OA17101A - Installation de liaisons fond-surface de type tour hybride multi-risers comprenant des conduites flexibles à flottabilité positive. - Google Patents

Abstract

Installation de liaisons fond-surface de type tour hybride multi-risers comprenant des conduites flexibles à flottabilité positive. La présente invention concerne une installation de liaison fond- surface comprenant un dit support flottant comprenant un touret (1a) comprenant : une pluralité de risers (10), dont les extrémités supérieures sont solidaires d'une structure porteuse (3a), et - une pluralité de conduites flexibles (4a-4b, 4a1-4a2, 4b1-4b2) s'étendant depuis ledit touret jusqu'aux extrémités supérieures (10a) des risers, dont - au moins deux dites premières conduites flexibles à flottabilité positive positionnées à des hauteurs différentes, et - des modules de guidage (20) solidaires d'un tendon et aptes à coulisser le long de flotteurs (11) desdits risers.

Description

INSTALLATION OF MULTI-RISER HYBRID TOWER TYPE BOTTOM-SURFACE CONNECTIONS INCLUDING FLEXIBLE POSITIVE BUOYANCY DUCTS

The present invention relates to an installation of multiple bottom to surface connections between submarine pipes resting on the seabed and a support floating on the surface, comprising a hybrid tower made up of a plurality of flexible pipes connected to a plurality of rigid risers. , or vertical risers, the lower end of the hybrid tower of which is integral with an anchoring device 10 comprising a base arranged at the bottom of the sea.

The technical sector of the invention is more particularly the field of the manufacture and installation of production risers for the underwater extraction of oil, gas or other soluble or fusible material or of production. a suspension of mineral matter from a submerged wellhead to a floating support, for the development of production fields installed in the open sea off the coast. The main and immediate application of the invention being in the field of petroleum production.

The floating support generally comprises anchoring means for remaining in position despite the effects of currents, winds and swells. It also generally comprises means for storing and processing the oil as well as means for unloading to removal tankers, the latter appearing at regular intervals to carry out the removal of the production. The common name of these floating media is the Angio-Saxon term Fioating Production Storage Offloading (meaning floating medium for storage, production and offloading), the abbreviated term FPSO of which is used throughout the following description.

The floating supports are:

- or of the fixed heading type, that is to say they have a plurality of anchors, generally located at each of the angles of said floating support λ

maintaining the latter in a fixed heading and allowing only roll, pitch and limiting swerves and yaw;

- either of the drum type, that is to say that all the anchors converge towards a cylindrical structure integral with the ship, but free to rotate along the vertical axis ZZ, the floating support then being free to rotate around said drum and position yourself in the direction of least effort resulting from the effects of wind, current and swell on the floating support and its superstructures.

The floating support is therefore either anchored at its four corners and it then keeps a substantially constant heading throughout the life of the Installations, or it is anchored at a single point, called a reel, generally located towards the front of the ship, usually in the forward third, or even outside the ship a few meters from the bow of the ship. The FPSO then turns around its reel and positions itself naturally in the direction of least resistance to the forces created by swell, wind and current. The bottom-surface connections are connected to the internal part of the drum which is substantially fixed with respect to the ground and rotary joints, known to those skilled in the art, transfer the fluids, the electrical power or the electrical signals between the dltes to the FPSO. bottom-surface bonds and said FPSO. Thus, in the case of an FPSO on a reel, the latter can rotate through 360 ° around the axis of its reel which remains substantially fixed with respect to the ground.

When conditions are severe, or even extreme, as in the North Sea, an advantageous floating support is of the drum type in which all the bottom-surface connections must converge towards a drum before joining the FPSO proper, through the intermediary of a rotary joint connection located at the axis of said reel. In general, the bottom-surface connection pipes are made by flexible pipes directly connecting the pipes resting on the seabed to the reel, the flexible pipes being generally organized radlalement or in a star uniformly distributed all around the axis of said reel. This type of bottom-surface connection is more particularly intended for depths of

200 to 750 m.

The present invention relates more particularly to a bottom-to-surface connection installation between a plurality of submarine pipes resting on the seabed and a support floating on the surface, comprising a hybrid tower made up of a plurality of flexible pipes connected to rigid risers. , or vertical risers, the upper end of said flexible pipes being integral with a reel pivoting freely at the front of the ship or within the ship, generally in the forward third of said ship.

There is a wide variety of bottom-surface connections making it possible to connect subsea wellheads to an FPSO-type floating support and in certain field developments, several wellheads are connected in parallel to the same bottom-surface connection of so as to limit the number of pipes connected to the reel of the FPSO, which simplifies the design of the reel, the latter then being mainly designed to take up the anchoring forces of the FPSO subjected to the effects of swell, wind and currents.

Many configurations have been developed and patent WO 2009/122098 of the applicant is known, which describes an FPSO equipped with such a reel and associated flexible pipes, more particularly intended for the extreme conditions encountered in the Arctic. Such a configuration is interesting for average water depths, that is to say from 100 to 350 m, or even 500-600 m. In particular, the implementation of flexible conduits over the entire height of the water section between the rigid conduits resting on the seabed and the floating support allows greater displacements of the floating support than when using rigid pipes. However, it is not possible in this type of bottom-surface connection between the reel of a floating support and pipes resting on the seabed, to implement said flexible pipes in the form of a plunging chain, it is - i.e. with a low point of inflection as described in hybrid tower type fund-surface connections comprising:

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- a vertical riser whose lower end is anchored to the seabed by means of a flexible joint, and connected to a so-called pipe resting on the seabed, and the upper end is stretched by a submerged float in subsurface to which it is connected, and

- a flexible connection pipe, between the upper end of said riser and a floating support on the surface, said flexible connection pipe taking, if necessary, by its own weight the shape of a plunging chain curve, it is that is to say descending widely below the float to then go up to the floating support, which plunging chain allows significant displacements of the floating support absorbed by the deformations of the flexible pipe, in particular the rise or fall of said low inflection point of the plunging chain.

It will be recalled that the essential function of plunging flexible pipes is to absorb at least in part the movements of the upper ends of rigid pipes to which one of their ends is connected and / or the movements of the floating support to which their other end is connected by mechanically decoupling the respective movements of the upper ends of rigid pipes to which they are connected and of the floating supports to which they are also connected at their other end.

In a known manner, a so-called flexible connecting pipe takes, by virtue of its own weight, the shape of a plunging chain curve, that is to say descending widely below its attachment points at each end with respectively the floating support and the upper end of the rigid pipe to which it is connected, provided that the length of said flexible pipe is greater than the distance between its point of attachment to the floating support and the upper end of said rigid pipe to which it is connected.

To connect the flexible pipes to said rigid pipe or riser on Intercale, devices of the gooseneck type known to those skilled in the art, an improved example of which is described in FR 2 809 136 in the name of the applicant.

But, as soon as the water depth reaches 1000 - 1500 m, or even 2000 - 3000 m, the cost of this multiplicity of flexible pipes becomes very high due to the developed length of each of said flexible pipes, because these Flexible pipes are very complex and very delicate to manufacture in order to achieve the levels of operational safety required to remain in operation for periods that can reach and exceed 20-25 years, or even more. In particular, the flexible pipes may interfere with each other and collide.

In WO 2011/144864, a fund-surface connection installation of a floating support equipped with a reel on which flexible pipes are fixed and secured via a guide structure has been described. This type of bottom-to-surface connection is at the same time not very bulky, mechanically reliable in terms of resistance over time while being relatively inexpensive and simple to produce.

In WO 2011/144864, said guide structure is maintained at the subsurface between said reel and said supporting structure and makes it possible to create a plurality of plunging chains extending (with regard to the center of the pipe) in substantially vertical planes passing by the vertical axis ZiZi of said guide structure on the one hand, and on the other hand, laterally spacing said plunging chains from one another in a horizontal perpendicular plane.

On the other hand, the guide structure makes it possible to guarantee that the curvature of said plunging chains at their low point of inflection always remains with a radius of curvature greater than a minimum radius of curvature below which the deformation of the flexible pipe. would become Irreversible and / or damage it.

In total, said guide structure of WO 2011/144864 makes it possible to implement with an optimum reduced size, a greater number of flexible pipes without these interfering with each other and in particular not colliding, in case of movement of said floating support linked to the swell, the current and / or the waves.

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However, in certain field developments, we are obliged to connect each of the wellheads individually to said FPSO and we then end up with a very large amount of bottom-surface connections, which requires increasing the dimensions of the reel and / or the guide structure such as described in WO 2011/144864 in order to be able to contain all the flexible connections without them interfering with one another and especially to have risers with multiple pipes because two said risers must be sufficiently apart to do not interfere with each other.

An aim of the present invention is therefore to provide an installation capable of comprising a greater number of bottom-surface links connecting a reel and pipes to the seabed in a reduced size and conditions of mechanical reliability and equally cost. optimal.

To implement a maximum of bottom-surface links from a single floating support in order to optimize the exploitation of oil fields, various systems have been proposed which can combine several vertical risers together in order to reduce the size of the field. 'operation and be able to implement a larger number of bottom-surface links connected to the same floating support. Typically, it is necessary to be able to install up to 30, or even 40 fund-surface links from the same floating support.

