Abstract:
An arrangement for providing fluid communication between an offshore hydrocarbon production and/or storage facility and an offshore loading system such as a CALM buoy is disclosed. A pipeline from a FPSO or production platform runs to a submerged flowline termination buoy which is positioned beneath and separated a short distance from a CALM buoy. The flowline termination buoy is separately anchored to the sea bed from the CALM buoy, and is at a depth below the turbulent zone of the sea. The end of the pipeline is suspended from the flowline termination buoy, and a marine hose fluidly connects the end of the pipeline to a stationary part of a fluid swivel on the CALM buoy. The arrangement isolates the end of the pipeline from fatigue inducing motions of the CALM buoy.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from U.S. Provisional Application Serial No. 60/221,239 filed on Jul. 27, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to an offshore loading system such as a CALM which serves as a. single point mooring (SPM) for a shuttle tanker or the like and a product transfer system for transferring hydrocarbon product via an associated product flowline arrangement between a production and/or storage facility and the SPM. 
     2. Description of Prior Art 
     In deep water operations, certain operational considerations make it desirable to offload hydrocarbons from a production and/or storage facility by running a pipeline to an offshore loading system, such as a CALM buoy, where a shuttle tanker may be moored and connected to a loading hose for filling its tanks with crude oil. Deep water installations, e.g., in depths greater than about 1000 feet, require that the pipeline be suspended between the production and/or storage facilities, such as a platform or FPSO and the CALM buoy rather than running the pipeline along the sea bed. The pipeline must be submerged at a depth deep enough so as not to interfere with shuttle tanker traffic. A problem exists in connecting the end of the pipeline directly to the CALM buoy, because as the buoy moves up and down and side to side, the end of the pipeline moves with it, and as a result is subject to fatigue failure. The term “pipeline” includes steel tubular pipelines as well as bonded and unbonded flexible flowlines fabricated of composite materials. 
     The problem identified above is inherent in prior offloading deep water CALM buoys which have pipelines attached directly to and supported from a CALM buoy. The pipelines are directly coupled to the CALM buoy such that motions of the CALM buoy are also directly coupled to the pipeline with resulting fatigue damage. Prior systems such as that described in U.S. Pat. No. 5,639,187 have provided a hybrid flowline including rigid (e.g., steel catenary risers) pipelines on the sea bed from subsea wells combined with flexible flowlines (e.g., marine hoses) at a submerged buoy which is moored to the sea bed by tension leg tether legs. The buoy is positioned at a depth below the turbulence zone of the water. Flexible hoses are fluidly connected to the steel catenary risers at the submerged buoy and extend upward through the turbulence zone to the surface. 
     Another prior system, described in British Patent GB 2335723 B, attempts to solve the problem identified above by suspending the end of a rigid steel tubular flowline (e.g., the pipeline) by a chain from the offloading buoy and fluidly connecting a flexible hose to the end of the rigid steel flowline below the turbulence zone of the sea. While eliminating a certain level of coupling of wave induced forces to the end of the rigid steel flowline which extends from a production and/or storage facility (FPSO or platform), nevertheless, a sufficient degree of coupling still exists to create a fatigue problem, and possible failure, for the pipeline. 
     IDENTIFICATION OF OBJECTS OF THE INVENTION 
     The primary object of the invention is to provide a product transfer system from a FPSO or platform via a pipeline (either rigid or flexible) to an offloading buoy and to a shuttle tanker while substantially eliminating coupling of wave induced motions of the offloading buoy with the end of the pipeline. 
     Another object of the invention is to provide a conventional CALM buoy for the product transfer system on which an above-water product swivel is placed so that in-situ servicing of the swivel and CALM buoy can be conducted. 
     Another object of the invention is to provide an offshore product transfer system that is suitable for use with large diameter, submerged, rigid (e.g., steel) or flexible (e.g., composite) pipelines in deep water. 
     Another object of the invention is to provide a product transfer system which decouples a submerged pipeline from a surface offloading buoy and its wave induced motions thereby reducing fatigue damage to the pipeline. 
     Another object of the invention is to provide a product transfer arrangement that allows for optimizing of pipeline diameter and buoyancy, because improved fatigue resistance allows for greater variability in the configuration of the submerged pipeline. 
     Another object of the invention is to provide a method for offshore installation of the product transfer system in staged steps for the pipeline hoses, the Flowline Termination Buoy, and the surface offloading buoy. 
     Another object of the invention is to provide a product transfer arrangement in which the surface offloading buoy can be replaced or repaired easily without disturbing the pipeline from the FPSO or platform with a resulting increase in overall system reliability. 
     Another object of the invention is to provide a product transfer system that meets the objects described above while employing a conventional surface offloading mooring and hydrocarbon transfer terminal. 
