Patent Publication Number: US-6983712-B2

Title: Offloading arrangements and method for spread moored FPSOs

Description:
RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/309,853, filed Aug. 3, 2001 by Roy H. Cottrell, Rick A. Hall, Brent A. Salyer, Caspar N. Heyl and Richard H. Gunderson and entitled “Offloading Arrangements and Methods For Spread Moored FPSOs”, which provisional application is incorporated by reference herein for all purposes. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to mooring systems for offshore terminals and in particular to offloading apparatus and methods for spread moored FPSOs (floating production storage and offloading vessels). 
   2. Description of the Prior Art 
   The spread mooring of FPSO vessels with offloading by tandem connection of a shuttle tanker is well-known in the prior art. Prior art tandem connection of a shuttle tanker to an FPSO for hydrocarbon offloading are characterized by several problems:
         (1) The limited sector available to the shuttle tanker for unloading at the bow or stem of the vessel (centerline dead astern or dead ahead to ±30 degrees to port or starboard).   (2) The proximity between the shuttle tanker and the FPSO required for tandem offloading, during approach and offloading with the possibility of collision in severe weather.   (3) The FPSO&#39;s inability to weathervane.   (4) The magnitude of potential damage in the event of collision.   (5) The cost of maintaining the shuttle tanker within the safe unloading zone during offloading.   (6) The cost of assisting the shuttle tanker during approach to the FPSO.       

   Summing up, prior offloading systems and methods for tandem offloading from a spread moored vessel to a shuttle tanker results in collision risk and unloading downtime risk. 
   To reduce risks, prior art systems are known which provide an SPM (Single point Mooring) terminal at a distance of 2000 meters from the FPSO for offloading. Such an arrangement permits weathervaning of the shuttle tanker, eliminates proximity to the FPSO, reduces the cost of collision between the shuttle tanker and the terminal, and minimizes the cost of shuttle tanker assistance. 
   Such prior art spread moored FPSO offloading systems and methods have provided an independent SPM for a shuttle tanker such as a CALM buoy located a long distance (usually about 2000 meters) from the FPSO in order that a shuttle tanker not contact the FPSO. A flow line, such as a steel pipeline, is run from the FPSO to the CALM buoy. A hose is then run, via a rotatable fluid coupling, to the shuttle tanker which is moored to the CALM buoy by means of a mooring hawser. Fatigue problems (due to constant movement of the sea surface) in the pipe line where it connects to the CALM buoy have been overcome by terminating the pipeline at a submerged Flowline Termination Buoy (FTB). A flexible hose is run from the pipeline end at the FTB to the CALM buoy. 
   Such prior art systems have provided complete independence of the SPM for the shuttle tanker due to the great distance between the tanker and the FPSO. In other words, the CALM buoy, to which the shuttle tanker is moored, is anchored to the sea floor without any mooring members connected to the FPSO. Unfortunately, in deep water, the cost of the mooring system, SPM terminal, and the fatigue resistant flow line from the FPSO to the FTB is very high and justified only for installations with high throughput and consequent high frequency of offloading with resulting higher risk. 
   Identification of Objects of the Invention. 
   The object of this invention is to provide arrangements and methods which overcome the disadvantages identified above. 
   Another object of the invention is to provide a single point mooring (SPM) for a shuttle tanker where the SPM is controlled directly or indirectly by linkage to a spread moored FPSO, with the result that the disadvantages identified above are overcome. 
   Another primary object of the invention is to provide a mooring system by which a shuttle tanker is moored to a SALM which is directly linked to a submerged yoke which is pendularly connected to the FPSO such that the shuttle (1) can be moored to an FPSO with the mooring being tolerant to surge conditions of the sea, (2) can accept connections of the shuttle tanker at angles to the longitudinal axis of the FPSO, and (3) can allow weathervaning angles of the shuttle tanker with respect to a spread moored FPSO up to about 300 degrees. 
   Another object of the invention is to provide a mooring system by which an LNG shuttle tanker is moored to buoyant columns secured to a submerged yoke which is pendularly connected to an LNG/FPSO such that the tanker is (1) surge tolerant, (2) can be moored at angles to the longitudinal axis of the LNG/FPSO and (3) can rotate in an arc about an end of the LNG/FPSO. 