In WO 00/49267 of the Applicant, a hybrid multl-riser tower has been described comprising an anchoring system with a vertical tendon consisting either of a cable or of a metal bar, or even of a pipe stretched to its upper end by a float. The lower end of the tendon is attached to a hub resting at the bottom. Said tendon comprises guide means distributed over its entire length through which passes a plurality of said vertical risers. Said base can be placed simply on the seabed and remain in place by its own weight, or remain anchored by means of batteries or any other device suitable for holding it in place. In WO 00/49267, the lower end of the vertical riser is able to be connected to the end of an elbow sleeve, mobile, between a high position and a low position, relative to said base, to which this sleeve is suspended and associated with a return means bringing it back to the high position in the absence of the riser. This mobility of the elbow cuff makes it possible to absorb variations in the length of the riser under the effects of temperature and pressure. At the head of the vertical riser, a stop device, integral with the latter, rests on the support guide installed at the head of the float and thus maintains the entire riser in suspension.

The connection with the submarine pipe resting on the seabed is generally carried out by a portion of pipe in the form of a pigtail or in the form of an S, called a "jumper", said S then being produced in a vertical plane. or horizontal, the connection with said underwater pipe generally being produced by means of an automatic connector.

To implement multi-riser hybrid towers as described in WO 00/49267, the bottom-surface connections are generally kept vertical thanks to a very large float, its buoyancy being able to reach 500 tonnes, or even 1000 tonnes for bigger. However, safety regulations require that the weather vane ship around its reel is never above such a large capacity float. The reason is that, in the event of rupture of the connection between said float and said riser, the sudden and uncontrolled rise of such a float constitutes an extremely dangerous projectile for any equipment present in the ascent zone. It is then necessary to move the foot away from the riser considerably, so that said float always remains largely outside the avoidance circle of the boat. This results in a considerable lengthening of the length of the flexible pipes connecting the top of the riser to the reel of the FPSO, which increases the cost considerably, since these high pressure flexible pipes are very expensive components. Large capacity FPSOs measure more than

300-350 m in length, and the over-length of the hose can reach and exceed 500 or even 750 m for each of them. In addition, by increasing the length of the flexible pipes, the forces generated by the swell and the various currents are correspondingly increased, the latter having repercussions on the reel, therefore on the anchoring, which goes against the stability sought for the FPSO.

Furthermore, in WO 2009/138609 by the applicant, a hybrid tower-type bottom-to-surface connection was described aimed at facilitating its manufacture and installation at sea, without a head float, consisting of a rigid riser embedded in foot in a foundation and connected to the FPSO by a flexible pipe equipped with buoyancy elements on an end portion of its length, the end portion of the flexible pipe with positive buoyancy being in continuity of curvature with said rigid riser and making it possible to avoid to implement a float at the head and also making it possible to avoid using a connection device of the swan-type connection between the riser and the flexible pipe. However, this type of hybrid tower from WO 2009/138609, capable of being manufactured and installed at sea in a simplified manner, represents only a single bottom-surface connection and is not suitable for the implementation of a multi-rlser hybrid tower comprising a plurality of risers around a tendon anchored at the foot.

In WO 2010/097528 and WO 2011/144864, hybrid multi-riser towers equipped with buoyancy and sliding guide modules have been described, comprising:

a) a vertical tendon integral at its upper end of a supporting structure capable of being suspended from a float at the top submerged in the subsurface, by means of a chain or cable, said tendon being integral at its lower end to a structure Lower guide and being able to be fixed to a base resting on the seabed or a foundation sunk into the seabed, preferably by means of a flexible joint, and

b) a plurality of vertical rigid pipe called riser, the upper end of which is integral with said supporting structure, the lower end of each said rigid pipe or riser being able to be connected to an underwater pipe resting on the seabed ,

c) a plurality of guide means for said risers, said guide means as well as said lower guide structure being able to maintain said risers arranged around said tendon, and

d) buoyancy elements cooperating with said tendon, distributed along said tendon, preferably buoyancy elements resistant to underwater hydrostatic pressure, more preferably buoyancy elements made of syntactic foam, and characterized in that said tower comprises a plurality of buoyancy and guide modules constituting a plurality of independent structures able to slide along said tendon and along said risers, said structure supporting said buoyancy elements and guiding said risers in a position preferably regularly and symmetrically distributed around said said tendon.

Said modules and therefore said buoyancy elements slide along the tendon below said supporting structure and are retained at the upper end of said risers and tendon by said supporting structure. The tension created by the sum of the buoyancies of the different modules is thus transferred to the top of the tendon via said supporting structure against which the upper buoyancy module abuts, the other modules being pressed against each other.

Thus, in this embodiment, due to the fact that the buoyancy modules slide on the risers and tendon, all the tension is brought back to the level of the upper supporting structure to which are fixed the upper ends of the risers and the structure of the modules as well as the connection between the risers and the upper load-bearing structure must take up considerable tensile forces representing the entire weight of the risers. Indeed, if the foundation is only subjected to the resulting force Tr exerted at the level of the head float, namely 10 to 50% of the total weight of the tower, the total weight of the tower is taken directly by the set of buoyancy modules, the latter exerting a vertical upward thrust directly on the underside of said supporting structure. More particularly, all of the buoyancy modules provide cumulative buoyancy ZF representing a tensile force of greater intensity than the total weight of the tower Pt, preferably from 102 to 110% of the total weight of the tower.

In addition, since crude oil travels over very long distances, several kilometers, we must provide them with an extreme and very expensive level of insulation to, on the one hand, minimize the increase in viscosity which would lead to a reduction in your hourly production. wells, and on the other hand to avoid blocking the flow by deposition of paraffin, or formation of gas hydrates as soon as the temperature drops to around 30-40 ° C. These latter phenomena are all the more critical, particularly in West Africa, as the seabed temperature is around 4 ° C and crude oils are of the paraffinic type. It is therefore desirable for the bottom-to-surface connections to be of reduced lengths and therefore for the bulk of the various connections connected to the same floating support to be limited, for this additional reason of thermal insulation.

In WO 2011/097528 and WO2011 / 144864, the buoyancy elements are sliding and cover only a part of the total length of the risers, so that they cannot ensure optimum thermal insulation.

An object of the present invention is therefore to provide a new type of installation of a large amount of multiple bottom-surface links and of various types in connection with an FPSO anchored on a reel, allowing preferably to individually connect a plurality of wellheads and subsea facilities Installed at the bottom of the sea at great depth, that is to say beyond 1000 m of water depth, and not having any dangerous buoyancy element , such as a large-sized tenslonnement float which can reach 500-1000 m 3, or even more, and overcoming the drawbacks of previous embodiments, in particular as described in WO 2010/097528 and WO 2011/144864.

It is also sought to provide an installation capable of using a plurality of bottom-surface connections of the tower-hybrid type of reduced size and movement from a single floating support, and which is also simpler to install. More particularly still, another problem posed according to the present invention is therefore to provide an installation with a multiplicity of bottom-to-surface connections from the same floating support, the installation of which and the installation of which make it possible. at a time :

• to reduce the installation distance between the different bottom-surface connections, that is to say allow a plurality of bottom-surface connections to be installed in the smallest possible space or in other words with a footprint on the ground, this in order, among other things, to increase the number of bottom-surface connections that it is possible to install on the reel of an FPSO, without the bottom-surface connections interfering with each other, and ,

- easy manufacture and installation by manufacturing on land, then towing on site and final installation after cabanage

- to optimize the installation of the risers, if necessary equipped with the various flexible links, the assembly remaining pending the future installation of the FPSO anchored on its reel.

Indeed, during the engineering phase of the development of an oil field, the oil reservoir is only incompletely known at this stage, production at full capacity then often requires reconsideration, after a few years, Initial production plans and organization of associated equipment. Thus, during the installation of the Initial system, the number of bottom-surface links and their organization is defined in relation to estimated needs, the dlts needs being almost systematically revised upwards after the field is put into production, either for the recovery of crude oil, or for the need to inject more water into the reservoir, or again to recover or reinject more gas. As the reservoir is depleted, it is generally necessary to drill new wells to reinject water or gas, or to drill production wells in new places in the field, so as to increase the overall recovery rate, which further complicates the set of bottom-surface connections connected to the FPSO reel.

Another problem posed according to the present invention is to be able to produce and install such bottom-surface connections for submarine pipelines at great depths, such as beyond 1000 meters for example, and of the type comprising a vertical hybrid tower and of which the transported fluid must be maintained above a minimum temperature until it reaches the surface, reducing to a minimum the components subject to heat loss, avoiding the disadvantages created by the inherent thermal expansion, or differential, of the various components of said tower, so as to withstand extreme stresses and phenomena of cumulative fatigue over the life of the structure, which commonly exceeds 20 years.

Another problem of the present invention is also to provide an installation of multiple bottom-to-surface connections with hybrid towers whose anchoring system is of high strength and low cost, and whose manufacturing and implementation methods. in place of the various constituent elements are simplified and also inexpensive, and can be achieved at sea with common installation vessels.

