Abstract:
System for transferring fluids between a floating installation ( 10 ) and a tanker ( 30 ), wherein the floating installation is permanently moored ( 31, 32 ) to the seabed ( 1 ), and comprising a buoy ( 20 ) being moored ( 21 A,  21 B) to the seabed and connected to the installation through a first mooring line ( 12 A) and a first fluid transfer line ( 12 B). Buoy ( 20 ) is adapted to be connected to the tanker ( 30 ) through a second mooring line ( 32 A) and a second fluid transfer line ( 32 B), and the buoy ( 20 ) is immersed in the sea in its normal operative position and is provided with a swivel for the second mooring line ( 32 A) and the second fluid transfer line ( 32 B).

Description:
BACKGROUND OF THE INVENTION 
     This invention is primarily directed to a system and methods for use in offshore oil and gas production, where there is in many cases a need for transferring fluids between a floating installation and a tanker. Thus, the floating installation (FPSO) serves for more or less temporary storage of hydrocarbon fluids being produced, with tankers being employed for the actual export of the products. 
     Systems for such purposes are known, wherein the floating installation is permanently moored to the seabed, and comprising a buoy being moored to the seabed and connected to the installation through a first mooring line and a first fluid transfer line, and being adapted to be connected to the tanker through a second mooring line and a second fluid transfer line. 
     A typical example of such a system is found in U.S. Pat. No. 5,065,687, describing a mooring buoy to be located on the sea surface and making possible the weathervaning of a moored tanker vessel through a certain, but limited arc of a circle. 
     The present invention has for an important object to provide a cost effective, reliable fluid offtake system with high operability, for the purposes indicated above. This is obtained by utilizing the principle of a mooring buoy being immersed under the sea surface, as will be described further below. 
     At this point it should be made clear that submerged mooring buoys as such are previously known, for example from U.S. Pat. No. 5,816,183. This and other examples, however, are not related to the particular arrangement where a large floating installation constitutes an essential component in the complete mooring and fluid transferring system. 
     The heart of the present system is the buoy, forming a base unit with buoyancy and having all required facilities. It is partly moored to the sea bottom and partly to the FPSO; thus the whole system can assist in mooring the FPSO. Equilibrium of the buoy is ensured by proper load and load attachment, represented mainly by mooring lines connected to the buoy. Means for mooring a shuttle tanker are provided on top of the buoy. All functions on the base unit or buoy may be controlled via an umbilical cable from the FPSO. 
     As will be seen from the following description the mooring of the buoy is asymmetric, for example by having four mooring lines directed away from the FPSO and two lines attached to the FPSO. 
     The system according to this invention, as defined in the claims, involves advantages as follows: 
     No collision danger, as will be present with surface buoys Significantly reduced risk of collision with the floating installation (FPSO). 
     Eliminates contribution to tanker hawser tension variation by buoy (negligible) movements. 
     Easy installation with dry (no diving) connections and installation of main components before immersing the mooring buoy. 
     Simple export hose arrangement from FPSO, with easy installation, inspection and replacement. 
     270 degrees normal weathervaning/full 360 degrees capability Eliminating polyester mooring line elongation problems. 
     As a typical example of dimensions and capacities in a practical embodiment of the system according to the invention, the following is given: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 FPSO (floating installation) of 
                 300 000 DWT 
               
               
                   
                 FPSO Length overall 
                 300 m 
               
               
                   
                 FPSO Breadth 
                  58 m 
               
               
                   
                 FPSO Draught 
                  10 m (loaded:23 m) 
               
               
                   
                   
               
             
          
         
       
