Abstract:
An underwater fluid transport apparatus ( 10 ) to transport a fluid between a sea bed ( 12 ) and the surface ( 14 ) of the sea vertically above the sea bed. A riser ( 20 ) is connected to a flexible pipe ( 32 ) leading to the sea surface ( 14 ). A retaining float ( 24 ) is installed around the riser in order to maintain the riser ( 20 ) in a stretched suspended position between the sea bed ( 12 ) and a subsurface region situated between the sea bed and the surface ( 14 ) of the sea. The flexible pipe ( 32 ) extends in a catenary curve between the riser ( 20 ) and the sea surface. An additional float ( 28 ) is installed between the riser ( 20 ) and the sea surface ( 14 ). The riser ( 20 ) is attached to the additional float ( 28 ) to increase the buoyancy of the riser.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a 35 U.S.C. §371 national phase conversion of PCT/FR2009/000717, filed Jun. 16, 2009, which claims priority of French Application No. 0803498, filed Jun. 23, 2008, the contents of which are incorporated by reference herein. The PCT International Application was published in the English language 
     BACKGROUND OF THE INVENTION 
     The present invention relates to an underwater fluid transport installation for transporting hydrocarbons for example, and to a method for positioning such an installation between a sea bed and a sea surface situated vertically above the sea bed. 
     Known installations make it possible to extract the hydrocarbons from underwater deposits. In addition to the difficulties associated with the pressure exerted by the marine environment on these installations, which increases as the depth of extraction increases, other difficulties result from the difference of the disturbances between the sea surface and the sea bed. Specifically, at the sea surface, the water is relatively turbulent to a variable depth of approximately thirty meters beneath the surface, while on the sea bed, it is much less turbulent because it does not sustain the influence of the wind and of the swell in particular. Therefore, such phenomena make it necessary to adapt the installations for extracting the hydrocarbons and for transporting them without disruption from the sea bed to the surface. Therefore, the installations comprise a riser, usually rigid, which extends between the sea bed and a subsurface zone situated beneath the sea surface, and more particularly beneath the aforementioned turbulent zone. This riser is fitted with one or more retaining floats which are installed around it, up to its end in order to keep it stretched in suspension vertically between the sea bed and the subsurface zone. Usually, these retaining floats are cylindrically symmetrical, and the riser passes through them axially. Therefore, the riser is held vertically in a relatively calm zone and its top end is then connected to a flexible duct which leads to a surface vessel floating on the sea surface. In this way, the flexible duct sustains the surface turbulence by deforming without being damaged. 
     Document US 200700 44 972 describes a system of the hybrid tower type in which one or more floats are mounted around the riser. These retaining floats are symmetrically cylindrical and the riser passes through them axially. However, this configuration is not satisfactory because considerable forces are exerted at the interface between the buoys and the riser, thus weakening the underwater installation. 
     Many hydrocarbon deposits are situated underground beneath sea beds that are relatively deep, for example more than 1500 meters, and the risers are consequently of increasing length. Therefore, they are increasingly heavy and the floats necessary to hold them in position vertically must be increasingly voluminous in order to increase their buoyancy. Therefore, bringing such floats in line with the hydrocarbon deposits is relatively difficult and requires a large amount of energy since they have to be towed. 
     Therefore, one problem that arises and that the present invention aims to solve is to propose an installation for the underwater transport of fluids, and precisely of hydrocarbons, which not only makes it possible to extract the hydrocarbons from relatively deep deposits but also which can easily be applied at an advantageous cost and has an acceptable behavior in terms of ageing and fatigue under the effect of the currents and the movements of swells. 
     SUMMARY OF THE INVENTION 
     For the purpose of solving this problem, the present invention proposes a fluid transport installation for transporting a fluid between a sea bed and a sea surface vertically above said sea bed, said fluid transport installation comprising a riser connected to a flexible duct, and a retaining float installed around said riser to keep said riser stretched in suspension between said sea bed and a subsurface zone situated between said sea bed and said sea surface, while said flexible duct extends like a catenary between said riser and said sea surface; according to the invention, the installation also comprises an additional float installed between said riser and said sea surface; and said riser is coupled to said additional float to increase the buoyancy of said riser. 