To do this, the present invention provides a bottom-to-surface connection installation between a plurality of underwater pipes resting on the seabed and a support floating on the surface and anchored to the seabed, comprising:

- a said floating support comprising a reel, and

- at least one hybrid-type tower comprising:

a) a multi-riser tower comprising:

al) a vertical tendon integral at its upper end with an upper supporting structure, said tendon being fixed at its lower end to a base resting on the seabed or an anchor, preferably of the suction anchor type, driven into the bottom of the sea, and

a.2) a plurality of vertical rigid pipes called risers, the upper end of each riser being integral with said supporting structure, the lower end of each said riser being connected or able to be connected to an underwater pipe resting on the bottom of the sea,

a.3) a plurality of means for guiding said risers capable of maintaining said risers arranged around said tendon at a substantially constant distance, preferably regularly and symmetrically distributed around said tendon, and

b) a plurality of flexible pipes extending from said reel to the upper ends of a respectively plurality of rigid pipes, of which at least one flexible pipe, hereinafter referred to as the first flexible pipe, comprises an end part of the flexible pipe , on the side of its junction at the upper end of said riser, equipped with floats called the first float giving it positive buoyancy, and at least the upper part of said vertical riser is fitted with floats called second floats giving it positive buoyancy, so that the positive buoyancies of said end part of the first flexible pipe and of said upper part of said vertical riser allow the tensioning of said risers in a substantially vertical position and the alignment or continuity of curvature between the end of said end part with buoyancy positive of said first flexible pipe and the upper part e of said vertical riser at their connection, said installation being characterized in that at least one said hybrid tower comprises:

- at least two said first flexible pipes with positive buoyancy, the ends of which are respectively fixed to two upper ends of two said risers, the two upper ends of the two risers arriving above said upper supporting structure at different heights, so that said first flexible pipes are positioned at different heights relative to each other, and

- Said risers equipped with second peripheral coaxial floats surrounding lesdlts risers and integral with said risers, lesdlts second coaxial floats being distributed, preferably continuously, over at least an upper part of at least 25% of the length of said risers below and at from said upper supporting structure, preferably a length of at least 50% of the length of said risers, more preferably over at least 75% of their length, all of said second coaxial floats compensating at least for the total weight of said risers , and

- lesdlts guide modules integral with said tendon and able to slide along said second floats of said risers, lesdlts guide modules being spaced and distributed, preferably regularly, over at least an upper part of at least 25% of the length of said tendon below and from said upper supporting structure, preferably over a length of at least 50% of the length of said tendon, more preferably over at least 75% of its length,

- said tendon and said upper supporting structure not being suspended from a float submerged in the subsurface, and said tendon being located at a distance from the vertical axis of the reel (ZZ) less than the distance between the axis of said reel and l end furthest from said floating support.

Lesdlts guide means are advantageously installed over the entire height of the tower and thus have the essential function of maintaining a constant positioning geometry of the risers with respect to each other and to the tendons, and thus to prevent buckling of said risers when those they are put into compression, in particular when they are filled with gas, the spacing between two successive guide means preferably being reduced in this zone subject to lateral buckling.

Due to the respective arrangement of said flexible pipes with positive buoyancy relative to each other, it is possible to implement on each multi-riser hybrid tower a plurality of flexible pipes with positive buoyancy, in particular from 2 to 8 flexible pipes at positive buoyancy, offset in height although close together in terms of lateral spacing, since all converge towards the same tower, that is to say near the same tendon.

Due to the plurality of flexible pipes with positive buoyancy in combination with a positive buoyancy distributed over a said upper part of the length of said risers and of the length of said tendon from said upper supporting structure, it is no longer necessary to put implement a float at the top of the tower to ensure tensioning of the tower. Thus, it is possible to bring the hybrid towers together inside the avoidance zone of the ship without risk of accident, as explained previously. And, it is thus possible to reduce the problems associated with the length of flexible pipes, thereby reducing the cost of manufacture and thermal insulation.

Due to the fact that all of said second coaxial floats compensate at least for the total weight of said risers, and more particularly that each of said second floats associated with the same riser at least compensates for the total weight of said riser, thus giving said audit! to said risers positive buoyancy even when said riser (s) are filled with sea water and the buoyancy elements of the tower do not slide along said risers and said tendon, the installation according to the invention has the following advantages:

- each of the risers being independent of its neighbors, the forces generated by the buoyancy of said riser is applied only to the upper load-bearing structure, then to the tendon, then to the foundation, and

- on the other hand, it is possible to combine thermal insulation and buoyancy by implementing buoyancy elements made of a material combining the properties of buoyancy and thermal insulation, in particular as described in FR 11 52574 in the name of the Applicant explained below.

Another advantage of an installation according to the invention is that it is possible to implement a plurality of hybrid towers whose flexible pipes are connected to the same reel but angularly and radially offset, so that the towers are arranged in a fan around said reel at identical or different distances from said reel, certain towers being able to be only partially installed and not yet comprise flexible conduits or only part of said rigid conduits which may be extended by said flexible conduits at their upper ends and / or connected to said underwater conduits resting on the seabed at their lower ends, rigid conduits lesdltes awaiting connection to wellheads as well as to the floating support, as explained below.

By “first flexible pipe” is meant here the pipes known under the name “flexible” well known to those skilled in the art and which have been described in the normative documents published by the American Petroleum Institute (API), more particularly. under the references API 17J and API RP 17 B. Such hoses are in particular manufactured and marketed by the company TECHNIP France under the brand COFLEXIP. These flexible pipes generally comprise internal sealing layers of thermoplastic materials associated with layers resistant to the pressure internal to the pipe, generally of steel or of composite materials made in the form of spiral strips, contiguous inside the pipe. thermoplastic pipe to withstand the internal bursting pressure and completed by external reinforcements above the thermoplastic tubular layer also in the form of contiguous spiral strips, but with a longer pitch, i.e. an angle d 'inclination of the lower propeller, in particular 15 ° to 55 °.

By "vertical" is meant that when the sea is calm and the installation is at rest, the connecting hoses to the FPSO not being installed, the tendon and the risers are arranged substantially vertically, it being understood that the swell, and the movements of the floating support and / or flexible pipes can cause the tower to deflect in an angle at the top preferably limited to 10-15 °, in particular due to the implementation of a junction piece and transition of inertia, or of a flexible articulation of the Roto-Latch® type at the foot of the tendon, at its point of attachment to said base or anchor.

The term vertical tower or riser is used here to account for the theoretical substantially vertical position of said risers when they are at rest, it being understood that the axes of the risers can experience angular movements with respect to the vertical and move in a cone of 'angle γ, the apex of which corresponds to the point of attachment of the lower end of the tendon to said base. The upper end of a said vertical riser may be slightly curved. The term “end portion of the first flexible pipe is therefore understood to mean substantially in alignment with the axis ZiZi of said riser that the end of the inverted chain curve of said first flexible pipe is substantially tangent to the end of said vertical riser. In any case, in continuity of variation of curvature, that is to say without singular point, in the mathematical sense.

By continuity of curvature between the upper end of the vertical riser and the part of the first flexible pipe having positive buoyancy is meant that said variation in curvature does not have a singular point, such as a sudden variation in the angle of inclination. of its tangent or an inflection point.

Preferably, the slope of the curve formed by the first flexible pipe is such that the inclination of its tangent with respect to the axis ZiZi of the upper part of said vertical riser increases continuously and progressively from the point of connection between the end top of the vertical riser and the end of said end part of the first flexible pipe of positive buoyancy, up to the point of inflection corresponding to an inversion of curvature between said convex end part and the first concave part of the first flexible pipe.

The installation according to the present invention therefore makes it possible to avoid the tensioning of the vertical riser by a surface or sub-surface float, from which its upper end would be suspended. This type of installation confers increased stability in terms of angular variation (γ) of the excursion angle of the upper end of the vertical riser with respect to a theoretical vertical rest position, because this angular variation is reduced in practice. at a maximum angle not exceeding 5 °, in practice of the order of 1 to 4 ° with the installation according to the invention, while, in the embodiments of the prior art, the angular excursion could reach 5 at 10 ° or more.

Another advantage of the present invention is that, due to this small angular variation of the upper end of the vertical riser, it is possible to implement, at its lower end, a rigid embedding on a second or nlème base resting at the bottom of the sea, without resorting to a transition piece of inertia of too large size and therefore too expensive. It is therefore possible to avoid the use of a flexible articulation, in particular of the flexible spherical ball joint type, provided that the junction between the lower end of the second or nlth riser and said recess comprises an inertia transition piece. .

Likewise, and in a known manner, it is understood that said upper supporting structure ensures the maintenance at constant geometry of the upper ends of said risers and of said vertical tendon ensuring the attachment between them at a constant distance.

In a known manner, said reel comprises a cavity within a structure offset in front of the floating support or integrated in or below the hull of the floating support, preferably said cavity passing through the hull of the floating support over its entire height. .

In a still known manner, said vertical tendon consists of a cable or a rigid bar, in particular metallic, or else of a pipe.

In a known manner, said end portion of the first flexible pipe extends over only a part of the total length of the first flexible pipe such that said first flexible pipe has an S-shaped configuration, with a first portion of the first flexible pipe. on the side of said floating support having a concave curvature in the form of a plunging chain and said remaining end part of said first flexible pipe having a convex curvature in the form of an inverted chain by virtue of its positive buoyancy. Here by "concave curvature" of said first portion of the first flexible pipe is meant a curvature with concavity facing upwards, and by "convex curvature" of said end portion of said first flexible pipe, a curvature with convexity facing upwards or concavity facing upwards. down.

It will be understood that the first flexible pipes positioned at different heights means that two points of respectively a first upper flexible pipe and a second lower flexible pipe, located in the same vertical direction, are always situated one above the other. , although a point of the first upper flexible pipe may be at a lower height than a point of the first lower flexible pipe, if the two points of the first two upper and lower flexible pipes are not vertically aligned.

It is also understood that the two said first flexible pipes are necessarily slightly offset laterally since their ends are connected on the one hand to the upper ends of said rlsers, which are laterally offset at said upper supporting structure on the one hand, and their points d The attachment to the reel are also slightly offset laterally at the level of the reel. In general, the height offset is greater than the lateral offset between the first two flexible pipes.

In practice and depending on the diameters of the flexible pipes with positive buoyancy, the minimum height offset of the upper ends of said risers to which the first flexible pipes are fixed and therefore the minimum distance in height between two said first flexible pipes arranged at different heights is d at least 3 m, preferably 5 to 10 m.

More particularly, a said tower comprises from 2 to 7 rigid pipes and 2 to 5 said first flexible pipes.

