Classifications

• • E—FIXED CONSTRUCTIONS
  • E21—EARTH OR ROCK DRILLING; MINING
  • E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
  • E21B17/00—Drilling rods or pipes; Flexible drill strings; Kellies; Drill collars; Sucker rods; Cables; Casings; Tubings
  • E21B17/01—Risers
  • E21B17/012—Risers with buoyancy elements

Definitions

• the present invention relates to a bottom-to-surface connection installation between a plurality of submarine pipes resting at the bottom of the sea and a floating support surface, comprising a hybrid tower called multi-riser, consisting of a plurality of connected flexible pipes each with a rising rigid pipe, or riser vertical, whose lower end is connected to an underwater pipe resting on the seabed.
• a hybrid tower called multi-riser, consisting of a plurality of connected flexible pipes each with a rising rigid pipe, or riser vertical, whose lower end is connected to an underwater pipe resting on the seabed.
• the technical field 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 a suspension of mineral material from wellhead immersed to a floating support, for the development of production fields installed offshore at sea.
• the main and immediate application of the invention being in the field of oil production.
• the floating support generally comprises anchoring means to remain in position despite the effects of currents, winds and waves. It also generally comprises oil storage and processing means as well as means of unloading to removal tankers, the latter being present at regular intervals to carry out the removal of the production.
• the common name of these floating supports is the Anglo-Saxon term "Floating Production
• a tower comprising: a) a vertical tendon secured at its upper end to a supporting structure suspended from a float called the sub-surface immersed float, preferably via a chain or cable, said tendon being secured at its lower end to a lower guide structure and a base resting at the bottom of the sea or a foundation anchor preferably of the suction anchor type driven to the bottom of the sea, preferably via a joint flexible, and b) a plurality of rigid vertical pipe called riser whose upper end is integral with said support structure, the lower end of each said rigid pipe or riser being connected to a said underwater pipe resting at the bottom of the sea, preferably via automatic connectors between said lower ends of the riser and the ends of the underwater pipes, and preferably by means of bent sleeves and / or connecting pipes, c) a plurality of means for guiding said risers, said guide means and said lower guide structure being able to hold said risers arranged around said tendon , preferably regularly and symmetrically distributed around said tendon, and d
• connection conduits preferably flexible connection lines between the upper ends of said risers and the floating support, preferably said flexible pipes in the form of plunging chains, said flexible pipes being connected to the upper end of said risers via gooseneck devices.
• Said connecting lines take, if appropriate, by their own weight in the form of a plunging chain curve, that is to say, descending widely below the upper end of the riser and then up to the support floating.
• a multiple hybrid tower comprising an anchoring system with a vertical tendon consisting of either a cable or a metal bar, or a tense conduct at its upper end by a float.
• the lower end of the tendon is attached to a base resting at the bottom.
• Said tendon comprises guiding means distributed over its entire length through which passes a plurality of said risers vertical.
• Said base can be placed simply on the seabed and stay in place by its own weight, or remain anchored by means of batteries or any other device to keep it in place.
• the lower end of the vertical riser is adapted to be connected to the end of a bent sleeve, movable, between a high position and a low position, with respect to said base, to which this cuff is suspended and associated with a return means bringing it up in the absence of the riser.
• This mobility of the bent sleeve makes it possible to absorb the length variations of the riser under the effects of the temperature and the pressure of the fluid flowing through it.
• a stop device integral with it, comes to rest on the support guide installed at the head of the float and thus maintains the entire riser in suspension.
• connection with the submarine pipe resting on the seabed is generally effected by a pig-shaped or S-shaped pipe portion, said S being then produced in a plane either vertical or horizontal, the connection with said underwater pipe being generally carried out via an automatic connector.
• the vertical tendon is connected at its lower end to the base by a flexible joint type laminated stopper marketed by TECH LAM France or the roto-latch ® type , available from OILSTATES USA, known to the man of the art.
• This embodiment comprising a multiplicity of risers maintained by a central structure comprising guide means is advantageous when it is possible to pre-manufacture the entire tower on the ground, before towing it at sea and then once on site. , cabaner for its final establishment.
• WO-2008-056185 which consists in suspending the upper ends of pipes to a carrier upper structure and in securing a plurality of buoyancy or insulation modules. buoyancy, at the central tendon, by means of a plurality of structural elements integral with the tendon and also acting as guiding of the various vertical ducts, thus allowing them to stretch freely downwards when they are pressurized or / and subjected to a high temperature (crude oil from wells).
