EP2401468B1 - Bottom-surface connecting installation of the multi-riser hybrid tower type, comprising sliding buoyancy modules - Google Patents

Description

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.

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 Storage Offloading" (meaning "floating medium of storage, production and unloading") which one uses the abbreviated term "FPSO" in the whole of the following description.

1. 1) une tour comprenant :
   1. a) un tendon vertical solidaire à son extrémité supérieure d'une structure porteuse suspendue à un flotteur dénommé flotteur au sommet immergé en sub-surface, de préférence par l'intermédiaire d'une chaine ou câble, ledit tendon étant solidaire à son extrémité inférieure à une structure inférieure de guidage et à une embase reposant au fond de la mer ou une ancre fondation de préférence du type ancre à succion enfoncée au fond de la mer, de préférence par l'intermédiaire d'une articulation flexible, et
   2. b) une pluralité de conduite rigide verticale dénommé riser dont l'extrémité supérieure est solidaire de ladite structure porteuse, l'extrémité inférieure de chaque dite conduite rigide ou riser étant reliée à une dite conduite sous-marine reposant au fond de la mer, de préférence par l'intermédiaire de connecteurs automatique entre lesdites extrémités inférieures des riser et extrémités des conduites sous-marine, et de préférence par l'intermédiaire de manchettes coudées et/ou de conduites de jonction,
   3. c) une pluralité de moyens de guidage desdits risers, lesdits moyens de guidage ainsi que ladite structure inférieure de guidage étant aptes à maintenir lesdits risers disposés autour dudit tendon, de préférence régulièrement et symétriquement répartis autour dudit tendon, et
   4. d) des éléments de flottabilité coopérant avec ledit tendon, répartis le long dudit tendon, de préférence des éléments de flottabilité résistants à la pression hydrostatique sous-marine, de préférence encore des éléments de flottabilité en mousse syntactique, et
2. 2) une pluralité de conduites de liaison de préférence des conduites de liaison flexible entre les extrémités supérieures desdits risers et le support flottant, de préférence encore desdites conduites flexibles en forme de chaînettes plongeantes, lesdites conduites flexibles étant reliées à l'extrémité supérieure desdits risers par l'intermédiaire de dispositifs de type col de cygne.

Background-to-surface bonding devices of an underwater pipe resting at the bottom of the sea are known, and a surface floating support of the hybrid tower type comprising:

1. 1) a tower comprising:
   1. a) a vertical tendon secured at its upper end to a supporting structure suspended from a float called a sub-surface submerged top float, preferably via a chain or cable, said tendon being secured at its lower end; 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 flexible joint, and
   2. 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 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 via bent sleeves and / or connecting pipes,
   3. c) a plurality of means for guiding said risers, said guide means and said lower guide structure being able to maintain said risers arranged around said tendon, preferably regularly and symmetrically distributed around said tendon, and
   4. d) 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
2. 2) a plurality of 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.

More particularly, in WO 00/49267 of the applicant, there is described a multiple hybrid tower comprising an anchoring system with a vertical tendon consisting of either a cable or a metal bar, or a pipe stretched 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. In WO 00/49267 , the lower end of the riser vertical is adapted to be connected to the end of a bend, movable cuff, between a high position and a low position, relative to said base, to which this cuff is suspended and associated with a means of return bringing it back 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. At the top of the vertical riser, 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.

The 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 hinge-type laminated stopper marketed by TECHLAM France or the roto-latch ^{® type} , available from OILSTATES USA, known to those skilled in 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.

In addition, since crude oil travels a great distance over several kilometers, it must be provided with an extreme and very costly level of insulation in order, on the one hand, to minimize the increase in viscosity which would lead to a reduction in hourly production. wells, and secondly to avoid the blockage of the flow by deposition of paraffin, or formation of gas hydrates when the temperature drops to around 30-40 ° C. These last phenomena are all the more critical, especially in West Africa, that the temperature of the seabed is of the order of 4 ° C and that the crude oils are of the paraffinic type. It is therefore desirable that the bottom-surface bonds are carefully insulated over their entire length.