In known manner, said reel comprises a cylindrical internal part capable of remaining substantially fixed with respect to the seabed inside said cavity when said floating support is driven in rotation around the vertical axis (ZZ) of said part internal or said cavity of the drum, said floating support being anchored to the seabed by lines fixed at their upper ends to said cylindrical internal part of the drum.

In a known manner, the lower ends of the risers are fixed to the ends of the submarine pipes resting on the seabed, preferably by means of automatic connectors between the lower ends of the risers and the ends of the submarine pipes, and / or by means of elbow sleeves and / or elbow junction pipes.

More particularly, an installation according to the invention comprises second flexible pipes of smaller diameters or of lower linear weight than the first flexible pipes, the second flexible pipes not including buoyancy elements and being connected to the upper ends of said riser by means of connection devices, preferably of the swan coi type, lesdltes second flexible conduits being located below lesdltes first flexible conduits.

Advantageously, buoyancy elements can be secured to the connection part and / or on the underside of said upper supporting structure to compensate for the weight of said second flexible pipes and of the various accessories such as goosenecks, structural reinforcement elements as well. than automatic connectors.

An installation according to the invention can also include other “flexible underwater pipe” such as a cable, an umbilical or a pipe capable of accepting significant deformations without generating significant return forces, in particular a flexible pipe. In particular, a control umbilical will include one or more hydraulic lines and / or electric cables for the transmission of energy and / or information.

More particularly, said tendon is fixed at its lower end to a base or anchor by means of a junction and inertia transition piece, the variation of the inertia of which is such that its inertia gradually increases from its upper end up to 'at the lower end of said junction piece performing the embedding of the lower end of said tendon at said base or anchor.

Inertia is understood here to mean the moment of inertia of said junction part and transition of inertia with respect to an axis perpendicular to the axis of said junction part and inertia transition, which reflects the stiffness in bending in each planes perpendicular to the vertical axis of symmetry of said junction and transition piece of inertia, this moment of inertia being proportional to the product of the section of material by the square of its distance from said axis of said piece of junction and transition of inertia.

In known manner, said junction and inertia transition piece has a cyllndro-conlque shape, and said junction piece is fixed at its base to a first tubular pile passing through a cylindrical cavity of said base or anchor so as to allow the embedding of said junction part in said base or anchor.

More particularly, an Installation according to the invention comprises third floats integral with said tendon at least in the spaces between lesdlts guide modules, lesdlts third floats providing positive buoyancy compensating at least for the weight of said tendon.

More particularly, lesdlts guide modules constitute a plurality of independent rigid structures spaced at least 5 m apart along at least the upper part of said tendon, each said rigid structure comprising a plurality of tubular rlser guide elements defining tubular orifices in which said rlsers, equipped with said second floats, can slide and a central tendon connecting element preferably defining a central orifice through which said tendon passes and which is secured to it in particular by welding.

More particularly still, said guide modules and lesdlts second floats extend over at least 50% of the length of the tower between said supporting structure at the top and the lower end of the tendon.

More particularly, the guide modules are spaced 2 to 20 m apart, preferably 5 to 15 m, and are at least 20, preferably at least 50 guide modules for a tower of at least 1000 m high.

More particularly, all of said first floats provide cumulative buoyancy representing a tensile force directed upwards, of greater intensity than the total weight of said rlsers, preferably than the total weight of the tower, preferably from 102 to 115%, preferably from 103 to 106%, of the total weight of said rlsers, more preferably of the total weight of the tower.

Thus, the resulting vertical upward tension at said upper supporting structure being 2 to 15% of the total weight of the tower, preferably 3% to 6% of the total weight of the tower.

Thus, said multi-riser tower is tensioned by said floats and said support is anchored so that the angle γ between the axis (ZiZi) of said tendon and the vertical remains less than 10 °, when the floating support is moved. from the agitation of the sea and / or the force of the wind despite its anchorage.

Preferably, said second coaxial floats are distributed continuously, over the entire length of said risers below and from said upper supporting structure, and said guide modules are distributed over the entire length of said tendon below and from said said tendon. upper load-bearing structure.

The positive buoyancies of the riser, of the first flexible pipes and of the tendon can be provided in a known manner by coaxial peripheral floats surrounding said pipes, or, preferably, in the case of the rigid pipe or vertical riser, by a coating of material. of positive buoyancy, preferably also constituting an insulating material, such as syntactlque foam, in the form of a sleeve shell enveloping said pipe. Such buoyancy elements resistant to very high pressures, that is to say to pressures of about 10 MPa per section of 1000 m of water, are known to those skilled in the art and are available. with BALMORAL (UK).

Advantageously, the buoyancy and insulation material will consist of a gum of microspheres of lower compressibility than that of sea water, as described in the applicant's patent application FR 11 52574 and described below.

Also preferably, said first, second and third floats are in the form of tubular sleeves, preferably in the form of two half-shells forming a tubular sleeve, made of a material resistant to submarine hydrostatic pressure, and at least said second floats and preferably said first and second floats are made of a material which also has thermal insulation properties.

More specifically, a thermal insulation and rigid buoyancy material consists of a mixture of:

- (a) a matrix of a homogeneous mixture of crosslinked elastomeric polymer and a liquid insulating plasticizer compound, said insulating plasticizer compound being chosen from compounds derived from mineral oils, preferably from hydrocarbons, and from compounds derived from of vegetable oils, preferably esters of vegetable oils, said insulating plasticizer compound not being a phase change type material at a temperature of -10 ° to + 150 ° C, the mass proportion of said plasticizer compound insulation in said matrix being at least 50%, preferably at least 60%, and

- (b) hollow beads, preferably glass microbubbles, dispersed within a matrix of said homogeneous mixture of said polymer and said insulating plasticizer compound, in a volume proportion of at least 35% of the total volume of the mixture of said beads with said matrix, preferably 40 to 65% of the total volume.

Such a material exhibits properties of thermal insulation, of buoyancy and of increased resistance to cracking as well as a lower cost compared to a syntactic foam material made up of the same constituents but without a plasticizing compound as will be explained below.

Hollow microbeads are added to an isolating gel of the type of WO 02/34809. This mixture of an insulating gel and hollow microbeads has an advantage in that its buoyancy does not decrease, or even increases with depth, whereas, on the contrary, the buoyancy of a syntactic foam material (similar material but without plasticizer compound) decreases very significantly with water depth. This increased buoyancy as a function of depth results from the fact that the compressibility modulus of said rigid insulating material according to the invention is greater than the compressibility modulus of water, namely greater than 2200 MPa, the compressibility modulus of water. water being around 2000 MPa. In other words, the increase in buoyancy of said material results from the fact that the density of water increases more than that of said material depending on the depth at which the material is found.

Consequently, a rigid insulating material according to the invention or GBG, hereinafter abbreviated as GBG (Glass Bubble Gum), that is to say gum of glass beads, is much more efficient in terms of high buoyancy. depth, in particular for depths of 1000 to 3500 m and beyond, in comparison with a syntactic foam of the prior art (similar material without plasticizing compound) whose compressibility modulus does not exceed 1600 MPa.

In addition, in this material the breaking of the microspheres occurs for a compression value, therefore a water depth of 15 to 30% greater than that of traditional syntactic foam.

Altogether therefore, the material according to the present invention provides better mechanical properties of resistance to cracking and increased buoyancy at great depth as well as a lower cost than a comparable syntactic foam material (similar components without plasticizing compound).

By thermal insulator is meant here a material whose thermal conductivity properties are less than 0.25 W / m / K and by positive buoyancy a density less than 1 with respect to sea water.

By rigid material is meant here a material which holds its shape by itself and does not substantially deform due to its own weight when it is preformed by molding or confined in a flexible envelope, and whose Young's modulus λ is greater than 200 MPa, unlike a gel which remains extremely flexible and whose Young's modulus is almost zero.

The term “mineral oil” is understood here to mean a hydrocarbon-based oil derived from fossil material, in particular by distillation of petroleum, coal, and certain bituminous shales and vegetable oil, an oil obtained from plants by extraction, in particular in the case of oils rapeseed, sunflower or soybean, and more particularly by treatment in the case of esters of these vegetable oils.

In a known manner, the hollow balls are filled with a gas and resist the hydrostatic external pressure underwater. They have a diameter of 10 µm to 10 mm, and, for microbeads, of 10 to 150 µm, preferably 20 to 50 µm and have a thickness of 1 to 2 microns, preferably about 1.5 µm. Glass microspheres are available from the company 3M (France).

More particularly, to produce an insulating buoyancy material resistant to 2,500 m, or approximately 25 MPa, advantageously used is a selection of microspheres whose Gaussian distribution is centered on 20 μm, while for a depth of 1,250 m, a distribution Gaussian centered around 40 pm is suitable.

The phase stability of the plasticizer compound according to the invention at temperature values of -10 ° to + 150 ° C makes it compatible with the temperature values of sea water and of petroleum production fluids at deep sea levels.

A rigid insulating material of this type, although relatively rigid within the meaning of the present invention, has a mechanical behavior in terms of compressibility which is similar to an elastomeric rubber due to the low value of its Young's modulus, whereas a syntactic foam behaves like a solid. The rigidity within the meaning of the present invention of the insulating material results essentially from the high mass content of said microbeads, said microbeads also providing a gain in buoyancy and thermal insulation compared to an insulating gel of the same composition.

More particularly, the rigid insulating buoyancy material has a density of less than 0.7, preferably less than 0.6, and a thermal conductivity of said material of less than 0.15 W / m / K, preferably less than 0.13 W / m / K, and a Young's modulus or triaxial compressive modulus of said material of 100 to 1000 MPa, preferably 200 to 500 MPa and a compressibility modulus of said rigid insulating material greater than 2000 MPa, preferably greater at 2,200 MPa, that is to say a compressibility modulus greater than that of water.

