Abstract:
For transferring a fluid between at least two floating supports or one floating and one fixed support, a rigid hollow transport line is immersed with a cable suspension system in the sea. A flexible connector links each end of the rigid transport line to one of the supports. The entire rigid transport line including its ends is immersed in the sea at a depth which is greater than the turbulent zone of the sea. Each connector provides continuity of oil flow between the two floating supports via the rigid transport line.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a device in an offshore oil production installation for transferring a fluid between at least two floating supports such as a production vessel producing a gas-free product (dead oil) from live crude and a loading buoy (CALM buoy) from which oil tankers are filled with the product to be transported to land (onshore). 
     The production vessel having an acronym FPSO (Floating Production Storage Offloading) is generally anchored in a zone where the live crude is produced and is separated from the CALM buoy by several kilometers of the order of 1 to 3 kilometers. 
     The device for transferring the dead oil from the production vessel to the CALM buoy consists of at least one line known as an export line, one end of which is connected to the production vessel and the other end of which is connected to the CALM buoy. These export lines consist of a flexible pipe or rigid tube as described in API 17B, 17J and 5CT (American Petroleum Institute). 
     When the export line is built rigid, the connections at its ends are provided by kinds of ball joints (flex joints) so as to allow the export line to follow, on the other hand, the relative movements of each of the floating supports and, on the other hand, to more or less absorb the influences of the swell and marine currents likely to be present down to a certain depth in the sea. It is known from GB 2 335 723 to replace the conventional ball joint by flexible connecting means connecting the end of the rigid transport line to one of the floating supports and ensuring the continuity of flow of the crude between the two floating supports via the rigid transport line. However, according to this reference, the flexible pipe which replaces the conventional connection has the same dimensions as that connection, on order of a few meters. The rigid pipe stays partially submerged in a turbulent zone, and, consequently, the ends of the pipe undergo vibrations due to high marine currents. These vibrations in combination with the tensile forces cause early fatigue of the rigid pipe. 
     As the floating supports concerned can move independently of one another, and in any arbitrary direction, over a distance which is considered to be approximately equal to about 10% of the water depth of the sea on which the said supports are afloat, the amplitude of the relative movement between the two structures may thus be of the order of 20% of the said depth. 
     In order to allow these relative moments which may represent from 10 to 50% of the distance between the floating supports, it is common practice to provide an export line of a length much greater than the distance separating the two floating supports. 
     Furthermore, dynamic loadings in bending and vibrations are generated in the standing part of the export line by the movement of the swell, the marine current and the relative displacements of the supports. In addition, tensions are also created at the ends of the export line, these tensions being due mainly to the weight of the said export line. 
     The combination of the dynamic loadings, of the vibrations and of the tensions leads to significant fatigue of the export line at the end connections, which significantly reduces the life of the export line. 
     In the case of a rigid tube and in order to reduce vibration, the zones subjected to significant vibrations are equipped with additional special-purpose means such as anti-vibration strakes, for example. However, a solution such as this leads to additional cost of manufacture of the export line. 
     In order to reduce the tension caused by the weight of the line and to limit the tension at the ends, buoys with positive buoyancy have been widely used to create a single or double wave between the two floating supports. The series (of which there may be more than one) of buoys corresponding to the waves formed along the length of the export line gives the export line an additional length between its ends, which makes it possible to absorb the differences in length that are due to the relative displacements of the floating supports and for this to be possible under the most unfavourable operating conditions, that is to say when the said floating supports are moving in opposite directions. 
     One disadvantage of having buoys of positive buoyancy on the export line lies in the fact that the cost of the said export line is increased significantly without in any way solving the problems associated with the bending moments generated by the dynamic loadings or those associated with the vibrations caused by marine currents in particular. 
     In addition, by reducing the apparent weight of the export line, the latter tends to move with not insignificant amplitudes of movement as a function of the marine currents. These repeated movements lead to significant fatigue, mainly at the connections with the floating supports. 
     Another solution consists in laying the rigid export line on the seabed and in connecting its ends to the floating supports by risers. However, the length of such an installation is entirely prohibitive and cannot really be envisaged for great depths. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to overcome the aforementioned drawbacks by dissociating the bending moments developed by the movements of the floating supports and the vibrations from the tensile loadings developed by the weight of the export line. 