The development of the Girassol field carried out in 1997-1999 off the Angolan coast by the plaintiff is known, in which it was sought to isolate the crude oil pipes with a syntactic foam also playing the role of buoyancy. To do this, the technique used is similar to that described in WO-2006-136960 and WO-2008-056185 which is considered to be the closest state of the art, and which consists in suspending the upper ends of pipes to a higher bearing structure and in securing a plurality of buoyancy or buoyancy-buoyancy modules, 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 lie 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.

In the case of the Girassol project, 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.

Indeed, as detailed in WO-2006-136960 , each of the buoyancy modules 220 ( figure 2 ) transfers to the central tendon 200 a vertical force 219 directed upwards through an element of guide and load transfer structure 212 integral with the tendon. Thus, in the towing phase, 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. After cabanage, 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. In total, the lower end of the tendon is pulled equal to the sum of the individual pulls F of each of the modules. As a result, the foundation at the base of the tendon must take up a tension corresponding to n XF, if "n" buoyancy modules are integral with the tendon, each exerting a float F.

Conversely, the upper bearing structure of the tower is subjected to a compressive force 218 corresponding to the weight of the tower, comprising the weight of the upper bearing structure and the weight of all the suspended pipes carried by said structure. It is therefore necessary to implement at the top of the tower a float 8-1 exerting traction on the carrier structure T1 = Pt-F, "Pt" being the total weight of the tower and "F" being the tension exerted by the only last upper module for the upper supporting structure of the tower.

As a result, in practice, the embodiment described in WO 2006/13696 requires the implementation of foundations and floats at the summit of considerable importance that make the process very expensive.

Finally, the compression stress levels are such that a lateral buckling of the central tendon will result on almost the entire height of said central tendon, whatever the mechanical characteristics of said tendon.

WO 2004/051051 discloses a multi-riser hybrid tower-type bottom-surface connection installation, comprising buoyancy modules sliding along said risers and a central tendon, without a lower guide structure capable of holding said risers arranged around said tendon.

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 a foundation having to take back all the 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.

1. 1) une tour comprenant :
   1. a) un tendon vertical solidaire à son extrémité supérieure d'une structure porteuse apte à être suspendue à un flotteur dénommé flotteur au sommet immergé en subsurface, de préférence par l'intermédiaire d'une chaine ou câble, ledit tendon étant solidaire à son extrémité inférieure à une structure inférieure de guidage et étant apte à être fixé à une embase reposant au fond de la mer ou une ancre fondation de préférence du type ancre à succion enfoncée au fond de la mer, de préférence par l'intermédiaire d'une articulation flexible,
   2. b) une pluralité de conduite rigide verticale dénommé riser dont l'extrémité supérieure est solidaire de ladite structure porteuse, l'extrémité inférieure de chaque dite conduite rigide ou riser étant apte à être reliée à une conduite sous-marine reposant au fond de la mer, de préférence par l'intermédiaire de connecteurs automatiques entre lesdites extrémités inférieures des riser et extrémités des conduites sous-marine, et de préférence par l'intermédiaire de manchettes coudées et/ou de conduites de jonction,
   3. c) une pluralité de moyens de guidage desdits risers, lesdits moyens de guidage ainsi que ladite structure inférieure de guidage étant aptes à maintenir lesdits risers disposés autour dudit tendon, de préférence régulièrement et symétriquement répartis autour dudit tendon, et
   4. d) des éléments de flottabilité coopérant avec ledit tendon, répartis le long dudit tendon, de préférence des éléments de flottabilité résistants à la pression hydrostatique sous-marine, de préférence encore des éléments de flottabilité en mousse syntactique,

caractérisée en ce que ladite tour comprend une pluralité de modules de flottabilité et de guidage constituant une pluralité de structures indépendantes aptes à coulisser le long dudit tendon et le long desdits risers, ladite structure supportant lesdits éléments de flottabilité et guidant lesdits risers en position de préférence régulièrement et symétriquement répartis autour dudit tendon.To do this, the present invention provides a hybrid multi-riser tower comprising:

1. 1) a tower comprising:
   1. a) a vertical tendon secured at its upper end to a bearing structure capable of being suspended from a float called a subsurface submerged float, preferably via a chain or cable, said tendon being integral at its end; lower than 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 via a joint flexible,
   2. 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 adapted to be connected to a subsea pipe resting at the bottom of the sea preferably via automatic connectors between said lower ends of the risers and ends of the underwater pipes, and preferably via bent sleeves and / or connecting pipes,
   3. c) a plurality of means for guiding said risers, said guide means as well as said lower guide structure being able to maintain said risers arranged around said tendon, preferably regularly and symmetrically distributed around said tendon, and
   4. d) 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,

characterized in that 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 can 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.