• the various structural elements and buoyancy elements are regularly spaced along the entire length of the vertical tendon, which allows towing the tower at sea, which floats thanks to its buoyancy elements distributed over its entire length.
• the dual function required of the syntactic foam ie buoyancy plus insulation, creates a problem of resistance over time, because if such foams hold correctly on the one hand at the pressure of bottom which is substantially 100 bar per 1000 m of water, and secondly at the desired maximum temperature, they are in fact sensitive to the combination of the maximum pressure associated with the maximum temperature, and very significant damage their thermal performance and buoyancy. Therefore, it is desired to dissociate the two functions, and it is preferred to use firstly a syntactic foam type buoyancy and a PiP type of insulation, as described in the prior patents in the name of the applicant, or gel type or phase change material type as described in the prior patents in the name of the applicant.
• the syntactic buoyancy foam is thus used to ensure the buoyancy of the tower during its on-site towing and cabin hoisting, but it can not be fully taken into account in its operational vertical configuration and must be assisted by a float at the top. considerable volume located at the top of said tower, as explained below.
• each of the buoyancy modules 220 transfers to the central tendon 200 a vertical force 219 directed upwards by means of an element of guide and load transfer structure 212 integral with the tendon.
• the total of the vertical tensile forces added to the various buoyancy modules 219 corresponds at least to the total weight of the total tower, so that the tower naturally floats flush with the surface of the water.
• the central tendon is arranged vertically and the vertical traction forces 219 of each of the different modules are transferred by the tendon so that the traction exerted on the attachment point of a given buoyancy module on the tendon corresponds the buoyancy of the module concerned plus the modules above it.
• WO 2006/13696 requires the implementation of foundations and floats at the summit of considerable importance that make the process very expensive.
• the object of the present invention is therefore to provide an improved multi-riser hybrid tower-type bottom-surface connection installation, in particular which does not require the use of floats at the top, nor of foundation having to take back the full weight of the tower for the float at the top and the totality of the buoyancy exerted on the tower as regards the foundation of the tower on the one hand, and on the other hand, which does not require the implementation of vertical central tendon in front of also undergo compressive stresses likely to cause lateral buckling of said central tendon.
• the present invention provides a hybrid multi-riser tower comprising:
• a tower comprising: a) a vertical tendon secured at its upper end to a supporting structure capable of being suspended from a float called the subsurface submerged top float, preferably by means of a chain or cable, said tendon being secured at its lower end to a lower guide structure and being adapted to be fixed to a base resting at the bottom of the sea or a foundation anchor preferably of the suction anchor type driven to the bottom of the sea, preferably by intermediate of a flexible joint, and b) a plurality of rigid vertical pipe called riser whose upper end is integral with said supporting structure, the lower end of each said rigid pipe or riser being adapted to be connected to a pipe underwater at seabed, preferably via automatic connectors between said lower ends of riser and ends of subsea pipes, and preferably by means of bent sleeves and / or connecting pipes,
• buoyancy elements cooperating with said tendon, distributed along said tendon, preferably buoyancy elements resistant to the underwater hydrostatic pressure, more preferably syntactic foam buoyancy elements, and
• said tower comprises a plurality of buoyancy and guiding 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 position preferably regularly and symmetrically distributed around said tendon.
• Vertical means that when the sea is calm and the installation is at rest, the connection hoses to the FPSO not being installed, the tendon and the risers are arranged vertically, it being understood that the swell, and the movements of the floating support and / or flexible ducts may cause the tower to travel in a vertex angle preferably limited to 10-15 °, in particular due to the implementation of a flexible articulation of rotary type. latch ® at the foot of the tendon, at its point of attachment to said base or anchor.
• the tower is able to remain vertical in the absence of the tensioning float at the top, whereas in the prior art, said float must be permanently present to prevent compression of the central tendon.
• This arrangement is particularly interesting for the installation phase and for the maintenance of the system,
• the head float and the foundation must exert respectively take up a voltage well above the total weight of the tower.
• a total buoyancy ⁇ F of 102 to 110% of all the buoyancy modules is necessary to maintain emerged approximately 2 to 10% of the volume of the tower when it is towed on the surface, so that the tower can be covered by the swell and be subjected to less mechanical stress including torsion and bending during towing on site.
