessel sags (or hogs . . . the opposite of sag) so load wheels  12  remain in contact with rail  26 B, thereby continuing to distribute the vertical load to all of the wheels  12 . The total deflection of the turret main frame  1  can be determined, because each of the deflections of the turret frame  100  described by reference to  FIG. 6A  (deflection due to horizontal forces F x ),  FIG. 6B  (deflection due to vertical loads) and  FIGS. 7A and 7B  (deflection due to hogging or sagging) are linear and can be added by the principle of superposition. 
     FIG. 7C  illustrates a preferred design or embodiment of the invention where the turret  100  includes a chain table  3  with pillars  2  connecting a turret main deck  1  of thickness h 3  and with the main turret deck  1  having a central hub  32  and spokes  31  connecting an outer ring  34 . The hub, spoke, outer ring design of the turret main deck  1 , combined with its thickness h 3  allows sufficient flexibility for upper rail  26 U to flex in conformity with the sagging shape of bottom rail  26 B when the vessel sags. The main deck  1  flexes due to the vertical force acting on it as illustrated in  FIG. 6B . 
     FIG. 8  is a diagram of wheel loads distributed around a circular track between upper rails  26 U and bottom rails  26 B with a linear load variation that occurs with the rails  26 U and  26 B in a perfectly flat condition and an external load “Fv” acting vertically at an eccentric location “e” from the center of the rails of the bearing. On side “B” of the bearing, the wheel load is a maximum of “Fw(max)” per wheel on one or two wheels, while all other wheel loads are smaller. At side “A” the load per wheel reaches the minimum value. The wheel load is linearly distributed along the centerline (C/L) of the vessel from point A to point B. 
   The diagram of  FIG. 9  shows wheel loads distributed around a circular track where the lower rail  26 B has been displaced, but the upper track rail  26 U is attached to a very rigid turret structure that is in a flat plane. The lower track rail  26 B has been deflected out of the flat plane into an exaggerated “sagging” deflection curve. This condition causes the maximum load per wheel  12  to reach higher values at locations “A” and “B” than occurs with both rails  26 U,  26 B in a flat plane, as illustrated in  FIG. 8 . Some of the wheel loads are reduced to near zero, or some wheels may even lift off of the track, while the maximum wheel load can reach two to five times the “Fw(max)” load per wheel shown in the  FIG. 8  flat rail condition. An eccentric load as shown in  FIG. 9  again causes the maximum load per wheel to occur near location “B”. 
     FIG. 10  demonstrates wheel loads distributed around circular track rails  26 U,  26 B that have been deflected by vessel hull bending. The lower rail  26 B attached to the vessel structure is deflected because of vessel “sagging”. In this case the turret structure  100  (See  FIGS. 12 ,  13 ) is sufficiently flexible so that the upper rail  26 U tends to conform to the out-of-plane shape of the lower rail  26 B due to the downward vertical force Fv, thereby more uniformly distributing the total load to all of the wheels  12 . This improved distribution reduces the maximum load per wheel to a significantly lower value than is the case for the conditions of  FIG. 9 . When the load is eccentric by an amount “e”, the maximum load per wheel again occurs at location “B.” 
     FIG. 11  and Table 1 below illustrate geometric proportions of the turret frame  100  that are provided to assure sufficient frame flexibility according to a preferred embodiment of the invention. 
   
     
       
         
             
             
             
           
             
               TABLE 1 
             
             
                 
             
             
               Dimension 
               Minimum 
               Maximum 
             
             
               Ratio 
               Ratio 
               Ratio 
             
             
                 
             
           
          
             
               D2/D1 
               1.00 
               1.30 
             
             
               D3/D1 
               0.40 
               0.70 
             
             
               D4/D1 
               0.15 
               0.25 
             
             
               D5/D1 
               0.70 
               1.20 
             
             
               D6/D5 
               0.60 
               0.80 
             
             
               A1/D1 
               0.05 
               0.15 
             
             
               A2/D5 
               0.05 
               0.15 
             
             
               L1/D1 
               0.70 
               2.00 
             
             
               W1/L1 
               0.06 
               0.15 
             
             
               T1/W1 
               0.01 
               0.03 
             
             
               δ/D1 
               0.0000 
               ±0.0010 
             
             
                 
             
          
         
       
     
   