     Therefore, a particularly advantageous feature of the invention lies in the use of both a retaining float which surrounds the riser and an additional float to which it is coupled. This increases the buoyancy of the riser which makes it possible to suspend increasingly heavy risers for increasingly deep seas. Furthermore, to a certain degree, the use of two floats, one directly secured to the riser, the other vertically above this riser, makes it possible to provide floats of smaller dimensions than a single float although the total volume of the two floats is greater than the volume of a single float used according to the prior art to suspend heavy ducts. 
     Moreover, said riser and said flexible duct are connected together by means of a gooseneck duct installed between said floats so as to be able to stretch the flexible duct as a catenary between the riser and a surface vessel floating on the sea surface. This specific configuration allows the installation according to the invention to provide great resistance to ageing due to the movements of swell and to the currents. 
     Advantageously, said riser and said additional float are coupled together by means of a post and, preferably, the gooseneck duct is installed and held inside said post. As will be explained below, the post has a foot to which the riser is connected and a head connected to the additional float. The gooseneck duct is secured to the foot of the post and extends to the head. 
     Advantageously, said retaining float is symmetrically cylindrical and said riser extends axially inside said retaining float. Moreover, and according to a preferred feature, said riser has a top end furnished with a collar forming a shoulder, while said retaining float has a bearing edge capable of receiving said collar resting on it in order to support said riser. In this way, the riser extends longitudinally inside the retaining float and the latter, oriented vertically, tends to be drawn to the sea surface and consequently to retain the riser which is trapped in the retaining buoy by means of its collar which for its part rests against the bearing edge. 
     Moreover, and according to a particularly advantageous embodiment of the invention, the installation comprises an elastically deformable spacer installed coaxially between said retaining float and said riser, which is free to move inside the retaining float according to a restricted amplitude of movement. In this way, the bending moments between the retaining float and the riser are attenuated. 
     Moreover, said floats advantageously have a diameter of less than 5 meters which makes it possible to transport these floats directly on the positioning vessels and to install them through the latter. Preferably the floats also have a total volume of more than 800 m 3  and can then support risers of more than 800 tons. 
     Advantageously, according to a particular embodiment of the invention, these floats consist of a plurality of caissons that are independent of one another so as to maintain the overall properties of the float when only one of the caissons is damaged and water enters it. 
     According to another aspect, the present invention proposes a method for positioning an underwater fluid transport installation as described above, said method being of the type in which a positioning vessel is provided having a central positioning well surmounted by a positioning tower, according to the invention, said method comprises the following steps in order: a) a tubular duct having a top retaining end is provided on said positioning vessel; b) said tubular duct is then submerged to form a riser and said top retaining end is held on said positioning vessel; then c) a retaining float is installed around said top retaining end in order d) to submerge said top retaining end surrounded by said retaining float through said central well; then e) an additional float is coupled to said top retaining end surrounded by said retaining float; and, finally, f) said additional float is submerged through said central well. 
     Therefore, the floats are no longer installed with a derrick as is the case according to the prior art, but through the central well of the positioning vessel which makes installation easier and less costly. 
     Advantageously, in steps a) and b), a plurality of duct sections are provided and said duct sections are connected successively while simultaneously submerging said duct sections connected section-by-section to form said tubular duct, so as to form a riser. Moreover, a gooseneck duct is preferably connected to said top retaining end between step d) and step e). Preferably, the gooseneck duct is installed inside a post which is also submerged through the central well and to which the additional float is directly connected as will be explained in greater detail below. 
