Abstract:
A system is described for use at offshore locations of large depth, for mooring a production vessel or floating unit ( 14 ) at a location over a hydrocarbon reservoir ( 26 ) and for connecting risers ( 101 ) that can be carrying hydrocarbons up from the sea floor to a production vessel that stores the hydrocarbons, flowlines for water injection, gas lift, gas export, umbilicals and mooring lines that moor the vessel. Both the mooring lines and the risers are disconnectably connected to the vessel though a connection buoy, or connector ( 16 ). The invention concerns a system that allows a connector ( 16 ) to be used that is of minimum mass and volume, to ease its handling especially during its connection and disconnection to and from a vessel.

Description:
CROSS-REFERENCE 
   Applicant claims priority from US Provisional Patent Application Ser. No. 60/934,230 filed 12 Jun., 2007. 

   BACKGROUND OF THE INVENTION 
   Hydrocarbons in an undersea reservoir lying at the bottom of a deep sea (over 500 meters) are commonly produced by an installation that includes risers for carrying the hydrocarbons up from the sea floor to a production vessel that stores the hydrocarbons. The connections to the sea floor can also include flowlines for water injection, gas lift, gas export, and umbilicals, and also mooring lines that moor the vessel. At times the vessel must sail away from a location over the region of the reservoir where the risers and mooring lines are located, as when a storm is approaching, or to carry the stored hydrocarbons to another station, or for another purpose. For this reason, the installation commonly includes a connection buoy, or buoyant connector that is connected to the upper ends of the risers and the upper ends of the mooring lines, and that is in turn, connected to the vessel in a manner that allows the connector to be disconnected and reconnected. When the connector is disconnected from the vessel, the connector sinks to a position that is at least 25 meters under the sea surface so the connector lies under most or all of the wave action zone. 
   When the vessel returns to the production installation, the connector must be raised and connected to the vessel by personnel on the vessel and/or divers. The less massive the connector, the easier it is to manipulate and move during disconnection and reconnection. The present invention is directed largely to making such installations so the connector is of minimum mass and volume and therefore easier to move, and so the connector is moved a minimum distance. The installations are used primarily for the production of hydrocarbons, but are useful wherever large quantities of hydrocarbons are to be transferred. 
   SUMMARY OF THE INVENTION 
   In accordance with one embodiment of the invention, an installation is provided for mooring a hydrocarbon transfer vessel that includes a buoyant connector that connects risers and mooring lines to a vessel, wherein the connector can be disconnected from the vessel to sink under much of the wave action zone, wherein the connector can be moved with minimum effort. The mooring lines have primarily vertical lower portions that extend up to mooring buoys and have upper portions that extend primarily horizontally from the mooring buoys to the connector. The risers have lower portions that extend from the sea floor up to riser buoy means, and the risers have upper portions in the form of jumper hoses that extend from the riser buoy means to the connector. In most cases, the riser buoy means are buoys that are separate from the mooring buoys, but in some cases the riser buoys are formed by the mooring buoys that also support the lower portions of the risers. According to the invention, the riser buoy means is not directly moored to the seabed, but is coupled to the mooring buoys. It should be noted that in this text, “coupled to the mooring buoys” includes attached to the mooring system in the vicinity of the buoy or to a junction element linked to the buoy. 
   There is no primarily vertical line or other weight-supporting connection between any riser buoy (or riser buoy means) and the buoyant connector. Flexible jumper hoses extend from the riser buoy to the connector, but the jumper hoses are buoyant in water and are too long and flexible to transfer weight from the riser buoy to the connector. As a result, the connector supports substantially only its own weight, and half of the weight of the jumper hoses. As a result, when the connector must be lifted from deep (e.g. 50 meters) under water to the vessel, the personnel must lift only the weight of the buoyant connector (minus its buoyancy), one end of each mooring line horizontal upper portion, and a portion of the jumper hoses of the risers. 
   The novel features of the invention are set forth with particularity in the appended claims. It should be understood that when referring to risers, applicant refers to risers carrying the hydrocarbons up from the sea floor to a production vessel that stores the hydrocarbons, as well as flowlines for water injection, for gas lift, for gas export (when needed) and umbilicals. The invention will be best understood from the following description when read in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1   a  is a side elevation view of a vessel and a hydrocarbon transfer installation of the invention, with the connector of the installation connected to the vessel. 