   Another object of the invention is to provide a mooring system by which a shuttle tanker is moored at one end to a hold back buoy which is indirectly linked to the FPSO by means of a tension member connected between an end of the FPSO and an opposite end of the shuttle tanker, such that the shuttle tanker (1) can move in an arc about the end of the FPSO (2) is prevented from contacting the FPSO by the hold back buoy and (3) can be quickly disconnected from the hold back buoy. 
   Another object of the invention is to provide a mooring system by which a shuttle tanker is moored to a mooring buoy spaced about 600 meters from the end of the spread moored FPSO, where the mooring buoy is anchored to the sea floor and linked to the FPSO by means of a catenary chain, such that the shuttle tanker can move in a three hundred sixty degree circle about the mooring buoy without contact with the FPSO. 
   Another object of the invention is to provide a mooring system by which a shuttle tanker is moored to a mooring buoy in the form of a SALM which is connected to a mooring leg group for the FPSO, where the SALM is spaced about 600 meters from the end of the spread moored FPSO, such that the shuttle tanker can weathervane in a three hundred sixty degree circle about the SALM without contact with the FPSO. 
   Another object of the invention is to provide a mooring system by which a shuttle tanker is moored to a mooring buoy in the form of a Dynamically Positioned buoy, indirectly linked to the FPSO by means of a remote control link and directly linked to the FPSO by means of a mooring line between the DP buoy and the FPSO, where the DP buoy is spaced about 600 meters form the end of the spread moored FPSO, such that the shuttle tanker can weathervane in a three hundred sixty degree circle about the DP buoy and the DP buoy can be positioned in an arc about an end of the FPSO. 
   SUMMARY OF THE INVENTION 
   The objects identified above, along with other features and advantages of the present invention, are provided in a mooring system for a shuttle tanker for offloading from a spread moored FPSO type vessel in deep water, where a mooring buoy linked directly and/or indirectly to the FPSO moors the shuttle tanker in close proximity (e.g., about 600 meters or less) from an end of the FPSO. According to a first FPSO offloading arrangement, a shuttle tanker is moored from a FPSO by a submerged yoke where a first yoke end is supported in dependent and moveable relation from an end of a FPSO and a second yoke end is supported in dependent relation from a SALM (Single Anchor Leg Mooring) buoy. The SALM is moored to a second end of the submerged yoke with a mooring hawser connected between the SALM and the shuttle tanker. 
   According to a second FPSO offloading arrangement for an LNG/FPSO, a submerged yoke is suspended in dependent relation from the LNG/FPSO by flexible links as in the first offloading arrangement. The submerged yoke is provided with spaced buoyant forward and aft columns which also serve as mooring elements to which the LNG/shuttle tanker can be moored. The bow of the LNG/shuttle tanker is moored to the forward buoyant column and the midships of the LNG/shuttle tanker can be moored to the aft buoyant column, with its LNG manifold being located immediately adjacent the aft buoyant column. The aft buoyant column is provided with a loading boom for controlled support and orientation of the LNG offloading hose. In this case, the flexible connection of the submerged yoke to the FPSO permits the submerged yoke and the LNG/shuttle tanker to weathervane about a significant arc even though the spread mooring system of the LNG/FPSO prevents it from weathervaning. This mooring arrangement is not strictly restricted to offloading of LNG products, but may be employed for offloading any of the usual products, for example, crude oil, distillate, etc., without departing from the spirit and scope of the present invention. 
   In situations where limited weathervane movement of a shuttle tanker is allowed and where controlled non-contact stationing of the shuttle tanker is necessary, a third mooring and offloading arrangement is provided within the scope of the present invention wherein an FPSO is spread moored in deep water. A compliant hold-back buoy, connected to an aft end of the shuttle tanker, is located a distance from one end of the FPSO by a dual diverging leg mooring arrangement and has an operative position and a rest position with respect to the FPSO, the operative position being established as the buoy is moved closer to the FPSO by traction or tension forces applied through this shuttle tanker itself by a traction hawser and traction winch mechanism connected between the FPSO and the bow end of the shuttle tanker. To permit offloading activity, a shuttle tanker is moved into position between the FPSO and the rest position of the hold-back buoy and one of the ends of the shuttle tanker, preferably the aft end, is connected to the hold-back buoy by an anchor chain. An opposite end of the shuttle tanker, typically the bow, is connected to the FPSO by a mooring chain. The mooring chain may be composed entirely of chain material or, if desired, it may have chain ends to permit ease of connection and disconnection, with the chain ends being connected to respective ends of a mooring hawser composed of cable, rope or any other desirable material of high tensile strength. During mooring connection, a pull-in or traction hawser is connected to the shuttle tanker and applies tension or traction force to the mooring chain to move the shuttle tanker slightly closer to the FPSO than the desired mooring position. The tension being applied to the anchor chain also moves the hold-back buoy, which is tethered to the shuttle tanker, from its rest position to an operative position nearer and in substantial alignment with the FPSO. After the shuttle tanker has been pulled to a position slightly closer to the FPSO than the desired offloading position, the mooring chain is connected between the FPSO and the shuttle tanker, and the tension of the traction winch is relaxed, permitting the mooring chain to accept the entire mooring load. In this moored condition, because the hold back buoy mooring is more compliant than the FLSP mooring, the shuttle tanker is allowed to weathervane slightly about its mooring point on the FPSO to remove the mooring loads induced on the system by waves, wind or current not aligned with the longitudinal axis of the FPSO. The traction winch and its traction or tension hawser may be used at any point to apply greater tension to the anchor chain. In this case, the tension that is applied to the anchor chain by the traction winch combined with the stiffness characteristics of the mooring legs determines the amount of weather and current compliant lateral excursion of the shuttle tanker from alignment with the center-line of the FPSO and the hold-back buoy. 