     
     Export tankers (shuttle tankers) for use with the system can be of sizes substantially corresponding to what is indicated above with respect to the FPSO. 
     Typical buoy floating depth 50-100 m. 
     Buoy net buoyancy 250-300 tonnes. 
     From the above example it will be understood that the mooring capabilities and properties of the system are of utmost importance for performing the tanker loading operations under varying conditions of wind and waves. Thus, the system according to the invention is capable of performing well in more severe conditions than most existing systems. A down time requirement of not more than 1% per year throughout 20 years life time, should be satisfied at the location given. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following description the invention will be explained more in detail with reference to the drawings, in which: 
     FIG. 1 schematically in elevation shows an embodiment of the system without any tanker moored thereto; 
     FIG. 2 shows the same arrangement as FIG. 1 in plan view; 
     FIG. 3 in elevation shows a complete arrangement including a tanker moored to the buoy incorporated in the system; 
     FIG. 4 in side view shows an embodiment of the buoy; 
     FIG. 5 is a mid sectional view of the buoy in FIG. 4; 
     FIG. 6 is a plan view of the same buoy; 
     FIG. 7 in plan view shows a variant of the mooring arrangement in FIG. 2; and 
     FIGS. 8A-D serve to illustrate important steps in the installation procedure for establishing a practical embodiment of the system according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIGS. 1 and 2 an FPSO—floating installation  10  is shown with moorings  31 ,  32 ,  33  and  34  of conventional design, connected to respective anchors at the seabed. The mooring of installation  10  is to be taken as “permanent”, i.e. when installed at a given location it is intended to be in service for a long period of time, such as several years. A buoy  20  has also moorings  21 A-D with corresponding anchors at the seabed, for example suction anhors. 
     Between installation  10  and buoy  20  there is provided at least one mooring line  12 A; however, two such mooring lines are preferred, as will be seen in FIG.  2 . Moreover, at least one fluid transfer line in the form of a flexible riser  12 B, is connecting buoy  20  to installation  10 , for exporting fluids from the latter. 
     It is important to note that seabed moorings  21 A-D for buoy  20  are all directed more or less away from installation  10 . Thus, for balanced or stable mooring of the buoy the one or two mooring lines  12 A are required. In this balanced system the immersion depth of buoy  20  can be determined or adjusted, as will be explained further below. It is also to be noted that during regular operations mooring line(s)  12 A as well as fluid transfer risers  12 B are immersed below the sea surface. 
     A number of pipelines  10 A and  10 B as indicated in FIG. 2 are provided for supplying produced fluids to installation  10  from subsea wells or stations (not shown) at the oil or gas field concerned. 
     In FIG. 3 showing the complete system in operation, suction anchors  22 A and  22 B in seabed  1  are provided for mooring lines  21 A and  21 B, respectively. Of course other types of anchors could be used instead of suction anchors, but the latter type is very suitable for the purpose. Tanker  30  is moored to buoy  20  by means of a mooring line  32 A in the form of a hawser. There is also a loading hose connecting buoy  20  to the tanker  30 , as will be explained more in detail with reference to FIGS. 4,  5  and  6 . 
     The main structure of buoy  30  comprises six buoyant compartments in a symmetrical arrangement. In FIG. 4 three of these compartments have been denoted  30 A,  30 B and  30 C, respectively. Attachment points are shown at the lower portion of the buoy for seabed moorings  21 A-D and the two mooring lines  12 A for connection to FPSO installation  10 . Centrally at the top of the buoy there is a swivel  25  for mooring line  32 A and loading hose  32 B. 
     Whereas in FIG. 4 the line or hawser  32 A is shown in an idle position, the line is in a tensioned condition as illustrated in FIG. 5, thus indicating that a tanker is moored to the buoy (see FIG.  3 ). Swivel  25  can be of a design being in principle known per se, with swivel rotation effected primarily by the force exerted by hawser  32 A when tensioned by a tanker. A guardrail  29  has been provided for hawser  32 A. 
     It is an advantage to have both loading hose  32 B and fluid transfer lines or risers  12 B of a relatively flexible, marine hose type, as commercially available. Preferably, the free end of marine loading hose  32 B is hauled adjacent to and stored at the floating installation  10  during periods when no tanker is being loaded. When a tanker is to be loaded, the hose  32 B is usually connected to a midship manifold on the tanker. 
     Turning now to FIG. 7 there is illustrated an arrangement where the mooring line  14  between installation  10  and buoy  20  has replaced one (group of) permanent mooring normally present with installation  10 . Moorings  31 ,  32  and  34  are as in the arrangement of FIG. 2, but mooring  33  is no longer required, since mooring line  14  to buoy  20  has taken over the function of mooring  33 . This results in a much simplified and less expensive system. The balanced configuration as explained above is still fully possible, with mutually advantageous mooring cooperation between buoy  20  and floating installation  10 . 
     The method of installing the system according to the invention comprises steps and operations to be explained with reference to FIGS. 8A-D, showing in a simplified manner certain important steps. These comprise permanently mooring the floating installation to the seabed and mooring the buoy to the seabed. The order of performing these two operations is not decisive. 
     In FIG. 8A a situation is shown, where a suction anchor  22 A has already been set in seabed  1 , and an assisting vessel is engaged in the mooring of buoy  20  by means of line  21 A. The latter can for the most part consist of a polyester rope, with a bottom chain portion near anchor  22 A as well as an upper chain portion near buoy  20 . 
     Buoy  20  being moored to the seabed  1 , FIG. 8B illustrates how the buoy is furnished with mooring hawser  32 A and floating hose  32 B, both being connected to swivel  25  on the buoy. The floating installation is not seen in FIGS. 8A-8B. 
     FIG. 8C illustrates the installation of mooring line(s)  12 A between buoy  20  and FPSO  10 , seabed mooring  31  and  32  of the latter being already provided for in a conventional manner. 
     Then finally FIG. 8D shows installation of two loading hoses or risers  12 B for the transfer of fluids from FPSO  10  to buoy  20 , as well as an umbilical cable  19  for control of the buoy. Still in the situation of FIG. 8D however, buoy  20  is floating in a surface position. 
     Turning now again to FIGS. 1 and 3 the buoy is brought to the immersed position shown therein, by means of winches on FPSO installation  10  that are operated to tension mooring  12 A. Proper balancing of all mooring lines incorporated in the system, and loads carried by buoy  20 , will secure the desired configuration also when a tanker is moored, as shown in FIG.  3 . It will be understood that mooring of a tanker  30  involves the tendency for buoy  20  to ascend somewhat from its idle position depth, but this can be taken into account when adjusting the mooring system as a whole. Supplementary adjustment can be effected any time by means of winches in FPSO  10 , as mentioned above. 
     In this connection it is contemplated that such FPSO equipment can be used for bringing the buoy from its immersed position to a surface position, by slackening or paying out of the mooring line  12 A to a suitable degree. This is an advantageous feature making possible easy inspection and maintenance of the buoy.