   The turret deflections at rail  26 U can be defined by a parameter d, a measurement of deviation of the elastic curve from a flat plane at the support rail  26 U as illustrated in FIG.  11 . Hull deflections can typically cause a δ in lower rail  26 B of about 15 millimeters with a moonpool diameter D 1  of twenty-nine meters. The expected range of upper rail  26 U deflections as a basis for this invention is a δ/D 1  ratio ranging from zero to 0.0010, where D 1  is the central diameter of the support wheel rails  26 U and  26 B. 
     FIG. 11A  provides graphs that define the allowable dimensional ratios, deflection ratios, and characteristic stress, for the flexible frame  100  of  FIGS. 11 and 7C . Characteristic stress is a numerical value based upon the nominal bending stress occurring in the turret main deck  1  outer ring  34 . A specific range of ratio of depth-of-turret-main-deck A 1  to support-rail-diameter, D 1 , A 1 /D 1 , is required to more uniformly distribute loads to the wheels  12  while assuring that stress is not large enough to cause metal fatigue failure in the turret main deck  1 . Region A in  FIG. 11A  is the desired range of turret main deck  1  proportions A 1 /D 1  as a function of deflection ratio δ/D 1  for the preferred embodiment of turret  100 . To achieve the objective distribution of load for the wheels  12 , a preferred design requires that the turret frame proportions be dimensioned to allow turret main deck  1  deflections that maintain at least 90% of the load wheels  12  in contact with their rails  26 B,  26 U while only a fraction of the total maximum vertical load is applied to any one wheel for the turret main deck  1 . 
     FIG. 12  is an enlarged view of the upper turret structure of the arrangement illustrated in  FIGS. 5 and 7C , with its bearing system, and equipment near turret main deck  1 . Main deck  1  includes outer ring  34  and center ring  32  connected together by radial beams  31 . Pump deck  6  is positioned below main deck  1  and is supported by pillars  2 . Pump deck  6  supports chemical tank  35  and chemical pump unit  36 . An assortment of ancillary modular equipment related to the fluids transfer system and control system can be located on deck  6 . 
   Components of the fluid transfer system that are supported by main deck  1  include manifold  7 , fluid swivel stack  10 , and flexibly supported piping  22 . Winch deck  8  has a support frame which is mounted on outer ring  34  of main deck  1  that allows main deck flexure without excessive stresses in the supports. In other words, the mounting of deck  8  on outer ring  34  is done so as not to stiffen outer ring  34  or the entire turret  100 .  FIG. 12  shows reeving of winch wire  21  from winch  9  to a centrally mounted rope sheave mounted on deck  6  for the purpose of pulling in a mooring leg. Winch wire  21  is reeved differently to pull in risers  18  using winch  9 . 
     FIG. 13  is an enlarged cross section view of the arrangement illustrated in  FIG. 5  showing the area at one side of the turret  100  at the bearing and riser supports. All loads from the turret  100  acting on the vessel  30  are transferred through load wheels  12 , radial wheels  13 , and uplift wheels  14 . Rollers  12 ,  13 , and  14  roll on rails  26  as illustrated in  FIG. 13 . Vertically acting loads are transferred through dual concentric rails  26 U,  26 B to turret insert tube  5  and hull structure  43  by means of the load equalizing effect of T-beam  27 . Radially acting loads are transferred to vessel hull structure  43  by means of radial wheels  13  held against radial rail  26 R by means of radial spring assemblies  15 . The action of spring assemblies  15  serves to distribute radial load to radial wheels  13  when bearing foundation  4  is deflected out of its initial circular shape by hull deflections. Uplift wheels  14  provide restraint of the turret against rails  26 B′ and  26 U′ in an unusual event that could cause uplift of main deck  1 . 
     FIGS. 13 and 5  also show that the riser tube  16  is positioned between main deck  1  and chain table  3  and is vertically supported by chain table  3 . A riser tube  16  encloses each riser  18  and provides protection of each riser  18  from accidental physical impact from moving objects and from accidental fire and explosion. Heat insulation material for fire protection can be applied to each riser tube  16  and to each pillar  2 . Riser tube slip joint  25  ( FIG. 13 ) horizontally restrains riser tube  16  while allowing small vertical displacements of guide tube  16  relative to main deck  1 . Seals  37  prevent leakage at the joint to the atmosphere of any accumulated gas from the interior of riser tube  16 . The weight of riser  18  is supported by means of riser support tube  33 , and riser support clamp  24  fitted onto riser end fitting  49 . This arrangement of riser tube support does not appreciable increase overall stiffness of the turret frame. Piping connections to the risers include safety valves  23 . 
     FIG. 13A  illustrates an alternative embodiment of riser tube  18  coupling to turret  100  wherein a hanging riser tube  45  is connected to turret main deck  1 . This feature is advantageous because it eliminates the need for chain table  3  to carry the weight of riser tubes  16  as shown in  FIGS. 13 and 5 . The weight of forty to one hundred twenty riser tubes can be in the range of several hundred to thousands to metric tonnes. Riser tube collar  48  is fastened to chain table  3 . Hanging riser tube  45  is a slip fit inside riser tube collar  48 , and end clearance is provided at the bottom end of riser tube  45  to allow small relative displacements of the riser tube  45  relative to chain table  3 . Riser tube  45  is arranged and designed so that it can flex without being overstressed at its connection to turret main deck  1 . Riser tube flange  44  of riser tube  45  is supported on deck  1 , and riser support clamp  24  is mounted on flange  44 . 