   A fourth offloading arrangement moors a shuttle tanker to a spread moored FPSO in deep water by locating a Single Point Buoy (SPM) a sufficient distance from the FPSO/SPM such that the shuttle tanker is permitted to weathervane 360 degrees about the SPM. The SPM can be moored by diverging hold-back mooring legs, or even a single hold-back leg, to ensure its minimum spacing with respect to the FPSO. The SPM is typically a buoyant column having its upper end provided with a loading boom or turntable for controlled support and positioning of the offloading hose or hoses through a rotatable coupling and the connection thereof to the fluid handling manifold of the shuttle tanker. A connection chain or other suitable connector links the SPM to the FPSO and maintains the position of the buoy or column and provides protection for an offloading riser between the FPSO and the shuttle tanker. The chain and riser have sufficient catenary shapes to permit the shuttle tanker to pass over them without any potential for contact or interference. 
   The present invention may take the form of a fifth offloading arrangement where one leg group of the spread mooring legs for the FPSO is modified to permit shuttle tanker mooring to a SALM buoy linked to the FPSO. At a distance sufficient to provide for 360 degree weathervaning movement of a shuttle tanker, a floating column or buoy type SALM is moored by a substantially vertically oriented mooring link, chain or line that is fixed intermediate the length of one of the typically four mooring leg systems of the FPSO. A production fluid flow line from the FPSO extends along and is tethered to the selected mooring leg system, with its remote end terminating at the SALM. The SALM is also provided with a mooring system for weathervane mooring of the shuttle tanker and is provided with handling and control equipment for one or more flow lines that extend, typically along the mooring hawser from the SALM to the flow control manifold system of the shuttle tanker through a rotatable coupling. 
   For stationing of SPM buoys relative to a moored FPSO, without using hold-back mooring or anchoring systems for the buoys, one or more dynamic positioning buoys, referred to here as DP buoys, are indirectly linked to the FPSO. According to the sixth offloading arrangement of the present invention, a DP buoy having independent on-board power systems and rotatable hawser and hose turntables is controlled directly on the DP buoy or is remotely controlled by the FPSO. A DP buoy may be stationed at a minimum distance (e.g., about 600 meters) from the FPSO that is sufficient to permit substantially 360 degrees rotation of the shuttle tanker about the DP buoy. Likewise, the DP buoy can be operated to be stationed at any location within an arc of about 180 degrees from the point of connection of its catenary mooring tether, line or chain, with the FPSO as urged by the action of wind, waves or currents. The catenary of the mooring line or chain permits the shuttle tanker to pass over it without contact by the shuttle tanker. A flow line or hose extends from the FPSO along the length of the mooring line or chain to the DP buoy and is protected against excess tension force by the mooring line or chain, because the chain is shorter than the flowline. When offloading of a shuttle tanker is not in progress or is imminently expected, the thrusters of the DP buoy can be deenergized, in which case the weight of the mooring line or chain and offloading hose draws the DP buoy to a rest station close to the FPSO. To provide for protection of the FPSO and the DP buoy when the buoy is located at its close-in rest station, the buoy is provided with one or more fenders. The fenders also provide protection for the shuttle tanker in the event of contact with the buoy. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described by reference to drawings of which: 
       FIGS. 1A and 1B  are side and plan views of a spread moored FPSO with a shuttle tanker moored by a submerged yoke to the stem of the FPSO; 
       FIGS. 2A and 2B  are side and plan views of a spread moored LNG/FPSO with a LNG/shuttle tanker and employing a submerged yoke for close mooring and for production offloading control; 
       FIG. 2C  is a view taken along lines  2 C— 2 C of  FIG. 2A  and showing the submerged yoke with buoyant columns and LNG offloading system of  FIGS. 2A and 2B  and showing an LNG shuttle tanker in relation to the surface of the seawater on which the FPSO offloading system is located; 
