Abstract:
An underwater buoy for hanging a duct between a sea bed and the surface, the buoy including a frame and a plurality of modular members mounted in the frame, the modular members extending between their two opposite modular-member ends, the frame including a longitudinal body for receiving the extended tubular duct, and retainers mounted on the body for maintaining the modular members substantially parallel to the body. The retainers include two retaining structures spaced from each other along the body. The retaining structures are maintained in a fixed position relative to each other in order to hold at least one modular member.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    The present application is a 35 U.S.C. §§371 national phase conversion of PCT/FR2008/000590, filed Apr. 24, 2008, which claims priority of French Application No. 0703086, filed Apr. 24, 2007, incorporated by reference herein. The PCT International Application was published in the French language. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to an underwater buoy with modular members for suspending tubular ducts for transporting hydrocarbons between a seabed and a surface installation. 
         [0003]    In order to raise hydrocarbons from an underwater well to the surface, tubular ducts are installed substantially vertically between the well and an underwater area situated below the surface of the water, after which these vertical tubular ducts are extended by generally flexible tubular ducts which join a surface installation. The substantially vertical tubular ducts are generally rigid and are held vertical by underwater buoys. The size of these underwater buoys and consequently the volume of air that they are able to trap can be adjusted as a function of the upward force that they must exert on the tubular duct in order to hold it vertical. This upward force also depends on the dimensions of the duct and its length, in other words the depth of water. Moreover, if the upward force to be exerted is relatively high, the volume of the underwater buoy must also be high. 
         [0004]    Furthermore, because it is difficult to transport buoys of large volume, transporting them in pieces has been envisaged, for example on laying ships, and then assembling them directly on site. 
         [0005]    Thus the buoys comprise a frame and modular members that form floats adapted to be mounted on said frame when the buoy is installed. The modular members extend longitudinally between two opposite modular member ends. Said frame has a longitudinal hollow body adapted to receive said extended tubular duct and radial retaining means mounted on said hollow body hold said modular members substantially parallel to said hollow body and around said hollow body. 
         [0006]    Reference may be made in particular to the document WO 03/064807, which describes one such underwater buoy. 
         [0007]    However, when the modular members are relatively bulky so that the underwater buoy exerts a relatively high upward force, the retaining means are intensely loaded and there is a risk of them breaking. 
       SUMMARY OF THE INVENTION 
       [0008]    Thus a problem that arises and that the present invention aims to solve is to provide an underwater buoy with modular members that are retained more strongly by the retaining means to prevent them breaking. 
         [0009]    With the aim of solving this problem, the present invention proposes an underwater buoy with modular members intended to suspend a tubular duct between a seabed and a surface. The buoy comprises a frame and a plurality of modular members forming floats adapted to be mounted in the frame. The modular members extend longitudinally between two opposite modular member ends. The frame has a longitudinal hollow body intended to receive the extended tubular duct and retaining means mounted radially on the hollow body to hold the modular member substantially parallel to the hollow body and around the hollow body. According to the invention, the retaining means comprise two facing retaining structures spaced longitudinally from each other on the hollow body. The retaining structures have respective pluralities of receiving areas each adapted to receive a modular member end. The said retaining structures are held in a fixed position relative to each other in a position in which the receiving areas face each other so as to trap at least one modular member when the opposite ends of the at least one modular member are engaged in respective facing receiving areas. 
         [0010]    Accordingly, one feature of the invention is the way the two retaining structures cooperate, trapping the modular members when they are brought into a position close to each other. As a result, when the tubular duct that rises from the seabed is suspended from the underwater buoy, the buoy is oriented so that the hollow body extends substantially vertically, just like the modular members. The modular members, which have a density lower than that of seawater, exert an upward force on one of the two retaining structures that is itself fastened to the hollow body. Also, these modular members abut against this retaining structure and are held in this position thanks in particular to the other retaining structure. 
         [0011]    Each modular member is advantageously of cylindrical shape with a circular directrix so that they can be fabricated industrially and at an advantageous cost. Given the symmetry of these modular members, their wall offers a much higher resistance to hydrostatic pressure despite a relatively small thickness compared to a modular member of parallelepiped shape, for example. What is more, thanks to this cylindrical symmetry, the modular members are more easily manipulated in the water in order to replace a modular member or to mount additional modular members in the frame. 