More particularly, said plasticizing compound has a compressibility modulus greater than that of said polymer, preferably greater than 2000 MPa, a thermal conductivity, as well as a density, lower than that of said polymer, preferably a thermal conductivity less than 0, 12 W / m / K and a density of less than 0.85, more preferably 0.60 to 0.82.

More particularly, an insulating material of this GBG type has the following characteristics:

- the mass ratio of said crosslinked polymer and of said insulating plasticizer compound is 15/85 to 40/60, preferably 20/80 to 30/70, and

the volume ratio of said microbeads relative to the volume of said matrix of crosslinked polymer and of said insulating compound is from 35/65 to 65/35, preferably from 40/60 to 60/40, more preferably from 45/55 to 57/43.

Beyond 85% of plasticizer compound in the matrix, the latter risks exuding outside it.

Also advantageously, said polymer has a glass transition temperature of less than -10 ° C., its phase stability thus being compatible with the temperature values of sea water and of petroleum production fluids at deep sea levels.

More particularly, these compressibility properties and thermal insulation properties and comparative density of said plasticizer compound and said polymer are observed when, in accordance with a preferred embodiment, said crosslinked polymer is of polyurethane type and said liquid plasticizer compound is a product. oil tanker, said to be light cut fuel type.

More particularly still, said plasticizer compound is chosen from kerosene, gas oil, gasoline and white spirit.

These fuels, with the exception of gasolines, also have the advantage of having a flash point above 90 ° C, thus eliminating any risk of fire or explosion in the manufacturing process.

Kerosene has a thermal conductivity of about 0.11 W / m / K.

In another embodiment, a plasticizing compound derived from vegetable oil of the biofuel type is used, preferably an oil ester of vegetable origin, in particular an alcoholic ester of vegetable oil, rapeseed, sunflower or soy.

More particularly, said polymer is a polyurethane resulting from the crosslinking of polyol and poly isocyanate, said polyol preferably being of the branched type, more preferably at least in a 3-branched star, and the polyisocyanate being an isocyanate prepolymer and / or polyisocyanate polymer.

More particularly still, said polyurethane polymer results from crosslinking by polyaddition of hydroxylated polydiene, preferably hydroxylated polybutadiene, and of aromatic polyisocyanate, preferably 4,4'-diphenyl-methane diisocyanate (MDI) or a polymeric MDI.

Preferably, the NCO / OH molar ratio of the two polyol and polyiscocyanate components is from 0.5 to 2, preferably greater than 1, more preferably from 1 to 1.2. An excess of NCO ensures that all of the OH has reacted and that the crosslinking is complete or, at the very least, optimal.

Advantageously, said rigid material is confined in a protective envelope.

The outer casing can be made of metal, such as iron, steel, copper, aluminum and metal alloys, but also can also be of polymeric synthetic material, such as polypropylene, polyethylene, PVC, polyamides , polyurethanes or any other polymer convertible into tubes, plates or envelopes, or else obtained by rotational molding of thermoplastic powders, or else into composite materials. The option of envelopes in polymeric materials mentioned above is an option which is all the more practical and effective as the solution of the invention, allowing the obtaining of an insulating material of rigid buoyancy according to the invention, makes possible 'use of envelope materials which are less rigid, lighter and less difficult to implement and, consequently, less expensive overall. The outer envelope may preferably be a more or less rigid thick layer, from a few millimeters to several centimeters in thickness, but may also be in the form of a flexible or semi-rigid film.

More particularly, said insulating material of rigid buoyancy is in the form of a pre-molded part, preferably suitable for being applied around an underwater pipe or an underwater pipe element to provide thermal insulation and / or protection. buoyancy and resistant to hydrostatic pressure underwater, preferably at a great depth of at least 1000 m.

More particularly, said positive buoyancy of said first floats of said first flexible pipes is regularly and uniformly distributed over the entire length of said end portion of said first flexible pipe and the buoyancy of said second floats distributed over at least said upper portion of rigid pipes , preferably over the entire length of said rigid pipes, provides a resulting vertical thrust of 50 to 150 Kg / meter over the entire length of said rigid pipes, and / or said first floats of the first flexible pipes confer positive buoyancy over a length corresponding to 30 to 60% of the total length of said first flexible pipes, preferably about half of its total length.

Also preferably, said tower comprises a cylindrical outer casing with circular horizontal section, made of plastic or composite material forming a rigid hydrodynamic protective screen surrounding (all of said rigid pipes at least in an upper part of the tower. This screen contributes in in addition to the thermal insulation of said rigid pipes.

More particularly, said outer envelope can be made of metal, such as iron, steel, copper, aluminum and metal alloys, but also can also be made of polymeric synthetic material, such as polypropylene, polyethylene, PVC, polyamides, polyurethanes.

More preferably, an installation according to the invention comprises a plurality of said multl-riser hybrid towers, preferably at least 5 towers, the flexible pipes of which are connected or capable of being connected to the same reel but extend in different directions. (YY ') angularly offset, so that said towers are arranged in a fan pattern around said reel at identical or different distances from said reel, some of said towers being able to be only partially mounted not yet comprising flexible pipes and / or only one part of said rigid conduits which can be extended by said flexible conduits at their upper ends and / or at least part of said rigid conduits not being connected to said underwater conduits resting on the seabed at their lower ends.

It is understood that said directions offset in (YY ') offset angularly are the horizontal directions between the vertical axis of the drum and the vertical axis of the tendon.

Said rigid pipes are thus awaiting subsequent connection to wellheads as well as to the floating support.

The vertical tendon can also be connected at its lower end to the base or anchor by a flexible articulation of the laminated stop type marketed by the company TECHLAM France or of the roto-iatch® type, available from OILSTATES USA, known to man. art.

This embodiment comprising a multiplicity of risers maintained by a central structure comprising guide means is advantageous when it is possible to pre-fabricate the entire tower on land, before towing it at sea, then once on site. , the caban for its final installation as explained below.

The present invention also provides a method for towing a so-called multi-rlser tower at sea and setting up an installation according to the invention comprising the following successive steps in which:

1) a said tower is prefabricated on land, connected at the head to said flexible pipes with positive buoyancy, the free end of which is connected to a fourth float, and

2) said tower is towed at sea in a horizontal position by a laying vessel, said tower floating on the surface thanks to its said second floats, and

3) a dead body is installed at the lower end of said tower and,

4) said tower is housed, the lower end of which is connected to the level of said base, and said fourth float connected to the free end of said flexible pipes with positive buoyancy being submerged in the subsurface and offset laterally with respect to the ZiZi axis of said tower so that said flexible pipes with positive buoyancy adopt a said S-shaped position, and

5) subsequently, the flexible pipe ends with positive buoyancy are disconnected in order to connect them to said floating support at said drum, and

6) simultaneously or subsequently, the connection of the lower ends of the risers with the ends of the pipes resting on the seabed is made.

According to another more particular aspect, the present invention relates to a method of operating an oil field using at least one installation according to the invention in which fluids are transferred between underwater pipes resting at the bottom. seabed and a floating medium, fluids comprising petroleum, preferably a plurality of said hybrid towers, in particular from 3 to 20 said towers connected to the same floating medium.

In a known manner, to connect the different pipes to one another, connecting elements are used, in particular of the automatic connector type, comprising the locking between a male part and a complementary female part, this locking being designed to be done very simply at the bottom of the sea using an ROV, a robot controlled from the surface, without requiring direct manual intervention by personnel.

Other characteristics and advantages of the present invention will become apparent in the light of the detailed description which follows, with reference to the following figures in which:

- Figure 1 is a side view of a hybrid tower type fund-surface connection installation 2 according to the invention, between the seabed 5 and a floating support 1 of the FPSO type anchored on the reel 1a, the foot of the multi-riser tower 3 being articulated 6a with respect to the foundation 5a,

- Figure 2 is a variant of Figure 1 in which the foot of the tower is embedded in the foundation 5a using a junction and inertia transition part 6b,

- Figure 3 is a cutaway side view of the substantially vertical portion of the tower consisting of rigid pipes or risers 10 and the tendon 6, detailing the various components constituting it, namely the upper ends lOa-lOb of the risers at- above the upper supporting structure 3a, the guide modules 20 and the second floats 11 of the risers 10 and third floats 21 of the tendon 6,

FIG. 3A is a cross section of one of the rigid pipes 10 detailing the assembly of the half-shells 11a providing insulation as well as buoyancy in the form of a sleeve 11,

Figure 3B is a cross section along the plane AA of Figure 3 detailing the positioning of four rigid pipes or risers 10 Insulated 11 installed around a central tendon 6 ensuring the connection with the foundation Sa,

Figure 3C is a cross section similar to Figure 3A, in which a small diameter pipe 10-1 for gas injection is positioned in contact with the main rigid pipe 10, all along the latter, the two half-shells lla-llb forming a common insulating sleeve for all of the two pipes 10 and 10-1,

FIG. 3D is a side view of a guide module with a guide element 20a in vertical section showing the second float 11 able to slide inside the orifice formed by the guide element 20a,

FIG. 4A is a horizontal sectional view of a multirlsers tower at the level of a guide module 20, hereinafter also referred to as “diaphragm”, playing the role of centralizing element and of guiding element of rigid pipes. Isolated 10,

Figs 4B and 4C are perspective views of a portion of a muiti-riser tower without an external envelope (FIG. 4B) and with an external envelope 22 (FIG. 4C),

Figure 5A is a side view detailing the on-site towing, cabanage and installation of a tower with flexible pipes,

FIG. 5B is a side view of a bottom-surface connection according to the invention, partly preinstalled on site before the installation of an FPSO, the flexible pipes 4 being held in the sub-surface by means of floats 7a and of cables 7b connected to dead bodies 7c,

- Figure 6 is a plan view of an FPSO anchored on a reel and connected to 4 towers 2, 2-1 to 2-4, a fifth tower 2-5 having been pre-installed but not connected to the FPSO by flexible pipes

4.