     SUMMARY OF THE INVENTION 
     The objects identified above along with other advantages and features are provided in the invention embodied in a product transfer system by which a rigid or flexible pipeline from a FPSO or platform or the like extends in the sea above the sea bed for about a nautical mile where it terminates close to a CALM buoy, and where it is fluidly coupled to a flexible hose at a Flowline Termination Buoy (FTB) which is positioned by anchor legs below the wave kinematic zone. The other end of the flexible hose is coupled to the piping leading to the stationary inlet of a product swivel mounted on a stationary portion of a single point mooring offloading buoy such as a CALM. A shuttle tanker is moored to the CALM buoy by a hawser secured to a rotatable portion of the CALM buoy. A hose from the rotatable output of the product swivel extends to the shuttle tanker to complete the product flow path from the (FPSO or platform) to the shuttle tanker. 
    
    
     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 an illustrative embodiment of the invention is shown, of which: 
     FIG. 1 is a schematic illustration of an arrangement of the invention where an end of a rigid or flexible pipeline from a FPSO or production platform is supported by a tethered submerged Flowline Termination Buoy (FTB) with a flexible marine hose fluidly connected between the end of the pipeline and a stationary inlet of a product swivel mounted on a deepwater CALM buoy; 
     FIG. 2 is a schematic illustration showing more detail of the suspension of the rigid or flexible pipeline with an illustration of a side view of the Flowline Termination Buoy and the fluid connection of the flexible hoses to the ends of the pipelines; 
     FIG. 3A (top view) and FIG. 3B (end view) illustrate a preferred embodiment of the Flowline Termination Buoy of the invention; 
     FIG. 4 illustrates a gooseneck connector, adapted for suspension by a chain from the Flowline Termination Buoy for connection between the end of the pipeline and the end of the flexible hose; and 
     FIGS. 5A-5H and enlarged illustrations of FIGS. 5A-1 to  5 F- 1  illustrate installation steps of the Flowline Termination Buoy with the pipeline and final connection to an offloading buoy. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The double buoy offloading arrangement of this invention is for deep water hydrocarbon offloading from offshore production platforms either fixed (e.g., Jacket structures), or floating (e.g., FPSOs, Semi-submersibles, or Spars). Conventional offloading arrangements provide a single offloading buoy located approximately 2 kilometers away from the platform, with a submerged flexible or steel pipeline(s) connected between them. With the prior arrangement, the surface offloading buoy requires a large displacement to support the submerged pipeline(s) and their product. Because of its size, the offloading buoy is subject to motions in response to the wave environment. These wave-frequency motions are coupled to the pipeline and affect its dynamic response, leading to fatigue damage to the pipeline over time. 
     The double buoy concept of this invention effectively eliminates the fatigue damage to one or more pipelines by decoupling the motion of the surface offloading buoy from the pipelines. This is accomplished by using a Flowline Termination Buoy (FTB) submerged beneath the sea surface (on the order of 50-125 meters). The FTB is independently moored and supports the pipeline. Because the FTB is effectively out of the range of the wave kinematics, it does not exhibit significant response to the wave field, thus reducing the fatigue damage to the pipeline. Offloading to shuttle tankers is performed through a conventionally sized CALM buoy system with its own anchor leg system. Standard marine hoses or flexible flowlines connect the CALM buoy to the pipelines supported by the FTB. 
     FIG. 1 shows the general arrangement  10  of the invention where one or more pipelines  12  are fluidly connected between a FPSO or platform  14  to a deepwater CALM buoy  16  via a Flowline Termination Buoy  18  (hereafter referred to as “FTB”). The pipelines may have buoyancy modules attached along the run of the pipeline and may achieve different depth profiles (as suggested by the illustration of FIG. 1) as a function of distance from the FPSO, if desired. Marine hoses or flexible flowlines  20  are fluidly connected to the pipelines at the FTB  18  and to the product swivel of CALM buoy  16 . Mooring legs  17  couple the CALM buoy  16  at the sea surface  4  to the sea floor  6 . The submerged FTB  18  is coupled to the seabed by anchor legs  19 . The anchor legs  19  are preferably taut, but not necessarily so, depending upon design considerations. 
     The pipelines  12 , preferably steel tubular members which have flotation attached to them along their path from FPSO  14  to the FTB  18  to prevent excessive sagging due to their heavy weight, do not touch the sea floor. The pipelines may be steel tubular members which are joined end to end by welding as is known in the art of pipeline construction. Alternatively, the pipeline may be fabricated with composite materials. They typically run at least one nautical mile to the vicinity of the CALM offloading buoy  16 , but are submerged beneath the sea surface  4  at a depth so that shuttle tankers can maneuver between the FPSO  14  and the CALM buoy  16  without fear of fouling the pipelines  12 . Steel pipelines are rigid in the sense that they are continuous steel tubular members, but of course such a steel pipeline has flexibility due to their great weight and the inherent flexibility of a long spaghetti-like steel tubular string. Although the FTB  18  is shown positioned between the FPSO  14  and the CALM  16  as in FIG. 1, it may be positioned to the far side of CALM buoy  16  as shown in FIG.  2 . 