     FIGS. 13B ,  13 C,  13 D, and  13 E, illustrate a preferred installation method for hanging riser tubes  45  into main deck  1 . If sufficient crane boom height is not available, riser tube  45  can be fabricated as one piece and lowered into the slip fit of collar  48  on chain table  3  and into its place resting on main deck  1  as in  FIG. 13A . If insufficient crane boom height is available for one-piece installation, the riser tube can be installed in two or more pieces wherein a first riser tube section  45   b  is lowered through tube hole  50  to rest on chain table  3  as in  FIG. 13B . Subsequently, riser tube  45   a  is lowered through hole  50  to rest on main deck  1  as shown in  FIG. 13C . In  FIG. 13D , welding fixture  46  is used to clamp tube sections  45   a  and  45   b  together in alignment for making weld joint  47 .  FIG. 13E  illustrates the completed riser tube  45 . 
     FIG. 14  illustrates winch deck  8  in a plan view (see also  FIGS. 5 and 12 ) where winch  9  is used to pull in all risers and anchor legs. A horizontal sheave  28 , and at least one moveable vertical sheave  29 , provide an arrangement for reeving winch line  21  to a point above any of risers  18  to provide vertical pull-in the risers.  FIG. 5  illustrates the arrangement where winch line  21  is reeved through a central sheave from which any of anchor chains  20  can be pulled in or let out during installation or readjustment of anchor leg tension. 
     FIG. 15  is a plan view of main deck  1  and illustrates structural components of main deck  1  that provide the required flexibility and strength of the preferred embodiment of turret frame  100  according to the invention. At least three (but preferably six) radial beams  31  connect and support center ring  32  to outer ring  34 . As mentioned above, a single cylindrical tube (See  FIG. 20 ) can be substituted for the pillars or columns  2 , but its flexibility must be designed in coordination with chain deck  3  and main deck  1 . Center ring  32  provides support for swivel stack  10  and its associated piping. Pillars or columns  2  (see  FIG. 5 ) are connected to the underside of radial beam  31  near the intersection of radial beam  31  and outer ring  34 . The large open space between radial beams  31  is advantageous for turret interior ventilation and minimizes internal pressure in the moonpool area in the event of a gas explosion. 
     FIG. 16  is a plan view of chain table  3  and illustrates a preferred embodiment of structural components that provide the required flexibility and strength of the turret frame of this invention. At least three, and preferably six pillars  2  (see also  FIG. 5 ) are attached by moment connection to chain table  3 . No brace members are provided between pillars or columns  2  so that a desired flexibility of the turret  100  may be achieved. The pillars  2  are spaced apart to provide clearance for pulling all mooring chains  20  radially toward the center of the turret frame. This arrangement of  FIG. 16  also provides open clear space diametrically across the interior of the turret frame, that can advantageously be used for launching underwater remote operated vehicles (ROVs), diver entry into the water at the center of the turret, or space for subsea well service equipment or well work-over equipment to operate out of the bottom of the turret. 
     FIG. 17  illustrates an alternative embodiment of the invention where a lower bearing  42  is provided to transfer horizontal load from chain table  3  into vessel hull structure  43  near keel level. This arrangement is advantageous for mooring conditions where large horizontal loads exist that tend to overturn the turret frame  100 . Lower bearing  42  comprises a plurality of lubricated individual bearing units which slide on a prepared corrosion resistant surface inside turret insert tube  5 . This arrangement takes advantage of the horizontal flexibility of the turret frame  100  to compensate for misalignment and non-concentricity of the upper and lower bearings thereby preventing consequential overload of either the lower bearing or the upper bearing. 
     FIG. 18  illustrates another alternative arrangement where elastomeric bumper pad  38  transfers horizontal load from chain table  3  into vessel hull structure  43  near keel level. This arrangement is advantageous for mooring conditions causing large but infrequent horizontal loads that tend to overturn the turret frame. A plurality of bumper pads  38  restrains chain table  3  when the elastic deflections of the turret frame exceed a desired limit such as about 100 millimeters. 
     FIG. 19  illustrates another embodiment of the turret frame where a plurality of riser tubes  16  provide structural connection of chain table  3  to main deck  1 . In this arrangement, riser tubes  16  transfer all loads from chain table  3  to main deck  1  thereby making the pillars  2  unnecessary. 
     FIG. 20  depicts another embodiment of the turret frame where a single central tube or column  53  connects chain table  3  to turret main deck  1  instead of multiple pillars  2  as shown in  FIG. 6 . This arrangement can be advantageous when the moonpool diameter D 1  is in the range of 14 meters to 20 meters.