       FIGS. 3A and 3B  are side elevation and plan views of a spread moored FPSO with a shuttle tanker having a hold-back buoy provided to reduce collision risk between the shuttle tanker and the FPSO and to permit environmental compliant lateral excursion of the shuttle tanker with regard to the tension being applied by a traction winch of the FPSO, and 
       FIG. 3E  illustrates connection and release mechanisms by which the shuttle tanker is connected or disconnected from the FPSO; 
       FIGS. 3C and 3D  are side elevational and plan views showing an alternative mooring arrangement with dual mooring legs arranged to either side of a buoy or SALM and in alignment with the center-line of the FPSO for permitting environmental compliant movement of the buoy while maintaining predetermined spacing with the FPSO; 
       FIGS. 4A and 4B  are side elevation and plan views of a SALM moored shuttle tanker arranged to weathervane 360 degrees about the SALM while being tethered in production offloading relation with the FPSO; 
       FIGS. 5A and 5B  are side elevation and plan views of a SALM moored shuttle tanker tethered to one of the spread mooring anchor leg groups of a FPSO; 
       FIGS. 6A and 6B  are side and plan views of a DP buoy with propulsion which can be dynamically positioned at a safe distance from the FPSO for mooring a shuttle tanker in offloading relation with the FPSO; 
       FIGS. 7A and 7B  are side elevation and plan views of a spread mooring of a FPSO utilizing the DP buoy of  FIGS. 6A and 6B  for dynamically positioning the buoy at a selected safe distance and position relative to the FPSO for 360 degree weathervaning mooring of a shuttle tanker in offloading relation with the FPSO; and 
       FIGS. 7C and 7D  are side elevation and plan views of the spread mooring system of  FIGS. 6A and 6B  and showing the rest position of the DP buoy being drawn close to the FPSO by the weight of the catenary mooring line or chain and the offloading riser. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
   As illustrated in  FIGS. 1A ,  1 B, a mooring arrangement  100  is illustrated where a submerged yoke  30  is hung from outriggers  13  located at the unloading end  11  of an FPSO, in a pendular fashion, and is supported at its opposite end by a fendered SALM  26 . A shuttle tanker  20  is moored to the SALM  26  by a mooring hawser  28  and loaded in the normal fashion through a floating hose  27  between the SALM  26  and a loading manifold of the shuttle tanker  20 . The FPSO&#39;s aft mooring legs  14  are keel mounted to avoid interference with the yoke  30  during partial weathervaning. 
   The submerged yoke  30  is preferably supported at the aft end of the FPSO by two vertical links  15  such as chains or other tension members. Links  15  are connected to outrigger porches  13  and allow the yoke  30  to twist about the end of the FPSO such that fendered SALM  26  can rotate in an arc A 1  during weathervaning conditions operating on shuttle tanker  20 . The shuttle tanker  20 , connected to SALM  26  by the mooring hawser  28 , is capable of rotation in an arc depicted as A 2  about the SALM  26  as the center of rotation. 
   The arrangement  100  of  FIGS. 1A ,  1 B is advantageous because it allows partial weathervaning of the shuttle tanker  20  about the SALM  26  and in turn, the yoke  30  about the unloading end  11  of the FPSO  10 . This arrangement increases the safe unloading sector from ±30 degrees of prior art systems to ±150 degrees and facilitates a reduction in offloading down time due to non-colinearity of the tanker with the FPSO. Non-colinearity is a term describing the condition where the longitudinal axis of the shuttle tanker  20  is not aligned with that of the FPSO  10  due to environmental force misalignment with the FPSO. A second advantage is that because of the depth of the yoke  30  below the water surface and the use of a fendered SALM  26  as the shuttle tanker  20  mooring point, the likelihood of damage to the shuttle tanker due to shuttle tanker surge into the FPSO is eliminated. Consequently, tug assist during shuttle tanker approach and loading required of prior tandem offloading system is greatly reduced, resulting in a system which is more passive and less costly to operate. 