     The present invention concerns a device for transferring fluid between two floating supports at the surface of the sea, wherein the sea has a turbulent zone determined over a given depth. The device comprises a rigid hollow transport line which is submerged catenary-fashion in the sea. Flexible connecting means connect each end of the rigid transport line to one of the floating supports. The said connecting means ensure continuity of flow of the crude between the two floating supports via the rigid transport line. The entire rigid transport line including its ends is submerged in the sea to a depth greater than the turbulent zone. 
     What happens is that for a given region of the exploited oil field, the specialists can quite easily determine the height of the layer of water (turbulent zone) beneath which the movements of the swell are relatively small and in which the marine currents are weak, that is to say, in practice, a maximal speed of the marine currents less than 1 m/s or even 0.5 m/s. According to the invention, the rigid pipe is submerged within a non turbulent zone, defined by these speeds. 
     Each flexible and deformable connection connecting one end of the export line to the corresponding floating support absorbs all the dynamic bending stresses and vibrations without the need for additional special-purpose equipment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other advantages and characteristics will become apparent from reading the description of a number of embodiments of the invention and from the appended drawings, in which: 
     FIG. 1 is a schematic depiction of the invention according to a first embodiment. 
     FIG. 2 is a schematic depiction of the invention according to a second embodiment. 
     FIG. 3 is a schematic depiction of the invention according to a third embodiment. 
     FIG. 4 is a schematic depiction of a fourth embodiment of the invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The device according to a first embodiment of the invention depicted in FIG. 1 comprises a transport line consisting of a rigid tube  1  which is connected by each of its ends  2  and  3  to a floating support  4 , 5  arranged at the surface  6  of the sea  7  the depth (P) of which depends on the underwater oil field to be exploited. The support  4  is a production vessel denoted by the acronym FPSO, in which the live crude is converted into another product. 
     The support  5  generally consists of a CALM buoy which is anchored to the bottom  8  of the sea  7  using appropriate means  9  which will not be described and which are well known to those skilled in the art. The production vessel  4  is separated from the CALM buoy  5  by a distance L of between a few hundred meters and several kilometers. The oil tankers, not depicted, are filled with the converted product from the CALM buoy which will also not be described because it is widely used by specialists. 
     Each floating support  4 , 5  can move laterally with respect to a position of equilibrium by a distance roughly equal to 10% of the depth P. The directions of relative lateral movements are indicated by the arrows S 1  to S 4 , the said lateral movements having a tendency to move the two floating supports closer together or further apart. The maximum amplitude of the relative movements between the two floating supports  4 , 5  may reach 20% of the depth P. 
     Each end  2 , 3  is connected to the corresponding floating support  4 , 5  by a connecting means  10  which, in its simplest form, consists of a flexible pipe which absorbs the dynamic stresses and takes up the tension due to the weight of the rigid pipe. In this configuration, the transport line or rigid tube  1  is curved with a radius of curvature which essentially depends on the distance L and on the relative lateral movements of the two floating supports  4  and  5 . Obviously, the minimum bend radius (MBR) that the rigid tube  1  might adopt cannot be smaller than the MBR for the said rigid tube. The angle α at the top, under static conditions, that the export line makes with the surface  6  of the sea is between 45° and 75°. 
     In all cases, the ends  2  and  3  of the rigid tube  1  and the entirety of the rigid tube  1  must be located beneath the turbulent zone given for the sea in question, that is to say the zone situated at the depth P1 beneath which the effects of the swell and the marine currents such as the orbital currents are relatively small. 
     By virtue of the present invention, the rigid tube  1  is subjected only to tensile loads at the ends  2  and  3 , which tensile loads are generated by the weight of the rigid tube and the dynamic stresses created by the relative lateral movements of the two floating supports  4  and  5 . The rigid tube  1  is practically no longer subjected to the vibrations likely to be generated by the marine currents because the ends  2  and  3  are submerged at a depth P1 which is greater than the depth of the turbulent zone. As to the effects of the swell, these are absorbed by the ability that the flexible means  10  have to bend in given directions and take up the tensile loads developed in the rigid tube  1 . Specifically, when the floating supports move apart in the opposite directions S1 and S4, the rigid tube is subjected to tensile forces and when they move closer together in the directions S2 and S3, bending forces are generated, which leads to the rigid tube  1  adopting a significant curvature as its ends are moved closer together. 