• Tl T R + Pt
• the own buoyancy Tl of the head float according to the prior art then represents at least about 110 to 150% of the total weight of the tower plus the additional buoyancy provided for towing on site ( ⁇ F-Pt).
• the foundation must resume the traction exerted by the cumulative vertical thrust of all buoyancy elements ⁇ F acting directly on the central tendon, added the resulting tension T R wanted at the float of head. Since ⁇ F is greater than the weight of the tower for surface transport, the foundation must resume a tension of at least about 110 to 150% of the total weight of the tower.
• the foundation is subjected only to the resultant force T R exerted at the level of the head float, namely 10 to 50% of the total weight of the tower, insofar as the total weight of the tower is taken directly by all modules buoyancy, the latter exerting a vertical thrust upward directly on the underside of said carrier structure.
• said buoyancy and guide modules extend over a length of 2 to 20 m and are at least 50, preferably 50 to 500, buoyancy modules for a tower of at least 1000 m. height.
• the various buoyancy modules extend over the entire height of the tower.
• said plurality of buoyancy and guide modules disposed against each other covers not more than 75%, preferably less than 50% of the length of the tower between said carrier structure at the top and the structure lower guide solidarity of the tendon.
• the various buoyancy and guidance modules are linked to each other by links able to prevent the first module from moving away from said carrier structure and that two consecutive modules deviate no more than a maximum distance preferably identical data between the different modules and, said links being of such length that the various modules are distributed substantially uniformly over the entire length of the tower between said carrier structure and lower structure, the first module being linked at the level of said carrier structure and the last module arriving at said lower structure, when said tower floats on the surface of the sea being towed by a ship, and said links do not prevent said modules from sliding upwards when said tower is cabanne and vertical positioning of operation in a so-called bottom-surface connection installation.
• the different modules are not connected to each other and cover the entire length of the tower between said supporting structure and said lower guide structure.
• each buoyancy and guide module comprises two flanges connected to each other by tie rods, and said cylindrical buoyancy elements locked and held in position between and by said two flanges, forming preferably a module having a circular cross-section, each flange having a central orifice and peripheral orifices, said peripheral orifices and said buoyancy elements preferably being of the same shape and arranged around said central orifice, preferably regularly and symmetrically distributed around said central orifice; , said orifices forming sleeves able to be traversed by said risers and a said tendon thus allowing sliding guidance of said modules.
• the two flanges are spaced from each other in the longitudinal direction of the tendon and that said section transverse corresponds to a cross section perpendicular to said longitudinal direction of the tendon, and that said circular section of the module corresponds to a circular section of the flanges and all the buoyancy elements assembled against each other between the two flanges.
• This embodiment makes it possible to contribute to a buoyancy load transfer of said buoyancy elements in their isostatic assembly, that is to say the most homogeneous over the entire cross section of the flanges and to facilitate the manufacture and installation of said elements of buoyancy.
• each flange comprises a plurality of flange portions attached to each other, preferably at least as many flange portions as said peripheral orifices, each flange portion being adapted to lock and maintain the longitudinal end of a flange. said cylindrical buoyancy element.
• This embodiment contributes to limiting the number of different parts to be molded, and to further facilitate the manufacture and installation of said flanges and buoyancy elements while maintaining the isostaticity of the charge transfer of said buoyancy elements on the flanges.
• said modules comprise first intercalated elastic elements, preferably in the form of plates, between the longitudinal ends of said buoyancy elements and said flanges at least at said longitudinal end of the module and preferably also second elastic elements on the external faces of at least one of the two flanges, preferably in the form of plates, so as to improve the isostaticity of the distribution of buoyancy forces and their transfer between two consecutive modules.
• the present invention also provides a bottom-to-surface bonding facility between a plurality of subsea pipes lying at the bottom of the sea and a surface floating support, comprising: 1) a tower according to the invention, and
• the present invention also relates to a buoyancy and guide module of a tower according to the invention, as defined above.
• the present invention also provides a method of towing at sea a tower and set up in an installation according to the invention, characterized in that said tower floats on the surface when it is towed by at least one surface vessel, said buoyancy and guide modules being distributed over its entire length, preferably regularly distributed and spaced from each other and after hoisting the tower said buoyancy and guide modules slide upwards to be plated underneath and against others, preferably on only a part of the height of the tower.