In Figure 1, there is shown in side view a floating support 1 of the FPSO type, anchored to a reel 1a by lines of anchors lb, said reel being located beyond the bow of the FPSO and connected to a link hybrid tower type fundurface 2 comprising 4 flexible pipes 4, 4a-4b and a multi-riser tower 3. Said flexible pipes 4 are connected to the top of the tower 3, each flexible pipe 4 being respectively connected to each of the rigid pipes 10 of said multirisers tower 3, as will be explained in more detail before in the description of the invention below.

Two first flexible pipes 4a, 4al-4a2 have on a part 4-3 of their length floats 4-5 which give it a positive buoyancy, thus ensuring a continuity of variation of curvature, directed downwards or bottom 5, up to at its connection with the upper end 10a of a substantially rectilinear rigid pipe 10 of the tower, the radius of curvature of which is therefore substantially infinite, ie its curvature is substantially zero. The first portion 4-4 of the first flexible pipe 4a between the drum and the part 4-3 is devoid of floats and therefore has an apparent weight in the water and its overall curvature has a concavity directed upwards in the form of a plunging chain. . The first portion 4-4 and terminal portion 4-3 of first flexible pipes 4a are separated by an inflection point 4-6, that is to say a change in the curvature of the pipe 4a, the terminal part 4 -3 positive buoyancy having a curved shape with convexity directed towards the surface. The whole of the first flexible pipe therefore has an S-shaped configuration.

Two second flexible pipes 4b, 4bl-4b2 of smaller diameter are each connected to a connection device of the gooseneck type 4c, the latter being connected to the upper end of a rigid pipe 10. corresponding tower

3. The curvature of the second flexible pipes 4b is with a concavity directed upwards in a plunging chain from its point of connection 4-1 with the reel to its point of connection 4-2 with the gooseneck 4c.

The horizontal forces generated by the flexible pipes in the chain configuration apply to the top of the tower 3 and cause it to tilt at an angle Y * with respect to the vertical.

In Figure 1, the bottom of the tower 3 is connected to a suction anchor-type foundation 5a driven into the seabed 5, by means of a flexible articulation 6a integral with the lower end of the tendon 6 located at the ZiZi axis of the tower 3 and taking up all of the vertical upward forces created by the various buoyancy elements 11 and 21 integrated into the tower as will be explained further in the detailed description of the invention below.

In FIG. 2, the lower end of the axial tendon 6 of the tower 3 is connected to the foundation 5a by means of a junction piece with variable inertia 6b, with increasing inertia towards said foundation, this piece of junction 6b being integral with a rod 6c driven into said foundation 5a. This results in an embedding of the axial tendon 6 of the tower 3 in its foundation 5a, thereby avoiding having to implement an extremely expensive flexible joint 6a as has been described with reference to FIG. 1. In fact , in the case of towers for very great depths, that is to say 2000-2500 m or even more, and comprising a large number of rigid pipes 10, the vertical forces that such junction pieces 6b or articulation must withstand flexible 6a without any mechanical failure during the lifespan of the installations, that is to say 20-25 years or more, are considerable and can reach and exceed 800 to 1000 tons or even more. Thus, the variable inertia junction part 6b is much more reliable because there is no

J has only one component and therefore no relative movement between several components as is the case for a flexible mechanical joint 6a. In addition, the latter remains very delicate and much more expensive to manufacture in order to achieve the same level of reliability. Such a variable inertia junction part 6b is detailed in the patents WO 2009/138609 and WO 2009/138610 of the applicant.

In Figures 1 and 2, said tendon 6 and said upper supporting structure 3a are not suspended from a submerged float in the subsurface. Thus said tendon 6 can be located at a distance from the vertical axis of the reel (ZZ) less than the distance between said axis of the reel and the end furthest from said floating support, that is to say at the inside the vessel's avoidance zone without risk to the vessel.

In Figures 1 and 2, a junction pipe 13 with multiple curvatures provides the connection by connectors 8 and 9 between the lower angled end 10c of the pipe 10 and a pipe 12 resting on the seabed joining the wellheads , known to those skilled in the art.

In Figure 3 is illustrated in side view and partially cut away the constitution of the tower 3 proper. It consists of an upper supporting structure forming an upper platform 3a to which are fixed a plurality of rigid pipes 10 extending over the entire height of said tower, each of the upper ends of said pipes comprises a connection flange 10a extending. above the supporting structure 3a so as to be able to be connected respectively to a flange at the end 4-2 of the first flexible pipe 4a, 4al-4a2 corresponding. In order to avoid interference between two first flexible pipes 4a1-4a2 adjacent in the area of connection with the tower and over their entire length, each of the flanges 10a, from left to right, is shifted upwards respectively by increasing values hl -h2-h3 with respect to the platform 3a as illustrated in this figure 3. Advantageously, the values of hl-h2-h3 depend on the type and on the number of first flexible pipes and are such that the values h3-h2 and h2-hl are between 2 m and 10 m, preferably 3 to 6 m.

As illustrated in FIG. 3A, each of the rigid pipes 10 is surrounded by tubular sleeves 11, preferably made of half-cylindrical half-shells 11 assembled together, so as to constitute not only an insulation of the pipe, but also an insulation. buoyancy that compensates for the dead weight of the running line. These sleeves 11 are installed continuously from the top of the rigid pipe, at the level of the upper flange 10a to the foot of the tower 3, at the level of the termination of the pipe 10 equipped with the male part 10 of a automatic connector 8a. The lower bent portion 10c as well as the upper portion 10b between the upper platform 3a and the flange 10a of the rigid pipe 10 are also equipped with insulation and buoyancy sleeves, not shown, similar to the sleeves 11 described above.

Each of the sleeves 11 is mechanically fixed on its rigid pipe 10 in a rigid manner, by means not shown, so that said sleeve does not slide axially on said pipe 10. Thus, if the buoyancy of the sleeve corresponds exactly to the weight in the water of the pipe portion 10 which it covers, then each meter of pipe fitted with its sleeve has zero weight in the water.

Advantageously, the linear buoyancy of all the sleeves 11 corresponds to 102 to 115%, preferably from 103 to 106%, of the dead weight of the entire pipe 10 Immersed in water and filled with water. Thus, the own weight of the pipe 10 filled with water is compensated along the said pipe 10 and a residual buoyancy then applies on the subface of the upper platform 3a corresponding to 2 to 15% respectively, preferably 3 to 6. % of the own weight of the water-filled pipe in the water. This buoyancy is transmitted to the upper platform 3a by means of the pipe 10, integral with said platform 3a. As a result, said pipe 10 is in a state of compression in the upper part close to said upper platform 3a. When the pipe 10 is filled with hydrocarbon, generally with a density of 0.8 to 0.9, the force transmitted to the upper platform 3a increases accordingly and the pipe portion 10 under compressive stress also increases. In addition, the compressive stress in the area close to said upper platform 3a also increases in the same proportions. Likewise, in the event of an arrival from the wells of a large pocket of gas, the interior of the vertical pipe 10 may be found entirely filled with gas, therefore empty of hydrocarbon. The pipe 10 is then completely light and the upper portion of the pipe 10 under compressive stress is then maximum, and the compressive stress in the zone close to said upper platform 10 3a is also maximum. Thus, a portion of 15 to 40% of the length of the vertical rigid pipe 10 may find itself, when it is filled with gas, under axial compressive stress, which creates a significant risk of lateral buckling (“lateral buckllng”). in English). To avoid this dreaded phenomenon, we install at regular intervals of the guide modules 20 consisting of a rigid structure comprising a central element 20b integral with the central tendon 6 and a plurality of guide elements 20a guiding and maintaining the vertical pipes 10 of the tower 3 at a constant distance from the central tendon 6, and therefore substantially in a straight line. The guide elements 20a are distributed on a plane substantially perpendicular to the axis ZiZi of the tower

3, and are arranged all around said central tendon 6, preferably at a constant distance from said central tendon and connected to element 20b by arms or structural elements 20c, preferably made of steel, the assembly thus constituting a diaphragm for guiding the pipes 10 isolated by the sleeves 11. Said guide element 20a forms a tubular orifice, preferably of circular section, the internal diameter of which is slightly greater than the external diameter of the buoyancy sleeves 11 of the corresponding rigid pipe 10. In this way, the pipe 10, isolated by the sleeves 11, is free to slide freely over the entire height below the upper platform 3a, under the effects of temperature, pressure or the reduction in length due to the pressure. compression (full pipe - empty pipe). All these variations in the length of the conduits 10 then have repercussions at the low level of the tower and generate movements which are absorbed by the conduits.

- ".

junction 13 with multiple curvature. Thus, due to the fact that each of the rigid pipes 10 is suspended from the upper platform 3a, it can extend or retract individually without modifying the behavior of the neighboring rigid pipes 10.

These guide modules or diaphragms 20 are arranged over the entire height of the tower 3, preferably at constant H intervals, but can advantageously be arranged closer to each other in the upper part so as to avoid the phenomena of buckling previously described. Thus, on a tower 1,600 m high, the guide modules 20 will advantageously be spaced from 5 to 7.5 m over a height of 150 m from the upper platform 3a, then 10 m over the following 300 m, and finally from 15 m over the rest of the height, up to the foot of the said tower.

