Patent Publication Number: US-6698372-B2

Title: Turret mooring system and method for installation

Description:
RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/241,694, filed Oct. 19, 2000. 
    
    
     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”) and Floating Production Storage and Offloading vessels (“FPSOs”) 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 on tanker hulls. FPSOs are the most flexible of FPUs in terms of water depth and sea conditions due to their variation in moorings and ship shape configurations. FPSOs are spread moored (anchored directly to the sea floor), attached via an internal or external turret, which is moored to the sea floor or detachably secured to a separately floating buoy that is moored to the sea floor. FPSOs have excellent storage and topside facilities configurations due to their large size and ship shape. Further, many modem FPSOs are turret moored. 
     FPSOs compete with other kinds of floating production units such as semi-submersibles, spars, and tension leg platforms. Their competitiveness depends on their advantages and disadvantages. 
     As mentioned above, the present invention is directed to a turret mooring arrangement. Prior turret mooring arrangements are known in the art. For example, U.S. Pat. No. 3,335,690 to Busking shows a permanently anchored turret which is rotatably supported from a frame that extends from the bow of the vessel. 
     U.S. Pat. No. 4,650,431 to Kentosh illustrates a turret which can be disconnected from a frame secured from the bow of a vessel. 
     U.S. Pat. No. 4,490,121 to Coppens illustrates a disconnectable turret which is rotatably secured from the bow of the vessel. 
     U.S. Pat. No. 4,955,310 to Pollack illustrates a bearing system for mounting a turret on the outer beams of a vessel. The bearings shown in this patent allow the turret to pivot about upper and lower horizontal axes. 
     U.S. Pat. No. 5,515,804 to Pollack illustrates a bearing system for a turret with a generally rigid upper mount and including a resiliently deflectable support structure that includes a plurality of elastomeric shear pads. 
     U.S. Pat. No. 5,468,166 to Breivik et al. shows a disconnectable buoy which is receivable into a submerged receiving space of the vessel. The outer portion of the buoy is latched to the vessel, but has a central member of smaller diameter which is rotatably mounted in the outer member and has a through-going passage for hydrocarbon to be transported via the buoy. A flexible joint is provided at the top end of the central member. The flexible joint is secured to an inner part of a fluid swivel. 
     Identification of Objects at the Invention 
     A primary object of the present invention is to provide a less expensive turret mooring arrangement for a FPSO. 
     Another object of the present invention is to provide a turret mooring arrangement with a smaller turret than conventional FPSOs. 
     An additional object of the present invention is to provide a less expensive turret mooring arrangement for rotatably mounting a turret on a vessel under conditions of a vessel ovaling and moment loading on the upper axial/radial bearing. 
     SUMMARY OF THE INVENTION 
     The objects identified above along with other features and advantages of the invention are provided with a turret configuration for a low cost internal turret in which the turret includes an upper section, a lower section and a flex joint coupled therebetween. 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 one mooring line which extends to the sea floor for anchoring the turret in a substantially geostationary position. 
     The upper section includes an axial/radial bearing assembly. This assembly permits the vessel to weathervane about the turret, yet resists other moment loadings caused by weather conditions, including sea conditions, causing the vessel to heave, pitch and yaw in the sea. 
     The flex joint is located just below the upper axial/radial bearing assembly. It is designed to minimize the effects of moment loading acting upon the upper axial/radial bearing assembly. 
