The present invention relates to an installation comprising multiple bottom-surface connections for at least two undersea pipes resting on the sea bottom, and in particular pipes situated at great depth, the installation comprising at least one hybrid tower. 
The technical field of the invention is more particularly the field of fabricating and installing rising production columns or “risers” for extracting oil, gas, or other soluble or fusible materials or a suspension of mineral material from under the sea via an immersed well head and up to a floating support, for the purpose of developing production fields installed off-shore, at sea.  The main and immediate application of the invention lies in the field of producing oil. 
In general, the floating support has anchor means for keeping it in position in spite of the effects of currents, winds, and swell.  It also generally has means for storing and processing oil, together with means for off-loading oil to oil-removal tankers, which call at regular intervals for the purpose of taking away the production.  Such floating supports are commonly referred to as floating, production, storage, off-loading supports as they are referred to below by the abbreviation “FPSO”. 
Bottom-surface connections are known for an undersea pipe lying on the sea bed, where such a connection may be of the so-called hybrid tower type, comprising:                a vertical riser having its bottom end anchored to the sea bottom and connected to a said pipe resting on the sea bottom, with its top end tensioned by a float that is immersed under the surface and to which it is connected; and        a connection pipe, generally a flexible connection pipe, between the top end of said riser and a floating support on the surface, said flexible connection pipe, where appropriate, taking up the shape of a plunging catenary under the effect of its own weight, i.e. a catenary that drops well below the float before subsequently rising up to said floating support.         
Bottom-surface connections are also known that are implemented by raising strong and rigid pipes made up of thick-walled tubular elements welded or screwed together continuously up to beneath the surface in a catenary configuration with curvature that is continuously varying along the entire length in suspension, and commonly referred to as steel catenary risers (SCRs), and also commonly referred to as “catenary type rigid pipes” or as “SCR type risers”. 
Such a catenary pipe can rise to a floating support on the surface or merely up to a float under the surface that tensions its top end, with said top end then being connected to a floating support by a plunging flexible connection pipe. 
Catenary risers of reinforced configuration are described in WO 03/102350 in the name of the Applicant. 
In WO 00/49267 in the name of the Applicant, proposals are made to use SCR type rigid pipes for a connection pipe between the floating support and the riser having its top tensioned by a float immersed under the surface, and the float is installed at the head of the riser at a considerable distance below the surface, and in particular at a depth of at least 300 meters (m) below the surface, and preferably at least 500 m.
In WO 00/08262 (D1) and WO 03/097990 (D2), proposals are made for installations having two rigid catenary pipes of the SCR type, each rising up to a respective float that is immersed below the surface, with the top ends of said rigid SCR type pipes being connected respectively to two plunging flexible pipes providing connections between said rigid SCR pipes and a floating support.  Those two bottom-surface connections are disposed symmetrically relative to each other and at substantially the same depth, both concerning the top end of the SCR and concerning the connection point and the curve formed by each of said flexible pipes. 
In WO 00/08262, the installation is particularly unstable and is subjected to large amounts of movements by swell and currents, such that such an installation can be envisaged only when the pipes are of the flexible pipe type and not of the SCR pipe type since they would present wear phenomena in their ground contact point zones, which are their zones subjected to the highest levels of stress, that are completely incompatible with operating for a lifetime of 10 years or even 30 years. 
In WO 03/097990, the installation is more stable since each of the floats is anchored to the sea bottom by a tie.  Furthermore, the horizontal connection between the floats is also constituted by a tie of considerable weight, and thus occupying a catenary configuration.  That creates horizontal tension that compensates the horizontal tension from each of the catenary-shaped SCR type pipes.  However the assembly thus presents a very high degree of flexibility, which is incompatible with long-term strength for a rigid SCR type pipe, in particular in the vicinity of the zone where it makes contact with the ground. 
In those two patents, the two bottom-surface connections, and in particular the two floats, are spaced apart from each other by a considerable distance in order to avoid any collisions during the movements of the two floats and the two flexible pipes in their respective movements under the effect of the displacements of the floating support and/or the swell, wind, and currents. 
It is desired to implement as many bottom-surface connections as possible from a single floating support in order to optimize the working of oil fields.  That is why various systems have been proposed enabling a plurality of vertical risers to be associated with one another in order to reduce the area or “footprint” occupied over the operating fields and in order to enable a larger number of bottom-surface connections to be connected to a single floating support.  Typically, it is necessary to be able to install up to 30 or even 40 bottom-support connections from a single floating support. 
In WO 00/49267, there is described a multiple hybrid tower comprising an anchor system with a vertical tendon constituted either by a cable, or by a metal bar, or indeed a pipe tensioned at its top end by a float.  The bottom end of the tendon is fastened to a base that is resting on the bottom.  Said tendon includes guide means distributed along its entire length and through which there pass a plurality of said vertical risers.  Said base may merely be placed on the sea bottom and stay in place under its own weight, or it may be anchored by means of piles or any other device suitable for keeping it in place.  In WO 00/49267, the bottom end of the vertical riser is suitable for being connected to the end of a bent sleeve that is movable between a high position and a low position relative to said base, from which said sleeve is suspended and is associated with return means urging it towards its high position in the absence of a riser.  This ability of the bent sleeve to move serves to absorb variations in the length of the riser under the effects of temperature and pressure.  At the head of the vertical riser, an abutment device secured thereto bears against the support guide installed at the head of the float and thus serves to keep the entire riser suspended. 