     The more detailed illustration of FIG. 2 illustrates a shuttle vessel  20  moored by hawsers  22  to the rotatable part of the single point mooring (e.g. CALM) offloading buoy  16 . The FTB  18  is submerged about 75 meters below the. sea surface  4  to prevent contact from tanker  20  and to reduce the wave forces on the FTB. FTB  18  is anchored by four anchor legs  19 . The centerline of the FTB  18  is about 80 meters from the centerline of the offloading buoy  16 . 
     The ends of pipeline  12  are terminated by gooseneck members  30  (see more detail in FIG. 4) which are suspended from FTB  18  by chains  26 . Wire or synthetic rope may be substituted for chains  26  as a suspension member. Alternatively, the ends of pipelines  12  may be attached directly to the FTB without the use of a suspending member. The attachment may be a rigid or elastic support. (This alternative direct connection is not illustrated.) In the preferred embodiment of FIG. 2, the flexible hoses  20  are connected between the goosenecks  30  and a stationary portion of a product swivel mounted on the stationary portion of CALM buoy  34 . One or more loading hoses  23  extend from the rotatable portion of product swivel  34  to vessel  20 . Clump weights  28  positioned on hoses  20  as illustrated in FIG. 2 provide for near-vertical entry of hoses  20  into the product swivel  34  on CALM buoy  16  with a curved section between clump weights  28  and gooseneck connections  32  providing further isolation of forces to the ends of pipelines  12  from wave, wind, and current induced motions of CALM buoy. Ball valves  30  and double closure breakaway couplings  36  provide for prevention of hydrocarbon spilling into the sea in case of repair or emergency disconnection of the hoses  20  from the pipeline  12 . 
     FIG. 3A (top view) and FIG. 3B (side view) show that a preferred embodiment of the FTB  18  includes three buoyancy tanks  50 ,  52 ,  54  mounted on a frame  56 . A side elevation of the FTB is shown in FIG.  2 . Four anchor legs  19  are connected to frame  56  via conductors  60 ,  62 ,  64 ,  66  and chain stoppers  61 ,  63 ,  65 ,  67 . Suspending chains  26  are connected between the seabed and FTB  18  via chain conductor  70 ,  68  and chain stoppers  69 ,  71 . 
     FIG. 4 illustrates gooseneck member  30  (one for each pipeline hose connection) to which a chain  26  is connected via a link  70  secured to a clevis  72  by a pin  71 . The ends of pipeline  12  are fluidly connected to inlet end  74 . The lower end of each hose  20  is connected to outlet end  76 . An extension member  78 , secured to the inlet section of gooseneck  30 , is connected to clevis  72 . A cross member  77  and shackle provides for handling of gooseneck member  30  during installation. 
     METHOD OF INSTALLATION 
     The preferred method for installing the arrangement of FIG. 1 includes providing FTB  18  (as shown in FIG. 5A) at a sea surface location and installing anchor legs  17 A and  17 B and adjusting their length to a final position in chain stoppers  61 ,  63 . The anchor chains  17 C,  17 D are connected to the FTB  18  with a length of additional installation chain connected to the top portion of the chain. A length of 100 meters of additional installation chain may be provided depending on the site. In this condition the FTB remains at the water surface  4 . 
     Next, a pipeline  12  is provided in one of two alternative ways. The pipeline  12  can be fabricated at an onshore location and towed to the FTB so that it extends from the hydrocarbon facility  14  to the FTB  18 . Alternatively, the pipeline  12  may be assembled in place at sea by J-laying or S-laying processes starting from the FTB  18  and running to the hydrocarbon facility  14 . A single pipeline  12  may be provided as illustrated in FIGS. 5A,  5 B, but two or more pipelines  12 A,  12 B may be provided as illustrated in FIGS. 5E-5H. 
     As shown in FIG. 5B, a first pipeline suspending chain  26 A is installed via FTB conductor  68  and chain stopper  71  to a desired length and is attached to gooseneck  30  at the end of a pipeline  12  which has buoyancy elements  100  added to it so that it floats on the sea surface  4 . The first pipeline  12 A is connected to the FPSO or platform  14 . It is flooded and excess buoyancy is removed to allow it to reach its desired suspended configuration as illustrated in FIG.  5 C. The FTB  18  supports the pipeline  12 A while remaining on the sea surface  4 . 
     As shown in FIG. 5D a second pipeline  12 B is connected to FTB  18  by means of a second suspending chain  26 B in the same way as described above. The pipeline  12 B is connected to platform  14 , and when the pipeline  12 B is flooded and allowed to reach its desired suspended configuration, the FTB  18  has sufficient buoyancy to remain on the surface as in FIG.  5 E. 
     FIG. 5F indicates that a submersible chain pulling device  100  such as chain jacks, rotary drives, etc. are installed on anchor legs  17 C,  17 D which are used to jack the FTB down to a desired depth by controlling progress on legs  17 C,  17 D. 
     As shown in FIG. 5G, the chain pulling devices  100  and excess installation chain at the top of legs  17 C,  17 D are removed. Next as illustrated in FIG. 5H, this CALM buoy  16  is installed and the marine hoses  20  are installed with gooseneck connections between pipelines  12 A,  12 B to a product swivel of CALM buoy  16 .