     In another embodiment depicted in FIG. 4, it is possible to use a rigid tube  1  submerged catenary-fashion more deeply in the sea  7 , so as to create relatively high tensions due to the higher weight of the rigid tube. This high weight of the rigid tube makes it possible to limit the influence that the marine currents have on the rigid pipe. Moreover, as the CALM buoy  5  is anchored to the seabed with a tension which is also high, the two types of tension due to the weight of the rigid tube and to the anchoring  9  of the CALM buoy, and optionally the additional anchoring  9 ′, achieve equilibrium. These high tensions make it possible to stabilize the CALM buoy and consequently limit its movements in all horizontal directions. In this case, it is preferable to use an angle α at the top, under static conditions, between  50 °and  65 °and preferably equal to  60 °. It should be noted, in this case, that only the other end of the export line is able to move in order to follow the movements of the floating support  4 . 
     In the embodiment depicted in FIG. 2, the connecting means  10  each consist, on the one hand, of at least one tether  11  which extends between the corresponding floating support and the end  2  or  3  of the rigid tube  1 , each end  2 , 3  consisting of a goose neck  12  and, on the other hand, of a length of flexible pipe  13 , one end  14  of which is connected to a connector  15  which, in turn, is connected to the corresponding floating support  4 , 5  and the other end of which is connected to the goose neck by appropriate means (connectors) to ensure the continuity of the flow of crude. 
     The tether  11  may consists of a chain, a textile cable, for example made of carbon, a steel cable or a nylon cord. 
     The tether  11  supports the weight of the rigid tube  1  and, by virtue of its flexibility, absorbs the effects of the swell, the marine currents not giving rise to any vibration because of the small diameter of the tether. The length of flexible pipe  13  allows the converted product to flow between the floating supports  4 , 5  and the rigid tube  1 . Because of the flexibility and of its ability to deform, the length of flexible pipe  13  is capable of following the movements of the floating support to which it is connected. 
     The length of the length of flexible pipe  13  is greater than the length of the tether  11 , the difference in length being of the order of 20%, so that it does not take any tensile force. 
     In one advantageous form, the length of flexible pipe is equipped, at least at one of its ends, with a bend limiter, for example vertebrae  16  or a stiffener, well known to those skilled in the art. 
     In all the embodiments of FIGS. 1 to  3 , the angle α at the top of the connecting means is between 45° and 75° under static conditions and between 20° and 85° under dynamic conditions. The angle α under dynamic conditions corresponds to the angle formed by the configuration during relative movements of the floating supports and rigid tube  1 . 
     The range from 20° to 85° under dynamic conditions is chosen so as to limit the horizontal component of the tension created in the rigid tube  1  when the amplitude of the relative movements of the floating supports is at a maximum and so as to avoid excessive curvature beyond the MBR and thus significant fatigue of the rigid tube  1  when the amplitude of the relative movements of the floating supports is minimum. 
     The non turbulent zone as mentioned earlier (and hence the turbulent zone) is defined by a zone or depth of water in which the marine currents have a maximum relative speed of between 0.5 m/s and 1 m/s. The person skilled in that art will know how to determine the depth of submersion as a function of the diameter of the rigid tube and of the effects of turbulence. For example, in the case of Brazil (a zone where the speed of the marine currents is high), the turbulent zone can be as deep as 300 m, or even 500 m (15% to 25% of the water depth) in certain fields. By contrast, in West Africa (a zone where the turbulences are weak), the turbulent zone can have a maximal depth in the order of 50 m (5% of the water deep). 
     In FIG. 3, a fixed production tower  20 , arranged over a well head, may be connected to the floating support  4  to constitute an oil production installation. In this case, the fixed tower  20  is connected to the said floating support  4  by connecting means such as those depicted in FIGS. 1 and 2 and by a rigid pipe  1  submerged catenary-fashion, the latter being entirely submerged at a depth P1 which is greater than the given turbulent zone of the sea. The length of each connecting means is greater than the depth P1. 
     This oil production installation is supplemented by a CALM buoy  5  (not shown in FIG. 3) which is connected to the floating support  4  by the means previously described. In this case, the live crude produced by the well head rising up into the fixed tower  20  is transferred to the floating production support  4 , the treated oil then being transferred to the CALM buoy  5  from which the oil tankers are supplied. 
     Of course, the floating supports may just as easily consist, for example, of an oil platform, a SPAR (the acronym for a Submersible Pipe Alignment Rig) or any other oil production surface entity.