       FIG. 1   b  is a side elevation view of an installation that differs from that of  FIG. 1   a  in that the mooring buoys are more tightly coupled to the riser buoys but less tightly coupled to the connector. 
       FIG. 1   c  is a view similar to that of  FIG. 1   a , but with the connector disconnected from the vessel and lying under the wave active zone. 
       FIG. 1   d  is a view similar to that of  FIG. 1   b , but with the connector disconnected from the vessel and lying under the wave active zone. 
       FIG. 2  is a side elevation view of a vessel and installation of another embodiment of the invention, wherein the mooring buoys serve as buoy means that also support the risers. 
       FIG. 3  is a plan view of the vessel and installation of  FIG. 1   a.    
       FIG. 4  is an end elevation view of a portion of the installation of  FIGS. 1   a  and  1   b.    
       FIG. 5  is a side elevation view of a vessel and installation of another embodiment of the invention wherein each riser (or group of risers that extend close together up from the sea floor) has a taut lower portion and the top of its lower riser portion is supported by a separate riser buoy. 
       FIG. 6  is a side elevation view of a vessel and installation similar to that of  FIG. 5 , but with primarily horizontal tether lines extending between each mooring buoy and riser buoy and between the riser buoys, and the riser lower portions have a catenary shape. 
       FIG. 7  is a side elevation view of a vessel and installation which combines the systems of  FIGS. 2 and 5 , with some of the riser lower portions supported by the mooring buoys and some of the riser lower portions supported by separate riser buoys. 
       FIG. 8  is a side elevation view of the system of  FIG. 7  but with the connector detached from the vessel and lying deep underwater. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1   a  illustrates a system  12  for mooring a vessel  14  such as an FPSO (floating, production, storage, and offloading) through a disconnectable turret buoy, or connection buoy, or buoyant connector  16 . The system includes risers (production lines, lines for water injection, gas lift, umbilicals)  101  whose lower ends  24  lead to well heads  25  that connect to a subsea hydrocarbon (oil and/or gas) reservoir  26 , and also includes mooring assemblies  30  that hold the vessel in position. The risers  101  and mooring or anchor assemblies  30  have upper ends connected to the connection buoy  16 , and lower ends connected to the sea floor  34 . Thus, all major connections of the vessel to the sea floor are made though the connection buoy  16 . There is no primarily vertical tensioned line that extends from the riser buoy  102  to the seabed  34 . The vessel sometimes sails away from the location over the reservoir, as when a large storm or iceberg is approaching, or if the vessel sails to a location where it unloads the hydrocarbons it has collected and stored. In those cases, the connection buoy  16  must be disconnected from the vessel  14  and allowed to sink to a height that is preferably below the bottom  70  of a wave action zone  40  of height A, and later picked up and reconnected to the vessel  14 . 
   During disconnection and reconnection of the connection buoy  16 , the buoy must be handled by personnel on the vessel and/or divers. The less massive the connection buoy, the easier it is to manipulate it and move it during such operations. The present invention is directed to designing the system so a connection buoy of minimal mass and volume can be used to reliably connect and disconnect the mooring and riser parts of the system to the vessel. 
   The mooring assemblies  30  include lines preferably made of steel wire or polyester ropes or combinations thereof which are of less weight than long steel chain mooring lines. Steel has a specific gravity of about 7 and if long steel chains were used their upper ends would have to be supported by a relatively large vessel or large buoy. 
     FIG. 1   a  also shows that the vessel has a turret that allows the vessel to weathervane, and that the buoyant connector  16  is connected to the bottom of the turret. A majority of the height of the connector  16  lies under the turret. The bottom of the vessel hull lies about 20 meters below the sea surface for the installation illustrated, and the top of the connector lies about 3 meters above the vessel hull bottom. As a result, the connector moves down about 33 meters in order to lie under the wave active zone  40  (which extends to about 50 meters under the sea surface, or to a depth between 25 and 75 meters under the sea surface), and the connector must be lifted about 33 meters in order to reconnect it to the vessel. If the connector lay fully in the vessel, then it would have to be moved up from a greater depth that is about 7 meters deeper for reconnection. In particular cases such as in seas where there are icebergs, the connector can move down about 100 meters in order to lie under icebergs. 