     The lower section includes a lower radial bearing assembly comprising a lower radial bearing and lower bearing outer housing. The lower bearing outer housing is strategically shaped to correspond with the hull of the vessel or an outboard device attached to the vessel. Because the lower bearing outer housing is not permanently connected to the vessel or outboard device, no requirements exist to integrally fabricate the assembly to the vessel or outboard device. Instead, such an assembly can be fabricated separately, thereby reducing costs. Moreover, the shape of the lower radial bearing assembly housing can be more easily customized as desired or necessary to facilitate capturing of the turret mooring arrangement by the vessel. For example, the outer profile of the lower bearing outer housing may be polygonally shaped, frusto-conically shaped, or any other shape desired. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The objects, advantages and features of the invention will become more apparent by reference to the drawings which are appended hereto and wherein like numerals indicate like parts and wherein an illustrative embodiment of the invention is shown, of which: 
     FIG. 1, showing one embodiment of the invention, illustrates an arrangement of a turret for a floating vessel with an upper turret coupled to a lower turret by a flex-joint/universal joint and with a lower bearing outer housing of frusto-conical shape, which requires installation of the lower bearing arrangement from below the vessel&#39;s keel; 
     FIG. 2, showing another embodiment of the invention, illustrates an arrangement of a turret for a floating vessel similar to that of FIG. 1 but with the lower bearing outer housing shown as a polygon which allows the turret to be installed by lowering the turret assembly into the moon pool from above the vessel&#39;s main deck; and 
     FIG. 3 is a cross-sectional view of the lower turret mooring arrangement across lines  3 - 3  in FIG.  2 . 
    
    
     While the invention will be described in connection with the preferred embodiment, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a first embodiment of the invention. The turret mooring arrangement  40  includes upper section  50 , lower section  70 , and flex joint  60  coupled therebetween. Upper section  50  includes first upper section end  41 , second upper section end  52 , upper axial/radial bearing assembly  53 , and an upper section cavity  6  within upper section  50 . The cavity  6  within upper section  50  permits risers  91  or other equipment, devices, tubulars, etc. to pass through upper section  50 . Riser tubes could alternatively be provided for the passage of risers  91  through the turret sections. Risers  91  are drawn upward with a winch and sheave arrangement  100 , known to those skilled in the art of offshore mooring arrangements. Upper section  50  is rotatably coupled via upper axial/radial bearing assembly  53  to rigid mounting rings  35  which are coupled to flexible/spring elements  22  which are in turn mounted to the vessel  30 . The upper axial/radial bearing assembly  53  is designed to allow the vessel  30  to weathervane about the turret mooring arrangement  40 . 
     Flex joint  60  may be a typical universal joint, e.g., Hooke&#39;s joint, or a tapered stress joint of metallic or composite construction, or a flex joint utilizing elastomeric or composite materials acting as the flexible element. Preferably, flex joint  60  provides two degrees of freedom of movement. 
     In one specific embodiment, flex joint  60  is located in close proximity to the horizontal plane of upper axial/radial bearing assembly  53 . Another way to describe the preferred location of flex joint  60  is that the height of upper section  50  is small compared to the height of lower section  70  whereby flex joint  60  is positioned a small distance from upper axial/radial bearing assembly  53  relative to the combined height of upper section  50  and lower section  70 . 
     By locating flex joint  60  close to the horizontal plane of upper axial/radial bearing assembly  53 , coupled with the release of two degrees of freedom by flex joint  60 , the moment loading experienced by upper axial/radial bearing assembly  53  is minimized, thereby reducing the capacity requirements of upper axial/radial bearing assembly  53 . Additionally, the reduction in moment loading greatly reduces the size of (or may completely eliminate the need for) flexible/spring elements  22  between upper axial/radial bearing assembly  53  and the structure of vessel  30  to account for vessel  30  ovaling due to environmental forces. This feature provides cost savings because the flexible/spring elements  22  required of conventional systems to provide this movement are costly and technically challenging. Locating the flex joint  60  near the horizontal plane of upper axial/radial bearing assembly  53  also reduces the horizontal load acting upon flex joint  60  as a result of reacting mooring loads. 
     Lower section  70  includes lower section shaft  71 , top end  72 , bottom end  73 , lower section cavity  7  running through lower section  70 , lower radial bearing  81 , and lower bearing outer housing  82 . The lower section cavity  7  running through lower section  70  permits risers  91  or other equipment, devices, tubulars, etc. to pass through lower section  70 . Lower section  70  is rotatably coupled via lower radial bearing  81  to lower bearing outer housing  82  which abuts against vessel  30  at a frusto-conically shaped recess  36 . This lower radial bearing assembly  81 ,  82  allows the vessel  30  to weathervane about the stationary turret mooring arrangement  40 . Additionally, support piece  27  may be fabricated (e.g. by welding) at recess  36  after the lower turret section  70  has been pulled upwardly into place to provide additional support to maintain lower bearing outer housing  82  in place against the inside surface of recess  36 . 