• FIG. 1 is a side view of a bottom-surface connection system 1 according to the invention comprising a central tendon 4 and at least two rigid vertical lines of the Riser 3-1,3-2 type in suspension, the assembly being maintained in a substantially vertical position by a buoyancy distributed over the entire height of the tower, as well as a float 8 secured to the top of the tower, a plurality of flexible 6-6-2 connecting the upper ends of said Riser at the head of said tower to an FPSO anchored to the surface nearby,
• FIG. 2 shows a side view of the tower being installed or maintenance, the head float not being installed
• FIG. 3 shows a side view towing the tower on site, and its cabanage and its connection to a foundation pile 11 suction anchor type
• FIG. 3A shows a variant of the tower of FIG. 3, in which the buoyancy modules are substantially uniformly distributed along the tower for towing, but spaced apart from each other in the towing position, collect in the portion high of the tower, by simply sliding along the central tendon, during the cabanage, on a height H2 of approximately 50% of the height of the tower,
• FIGS. 4A-4B show a side view of a tower according to the prior art, without float (4A) and with float (4B),
• FIGS. 5A and 5B show a side view of a tower according to the invention, equipped with buoyancy modules sliding along said vertical central tendon (Figure 5A) and without the modules (Figure 5B),
• FIG. 6 represents a perspective view of a section of the tower in a plane perpendicular to its axis, said tower being in a vertical position
• FIG. 7A represents a perspective view of a buoyancy module according to the invention without its buoyancy elements
• FIG. 7 represents a perspective view of two identical buoyancy modules contiguous to one another, with buoyancy elements being installed in one of the modules,
• FIG. 8 represents a perspective view of a pipe guide flange of a buoyancy module according to the invention, seen from the outside, that is to say seen from the interface plane with a module. neighbor buoyancy,
• FIG. 9 represents a perspective view of the flange of 8, seen from the inside, that is to say the side of the interface plane with the syntactic foam buoyancy elements held by said guide flange at each end.
• FIG. 1 shows a bottom-surface connection installation 1 connecting two underwater lines 2-1, 2-2 resting on the bottom of the sea 12 to a floating support of the FPSO type 10 moored by lines of anchor 10a.
• the bottom-surface connection consists of a central vertical tendon 4 connected to a suction anchor-type foundation 11 via a hinge 5a allowing the deflections of the tower in an angle cone at the top preferably less than or equal to 5 °.
• the tower comprises a plurality of pipes, for example four pipes 3-1, 3-2, as shown in the perspective views 6 and 7, preferably symmetrically distributed around the axis ZZ 'of the tower, the latter being coaxial with the central tendon 4.
• the lines 3-1,3-2 are each connected at the bottom to a subsea line 2-1,2-2 resting on the bottom of the sea by the intermediate of a bent junction sleeve 2a by means of automatic connectors 9a-9b, known to those skilled in the art.
• These ducts 3-1, 3-2 are suspended at the top of the tower to a superior carrier structure 4a integral with the central vertical tendon 4 and each connected by a gooseneck 7-1, 7-2 at the end of a flexible pipe 6-1,6-2 connecting said goosenecks to an FPSO 10.
• a float 8 connected via a chain 8a to the central tendon 4 exerts a complementary vertical tension on the tower.
• buoyancy of the tower 3 is ensured by a plurality of buoyancy modules 20 cooperating in sliding with the central tendon 4.
• These buoyancy modules 20 described hereinafter comprise guide elements 22 called flanges comprising 23,23-1,23-4 orifices, for guiding said central tendon and ducts 3-1,3-2.
• each of the buoyancy modules 20 slides freely on the one hand around the central tendon 4 and on the other hand, around each of the ducts 3-1,3-2 suspended from the support structure 4a located at the top of the tower. And, therefore, the entire buoyancy thrust ⁇ F of all the buoyancy modules 20 is directly transmitted to the upper carrier structure 4a, the latter supporting on the other hand the entire weight P of the tower.
• the float 8 at the top must be limited to exerting an additional upward tension equal to about 10 to 20% of the total weight of the tower so as to exert a vertical return force when the flexible connecting lines 6- 1.6-2 with the PSO are in place, which exert horizontal recall efforts when the sea is rough.