         [0012]    Moreover, the retaining means preferably further include spacers mounted on the hollow body to hold the modular members away from the hollow body, and the modular members bearing against these spacers. They also stiffen the connections between the modular members and the hollow body. Moreover, the spacers have respective semicircular recesses to receive the modular member and thus prevent lateral movement of the tubular member in directions substantially parallel to a plane tangential to the hollow body. 
         [0013]    In a preferred embodiment of the invention, the retaining structures have a central portion fastened to the hollow body and radial portions in which the receiving areas are provided. For example, the retaining structures thus have eight radial portions diametrally opposed in pairs in which respective receiving areas are provided. 
         [0014]    Moreover, the retaining structures define a mean plane that is substantially perpendicular to the hollow body. At least one of the retaining structures, that which is at the surface end when the underwater buoy is in position, is equipped with complementary immobilizing means in the receiving areas to immobilize the modular member in all directions substantially parallel to the mean plane. Thus, when the underwater buoy is in the normal working position, the hollow body is oriented vertically and the modular members, having density lower than that of water, tend to rise toward the surface and to exert upward forces precisely on the at least one of the retaining structures. Also, thanks to the additional immobilizing means, in the receiving areas of this retaining structure, which means absorbs the high upward forces, the ends of the modular members are totally fastened to the retaining structure. Because of this, the modular members are totally fastened to the frame. 
         [0015]    The at least one of said retaining structures, at the surface end, advantageously reinforcing means for increasing the stiffness of the at least one of the retaining structures to better resist the upward forces produced by the modular members. 
         [0016]    At the opposite end, the hollow body has in the vicinity of the other of the retaining structures means for attachment to the tubular duct to absorb the forces exerted by the tubular duct that rises from the seabed and tends to drag the underwater buoy toward the seabed. Thus these forces are exerted directly on the hollow body and are absorbed and compensated by the modular members via the retaining structure. Of course, the other retaining structure, facing the seabed, includes means for locking the modular members in order for them to be totally fastened to the hollow body. 
         [0017]    Other features and advantages of the invention will emerge from a reading of the description of particular embodiments of the invention given hereinafter by way of nonlimiting illustration and with reference to the appended drawings, in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a diagrammatic perspective view of a frame of an underwater buoy of the invention; 
           [0019]      FIG. 2  is a diagrammatic perspective view of an underwater buoy of the invention; 
           [0020]      FIG. 3A  is a diagrammatic view of the underwater buoy shown in  FIG. 2  in axial section on the plane III-III; 
           [0021]      FIG. 3B  is a diagrammatic view of different embodiment of an underwater buoy of the invention in axial section in a vertical plane; 
           [0022]      FIG. 4  is a diagrammatic plan view of the underwater buoy represented in  FIG. 3A  in the direction of the arrow IV; and 
           [0023]      FIGS. 5A to 5C  show diagrammatically in cross section the assembly of an underwater buoy as shown in  FIG. 2 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0024]      FIG. 1  shows the frame  10  of an underwater buoy of the invention. This frame  10  has a hollow body  12  between 30 and 40 meters long, for example 35 meters long, and an upper retaining structure  14  and a lower retaining structure  16 . Moreover, spacers  18  to be described hereinafter are installed along the hollow body  12 . This hollow body  12 , which is of circular symmetry about an axis A, has an upper end  20  and a lower end  22  and its relatively constant diameter is between 1 meter and 2 meters, for example 1.5 meters. The upper retaining structure  14  has a central portion consisting of a first interior ring  24  at least partially sleeved into the upper end  20  of the hollow body  12  and eight radial portions consisting of first branches  26  that extend radially from the first interior ring  24  and are offset from each other at an angle close to 45°, these first branches  26  being also fastened to a first exterior ring  28  of octahedron shape. This first exterior ring  28  constitutes in particular means for stiffening the upper retaining structure  14 . 