    
    
     
       Other particular features and advantages of the invention will emerge on reading the description made below of a particular embodiment of the invention, given as an indication but not being limiting, with reference to the appended drawings in which: 
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of the installation according to the invention; 
         FIG. 2  is a detailed schematic view of a part of the installation illustrated in  FIG. 1 ; 
         FIG. 3  is a detailed schematic view of a portion of said part illustrated in  FIG. 2 ; 
         FIG. 4  is a detailed schematic view of another portion of said part illustrated in  FIG. 2 ; 
         FIGS. 5A to 5C  are schematic views illustrating a method for positioning the installation according to the invention. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows schematically an installation for the underwater transport of hydrocarbons  10  between a sea bed  12  and a sea surface  14 . It will be observed that the hydrocarbons thus extracted usually also contain water and various gases. On the sea bed  12  a foundation is installed furnished with means  16  for anchoring the riser which is connected to an underground deposit and, on the sea surface  14 , floats a surface installation  18  inside which a hydrocarbon is capable of being collected. The installation  10  comprises a rigid riser  20  which extends from the sea bottom  12  to a top end  22 . This rigid riser  20  is fitted with a retaining float  24  to which it is secured at the top end  22 . The installation  10  also comprises connection means  26  that will be explained in detail below with reference to  FIG. 2 , and an additional float  28  connected to the connection means  26  via a flexible link  30 . Moreover, the connection means  26  and the surface installation  18  are connected together by means of a flexible duct  32  which extends as a catenary and which makes it possible to connect in a sealed manner the rigid riser  20  and the surface vessel  18  for conveying the hydrocarbon. 
     The arrangement of the two floats  24 ,  28  will be examined below in greater detail with reference to  FIG. 2 . First of all, the retaining float  24  which is symmetrically cylindrical has at its center  34  a longitudinal passageway inside which the riser  20  extends. The retaining float  24  consists of a plurality of caissons that are independent and sealed from one another.  FIG. 2  shows, as an example, 15 caissons  36  which are 2.5 meters high, or a total height of 37.5 meters. Moreover, these caissons have a diameter of approximately 3.7 meters. Consequently, the total volume of the retaining float is approximately 400 m 3 , which corresponds to a suspension capability of 400 tons deducted from the total weight of the retaining float  24 . 
     Before describing in detail the other elements shown in this figure, a description will first be given, with reference to  FIG. 3 , of the method for attaching the retaining float  24  and the riser  20  and then, with reference to  FIG. 4 , of a method of damping their relative movements. 
     Therefore  FIG. 3  shows the top end  22  of the riser  20 . This top end  22  has a connection flange  38  and, set back, a locking collar  40 . This locking collar  40  forms a shoulder  42  oriented toward the sea bottom  12 . Furthermore, the retaining float  24  has, at the first caisson  36 , a circular bearing edge  44  against which the shoulder  42  of the locking collar  40  rests. Therefore, the riser  20 , which is drawn toward the sea bed  12  under the effect of its own weight, is capable of being retained by means of the retaining float  24  at its locking collar  40 . 
     Moreover, it will be observed that the passageway at the center  34  of the retaining float  24  has a diameter substantially greater than that of the rigid duct D which, for its part, is for example of the order of 40 cm. Therefore, the rigid duct  20  has a range of movement inside the retaining float  24 , quite clearly according to relatively small amplitudes. 
     At the bottom end of the retaining float  24  shown in  FIG. 4 , an elastically deformable spacer  46 , or flexible seal, is mounted around the rigid duct and concentrically inside the retaining float at the last caisson. In this manner, the possible movements of the riser  20  relative to the retaining float  24  are damped by means of this spacer  46 . Consequently, the bending moments between the retaining float  24  and the riser  20  are attenuated at this level by the deformation of the spacer  46 . 
     Reference will again be made to  FIG. 2  which shows, in addition to the elements already described above, the connection means  26  suspended from the additional float  28  by means of the flexible link  30 . These connection means  26  comprise a post  50  having a foot  52  to which the top end  22  of the riser  20  is attached, and a head  54  secured to the flexible link  30 . Moreover, a gooseneck duct  56  extends inside the post  50 , from the foot  52  to the head  54 . This gooseneck duct  56  has a free end  58  to which the illustrated flexible duct  32  can be connected. The gooseneck duct  56  is quite clearly connected in a sealed manner to the riser  20 . 
     Moreover, and it is an advantageous feature of the invention, the post  50  is coupled to the additional float  28  by means of the flexible link  30  comprising a chain so as to further increase the buoyancy of the riser  20 . Moreover, the additional float  28  has dimensions comparable to those of the retaining float  24 , and notably in terms of diameter, which has a considerable advantage for the installation as will be explained below. 
     In the example shown in  FIG. 2 , the additional float  28  no longer comprises 15 caissons, but 17 caissons  60  each 2.5 meters high, identical to the caissons  36  of the retaining float  24 . Consequently, the lift force procured by this additional float  28  is substantially 50 tons more than those of the retaining float  24 . 