The central tendon 6 is itself provided with buoyancy elements or third floats 21 over its entire height. In Figure 3, there is shown, for a better understanding of the figure, a single floating element 21 between two guide modules 20. The buoyancy of each of the elements 21 is adjusted to compensate for the weight in water of the tendon 6. itself, as well as the self-weight proportion of the corresponding guide module. Thus, the buoyancy element 21 as drawn in FIG. 3 compensates for the weight in water of the height H of tendon 6 as well as the dead weight in water of a complete guide module 20.

The second flexible pipes 4b are lighter than the first pipes 4a and their weight can be taken up by the upper platform 3a. The same applies to goosenecks 4c and various structural elements, not shown. However, buoyancy elements, not shown, can compensate for the own weight of all of the second pipes 4b of said flexible pipes, of their respective gooseneck type devices as well as the own weight of said upper platform 3a, the whole which can represent several tens of tonnes in total.

Advantageously, the second flexible pipes 4b will be of smaller diameter and of lower weight in water than the first pipes 4a, so as not to unnecessarily increase the additional buoyancy required at the level of the upper platform 3a. In addition, the first heavier or larger diameter flexible pipes 4a have their own buoyancy 4-5 over a part 4-3 of their length, as explained previously.

Thus, the vertical tension exerted on the foundation 5a corresponds substantially to the resultant of the forces directed upwards at the level of the upper platform 3a, therefore to the sum of all the vertical forces directed upwards of each of the rigid pipes 10, whether they are full of water, crude oil, or gas, as previously described. ·

When all of the pipes 10 are in production, i.e. 15 normally full, the tension on the foundation is minimal, but as soon as some are accidentally filled with gas under pressure or at atmospheric pressure, this tension increases significantly. significant. In the unlikely event that all of the production pipes 10 would be in gas, the tension exerted on the foundation 5a is then doubled, 20 or even quadrupled, thus passing for example from 100-150 tonnes in normal operation to 400-800 tonnes or even more in extreme configuration, the basis on which regulations and oil operators require that installations be sized. It thus appears that a transition piece with variable inertia 6b only requires an additional material, generally steel or titanium, and its complexity is not really changed. On the other hand, a mechanical articulation 6a which is very delicate and expensive to manufacture sees its cost increase considerably because it must be over-dimensioned for extreme forces which in fact will never occur, but which, from a safety standpoint, are considered to be ahead. be the maximum value of the forces to be taken into account, apart from the usual safety coefficients.

FIG. 4A is a sectional view from below along the plane AA of FIG. 3 of a guide module 20, detailing:

- the position and the connection at 20d, for example by welding, of the guide elements 20a around the central element 20b of the module 20, and

- the position of the insulation elements 11 of the pipes 10 inside the tubular orifices of the guide elements 20a,

- The solldarlsatlon in 20c, for example by welding, of said central element 20b of the module 20 to the central tendon 6.

Five pipes 10 are thus shown, including three simple pipes according to FIG. 3A and two "plggyback" pipes according to FIG. 3C, a small pipe 10-1 being a gas injection pipe in the corresponding large pipe 10, the injection mode, known to those skilled in the art, being carried out at the bottom of the tower and used to accelerate the rate of rise of the crude oil towards the FPSO.

FIG. 4B is a perspective view of a tower 3, the cross section of which corresponds to FIG. 4A, illustrating three guide modules 20 as well as five pipes 10 equipped with their insulation and buoyancy elements or first floats 11.

In FIG. 4C, an outer envelope 22 of circular section made of composite or plastic material, preferably of polyethylene or polypropylene, constitutes a rigid hydrodynamic protective screen making it possible, on the one hand, to reduce the forces generated on tower 3 by the current. , and where appropriate by the swell in the upper portion of said tower. These screens 22 are advantageously manufactured in two demlcoqullles and have a length corresponding substantially to the distance H between two said guide modules 20. They are then assembled directly between two modules 20 and mechanically fixed to the latter. In addition, these screens 22 confine the interior volume 23 between two said guide modules 20 and said external envelope 22, thus limiting heat transfers with the environment 24 and reducing heat losses at the level of the insulation sleeves * ”, rigid pipes 10. By thus confining the interior 23 with respect to the exterior 24, the temperature ti inside 23 will always be higher than the temperature to outside 24. This results in a lower temperature differential between the pipes 10 and the interior 23, and therefore substantially reduced heat losses.

FIG. 6 is a top view of an FPSO 1 anchored to a reel 1a and connected to four hybrid towers 3-1, 3-2, 3-3, 3-4, by a plurality of flexible pipes 4a. A fifth multl-rlsers 3-5 tower has been pre-installed, but will not be connected until later when the oilfield is extended. On multl-rlsers 3-1, 3-2 towers, the four rigid pipes 10 are connected on the one hand to FPSO 1 by four first flexible pipes 4a, and on the other hand, at the bottom of the tower, to four rigid pipes 12 resting on the seabed. On tower 3-3, only two rigid conduits 10 are connected to the FPSO by two flexible conduits 4a and to rigid conduits 12 resting on the bottom, two conduits 10 awaiting connection to wellheads, as well as the FPSO. Likewise, the tower 3-4 only has three rigid pipes 10 connected to the FPSO by 3 flexible pipes 4a as well as to rigid pipes 12 resting on the bottom.

Such a fan-shaped installation makes it possible to install at least part of the multl-rlser towers 3 in the avoidance zone of the floating support 1 in order to increase the number of bottom surface connections of hybrid tower type 2 and to reduce the length. flexible pipes 4.

In Figures 1 to 6, the bottom-surface connection installation between a plurality of underwater pipes (12) resting on the seabed (5) and a floating support (1) on the surface (the) and anchored ( lb) at the bottom of the sea, includes:

a said floating support comprising a reel (la) comprising a cavity within a structure offset at the front of the floating support or integrated in or below the hull of the floating support, preferably said cavity passing through the hull of the floating support on all its height, and

- at least one tower of the hybrid type (2), in particular from 3 to 20, comprising:

a) a multi-riser tower (3) comprising:

al) a vertical tendon (6) integral at its upper end with an upper supporting structure (3a), said tendon being fixed at its lower end to a base resting on the seabed or an anchor, preferably of the anchor type. suction (5a), sunk to the bottom of the sea, said tendon (6) and said upper supporting structure (3a) not being suspended from a float submerged in the subsurface, and said tendon being located at a distance from the vertical axis of the reel (ZZ) less than the distance between said axis of the reel and the end furthest from said floating support, and

a.2) a plurality of vertical rigid pipes (10) called risers, in particular from 2 to 8 rigid pipes, the upper end (10a) of each riser extending above said supporting structure (3a), integral with the latter, the lower end (10b) of each said riser being connected or able to be connected to an underwater pipe (12) resting on the seabed, and said risers being equipped with coaxial second floats (11) peripherals surrounding said risers and integral with said risers, said second coaxial floats being distributed, preferably continuously, over at least an upper part of at least 50% of the length of said risers below and from said upper supporting structure, from preferably over the total length of said risers, said second coaxial floats associated with a riser compensating at least for the total weight of said riser full of water, and in any event all of said second coaxial floats compensating to m anoint the total weight of said risers full of water,

a.3) a plurality of guide modules (20) of said risers, said guide modules being able to maintain said risers arranged around said tendon at a substantially constant distance, preferably regularly and symmetrically distributed around said tendon, said guide modules (20) being integral with said tendon and able to slide along said second floats (11) of said risers, said guide modules being spaced apart and distributed over at least one said upper part of at least 50% of the length of said tendon at -below and from said upper supporting structure, preferably over the total length of said tendon, and

b) a plurality of flexible conduits (4a-4b, 4al-4a2, 4bl-4b2) extending from said reel to which their upper ends (4-1) are connected, to the upper ends (10a) of a respectively plurality of rigid conduits (10) to which the other ends (4-2) of said flexible conduits are connected, of which at least two flexible conduits, hereinafter referred to as first flexible conduits, each comprise an end part (4-3) of the flexible pipe, on the side of its junction at the upper end of said riser, equipped with floats called the first float (4-5) giving it positive buoyancy, and at least the upper part of said vertical riser is fitted with floats called second floats (11) giving it a positive buoyancy, so that the positive buoyancies of said end part (4-3) of the first flexible pipe and of said upper part of said vertical riser (9) allow the tensioning of said rlsers in a substantially vertical position and alignment or continuity of curvature between the end (4-2) of said positive buoyancy end portion (4-3) of said first flexible pipe and the top of said vertical riser at their connection, said end part (4-3) of the first flexible pipe (4), extending over a part of only 30 to 60% of the total length of the first flexible pipe so that said first flexible pipe (4a) has an S-shaped configuration, with a first portion (4-4) of a first flexible pipe on the side of said floating support (1) having a concave curvature in the form of a plunging chain and said end part (4-3) remaining of said first flexible pipe (4a) having a convex curvature in the form of an inverted chain due to its positive buoyancy, the at least two said first flexible pipes with positive buoyancy (4a, 4al-4a2) of which the the ends (4-2) are respectively fixed to two upper ends (10a) of two said risers (10), the two upper ends (10a) of the two risers arriving at 17101 above said upper supporting structure (3a) at different heights (hl, h2, h3) so that said first flexible pipes are positioned at different heights (4a1) relative to each other (4a2).