         [0025]    The first branches  26  each have a free end  30  and an arcuate first recess  32  near the free end  30 . This arcuate first recess  32  is oriented toward the lower end  22 .  FIG. 3A  shows the frame  10  comprising the hollow body  12  and the upper retaining structure  14 ; the axial section plane III-III intersecting two diametrally opposite first branches  26 , their respective arcuate recesses  32  are seen to be symmetrical with respect to an extremum (maximum point)  34  and symmetrical to each other with respect the axis of symmetry A. Moreover, the arcuate first recesses  32  are spaced from the hollow body  12 . 
         [0026]    Seen in  FIG. 1  is the first exterior ring  28 , which is of octahedron shape and connects the first branches  26  together at the level of the arcuate first recesses  32 . Each of the eight substantially plane portions of the first exterior ring  28  intersects substantially perpendicularly a first branch  26 . Moreover, an arcuate second recess  36  is produced in each of these plane portions of the first exterior ring  28 . This arcuate second recess  36  is of substantially identical curvature to the first recess  32  and has an extremum substantially coinciding with the extremum  34  of the arcuate first recess  32 . 
         [0027]    Thus, thanks to the arcuate recesses  32 ,  36 , the plane portions of the first exterior ring  28  and the corresponding first branch  26  together define a receiving area  38  which is oriented toward the lower end  22  and defines a spherical ring the function of which is explained hereinafter. 
         [0028]    First, the lower retaining structure  16  is described with reference to  FIG. 1  and to  FIG. 3A . The latter structure has a second interior ring  40  at least partly sleeved into the lower end  22  of the hollow body  12 . It also has second branches  42  symmetrical to respective first branches  26  with respect to a plane of symmetry intersecting the hollow body  12  perpendicularly half way between the lower end  22  and the upper end  20 . These second branches  42  are connected to each other by an exterior second ring  44 . On the other hand, the second branches  42  have respective notches  46  rather than an arcuate recess like the opposite first branches  26 . On the other hand, the notch  46  has a first portion located close to the second interior ring  40  substantially symmetrical to a first part  32  of an arcuate recess extending from the extremum  34  toward the first interior ring relative to the aforementioned plane of symmetry intersecting the hollow body  12  perpendicularly. However, a second portion of the notch  46  extends substantially radially toward the free end of the second branch  42 . 
         [0029]    Moreover, the star-shaped spacers  18  shown in detail in  FIG. 1  each define a mean plane substantially perpendicular to the hollow body  12  and are formed from a circular ring in which eight semicircular recesses  48  are produced. The semicircular recesses  48  of each of the circular rings are aligned with each other along an axis parallel to the axis of symmetry A of the hollow body  12  and each intersects facing first and second branches  26 ,  42 . 
         [0030]    As shown in  FIG. 2 , modular members  50  forming floats are engaged in each of the eight housings that extend between the upper retaining structure  14  and the lower retaining structure  16  and are defined by the semicircular recesses  48  of the spacers  18 , the receiving areas  38  and the opposite notches  46 .  FIG. 2  shows the upper retaining structure  14  and the lower retaining structure  16  connected together by the hollow body  12 , here concealed by the modular members  50 . The latter are of cylindrical shape with a circular directrix and each has two opposite free ends, an upper free end  52  and a lower free end  54 . Their diameter is between 2 and 3 meters, for example 2.4 meters, and their length is between 30 meters and 40 meters, for example 34 meters. 
         [0031]    The two free ends have a rounded shape defining a substantially spherical surface adapted to coincide with the receiving area  38 . Thus the upper free end  52  of each of the two modular members  50  is engaged in the receiving area  38 , the lower free end  54  bears against the corresponding second branch  42 , and the body  56  of each of the tubular members  50  bears against the spacers  18 , passing through their respective semicircular recesses  48 . Note that when assembling the underwater buoy the upper free end  52  of the modular members  50  is first engaged in the receiving area  38 , the modular members  50  being inclined relative to the hollow body  12 , and the tubular body  50  is then tilted toward the hollow body  12  into bearing engagement with the spacers  18 , with the lower free end  54  abutted against the second branches  42 . The modular members  50  are retained in this position either by locking members  58  attached to the free end of the second branches  42 , seen in more detail in  FIG. 3A , or by a buoy clamp, not shown, that surrounds and grips the eight modular members  50  in the vicinity of the lower retaining structure  16 . 