   An alternative addition to the arrangement to that of  FIGS. 1A ,  1 B is a single leg  5  stayed or tethered between the yoke  30  and the sea floor for directional stability of the yoke  30  between the FPSO  10  and the SALM buoy  26 . 
     FIGS. 2A ,  2 B, and  2 C illustrate an alternative mooring arrangement  200  of a LNG/FPSO processing vessel  210  with an LNG/shuttle tanker  220 . This alternative arrangement is similar to that of  FIGS. 1A and 1B  in that a submerged yoke  230  is suspended from the end of LNG/FPSO  210  by flexible links  215  which allow an end of the yoke  230  to rotate in an arc  231 . Two buoyant vertical columns  261 ,  262  are mounted on submerged yoke  230  and project above the water surface S to provide for LNG/shuttle tanker  220  mooring and LNG/FPSO  210  offloading. The buoyant column  261  provides a mooring structure to which one end, typically the bow, of an LNG/shuttle tanker  262  is moored when positioned for offloading. The buoyant column  262  is sufficiently spaced from the buoyant column  261  as to provide for mooring of the midship section of the LNG shuttle tanker  262  thereto. Such positioning causes the buoyant column  262  to be located immediately adjacent the midship section of the shuttle tanker  220 . An LNG loading boom  272  is mounted on the buoyant column  262 . The boom  272  provides support and control for the offloading arm or arms and hose or hoses extending from the LNG/shuttle tanker  220  and along the submerged yoke  230 . 
   The LNG/shuttle tanker  220  is moored by securing bow lines  233  to forward column  261 , and aft mooring lines  234  secure the tanker  220  to rear buoyant column  262 . A mid ship LNG manifold  270  accepts product via hose  280 , shown in  FIG. 2C , via LNG loading boom  272  which is in fluid communication with a fluid flow path (not illustrated) via the submerged yoke  230  to the LNG/FPSO  210  or with a marine loading arm (not illustrated). With the shuttle tanker tethered in substantially immovable relation with the submerged yoke  230 , the pendent link tethered relationship of the yoke to the FPSO permits the shuttle tanker  220  to weathervane in an arc  231  in the order of about 160 degrees. Thus, the LNG/shuttle tanker  220  has the capability for substantial arcuate excursion relative to the center-line of the LNG/FPSO  210 , while maintaining efficient fluid offloading connection with the LNG/FPSO  210  via the product offloading hose  280  or a marine loading arm. 
   The spread mooring arrangement  300  of  FIGS. 3A and 3B  illustrates a moored hold-back buoy  330  for mooring a shuttle tanker  301  between the buoy  330  and a FPSO  302 . The hold-back buoy  330  is moored to the sea floor at a predetermined distance away from the FPSO  302  in the direction generally down stream from the prevailing source of environmental forces. A pair of diverging mooring legs  303  and  304  permit the holdback buoy  330  to be stabilized against inadvertent movement. The shuttle tanker  301  or hold-back buoy  330  is fitted with a remotely actuated quick disconnect mooring point, such as shown at  305  or  306 , so that the shuttle tanker  301  can be quickly released from its mooring connection with the hold-back buoy  330  if desired. Also, when released from the shuttle tanker  301 , the hold-back buoy  330  is moved away from the FPSO  302  by the weight induced force of the mooring legs  303  and  304  or by environmental forces or both and assumes a “rest” position as shown in broken line at  307 . The FPSO  302  is fitted with a pull-in winch or traction winch  308  with a hawser storage reel  309  for applying tension or traction force to the mooring hawser  310  and thus pulling the shuttle tanker  301  toward the FPSO  302  after connection of the shuttle tanker  301  to the hold-back buoy  330 . 
   Shuttle tanker loading is typically accomplished by establishing a mooring connection at one end, typically the stem of the shuttle tanker  301  to the hold-back buoy  330 , with the hold-back buoy at its rest position  307 . The shuttle tanker can then move or be moved toward the FPSO  302 , thus causing the mooring legs  303  and  304  of the hold-back buoy  330  to assume the positions shown in  FIG. 3B , thus stabilizing one end of the shuttle tanker  301  and permitting its compliant movement within limits determined by the force being applied by the traction winch  308  and the stiffness characteristics of legs  303  and  304 . 