• the buoyancy modules 20 are schematically represented in FIGS. 1 to 3 and in FIG. 5, and in more detail in FIGS. 6 to 9.
• FIG. 6 is a perspective view of a section of the tower 1 in a vertical position, a section made above a buoyancy module 20.
• the central tendon 4 At the axis of the buoyancy module 20 is the central tendon 4 and at the periphery, PiP-type 3-1.3-2 pipes (pipe in pipe) comprising an external pipe 3a and including a production pipe 3b slightly off-center, so as to leave room for a water injection pipe or pipe gas injection 3c, as well as two water injection lines 3-3,3-4.
• FIG. 7 shows in perspective a section of the tower in the manufacturing position, showing two contiguous buoyancy modules 2 0 n , 20 n + 1, the module 20 n + 1 not being completely assembled, two buoyancy elements under syntactic foam block shape
• the flange 22 of buoyancy module respectively in external view ( Figure 8), that is to say seen from the interface side with the buoyancy module adjacent, and in internal view ( Figure 9), that is to say seen from the interface side with the buoyant elements 21 syntactic foam.
• the flange is preferably made of plastic, for example polyethylene, polypropylene or any other resistant thermoplastic material, charged or not.
• the flange 22 is in fact preferably composed of several independent parts 22-1 to 22-4, identical, assembled together by simple bolting.
• the flanges 22 comprise sheaths 23-1 to 23-4 generally not having the same diameter, the diameter of each of the sheaths 23-1 to 23-4 is adjusted so that it is slightly greater than the diameter of the corresponding pipe and can thus let it slide freely.
• buoyancy elements 21 all of the same cylindrical shape, are inserted into complementary shapes 22a of the inner face of the flanges 22, as shown in FIG. 9, and are preferably uniformly distributed around the periphery of the module 20.
• Figures 7 and 9 there is shown between two adjacent pipes two buoyancy elements 21 installed side by side. But, we could have installed a single element 21 of double section.
• the manufacture of large syntactic foam elements being very delicate, it is preferred to reduce the transverse thickness of the various elements and thus adopt the configuration shown in Figures 7 and 9.
• each module 8 blocks of syntactic foam 21 of the same cylindrical shape are thus introduced whose cross-sectional shape fills the spaces 22a delimited by the parts of cylindrical walls 23c-23d of the sleeves 23-1 to 23-4 and elements of separation 23a-23b.
• the buoyancy elements 21 have in cross section a circular outer circumference 21a likewise radius as the radius of the circumference of the flanges 22.
• buoyancy elements are 8 in number, they form angular portions having a flat side face 21b in contact with the lateral separation elements 23a on the inner surface of the flanges 22 , also having a flat surface abutting on the other separating element 23b on the inner face of the flange 22 and finally having an opposite side face of circular portion 21c in cross section bearing against the cylindrical walls of the sleeves 23c-23d.
• each block would have two opposite side surfaces having the same shape of circular portion in cross section, circular portion section bearing against each two lines 3-1,3-2 side by side between which the block of syntactic foam 21 is introduced.
• the thickness 23c of the wall of the sheath 23-3,23-4 is greater than the thickness 23d of the wall of the sheath 23-1, 23-2, such that the external rays R of the walls of the two sheaths are identical and correspond to the internal diameter of the circular portion 21c of the buoyancy elements
• tie rods 24 connecting the two flanges 22 and providing a prestressing of the buoyancy elements 21 between two flanges 22, and by strapping elements 26.
• All the buoyancy elements of the same module 20 have an identical length so that once the module 20 assembled, the two outer faces of the flanges 22 of said module are parallel to each other.
• a rubber plate 25a advantageously inserted between each of the buoyancy elements 21 and its housing 22a in the flange 22, a rubber plate 25a, of preferably of high stiffness neoprene, for example Shore hardness between A50 and A95 and 3 to 15 mm thick, so as to improve the isostaticity of the distribution of buoyancy forces and their transfer to the upper module.
• neoprene plates 25b preferably of identical characteristics to the plates 25a, are placed outside the flanges, said plates 25b being inserted between two adjacent buoyancy modules 20.
• FIG. 3A shows the towing and cabanage of a tower 3, the buoyancy of which is evenly distributed along the tower 3 for its towing, the modules 20 being pre-assembled 20a, here 3 by 3 and connected between 31 at 31 by cables 30, a cable 30 being integral at one end of the upper support structure 4a and at the bottom of a fixed floating element on the lower structure 5.