   A spring buoy  50  (a buoy with springs extending down from the buoy) is shown in  FIG. 1   a  lying under the wave action zone  40 , and is attached to the upper end of each primarily vertical lower mooring line parts  44 . Short lengths  52  of steel chain extend from the spring buoy to each lower line part  44 . Two or more primarily horizontal upper polymer or polymer-and-steel cable line parts  56 , which constitute upper mooring line parts, extend from the spring buoy to the connection buoy  16 . Applicant prefers to use at least two upper line parts for redundancy reasons, so to ensure continued mooring even if one upper mooring line part breaks. 
     FIG. 1   a  also shows risers  101  formed by steel catenary riser (SCR) lower riser parts  100  and flexible jumper hoses  64 , with a common riser buoy  102  connected by primarily horizontal lines  104  to the spring buoys or mooring buoys  50 . The riser buoy  102  lies closer to the sea surface than to the sea floor. The riser buoy  102  is not directly moored to the seabed but follows the movements and displacements of the spring buoys  50 , as they are interconnected. The common riser buoy  102  could also refer to a bundle of several smaller buoys (as shown in  FIG. 1   b ), one buoy supporting one riser lower part  100 . 
   Further, it can be seen in  FIG. 1   a  that the connection buoy  16  supports one end of each of the primarily horizontal upper mooring line parts  56 . These mooring line parts  56  have a specific gravity only moderately greater than water. The connector buoy  16  also supports some of the weight of the riser upper portions that are formed by the jumper hoses  64 . The jumper hoses are very flexible and do not support any weight other than their own weight. There is no primarily vertical weight-supporting line that extends from the common buoy  102  to the seabed. As there is no tensioned line between the buoyant connector  16  and the common buoy, or riser buoy means  102 , the common buoy  102  is not moved up or down appreciably (by at least 10% of common buoy vertical movement) when the connector is moved vertically. Thus, when the disconnected connector  16 A of  FIG. 1   c  must be reconnected to the vessel  14 , personnel have to lift and manipulate the mass of the connector  16 A, the mass of about half the weight in water of the upper mooring line parts  56 A, and a part of the mass of the jumper hoses  64 A. When the connector  16 A is lifted, it does not lift the common buoy  102 A or the weights of the riser lower portions  100 A that hang from the buoy  102 A. 
     FIG. 1   b  shows an alternative embodiment, where the mooring buoys  50  are connected to the common buoy via taut lines  104 , and the common buoy is a bundle of small buoys  102 , with one small buoy per riser  100 . In this configuration the pretension is shared between the mooring lines lower parts  44  and the primarily horizontal taut lines  104  that extend between the mooring buoys  50  and the small buoys  102 . The upper mooring line parts  56  do not have any net tension (other than that caused by their weight in water). The mooring buoys  50  could also be connected one to the other via a taut line  105  in addition to lines  104  (for redundancy or when there are no risers). In this configuration the mooring line upper parts  56  are very light and slack, so the mooring line weight supported by the connector buoy  16  is small. Hence, it creates an artificial water depth and hence the mooring line upper parts  56  and the jumper hoses  64  are independent from the pretension applied on the system, the connector  16  moving with jumpers  64  and the mooring lines upper parts  56 . The artificial water depth enables applicant to use upper mooring line parts  56  and jumper hoses  64  of short length which minimizes the suspended weight. Therefore, the design of the connector buoy can be simplified as it is less buoyant, smaller and lighter. 
     FIG. 1   c  shows the system of  FIG. 1   a  when the connection buoy  16 A has been disconnected from the vessel. The buoy  16 A is buoyant, while the upper line parts  56 A and jumper hoses  64 A connected to the buoy tend to sink in water. As the buoy sinks, it supports smaller portions of the jumper hoses  64 A until the buoy reaches a stable depth. It should be noted that all weight-carrying upper parts of the mooring system and the fluid transfer system are horizontally coupled so they all tend to move horizontally together. Thus, when the connection buoy at  16  or  16 A is horizontally displaced, the spring buoys  50  and riser buoy  102  will be horizontally displaced, because they all are horizontally coupled. 