     In this manner, the retaining float  24  and the additional float  28 , in this instance shown as an example, make it possible to exert a lift tension on the riser  20  of the order of 850 tons deducted from the weight of these floats, which makes it possible to hold in vertical position risers of a greater length and of a greater weight than those of the prior art for applications at shallower depth. Moreover, because of their small diameter, these floats have advantages in terms of use of the installation as will be explained below with reference to  FIGS. 5A to 5C  illustrating a possible method of installing the underwater installation according to the invention. 
     Usually, said positioning vessel is supplied with a tubular duct having a top retaining end and said tubular duct is submerged to form a riser while keeping said top retaining end  22  on said positioning vessel. Then, while the top end  22  is held, the retaining float is installed around the top retaining end  22 , then the top retaining end surrounded by the retaining float is submerged through said central well. An additional float is then attached to said top retaining end surrounded by said retaining float. Finally, said additional float is submerged through said central well. 
     Partially shown in  FIG. 5A  is a positioning vessel  62  in longitudinal section, which shows a central positioning well  64  bordered by a work table  65  and a positioning tower  66  terminated by an installation post which extends vertically above the central positioning well  64 . 
     First, the positioning vessel  62  is loaded with riser elements, not shown, designed to be welded together to form riser sections, themselves welded together to form finally the riser. The rigid riser is thus positioned according to a method called “J-Lay”. Therefore, according to this method, the duct elements and the duct sections are assembled to form a single continuous riser which is submerged step by step, gradually as the sections are assembled, through the central positioning well  64  while retaining the last section by means of a retaining sling from the installation post  68 . When a last section  70  of riser is extended vertically along the positioning tower  66  and assembled to the penultimate section which for its part is already at least partially submerged, it is fitted on the one hand with the retaining float  24  that is shown here around this last section  70  of riser and, on the other hand, with a gooseneck duct  72 . This retaining float  34 , in this instance shown in a single piece, has a diameter of less than four meters for example, and can consequently pass through the central positioning well  64  which has a larger diameter, for example of the order of five meters. Then, by virtue of the positioning tower  66 , the riser fitted with its retaining float  24  is submerged in its turn and is drawn in translation through the central positioning well  64 .  FIG. 5B  shows in cross section the positioning vessel  62  and the positioning tower  66  which submerges the last section  70  of riser surrounded by its retaining float  24 . After the last section  70  of riser has been submerged with the retaining float  24 , the top end of the riser  22  remaining at the work table  65  as illustrated in  FIG. 5 , this top end  22  is then fitted with the post  50  not shown here and the additional float  28 , also not shown, is installed and held vertically on the positioning tower  66 . It is then connected to the post  50  and then the assembly is submerged through the central positioning well  64  just like the retaining float  24 . The additional float is then held by the positioning tower  66  by means of the sling, so as to be able to adjust the riser to the desired position between the sea surface and the sea bed  12 . 
     Therefore, by virtue of the geometry of the floats, that is to say a great length relative to a small diameter, and also their assembly, one being a retaining float  24  around the riser  20 , the other  28  being located vertically above the riser and coupled by means of the post  50 , the assembly is capable of being submerged through the central positioning well  64  of the positioning vessel  62  without requiring an additional derrick for the installation. 
     Moreover, it will be observed in  FIG. 2  that the flexible link  30 , in this instance consisting of a chain, is extended at each of its ends by a metal rod; one top rod connecting the chain and the additional float  28 , the other a bottom rod, connecting the chain and the head  54  of the post  50 . The bottom metal rod makes it possible to retain the catenary consisting of the riser during installation, at the work table  65  by means of a clamping tool making it possible to clamp said bottom metal rod and to temporarily hold the duct in fixed position relative to the positioning vessel  62  during installation of the float. 
     In the same manner, provision is made to temporarily weld the top metal rod to the top end of the additional float  28  so as to be able to retain the assembly. 
     Clearly, such a positioning method could very well be applied according to the techniques called “rigid roll-out”. In this case the rigid duct is previously rolled up in a single piece on a suitable drum and it is paid out through the central well to form the riser.