         [0032]    Furthermore, in another embodiment, the modular members  50  are held in bearing engagement against the spacers  18 , independently of each other, by independent spacer clamps, which clamp the modular members  50  into their corresponding semicircular recesses  48 . The spacer clamps are mounted on each of the projecting ends of the spacers  18  and are adapted to be connected to another contiguous projecting end surrounding a modular member  50 . 
         [0033]    Moreover, in one particular embodiment of the invention shown in  FIG. 3A , the upper free end  52  has an axial slot  64  in which engages a projecting extension  66  of the plane portions of the first exterior ring  28  at the level of the arcuate second recess  36 . As a result, the upper free end  52  of the modular members  50  is perfectly fastened to the upper retaining structure  14  because it is perfectly immobilized against movement in directions substantially parallel to the mean plane P defined by the upper retaining structure  14 . Furthermore, the lower free end  54  is immobilized against movement in radial translation by the locking member  58  and the body  56  of the modular member  50  is immobilized against movement in translation in a perpendicular direction. 
         [0034]      FIG. 4  is a plan view of the underwater buoy of the invention. It shows each of the eight modular members engaged in its housing and the upper retaining structure  14  comprising the first interior ring  24 , the first branches  26  and the first exterior ring  28 . 
         [0035]    Thus the underwater buoy represented is relatively easy to assemble before being loaded onto a laying ship or on the ship or directly in the water. Moreover, it has eight modular members  50  here, but it could have only one in two of them, i.e. four modular members  50 . This would reduce its buoyancy. 
         [0036]      FIGS. 5A to 5C  show the assembly of the underwater buoy. First, two first modular members  50  are placed horizontally and parallel to each other on supports  70  and spaced by a particular distance. Then, as shown in  FIG. 5B , a hollow body  12  equipped with its spacers  18  is fitted to these first two modular members  50 . The latter are then fastened to the spacers  18  by spacer clamps as mentioned above. Then, as shown in  FIG. 5C , two more modular members  50  are mounted on the hollow body in the position diametrally opposite the first two modular members  50 . Finally, the assembly, equipped with four modular members  50 , is first tilted onto two first modular members  50  on supports identical to the supports shown in  FIG. 5A  and situated alongside those supports  70 , and then two other modular members  50  are installed on the last two remaining places on the hollow body  12 . 
         [0037]    The upper retaining structure  14  and the lower retaining structure  16  are then mounted on the ends of the hollow body  12 . 
         [0038]    As shown in  FIG. 3A , the underwater buoy of the invention is attached by means of a clamp  62  to a tubular duct  60  for transporting hydrocarbons. As a result, the tubular duct  60  is suspended from the underwater buoy, which tends to draw it in the direction S of the surface in an underwater area situated below the surface. Furthermore, the tubular duct  60  is connected to a flexible tubular duct  63  that passes through the underwater buoy and exits it at the top beyond the upper retaining structure  14  to join a surface installation. 
         [0039]    Thus the traction forces to be exerted on the tubular duct  60  can be adapted by adjusting the number of modular members  50  attached to the frame  10 . 
         [0040]    In another embodiment, not shown, the tubular duct  60  is connected to the underwater buoy by a frame itself suspended from the lower retaining structure  16  and the tubular duct  60  is connected to a flexible tubular duct, which no longer passes through the underwater buoy but instead passes around it to join a surface installation. 
         [0041]    In a variant of the invention represented in  FIG. 3B  and in said other embodiment the aforementioned hollow body is replaced by a succession of six independent floats, comprising four identical floats  72 ,  74 ,  76 ,  78  and two end floats  80 ,  82  stacked one on the other. Furthermore, the modular members forming floats are respectively replaced here by two modular half-members  84 ,  86  arranged in alignment with each other. The result of substituting the independent cylindrical float  72 ,  74 ,  76 ,  78 ,  80 ,  82  for the hollow body is to increase the overall buoyancy of the underwater buoy. Moreover in some particular embodiments, for the same buoyancy, the modular members are smaller. Moreover, increasing the number of floats that are decoupled from each other avoids the risk if one of them is damaged. The independent cylindrical floats  72 ,  74 ,  76 ,  78 ,  80 ,  82  are nevertheless adapted to receive water inside them so as to be able to submerge the underwater buoy, whereas the modular half-members are sealed and do not receive water. This water can then be evacuated from the independent cylindrical floats to confer on the underwater buoy its full buoyancy. For filling the independent cylindrical floats  72 ,  74 ,  76 ,  78 ,  80 ,  82  with water, they are equipped in their base with a first opening extended by a first pipe. The first pipes of all the independent cylindrical floats  72 ,  74 ,  76 ,  78 ,  80 ,  82  converge toward a common filler valve. To substitute a gas and in particular nitrogen for the water in the independent cylindrical floats  72 ,  74 ,  76 ,  78 ,  80 ,  82 , they have a top opening extended by a second pipe. The second pipes converge toward a common nitrogen feed valve. 