Said modules and therefore said buoyancy elements slide along the tendon below said carrier structure and are retained at the upper end of said risers and tendon by said carrier structure. Thus the tendon is in a substantially uniform tension, neglecting the tension differential due to its own weight, over its entire height, insofar as the tension created by the sum of the buoyancy of the different modules is transferred to the vertex of the tendon by the intermediate of said support structure against which just abuts the upper buoyancy module, the other modules being plated under and against the others.

As a result, the float and the suction anchor according to the present invention must exert and respectively take up a voltage lower than that required in the prior art and in particular less than the total weight of the tower.

In the present invention, 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,

In the prior art, the head float and the foundation must exert respectively take up a voltage well above the total weight of the tower.

Indeed, if we consider that the flexible pipes in the configuration of chains exert on the top of the tower a horizontal tension proportional to their linear mass, this horizontal tension tends to tip the tower in a cone half-angle to summit α. To limit this angle α and to contain the tower in a cone of half-angle at the top α of 5 to 15 °, preferably of 3 to 5 °, a resultant vertical tension should be exerted at the level of the head float T _R corresponding in practice to 10 to 50% of the total weight of the tower Pt, according to the weight of said flexible pipes and according to the desired stiffness of the system. Thus for lightly agitated seas and relatively light flexible pipes allowing excursions of the upper structure with an angle at the top of the tower with respect to the vertical in an angle cone at the top of 5 to 8 °, a resulting tension TR 10% of the weight of the tower will be sufficient, whereas for relatively heavy flexible pipes and reduced top expansions with an apex angle of the excursion cone less than 5 °, the resulting tension T _R may rise to at 50% of the total weight of the tower.

Furthermore, a total buoyancy ΣF 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. Thus, according to the invention, the additional traction force or buoyancy T1 brought by the float at the summit must it correspond to the desired resultant tension T _R increased by the differential between the cumulative buoyancy ΣF and the weight of the tower Pt [ T1 = TR + (ΣF-Pt)].

In contrast, in the prior art according to WO 2006/136960 , the head float must have a much higher inherent buoyancy (T1 = T _R + Pt) corresponding to said resultant tension T _R of 10 to 50% of the weight of the tower plus the compressive stress exerted on the central tendon at the level of the upper structure at the top of the tower, which corresponds to taking up at least the entirety of the self weight of the tower Pt. The own float T1 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 additional buoyancy provided for on-site towing (ΣF-Pt).

Similarly, in the prior art the foundation must resume the traction exerted by the cumulative vertical thrust of all buoyancy elements ΣF acting directly on the central tendon, added the resulting tension T _R wanted at the float of head. Since ΣF 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.

On the other hand, according to the present invention, 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.

More particularly, therefore, all of the buoyancy modules provide a cumulative buoyancy ΣF representing a traction force of intensity greater than the total weight of the tower Pt, preferably from 102 to 110% of the total weight of the tower, and said float at the top provides a clean buoyancy T1 such that [T1 = T _R - (ΣF - Pt)], and said foundation must take at least the resulting traction T _R at the top of the tower, T _R representing the resulting vertical tension upwards at the level of the top float equal to 5 to 50% of the total weight of the tower Pt, preferably from 10% to 20% of the total weight of the tower.

More particularly, 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.

In a first embodiment, the various buoyancy modules extend over the entire height of the tower.

In a preferred embodiment, 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.

To do this, it will suffice if necessary to oversize the diameter of all the buoyancy elements in cross section so as to reduce the size of said buoyancy elements in the longitudinal direction.

Thus, it is possible to implement less expensive buoyancy elements that would not withstand hydrostatic pressure at a water depth below the depth at which the last module or lower module arrives.

Advantageously, 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 caban and vertical positioning of operation in a so-called bottom-surface connection installation.

In another embodiment 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.

In an advantageous embodiment, 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.

It is understood that 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.

Preferably, 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.

More preferably, 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.