The connection with the undersea pipe resting on the sea bottom is generally provided by means of a portion of pipe having the shape of a pig's tail or of the letter S, with said S-shape then being made either in a vertical plane or in a horizontal plane, the connection with said undersea pipe generally being made via an automatic connector. 
That embodiment having a multiplicity of vertical risers held by a central structure comprising guide means is relatively expensive and complex to install.  Furthermore, the installation needs to be prefabricated on land before being towed out to sea, and then once on site, it needs up-ending in order to be put into place.  In addition, maintenance thereof also involves relatively high operating costs. 
In WO 02/066786 and WO 02/103153 in the name of the Applicant, hybrid towers are described having multiple risers with vertical riser anchor systems suitable for receiving two risers side by side from a common anchor base, with the floats at the top of said risers being secured and fastened together by means of a hinged structure in the form of a parallelogram.  The two risers are also connected together with the help of tubular collars fastened on one of the risers and connected by rings that slide freely around the second riser so that the two risers can perform substantially the same lateral movements while being relatively more independent when performing vertical movements. 
When it is desired to associate a plurality of risers with a common floating support, a problem of interference occurs between the movements of said risers, which risers are subjected to the same movements as their respective top tensioning floats under the effect of the displacements of the floating support on the surface that is subjected to swell, wind, and currents. 
The embodiments described above are relatively effective, but they are still too complicated concerning their methods of laying and their maintenance constraints when in use, particularly in the vicinity of the anchor system and also in terms of associating the risers with one another.  In addition, and above all, that type of hybrid tower having multiple risers needs to be prefabricated on land before being installed at sea. 
Furthermore, when a multiplicity of bottom-surface connections of the hybrid tower type are implemented, each comprising a single vertical riser, it is necessary in practice to space the various connections apart from one another for at least the following two reasons:
1) firstly, the respective bases of two hybrid towers, when anchored by suction anchors engaged in the sea bottom, need to be spaced apart by a distance that is not less than 5 times, and preferably not less than 10 times, the diameter of said anchors in order to avoid interference in terms of the strength of the sea bottom and in order to guarantee reliable anchoring; and
2) furthermore, the floats at the tops of the risers are subjected to displacements within respective cones having their vertices situated at the anchor systems (see FIG.  1) and presenting angles that require sufficient distance to be provided between the various top floats for the vertical risers to ensure that they do not strike one another. 
These constraints imply spreading out over the working zone, and they thus limit the number of bottom-surface connections that can be connected to a single floating support, over its sides, in order to avoid interference between the various connections. 
Furthermore, the crude oil is conveyed over long distances, several kilometers, so the risers need to be provided with extreme levels of insulation that are very expensive, firstly in order to minimize any increase in viscosity, which would lead to a drop in the hourly production rate from a well, and secondly to avoid the flow becoming clogged by paraffin being deposited, or by hydrates forming on the temperature dropping to around 30° C.-40° C. These phenomena are particularly critical, especially off West Africa, where the temperature at the sea bottom is about 4° C. and the crude oils are of the paraffin type.  It is therefore desirable for the bottom-surface connections to be short in length, and thus for the footprints of the various connections connected to a single floating support to be small. 
That is why it is desirable to provide an installation that can operate a plurality of bottom-surface connections of the hybrid tower type from a single floating support, with the connections presenting small footprints and moving little and also being simpler to lay and suitable for being fabricated at sea from a pipe-laying vessel, so as to avoid prefabrication on land followed by towing out to site and up-ending in order to put the installation finally into place. 
In published application US 2004/0129425, proposals are made to implement a single float connected to a plurality of vertical risers and/or SCR type pipes anchored or resting on the sea bottom, as the case may be.  However the system proposed in that patent presents several drawbacks:                firstly, for reasons associated with putting the installation into place, the float needs to present buoyancy characteristics that vary so as to be capable of becoming more and more buoyant as the various SCR riser type pipes plus the flexible pipe are connected sequentially thereto; and        secondly, connecting the top of a second vertical riser and/or SCR pipe to a float at the top of a first vertical riser leads to problems of a practical nature that have not been solved and are not even mentioned in that US published patent application.         
In this respect, the teaching of that patent application is thus incomplete and speculative.  When a vertical riser is put into place at sea on site from a surface vessel fitted in particular with a J-lay tower, which is the most advantageous method, the individual pipe elements or strings are connected one to another on board the vessel and the pipe that is being built up is lowered progressively to the sea bottom, with the first string being anchored to the sea bottom and with the last string including a float that enables the riser to be stabilized in a vertical position. 
It is therefore very difficult or even impossible to secure the top end of a second riser (that has not yet been stabilized) to a float that is already installed at the top of a first vertical riser that has already been laid.  In addition, in depths of water of 1500 m, differential expansion between said two risers, one of them cold and the other one hot, can amount to several meters, thus making it practically impossible for two of said risers to be put into parallel in the absence of special arrangements either at the tops or at the bottoms of said risers. 
Finally, another drawback of the system proposed in that patent application US 2004/0129425 is that the float needs to be pre-dimensioned as a function of some determined maximum buoyancy suitable for tensioning a predetermined number of pipes.  However in practice it is desirable to be able to put the various bottom-surface connections into place in a manner that is spread out over time without limits on the final number of pipes being set during initial installation of the system, since that number is generally not known in certain and accurate manner from the beginning. 
That multi-pipe tower system needs to be pre-fabricated on land before being put into place at sea.  However an object of the present invention is to provide a multi-pipe tower system that can be fabricated at sea from a pipe-laying vessel fitted with a laying tower.