• the buoyancy modules 20 grouped by 3 can not move axially beyond a constant distance given by the cables 30 between the different groups of buoyancy modules 20. This ensures a buoyancy distributed over the entire length of the tower 30 when the latter is towed on the surface by at least one vessel 10-1, 10-2 using cables 15-1 connected to the upper structure 4a.
• This configuration is interesting for very deep, because it allows to use syntactic foam of lower quality, because the deepest elements are at a depth H 2 and not at the depth.
• This configuration further illustrates the difference of the device according to the invention with respect to the device of the prior art, described inter alia in WO-2006-136960 in which each of the buoyancy modules is rigidly connected. at the central tendon.
• the buoyancy modules 20 due to their sliding along the tendons 4 and 3-1,3-4 conduits transfer all of their buoyancy ⁇ F to the upper structure 4a.
• TR resultant traction
• the sum of the buoyancy ⁇ F represents 102 to 110% of the total weight of the tower according to the sea states.
• the foundation 11 must take a resultant traction T R which is also exerted on the float at the top 8, namely 10 to 50% x Pt (total weight of the tower).
• FIGS. 4A and 4B The prior art is shown in side view in FIGS. 4A and 4B.
• the buoyancy modules 40 are integral with a central tendon 4 at cleats 41.
• the buoyancy elements 40 transmit their buoyancy F directly to the central tendon 4 via the cleats. 41 integral with the central tendon.
• the buoyancy elements 40 provide a total buoyancy ⁇ F of 102 to 110% x Pt (total weight of the tower) to allow its towing on the surface.
• the buoyancy modules 40 do not contribute any more to the buoyancy of the tower.
• the central tendon 4, at the upper latch 41 is subjected to upward traction equal to F.
• This upward traction F is transmitted to the level of the cleat located just in underneath: - the central tendon is then subjected to an upward pulling force equal to 2F, which is thus transmitted step by step to the last stop and to the foundation 11 which are then subjected to traction equal to ⁇ F to the top.
• Pt weight of the tower
• the upper portion of the tendon 4 located just below the upper structure 4-1 is subjected to a resultant compressive force substantially equal to Pt - F. And so, the top float 8-1 must provide an equal buoyancy Tl at T R + (Pt - F).
• the number of modules can range from 100 to 400. It is therefore possible to approximate F and consider that the buoyancy Tl brings the float to the vertex 8-1 is (T R + Pt) and must represent about 110 to 150% of the total weight of the tower.
• the foundation 11a must take up all the pulls exerted on the vertical tendon 4, along its height, namely the resulting tension T R is equal to 50% x Pt at the top of the tower, added to the total float F ( ⁇ F), (TR + ⁇ F).
• the foundation 11a must therefore recover from 112 to 160% x Pt, while according to the present invention, the float 8 at the top provides a clean buoyancy Tl [(T R - ( ⁇ F - Pt)] is (8 to 58% x Pt) and that the foundation 11 must take only the resulting tension T R at float 8, namely 10 to 50% of Pt.
• the float at the top 8 according to the invention must therefore provide a much lower buoyancy than according to the prior art and the foundation 11 according to the present invention must also take a tensioning force much less than that of the prior art.
• the total weight of the tower is deduced from the buoyancy provided by any fixed fixed buoyancy elements integrated in certain elements of the structure of the tower, namely the weight of the upper structure 4a, that of the 3-1,3-4 pipes suspended from the upper carrier structure 4a including the weight of the devices of the gooseneck type 7-
• the tower is able to remain vertical in the absence of the vertex tensioning float 8, as shown in FIGS. 2 and 3A, whereas in the prior art, said float must be permanently present to avoid any compression of the central tendon.
• This arrangement is particularly interesting for the installation phase and for the maintenance of the system, because in the event of an incident on said head float 8, as represented in FIG. 2, it is sufficient to purge the vertical lines and the flexible lines, to disconnect the hoses from the FPSO and to maintain them in sub-surface thanks to a small buoy connected to a dead body, so as to reduce considerably the horizontal force, thus the angle ⁇ of the tower. Due to the additional buoyancy of the tower alone, the latter remains substantially vertical, and it is then possible to disconnect said head float to repair, for example, a buoyancy compartment that would show a leak, so a loss of buoyancy.

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