1. 1) une tour conforme à l'invention, et
2. 2) une pluralité de conduites de liaison de préférence des conduites de liaison flexible entre les extrémités supérieures desdits risers et le support flottant, de préférence encore desdites conduites flexibles en forme de chaînettes plongeantes, lesdites conduites flexibles étant reliées à l'extrémité supérieure desdits risers par l'intermédiaire de dispositifs de type col de cygne.

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. 1) a tower according to the invention, and
2. 2) a plurality of connecting pipes, preferably flexible connecting pipes between the upper ends of said risers and the floating support, more 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.

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.

• la figure 1 est une vue de côté d'une installation de liaison fond-surface 1 selon l'invention comportant un tendon central 4 et au moins deux conduites rigides verticales de type Riser 3-1,3-2 en suspension, l'ensemble étant maintenu en position sensiblement verticale par une flottabilité répartie sur toute la hauteur de la tour, ainsi que par un flotteur 8 solidaire du sommet de la tour, une pluralité de flexibles 6-1,6-2 reliant les extrémités supérieures desdits Risers en tête de ladite tour à un FPSO ancré en surface à proximité,
• la figure 2 représente en vue de côté la tour en cours d'installation ou de maintenance, le flotteur de tête n'étant pas installé,
• la figure 3 représente en vue de côté le remorquage de la tour sur site, ainsi que son cabanage et son raccordement à une pile de fondation 11 de type ancre à succion,
• la figure 3A représente une variante de la tour de la figure 3, dans laquelle les modules de flottabilité sont sensiblement uniformément répartis le long de la tour pour le remorquage, mais espacés les uns des autres en position de remorquage, se rassemblent dans la partie haute de la tour, par simple coulissement le long du tendon central, lors du cabanage, sur une hauteur H2 d'environ 50% de la hauteur de la tour,
• les figures 4A-4B représentent en vue de côté une tour selon l'art antérieur, sans flotteur (4A) et avec flotteur (4B),
• les figures 5A et 5B représentent en vue de côté d'une tour selon l'invention, équipée de modules de flottabilité coulissant le long dudit tendon central vertical (figure 5A) et sans les modules (figure 5B),
• la figure 6 représente une vue en perspective d'une coupe de la tour selon un plan perpendiculaire à son axe, ladite tour étant en position verticale,
• la figure 7A représente une vue en perspective d'un module de flottabilité selon l'invention sans ses éléments de flottabilité,
• la figure 7 représente une vue en perspective de deux modules de flottabilité identiques accolés l'un contre l'autre, avec des éléments de flottabilité en cours de pose dans l'un des modules,
• la figure 8 représente une vue en perspective d'un flasque de guidage de conduites d'un module de flottabilité selon l'invention, vu de l'extérieur, c'est-à-dire vu du plan d'interface avec un module de flottabilité voisin,
• la figure 9 représente une vue en perspective du flasque de guidage de la figure 8, vu de l'intérieur, c'est-à-dire du côté du plan d'interface avec les éléments de flottabilité en mousse syntactique maintenus par ledit flasque de guidage à chaque extrémité.

Other features and advantages of the present invention will become more apparent in the light of the detailed description which follows, given in an illustrative and nonlimiting manner, with reference to the drawings in which:

• the figure 1 is a side view of a bottom-surface connection installation 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 substantially vertical position by a buoyancy distributed over the entire height of the tower, and by a float 8 secured to the top of the tower, a plurality of flexible 6-1,6-2 connecting the upper ends of said risers head of said tower to an FPSO anchored to the surface nearby,
• the figure 2 represents in side view the tower being installed or maintained, the head float not being installed,
• the figure 3 represents in side view the towing of the tower on site, as well as its cabanage and its connection to a foundation pile 11 suction anchor type,
• the figure 3A represents a variant of the tower of the figure 3 , wherein the buoyancy modules are substantially uniformly distributed along the tower for towing, but spaced apart from one another in the towing position, gather in the upper part 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,
• the Figures 4A-4B represent in side view a tower according to the prior art, without float (4A) and with float (4B),
• the Figures 5A and 5B represent 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 )
• the figure 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,
• the Figure 7A represents a perspective view of a buoyancy module according to the invention without its buoyancy elements,
• the figure 7 represents a perspective view of two identical buoyancy modules placed side by side with buoyancy elements being installed in one of the modules,
• the figure 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 neighboring buoyancy module,
• the figure 9 represents a perspective view of the flange of guiding the figure 8 seen from the inside, that is to say on the interface plane side with the syntactic foam buoyancy elements held by said guide flange at each end.