     FIG. 1   d  shows the system of  FIG. 1   b  when the connection buoy  16 B has been disconnected from the vessel. Once disconnected, the connector at  16 B lies underneath the riser buoy  102 B and the mooring buoys  50 . Thanks to this configuration the relative movement of riser lower portions  100 B is decreased. In  FIG. 1   d , the spring buoys  50  and riser buoy  102 B will be horizontally displaced, because they all are horizontally coupled via taut lines  104 B and  105 . Further, as the configuration of  FIG. 1   b  enables a vertical decoupling of the connector  16 B and the buoys ( 50 ,  102 B), it creates an artificial water depth, the relative movement of riser lower portions  100  is decreased and the connector supporting portions of the jumper hoses  64 B and mooring lines upper parts  56 B will reach a stable depth, which is deeper than the one of the configuration of  FIG. 1   a  shown sunk in  FIG. 1   c . A deeper depth of connector  16 B occurs because buoys  50  do not move further apart as the connector  16 B moves down. 
   Applicant places the interconnected spring buoys  50  and riser buoys  102 B of  FIG. 1   d  closely under the wave action zone  40 , and preferably with their center placed less than the distance A below the bottom  70  of the zone. 
     FIG. 2  shows risers  91  with steel catenary riser lower parts  90  that extend up to the spring buoys  50  and jumper hoses  92  that extend to the connection buoy. In the system of  FIG. 2 , the hoses that form upper portions of the risers are connected to spring mooring buoys  50  to be supported. The systems of  FIGS. 1   a  and  1   b  and  FIG. 2  can be used with steel catenary risers  91 ,  101  and also can be used with flexible risers and umbilicals. 
   In  FIG. 2  the mooring buoys  50  that keep the lower mooring line parts  44  taut and that support one end of each upper mooring line part  56  are part of riser buoy means that also supports one end of each jumper hose  92 . This avoids the need for at least one additional buoy. 
     FIG. 3  shows a top view of the vessel  14  and the system  12 , with the vessel shown in phantom lines. The particular illustrated system has three sets of mooring assemblies  30  angled 120° apart that each includes three primarily vertical line lower parts  44  made of steel wires or polyester ropes. For each set, applicant provides a plurality (preferably at least three) of vertical line lower parts  44  extending at slightly different (typically about 4°, that is, at 2° to 8°) compass headings. This provides redundancy to assure that there will be adequate mooring even if one of three lower mooring lines breaks or its foundation is damaged. 
   It is clearly shown that the risers and the riser buoys  102  lie in between the 120 degrees-separated mooring assemblies  30 .  FIG. 3  shows that the riser buoys  102  and the spring buoys  50  are interconnected. (For the embodiment described in  FIG. 1   b  a connection line  105  can be added, in addition to lines  104 , between the mooring buoys  50 ). 
   Mooring lines made partly of polyester materials are advantageous to minimize the weight that must be supported in deep waters (e.g. over 500 meters). In fact, polyesters materials have specific gravities of 1.1 to 1.4 so they require only a relatively light support. 
     FIG. 4  shows a side view of the configuration of a buoy  102 , jumper hoses  64 , and attached riser lower parts  100  of  FIGS. 1   a ,  1   b  and  3 . The jumper hoses  64  each extends in a catenary curve and have different lengths so as to avoid congestion. The lowest jumper hose  64   c  of  FIG. 4  has a length about 20% (10% to 35%) greater than the upper hose  64   a . This results in a vertical separation L 1  between the uppermost and middle hose  64   b  and a separation L 2  between the uppermost and lowermost hoses. The difference between lengths of adjacent hoses is preferably at least 5% and is preferably no more than 15%. As there is only a limited horizontal space in the congested area between the mooring lines near the vessel, the distances between the jumper hoses is primarily vertical by variation of the length of each jumper hose. This avoids the jumper hoses rubbing against each other in the limited and congested space between the mooring lines, which usually lies in the wave active zone. Each jumper hose extends in a J-curve, with a primarily vertical portion extending down from the connector  16 , and with a large curve extending down from the primarily vertical portion  100  and up to the buoy  102 . An alternative would be to have jumper hoses extending in a wave curve or S curve when the jumper is not buoyant. 
     FIGS. 5-8  show additional possible features of the invention with risers  20  each including a rigid lower riser part  60  that extends up from the sea floor to a riser buoy  62 , and a flexible upper riser part, or jumper hose  64  that extends in a catenary curve up to the connection buoy  16 . 