In FIG. 5A, a side view is shown of the process for installing the tower on site with flexible pipes, in which:

- tower 3 is prefabricated on land, pipes 10 are filled with either water or air, then tower 3 is launched into the sea,

- the tower is towed in flotation on site with at least one lead ship 31, the pipes 10 partially or totally filled with air give the tower a largely positive buoyancy,

- On site, the tower being in a horizontal position 33a, all or part of the pipes 10 are filled with sea water and, where appropriate, a dead-man 32 is installed at the lower end of the tower. A first cable 32a connects said lower end of the tower to a winch located on the ship 30, likewise a second cable 32b connects this same end to a winch located on a second ship 31.

- the cabanage 33b of the tower is controlled by controlling the length of the cables 32a and 32b, then the tower is secured to its foundation 5a,

- after disconnection of the cables, the tower as described above with its pipes full of sea water and with all of its buoyancy elements has positive buoyancy and naturally remains in a vertical position 33c,

- If necessary (figure SB), the ends 4-1 of the flexible pipes are then connected which are placed on standby on a buoy 7a connected to a mooring 7c by a cable 7b.

In figure SB, a side view is shown of the tower 2 preinstalled before the installation of the FPSO, the various hoses are temporarily connected to floats 7a connected by cables 7b to dead bodies 7c resting on the bottom of the sea 5.

Claims (6)

1. Installation of bottom-to-surface connection between a plurality of underwater pipes (12) resting on the seabed (5) and a floating support (1) on the surface (the) and anchored (lb) to the seabed , comprising: - a said floating support comprising a reel (la), and • at least one tower of the hybrid type (2) comprising: a) a multi-riser tower (3) comprising: al) a vertical tendon (6) integral at its upper end with an upper supporting structure (3a), said tendon being fixed at its lower end to a base resting on the seabed or an anchor, preferably of the anchor type with suction (5a), sunk to the bottom of the sea, and a.2) a plurality of rigid vertical pipes (10) called risers, the upper end (10a) of each riser being integral with said supporting structure (3a), the lower end (10b) of each said riser being connected or suitable for being connected to an underwater pipe (12) resting on the seabed, a.3) a plurality of guide means (20) capable of maintaining said risers arranged around said tendon at a substantially constant distance, preferably regularly and symmetrically distributed around said tendon, and b) a plurality of flexible pipes (4a-4b, 4al-4a2, 4bl-4b2) extending from said reel to the upper ends (10a) of a respectively plurality of rigid pipes (10), at least one of which flexible pipe, hereinafter referred to as the first flexible pipe, comprises an end part (4-3) of the flexible pipe, on the side of its junction at the upper end of said riser, equipped with floats called the first float (4-5). conferring positive buoyancy, and at least the upper part of said vertical riser is equipped with floats called second floats (11) giving it positive buoyancy, so that the positive buoyancies of said end part (4-3) of · * 't . "The first flexible pipe and of said upper part of said vertical riser (9) allow the tensioning of said risers in a substantially vertical position and the alignment or continuity of curvature between the end (4-2) of said buoyancy end part positive (4-3) of said first flexible pipe and the upper part of said vertical riser at their connection, said installation being characterized in that at least one said hybrid tower (2) comprises: - at least two said first flexible pipes with positive buoyancy (4a, 4al-4a2) whose ends (4-2) are respectively fixed to two upper ends (10a) of two said risers (10), the two upper ends (10a ) of the two risers arriving above said upper supporting structure (3a) at different heights (hl, h2, h3) so that said first flexible pipes are positioned at different heights (4al) with respect to each other (4a2) , and - Said risers equipped with second peripheral coaxial floats (11) surrounding said risers and integral with said risers, said second coaxial floats being distributed, preferably continuously, over at least an upper part of at least 25% of the length of said risers at- below and from said upper supporting structure, preferably a length of at least 50% of the length of said risers, more preferably over at least 75% of their length, the set of said second coaxial floats compensating for at least the total weight of said risers, and - said guide modules (20) integral with said tendon and able to slide along said second floats (11) of said risers, said guide modules being spaced apart and distributed, preferably regularly, over at least an upper part of at least 25 % of the length of said tendon below and from said upper load-bearing structure, preferably over a length of at least 50% of the length of said tendon, more preferably over at least 75% of its length, " " - said tendon (6) and said upper supporting structure (3a) not being suspended from a float submerged in the subsurface, and said tendon being located at a distance from the vertical axis of the reel (ZZ) less than the distance between l axis of said reel and the end furthest from said floating support. 2. Installation according to claim 1, characterized in that the minimum height offset of the upper ends of said risers to which said first flexible pipes are fixed and therefore the minimum distance in height between two said first flexible pipes arranged at different heights is of at least 3 m, preferably 5 to 10 m. 3. Installation according to claim 1 or 2, characterized in that a said tower comprises 2 to 7 rigid pipes and 2 to 5 said first flexible pipes (4a). 4. Installation according to one of claims 1 to 3, characterized in that it comprises second flexible pipes (4b) of smaller diameters or of lower linear weight than said first flexible pipes, said second flexible pipes (4b) not comprising any buoyancy elements and being connected to the upper ends of said riser by means of connection devices, preferably of the gooseneck type (4c), said second flexible pipes being located below said first flexible pipes. 5. Installation according to one of claims 1 to 4, characterized in that said tendon is fixed at its lower end to a said base or anchor (5a) by means of a junction and inertia transition piece ( 6b) whose variation in inertia is such that its inertia increases progressively from its upper end to the lower end of said junction piece making the lower end of said tendon embedded at said base or anchor (5a). 6. Installation according to one of claims 1 to 5, characterized in that it comprises third floats (21) integral with said t S ”tendon (6) at least in the spaces between said guide modules, said third floats (21) providing positive buoyancy compensating at least for the weight of said tendon. 7. Installation according to one of claims 1 to 6, characterized in that said guide modules (20) constitute a plurality of independent rigid structures spaced at least 5 m along at least the upper part of said tendon, each said rigid structure comprising a plurality of tubular riser guide elements (20a) defining tubular orifices in which said risers, equipped with said second floats, can slide and a central tendon connecting element (20b) preferably defining a central orifice crossed by said tendon which is secured thereto in particular by welding (20c). 8. Installation according to one of claims 1 to 7, characterized in that said guide modules (20) are spaced from 2 to 20 m, preferably from 5 to 15 m, and are at least 20 in number, preferably at least 50 guide modules for a tower at least 1000 m high. 9. Installation according to one of claims 1 to 8, characterized in that the set of first floats provides cumulative buoyancy representing a tensile force of greater intensity than the total weight of said risers, preferably the total weight of the tower. , preferably from 102 to 115%, preferably from 103 to 106%, of the total weight of said risers, more preferably of the total weight of the tower 10. Installation according to one of claims 1 to 9, characterized in that said second coaxial floats are distributed continuously over the entire length of said risers below and from said upper supporting structure, and said guide modules are distributed along the entire length of said tendon below and from said upper supporting structure. 11. Installation according to one of claims 1 to 10, characterized in that said first, second and third floats I are in the form of tubular sleeves, preferably in the form of two half-shells forming a tubular sleeve, made of a material resistant to submarine hydrostatic pressure, and at least said second floats and preferably said first and second floats are made of a material which also has thermal insulation properties. 12. Bottom-surface connection installation according to one of claims 1 to 11, characterized in that said positive buoyancy of said first floats of said first flexible pipes (4a) is regularly and uniformly distributed over the entire length of said. end part (4-3) of first flexible pipe and the buoyancy of said second floats distributed over at least said upper part of the rigid pipes, preferably over the entire length of said rigid pipes, provides a resulting vertical thrust of 50 to 150 Kg / meter over the entire length of said rigid pipes, and / or said first floats (4-5) of the first flexible pipes (4a) provide positive buoyancy over a length corresponding to 30 to 60%, of the total length of said first pipes flexible, preferably about half of its total length. 13. Installation according to one of claims 1 to 12, characterized in that said tower comprises a cylindrical outer casing (22) with a circular horizontal section, made of plastic or composite material forming a rigid hydrodynamic protective screen surrounding all of said. rigid pipes at least in an upper part of the tower. 14. Installation according to one of claims 1 to 13, characterized in that it comprises a plurality of said multi-riser hybrid towers (3, 3-1, 3-2, 3-3, 3-4, 3- 5), preferably at least 5 turns, the flexible pipes (4) of which are connected or capable of being connected to the same reel but extend in directions (YY ') angularly offset, so that said turns are arranged in a fan around said reel at identical or different distances from said reel, certain said towers being able to be only partially mounted not yet comprising flexible pipes and / or only part of said rigid pipes being able to be extended by said flexible pipes at their upper ends and / or at least part of said rigid pipes not being connected to said underwater pipes resting on the seabed at their lower ends. 15. A method of towing a multi-riser tower at sea (3) and setting up an installation according to one of claims 1 to 14, characterized in that it comprises the following successive steps in which: 1) a said tower is prefabricated on land, connected at the head to said flexible pipes with positive buoyancy (4a), the free end (41) of which is connected to a fourth float (7a), and 2) said tower is towed at sea in a horizontal position by a laying vessel (30), said tower floating on the surface (le) thanks to its said second floats (11), and 3) a dead body (32) is installed at the lower end of said tower and, 4) said tower is housed, the lower end of which is connected to said base (5a), and said fourth float connected to the free end of said flexible pipes with positive buoyancy being submerged in the subsurface and offset laterally with respect to the ZiZi axis of said tower so that said flexible pipes with positive buoyancy adopt a said S-shaped position, and 5) subsequently, the ends (4-1) of the first flexible pipes with positive buoyancy are disconnected in order to connect them to said floating support at the level of a said reel, and 6) simultaneously or subsequently, the connection of the lower ends of the risers (10) with the ends of the pipes (12) resting on the seabed (5) is made.