In the figure 1 there is shown a bottom-surface connection facility 1 connecting two underwater lines 2-1,2-2 resting on the bottom of the sea 12 to a floating support type FPSO 10 moored by anchor lines 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. On the figure 1 , 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 means of an angled 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.

As shown on the figure 5 , the 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 having orifices 23, 23. 1.23-4, 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, as a result, the entirety of the buoyancy thrust IF 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 . As a result, 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 FPSO are in place, which exert horizontal recall efforts when the sea is rough. For clarity of explanation, the buoyancy modules 20 are schematically represented on the Figures 1 to 3 and on the figure 5 , and in more detail on Figures 6 to 9 .

On the figure 6 is shown in perspective a section of the tower 1 in vertical position, cut above a buoyancy module 20. To the axis of the buoyancy module 20 is the central tendon 4 and the periphery, the lines 3- 1.3-2 type PiP (pipe in pipe) comprising an outer pipe 3a and including a production line 3b slightly off-center, so as to leave room for a water injection pipe or pipe injection of gas 3c, as well as two water injection lines 3-3,3-4.

On the figure 7 , there is shown in perspective a section of the tower in the manufacturing position, showing two contiguous buoyancy modules 20 _n , 20 _{n + 1} , the module 20 _{n + 1} not being completely assembled, two buoyancy elements in the form of block of syntactic foam 21 being ready to be inserted before the blocking locking flanges 22 are joined by ties 24 to constrain the buoyancy elements between two flanges 22, thus ensuring the overall rigidity of said buoyancy module 20.

On the Figures 8 and 9 , there is shown in perspective a flange 22 of buoyancy module respectively in external view ( figure 8 ), ie seen from the interface side with the buoyancy module adjacent, and in internal view ( figure 9 ), that is to say viewed from the interface side with the buoyancy 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. This provides a clearance of 5 to 15 mm, so an inner diameter of the upper sheath 10 to 30mm to the outer diameter of said pipe 3a concerned. Similarly, the central sheath 23 corresponding to the central tendon 4 will have the same increase in diameter relative to the outer diameter of said tendon.

The buoyancy elements 21, all of the same cylindrical shape, are inserted into complementary shapes 22a of the internal face of the flanges 22, as shown in FIG. figure 9 , and are preferably evenly distributed around the periphery of the module 20. On the figures 7 and 9 two adjacent buoyancy elements 21 are shown between two adjacent ducts 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 to adopt the configuration shown in the drawings. figures 7 and 9 .

In 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. Because these 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.

If it had been planned to introduce only four blocks of syntactic foam between the four pipes, it is understood that 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.

To have only one type of buoyancy element, that is to say buoyancy elements 21 of the same shape, 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 outer 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 21 applied against the walls of the sleeves, as shown on the figure 7 . The buoyancy elements are locked between two flanges 22 by tie rods 24 connecting the two flanges 22 and providing 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. To compensate for minimal variations in length, 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. In the same manner and for the same purpose, 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.

On the figure 3A the towing and hoisting of a tower 3, the buoyancy of which is evenly distributed along the tower 3 for its towing, is shown, the modules 20 being pre-assembled 20a, here 3 by 3 and interconnected at 31 by cables 30, a cable 30 being secured to one end of the upper support structure 4a and at the bottom of a fixed floating element on the lower structure 5. Thus, throughout the towing, 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 it is towed on the surface by at least one vessel 10-1, 10-2 using cables 15-1 connected to the upper structure 4a. Then, after having disconnected the cable 30 from the lower structure 5, when the cabin is made according to a known method from the native 10-2 and a cable 15-2 connected to the lower end 5a of the tendon 4, which is weighted with a weight 16, by unscrewing the cable 15-2 from the ship 10-2. As soon as the inclination of the tower 3 is sufficient, the modules 20, in groups of 3, tend to slide along the central tendon upwards and come into contact with each other and thus gather towards the top of the tower on the underside of the carrier structure 4a, the cables 30 adopting a relaxed shape, loose. 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 H2 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 FIG. WO-2006-136960 wherein each of the buoyancy modules is rigidly connected to the central tendon.