     Upper section  50  and lower section  70  may have any shape desired or necessary to facilitate capture of upper section  50  and lower section  70  by vessel  30 . Preferably upper section  50  and lower section  70  have a circular cross-section and the diameter of upper section  50  is larger than the diameter of lower section  70 . 
     While lower bearing outer housing  82  may have any shape desired or necessary to be sufficiently captured by hull  31  of vessel  30 , or by an outboard frame connected to the vessel, to stabilize turret mooring arrangement  40 , lower bearing outer housing  82  in the embodiment of FIG. 1 is frusto-conical in shape. Accordingly, vessel  30  has a corresponding frusto-conically shaped recess  36  for lower bearing outer housing  82 . Therefore, lower bearing outer housing  82 , and thus turret mooring arrangement  40 , must be captured by vessel  30  from below keel  32  of vessel  30 . For example, vessel  30  includes turret insert tube  37 , i.e., moonpool  37 , for receiving turret mooring arrangement  40 . Disposed along keel  32  of vessel  30  is recess  36  that is in communication with moonpool  37 . Turret mooring arrangement  40  is pulled into vessel  30  from the bottom of the vessel. 
     Vessel  30  may also capture turret mooring arrangement  40  through an outboard structure and is designed to capture lower bearing outer housing  82  in the same manner as described above. Such outboard arrangements are known in the art of offshore vessel mooring design. 
     FIG. 2 shows another embodiment of the invention where an arrangement of a turret for a floating vessel is similar to that of FIG. 1 but with the lower bearing outer housing  82  shown as a polygon which allows the turret to be installed by lowering the turret assembly into the moonpool  37  from above the vessel&#39;s main deck. This embodiment also includes an upper section  50 , lower section  70 , and flex joint  60  coupled therebetween. 
     The upper portion of the lower section  70  functions in an identical manner to that of FIG.  1 . The lower bearing outer housing  82  is polygonally shaped. Accordingly, vessel  30  has a corresponding polygonally shaped recess  36  for receiving lower bearing outer housing  82 . Therefore, lower bearing outer housing  82  permits turret mooring arrangement  40  to be installed by lowering turret mooring arrangement  40  into moonpool  37  from above deck  34  of vessel  30 , i.e., in the direction of from deck  34  to keel  32 . The lower radial bearing  81  allows the vessel  30  to weathervane about the stationary turret mooring arrangement  40  when the vessel  30  abuts against and rotates the lower bearing outer housing  82 . 
     As with the embodiment shown in FIG. 1, it is to be understood that vessel  30  may also capture turret mooring arrangement  40  through an outboard mechanism and is designed to capture lower bearing outer housing  82  in the same manner as discussed above. Such outboard mechanisms are within the skill of routineers in the art of offshore mooring systems. 
     FIG. 3 is a cross-section looking down from line  3 — 3  of FIG.  2 . Lower radial bearing assembly  80  includes lower bearing outer housing  82 , a continuous or segmented bushing, or inner sliding surface  83  constructed of either metallic or a composite material that is mounted on lower bearing outer housing  82 , and outer profile  84 . Outer profile  84  is arranged and designed to cooperate with a complementary profile in the interior of turret insert tube  37  (FIG. 2) so that lower bearing outer housing  82  is turned by the interior wall of turret insert tube  37  (FIG. 2) when vessel  30  rotates about turret mooring arrangement  40  (FIG.  2 ), without fixed attachment for lower bearing outer housing  82  to the wall of turret insert tube  37  (FIG.  2 ). 
     The lower bearing assembly  80  also includes lower radial bearing or journal  81  that is permanently secured to lower turret shaft  71 . Lower radial bearing  81  includes outer sliding surface  85 . Inner sliding surface  83  and outer sliding surface  85  are arranged and designed to rotate with respect to each other thereby providing sliding radial support of the lower turret section  71  with respect to the vessel and turret insert tube  37 . 