         [0042]    Another aspect of the invention relates to a method of installing an underwater riser column for transporting hydrocarbons between a seabed and a surface by means of an underwater buoy with modular members as described above. The method is of the type wherein: a seabed installation is anchored to said seabed; a tubular duct is provided having a connecting end intended to be connected to said seabed installation and an opposite end equipped with an underwater buoy with submersible floats; then water is allowed to enter said submersible floats to submerge said underwater buoy and said tubular duct vertically above said seabed installation, while said underwater buoy and said duct are retained by a suspension line from a surface vessel, said suspension line supporting traction forces corresponding to the weight of said underwater buoy and said duct; a traction cable is then provided and direction-changing means are installed on said seabed installation so as to be able to connect said traction cable to said connecting end and to draw said cable through said direction-changing means and simultaneously to draw said connecting end toward said seabed installation; according to the invention, a submerged pulling buoy is attached to the traction cable to exert additional traction forces on said suspension line; a gas is then substituted for the water in said submersible floats to compensate on the one hand traction forces corresponding to the weight of said underwater buoy and said duct and on the other hand at least some of the additional traction forces; and, finally, said pulling buoy is moored to said seabed installation and said suspension line is progressively released so that said seabed installation absorbs said additional traction forces exerted by the pulling buoy, while said underwater buoy exerts said other part of the additional traction forces on said duct to hold it vertical. 
         [0043]    Accordingly, additional traction forces can be exerted on said suspension line by the use in accordance with this other aspect of the invention of the pulling buoy when submerged below the surface, i.e. between the seabed and the surface, to be more precise near the seabed, which pulling buoy is attached to the traction cable and then released from it. When released, the pulling buoy, which contains a gas lighter than water, exerts traction on the traction cable in a direction that is reversed by the direction-changing means at the connecting end of the duct and therefore on the suspension line that joins the surface vessel. Thus an additional traction force is exerted on the suspension line in addition to the weight of the duct and the underwater buoy. 
         [0044]    Then, by mooring said pulling buoy to said seabed installation and then releasing or progressively paying out said suspension line from the surface vessel, said underwater buoy and said tubular duct descend progressively toward the seabed installation, because the traction cable is drawn through the direction-changing means by the pulling buoy which is itself drawn toward the surface. However, the buoy is retained by the mooring line that connects it to the seabed installation. From this time onward, the forces exerted on the suspension line between the underwater buoy and the surface vessel cancel out. The benefit of this is precisely that, as soon as the forces exerted on the suspension line tend toward zero and, for example, the surface vessel is lifted by the swell in a vertical direction away from the seabed, the forces exerted on the assembly comprising the chain, the suspension lines, the underwater buoy, the duct and the traction cable are then transferred to the pulling buoy, which is therefore drawn toward the seabed. This obviously retains all the elements of the aforementioned chain assembly, as the pulling cable is not anchored to the seabed, as is the case in the prior art. 
         [0045]    Said gas lighter than water is advantageously substituted for the water in said submersible floats to compensate the traction forces corresponding to the weight of the underwater buoy and to substantially half said additional traction forces exerted by means of the pulling buoy. 
         [0046]    Moreover, in one particular embodiment of the invention, to connect the connection end to said seabed installation, said pulling buoy is released from said seabed installation so that it rises toward said surface so as to draw said connecting end in the opposite direction, toward said seabed installation. To do this, damper means are provided for receiving the connecting end when, on descending, it approaches the seabed installation.