   The FPSO offloading and tanker loading system  300  is designed so that shuttle tanker surge is limited while partial weathervaning of the shuttle tanker about the loading connection at the FPSO is permitted by the compliance of the hold-back buoy mooring configuration. Also, the traction winch tension on the mooring hawser  310  can be simply and efficiently controlled to adjust system reaction to weather or environment induced lateral compliant movement of the shuttle tanker as evidenced by compliant movement arcs  311  and  312 . In this way, the hold-back buoy  330  eliminates the need for costly tugs that are normally employed for shuttle tanker hold-back and control during FPSO unloading. Loading the shuttle tanker  301  is accomplished with a floating hose  315  between the FPSO  302  and the shuttle tanker  301  or through a catenary shaped hose  321  suspended from FPSO  302  to shuttle tanker  301 . 
   As the shuttle tanker  301  connects to the hold-back buoy  330  during its approach to the FPSO  302 , hold back force with resulting shuttle tanker  301  position control increases as the shuttle tanker  301  nears the FPSO  302 . Such control reduces the risk of collision during approach. To pull the shuttle tanker into offloading position, the FPSO traction winch  309  pulls the shuttle tanker  301  toward the FPSO  302 . The tension can be released at any time during pull-in to allow the hold back buoy  330 , acting in response to the forces of its mooring legs  303 ,  304 , to pull the shuttle tanker  301  away from the FPSO  302  to a safe distance. The hawser  310  connecting the FPSO  302  to the shuttle tanker  301  has a chain section  316  at the FPSO end and a chain section  318  at the shuttle tanker end, such that upon arrival of the shuttle tanker  301  to the desired position relative to the FPSO  302 , a hook or stopper  317  on the FPSO  302  is readily connected to the hawser chain  316 . The chain section  318  is connected to hook  319  of the shuttle tanker  301 . The FPSO winch  309  then slacks off, transferring the load to the chain  316 -hawser  310 -chain  318  section of the pull-in line. The hook or stopper  317  can be released at any time, enabling the hold-back buoy  330  to pull the shuttle tanker  301  away from the FPSO  302 , to a distance of greater safety. The shuttle tanker  301  can be released normally at releasable hook or stopper  319  on shuttle tanker  301  or in an emergency by disconnecting link  313  from hook or stopper  317 . See  FIG. 3E . 
   An alternative spread mooring arrangement  300 ′ is shown in  FIGS. 3C and 3D  where a buoy or SALM  330  is moored by two mooring legs  321  and  322  which have anchor points  323  and  324  with the sea bottom B, the anchor points being in substantial alignment with the center-line  325  of a FPSO  326 . This arrangement permits substantial environment compliant movement as evidenced by compliant arrows  327  and  328 , while maintaining predetermined minimum spacing of the buoy  330  from the FPSO  326 , sufficient for greater lateral movement of a shuttle tanker with respect to the FPSO  325 . Mooring with one anchor leg positioned toward the FPSO  326  and a second anchor leg  322  directed away from the FPSO  326  provides for greater compliance in yaw and greater stiffness in surge. 
   Alternative configurations (not illustrated) to the arrangements of  FIGS. 3A ,  3 B,  3 C,  3 D include,
         (1) single anchor leg (rather than the two diverging anchor legs shown in  FIGS. 3A ,  3 B, and in  FIGS. 3C ,  3 D in the desired direction of unloading for lower loads and greater compliance;   (2) A hold-back buoy  330  which is submerged in operating conditions; and   (3) Multiple buoys, rather than the one hold back buoy of  FIGS. 3A ,  3 B, or  3 C,  3 D with the FPSO anchor legs serving as multi-buoy connection points.       

   The spread mooring and FPSO offloading arrangement  400 , in  FIGS. 4A and 4B  includes two mooring legs or groups of legs  401  and  402  between a single point mooring (SPM) terminal  403  to anchors  404  and  405  at the sea floor F, and a third mooring leg or groups of legs  406  connected to the FPSO  407 . The mooring leg or groups of legs  406  includes one or more chains  408  which is (are) shorter than an unloading hose or riser  409  and consequently are located over the unloading flow lines or hoses  409 . Alternatively, a single sea floor anchor leg group may be provided to the SPM buoy  403 . In such case, the single mooring leg  406  and its anchor will be aligned with the center-line of the FPSO. A shuttle tanker  410  is tethered by a hawser  412  to the SPM  403  and product hoses extend from the SPM to the shuttle tanker  410  for controlled offloading of the FPSO. 