     While inner sliding surface  83  and outer sliding surface  85  are described as sliding surfaces, it is to be understood that any surface that permits movement along inner sliding surface  83  and outer sliding surface  85  to facilitate relative rotation may be considered to be a sliding surface. For example, one or both of inner sliding surface  83  and outer sliding surface  85  may include rollers to facilitate rotational movement. Alternatively, one or more of inner sliding surface  83  and outer sliding surface  85  may include ball bearings to facilitate relative rotational movement. 
     FIGS. 1 and 2 
     As shown in FIGS. 1 and 2, lower bearing outer housing  82  is not mounted to vessel  30 . Instead, the design of lower bearing outer housing  82  profile is such that its shape is polygonal (FIG. 2) or frusto-conical (FIG. 1) as discussed above. These shapes permit lower bearing outer housing  82  to rotate with, and transmit a radial load onto, vessel  30  without a fixed or permanent attachment of lower bearing outer housing  82  to vessel  30 . Therefore, an inexpensive turret installation may be achieved by reducing the fabrication work required to hull  31  of vessel  30 . The configuration also permits a less complex construction approach to the components of lower radial bearing assembly  80 , because all of the pieces can be made and fitted together prior to installation. 
     As illustrated in FIGS. 1 and 2, turret mooring arrangement  40  includes chain table  90  having a cavity  94  through which risers  91  pass. Mooring lines  92  are secured to chain table  60  and to the sea floor (not shown) by any method, and through any means, known to persons skilled in the art, provided that mooring lines  92  anchor turret mooring arrangement  40  is an substantially geostationary position. As shown in FIGS. 1 and 2, mooring lines  92  are secured to chain table  90  by attaching mooring lines at points  93  as known in the art of anchoring systems. Mooring lines  92  (e.g., chains, wire rope, synthetic rope, etc.) do not have to be secured directly to the sea floor. For example, mooring lines  92  may be secured to submerged buoys that are secured directly to the sea floor. 
     Risers  91  (FIGS. 1 and 2) in communication with one or more sources of hydrocarbon fluid pass through lower section  70  and upper section  50  and are in communication with deck equipment, e.g., winches, production equipment, etc. Accordingly, turret mooring arrangement  40  permits at least one riser  91  (e.g., two risers  91  as shown in FIGS.  1  and  2 ), to be in fluid communication with the vessel. The vessel  30  is capable of weathervaning about the axis of the mooring arrangement  40  because of the rotational support of axial/radial bearing assembly  53  and lower radial bearing assembly  81 ,  82 . All other moments are reduced via flex joint  60 . 
     Method of Installation 
     In another aspect of the present invention, methods of installing a turret mooring arrangement  40  include the steps of fixing lower radial bearing  81  and lower bearing outer housing  82  to the bottom end of the turret mooring arrangement  40  and installing turret mooring arrangement  40  inside turret insert tube  37 . 
     In one embodiment of the method (e.g., the method of installing the structure of FIG.  1 ), the turret mooring arrangement  40  is installed inside turret insert tube  37  by passing turret mooring arrangement  40  first through deck  34  of vessel  30 , i.e., in a direction from deck  34  to keel  32 . In another embodiment of the method (e.g., the method of installing the structure of FIG.  2 ), lower radial bearing  81  and lower bearing outer housing  82  are installed inside turret insert tube  37  by passing turret mooring arrangement  40  first through keel  32  of vessel  30 , i.e., in the direction of from keel  32  to deck  34 . 
     It is to be understood that the invention is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as obvious modifications and equivalents will be apparent to one skilled in the art. For example, the outer member  37  may have any shape desired or necessary to facilitate stable capture of the turret mooring arrangement. Further, flex joint  60  may be any of the examples identified herein or any other device known to persons of ordinary skill in the art that permits relative movement about two orthogonal horizontal axes between upper turret section  50  and lower turret section  71 . Moreover, the components of the turret mooring arrangement may be manufactured out of any material and through any method known to persons of ordinary skill in the art. Accordingly, the invention is therefore to be limited only by the scope of the claims.