     FIG. 5  shows an installation similar to that of  FIG. 1   a , except that a separate riser buoy  62  is used to support each riser lower part  60 . This allows each lower riser part to extend tautly in a straight line that is primarily vertical, from the sea floor up to a buoy  62 , instead of having each riser lower part extend in a curve. The installation is otherwise similar to that of  FIG. 1   a  except that no stabilization line extends from the mooring buoys  50  to the riser buoys  62 . In  FIG. 5  each riser buoy  62  is placed to lie a short distance under the wave action zone  40 , with the distance (to the middle of each buoy  12 ) preferably being no more than the height A of the wave action zone. A typical wave action zone has a height of 50 meters, which is of the same order of magnitude as the height of about 35 meters of the particular FPSO vessel  14 . When disconnected from the vessel, the connection buoy  16  should lie at least 25 meters under the sea surface to lie under the upper half of the wave zone, where water movement is greatest, and preferably should lie under the entire wave zone height of about 50 meters (or even deeper if icebergs need to be avoided).  FIG. 5  also shows the connection buoy at  16 C after it has been disconnected from the vessel. The connection buoy  16  is buoyant, while the mooring upper line parts  56  and jumper hoses  64  connected to the buoy tend to sink in water, so the buoy moves down until its buoyancy equals the downward weight on it of the parts  56  and jumper hose  64  (and tension forces of upper line parts  56 ). 
     FIG. 6  shows an installation similar to that of  FIG. 5 , except that a primarily horizontal stabilization line  72  extends from each mooring buoy  50  to each riser buoy. A stabilization line such as a cable or chain  72  extends between each spring buoy and a riser buoy, to reduce their relative horizontal movements. This stabilization line is needed as the system has catenary lower riser parts  60  instead of taut vertical lower riser parts. 
     FIG. 7  shows an installation that combines the systems of  FIGS. 2 and 5 , with some riser lower parts  80  each extending to a spring buoy which also serves as a riser buoy means, and with some risers each extending to a separate riser buoy. In  FIG. 7 , an umbilical riser lower part  80  is provided that extends from the connection buoy  16  to each spring buoy  50  and from there to the well head  82  to carry tools. 
     FIG. 8  shows the installation when the connector  16  is connected and sinks to a height below (its center is below) the wave active zone. It shows the system of  FIG. 7  with the connection buoy at  16 B released to sink while a pickup buoy  84  remains at the surface. 
   The systems shown in  FIGS. 5 ,  7  and  8  also can be provided with stabilization lines between the secondary buoys  50 ,  62 , depending on environmental conditions. When the connection buoy  16  (e.g.  FIG. 5 ) is disconnected, the mooring buoys  50  and riser buoys  62  will support any additional weight of the upper mooring line parts  56  and jumper hoses  64 . Both spring buoys  50  and riser buoys  62  are designed to take this weight variation between the connected and disconnected positions of the connection buoy  16 . 
   Thus, the invention provides an improved installation that includes a connector buoy, or connector that connects mooring lines and risers to a vessel. The mooring lines have lower parts that extend primarily vertically to mooring buoys and have primarily horizontal upper parts that extend primarily horizontally to the connector to hold the vessel from drifting far away from a central location. The risers have lower parts that extend primarily vertically up to riser buoy means that may comprise a common buoy, individual buoys, or the mooring line buoys, and flexible jumper hoses that extend up to the connector. There is a vertical decoupling between the riser buoy means and the connector, or between any of the riser buoys or mooring buoys so the connector would not cause the riser buoy or mooring buoy to move appreciably vertically (more than 10% of connector vertical movement) with the connector. This minimizes the mass that has to be moved up when the connector is lifted for reconnection to the vessel. 
   The connector usually, but not always lies above the riser buoys (see embodiment shown in  FIG. 1   d ) when the connector is disconnected. The connector is connected to the lower riser part and to the riser buoy, by a flexible jumper hose that extends in a J-curve, so the jumper hose extends down from the connector to a height below the riser buoy and then extends in a curve up to the riser buoy. 
   The figures only show embodiments where the floating unit is a vessel such as an FPSO but it can also be any type of vessel (Floating storage and offloading unit (FSO), Floating storage and regassification unit . . . ) and any type of floating unit such as SPARs and floating production units (FPU). 
   Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art, and consequently, it is intended that the claims be interpreted to cover such modifications and equivalents.