   The arrangement  400  of  FIGS. 4A and 4B  allows 360 degree weathervaning of the shuttle tanker  410  at a distance on the order of 10 times greater than in the case of prior art tandem offloading, but if placed at about 600 meters from the FPSO  407 , the shuttle tanker  410  is less than one third of the 2000 m distance between the SPM and FPSO of current SPM terminal system designs. As a result, approach collision risk, offloading collision risk and offloading down time due to non-colinearity are all minimized. These advantages are achieved without, or with reduced, costly support tug assistance during unloading. Due to the reduced distance from the FPSO  407  to the SPM terminal  403  as compared with 2000 m distant SPM terminals, the flow lines are economically made of flexible material to eliminate fatigue concerns inherent in the larger diameter steel flow lines needed to keep head losses at reasonable levels with SPM terminals located 2000 m from the FPSO. 
   An alternative configuration to the spread mooring arrangement  400  illustrated in  FIGS. 4A and 4B  includes orientation of the SPM buoy  403  in the direction of the prevailing environment rather than being aligned with the centerline C/L of the FPSO or to the side of the FPSO to facilitate parallel approach in the case where the FPSO is aligned with the prevailing environment. 
   The spread mooring and FPSO offloading arrangement  500  in  FIGS. 5A and 5B  has a FPSO  501  that is moored by a plurality of mooring legs  502 ,  503 ,  504  and  505 . A SPM terminal  506  in the form of a SALM is tethered to one of the spread moor anchor leg groups  505  at a distance somewhere between the extremes of tandem (80 m) and CALM (2000 m) distance connections. The mooring leg group  505  includes a plurality of mooring leg sections  507 ,  508  and  509  having ends thereof received by an intermediate mooring connector  510 . The mooring connector  510  is linked to FPSO  501  by a single mooring line member or group of members  511  and is located at a distance of at least 600 m with respect to the FPSO  501 . A mooring leg or multiple mooring legs  514  extends from the intermediate connector  510  to an appropriate mooring connection of the SPM terminal  506 . A mooring hawser  516  establishes releasable mooring connection of the shuttle tanker  515  with the SPM terminal  506  and a product loading conduit  517 , which may be in the form of a flexible hose, provides a rotatable fluid flow connection of the SPM terminal  506  with a loading manifold of the shuttle tanker  515 . To permit 360 degree weathervane movement or rotation of a shuttle tanker  515  about the SPM buoy (or SALM)  506 , the single mooring line or link or multiple of mooring lines or links  511  has a length in the order of about 600 m so that the maximum shuttle tanker weathervaning radius permits the shuttle tanker  515  to remain well clear of the FPSO regardless of its weathervaned position. A product flow line or hose  512  from the FPSO  501  to the terminal  506  is routed along the single mooring line or link or multiple of mooring lines or links  511  of the spread moor anchor leg  505  and may be secured to the single mooring line or link by a plurality of retainer elements  513 . 
   The mooring link  514  is of a length such that the buoyancy of the SALM applies an upwardly directed force to the intermediate connector  510 , thus stabilizing the location of the SALM  506  with respect to the FPSO  501  to ensure efficiently controlled positioning of the shuttle tanker  515  relative to the FPSO  501  under all conditions of environmental positioning. 
   The mooring arrangement of  FIGS. 5A ,  5 B, similar to the FPSO tethered buoy of  FIGS. 4A ,  4 B, allows weathervaning and approach distances far greater than traditional tandem offloading, with flexible fatigue resistant flow lines. These advantages are achieved without, or with reduced, costly support tug assist during unloading. 
   An alternative arrangement to that illustrated in  FIGS. 5A ,  5 B includes mooring the SPM terminal from the anchor legs off to the side of the FPSO in the athwartships direction. 
     FIGS. 6A and 6B  illustrate a Dynamically Positioned buoy, shown generally at  600 , having an onboard propulsion system having sufficient directional controlled thrust for moving a shuttle tanker or for counteracting environmental forces. The DP buoy  600  is therefore capable of being dynamically positioned by its propulsion system at a selected distance from the FPSO  620 , shown in the operational plan and elevational views of  FIGS. 7A and 7B  and thus permit control of the character and location of shuttle tanker mooring that is desired. The DP buoy  600  also permits the position of the shuttle tanker  617  to be controlled with respect to changes in the environment. The DP buoy  600  includes a buoyant body  601  which positions the buoy at the water surface S. A turn-table  603  having a rotary mounting section  604  is rotatably supported by the buoyant body  601 , thus permitting the buoyant body  601  to be selectively rotatably positioned relative to the turn-table  603 . The rotary mounting section  604  is of generally cylindrical configuration and has a lower conduit connector  605  having a connection extension  606  to which under-buoy FPSO product hoses  607  are connected. A catenary tether  608 , which is preferably in the form of a mooring chain, is connected to an FPSO  620  and the DP buoy  600  and assumes a catenary configuration as shown in  FIG. 7A  to permit a weathervaning shuttle tanker  617  to pass over it in response to environmental changes. The submerged product hoses  607  have sufficient length to accommodate the minimum 600 m spacing of the buoy  600  from the FPSO  620  and to accommodate the catenary that is required to permit a shuttle tanker to pass over the product hoses  607  and the catenary tether  608 . 
   The turntable  603  is provided with a hose connector extension  609  which provides for support, orientation and connection of floating hose  611  which extend to the loading manifold of a shuttle tanker  617  being moored from the buoy  600 . One or more hawser members  613  are provided on a turntable extension  612  of the buoy  600  to permit connection of shuttle tanker hawsers  613  for mooring of a shuttle tanker  617  during FPSO  620  offloading and shuttle tanker  617  loading. 
   The DP buoy  600  is powered by twin z-drive propulsion units  614  that are locally controlled on the DP buoy  600  itself or are remotely controlled from the FPSO. Remote control units are schematically indicated by controller  630  with antennae for remote communication between FPSO and DP buoy as illustrated in  FIGS. 6A and 7A . The catenary tether  608  of the DP buoy  600  to the FPSO is connected to an under buoy turntable  605 , which also houses the connection of under buoy loading hoses  607 . The shuttle tanker is moored through hawsers  613  to a deck-mounted turntable  603  and loaded through typical floating hose or hoses  611  connected to the same turntable assembly. The floating hoses  611  and under buoy hoses  606  fluidly communicate through a product swivel  615  located at the center of the body  601 . The buoy  600  also includes one or more fenders  616  which provide protection for the buoy  600 , the shuttle tanker  617  and the FPSO  620  in the event of contact. 
   In operation, the DP buoy  600  is free to weathervane about the FPSO  620  on its catenary tether  608  as evidenced by the buoy position arc  618  of  FIG. 7B . The shuttle tanker  617  is, in turn, free for 360 degree weathervaning about the DP buoy  600  within a maximum shuttle tanker radius  619  that permits the shuttle tanker to pass over and well clear of the catenary tether  608  and the submerged FPSO product hoses  607  during weathervaning movement. As mentioned above, the DP buoy  600  is fitted with twin z-drive propulsion sets  614 , which exert force away from the FPSO in the event of a sudden change in prevailing environment forces which might put the shuttle tanker  617  in jeopardy of collision or interference with the FPSO  620 . Used with an FPSO having its mooring legs connected at keel level, the safe unloading zone of the FPSO  620  is increased from ±30 degrees to ±90 degrees, thereby minimizing the frequency and magnitude of DP buoy propulsion system use. 
     FIGS. 7C and 7D  are elevational and plan views which illustrate positioning of the DP buoy  600  when it is not in use. After loading of a shuttle tanker has been completed, the shuttle tanker disconnects from its product loading connection and its mooring connection with the DP buoy. At this point, the propulsion system may be activated to move the buoy  600  from its operative position on the arc  618  to close proximity with the FPSO as shown in  FIG. 7C , with the floating product loading hose or hoses  611  remaining on the water surface and available for connection with the next shuttle tanker to be loaded. Alternatively, the DP buoy may be deenergized, causing the weight induced forces of the catenary tether  608  and the submerged hose or hoses  607  to pull the DP buoy  600  to a position near the FPSO, with the catenary tether  608  and the submerged hose or hoses  607  settling toward the sea bottom and generally assuming the configuration shown in the elevational view of  FIG. 7C . 
   In view of the foregoing it is evident that the present invention is one well adapted to attain all of the objects and features hereinabove set forth, together with other objects and features which are inherent in the apparatus disclosed herein. 
   As will be readily apparent to those skilled in the art, the present invention may easily be produced in other specific forms without departing from its spirit or essential characteristics. The present embodiment is, therefore, to be considered as merely illustrative and not restrictive, the scope of the invention being indicated by the claims rather than the foregoing description, and all changes which come within the meaning and range of equivalence of the claims are therefore intended to be embraced therein.