As shown on the Figure 5A according to the invention, the buoyancy modules 20, due to their sliding along the tendons 4 and 3-1,3-4 conduits transfer all of their IF buoyancy to the upper structure 4a. On the other hand, no compression force is transmitted to the central tendon 4 by the upper structure 4a, the latter supporting the entirety of the total weight of the pipes in suspension. Thus, according to the present invention, the upper structure 4a is subjected to a resultant traction (TR = ΣF - Pt) corresponding to the sum of the floats of the various modules 20 less the total weight Pt of the tower. In practice, to allow towing of the tower under satisfactory conditions, it is necessary to provide that the sum of the IF buoyancy represents 102 to 110% of the total weight of the tower according to the sea states.

Furthermore, it is necessary to provide an 8-1 float at the top so that the resulting traction T _R exerted at the top of the structure 4a at the top of the tower 3 corresponds to at least 10%, generally from 10 to 50% of the total weight of the tower Pt, so as to compensate for the weight of the flexible pipes 6-1,6-2 which will be connected to it and the horizontal restoring forces caused by the flexible pipes in case of deflection in inclination of the tower 3 of an angle α.

Depending on the stiffness that we want to give to the system, we will provide a resultant traction brought by the float of up to 50% of the total weight of the tower for a system with high stiffness, that is to say with very heavy flexible pipes and an apex angle of less than 5 ° and a vertical tensile result of about 10% of the weight of the tower for relatively light flexible pipes and major corners of the tower inclination of 5 at 8 ° to the vertical.

The float at the summit must therefore bring a specific buoyancy T1 equal to [T1 = TR - (ΣF -Pt)], ie [(10 to 50% x Pt) - (2 to 10% x Pt)].

Finally, according to the present invention, 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).

The prior art is shown in side view on the Figures 4A and 4B . In the prior art, the buoyancy modules 40 are integral with a central tendon 4 at cleats 41. Figure 4A , the buoyancy elements 40 transmit their buoyancy F directly to the central tendon 4 by means of the cleats 41 integral with the central tendon. In this configuration, the buoyancy elements 40 provide a total IF buoyancy of 102 to 110% x Pt (total weight of the tower) to allow its towing on the surface.

On the other hand, in the vertical operating position, the buoyancy modules 40 do not contribute any more to the buoyancy of the tower.

Thus, without taking into account the weight 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 ΣF to the top. If now we consider the self-weight of the pipes 3-1,3-2,3-3,3-4 and the structure 4a, it is then exerted at the level of the upper bearing structure 4-1 a force downward compression ratio equal to Pt (weight of the tower). Thus, 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 thus, the vertex 8-1 must bring an equal buoyancy T1 at T _R + (Pt - F).

In practice, for a tower of about 1,000 meters and putting 2 to 10 m long buoyancy modules, the number of modules can range from 100 to 400. We can therefore, by approximation, neglect F and consider that the buoyancy T1 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.

In addition, the foundation 11a must take up all the tractions which are 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 total float F (IF), (TR + IF). 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 T1 [(T _R - (ΣF - 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.

Thus, for Pt = 1000 T, with 204 buoyancy modules exerting a unit traction of F = 5 Tonnes, ΣF = 1020 T and with a vertical tension at the top R = 20% of the weight of the tower, ie 200 T, the foundation 11a of the prior art is subjected to the global force (ΣF + T _R ), ie 1200 T and the head float 8-1 must have a buoyancy of about 1200 T also.

On the other hand, according to the invention, the buoyancy of the float at the head is [T1 = TR - (ΣF - Pt)], ie about 180 T, and the foundation 11 is subjected substantially only to the sole force of restoring tension T _R of 200 T, that is, float buoyancy forces T1 and tension take-up by the foundation (T _R ) approximately 6 times less than in the prior art.

In the description of the present invention, 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 lines 3-1,3-4 suspended to the upper carrier structure 4a including the weight of the gooseneck devices 7-1,7-2, the lower bends 3a and the automatic connectors 9a-9b, and that of the flexible foot joint 5a and said lower structure 5, but deduction made of the possible buoyancy provided by the integrated fixed buoyancy elements such as, where appropriate, at the level of said goosenecks, said upper structures 4a and lower 5, said automatic connectors 9a-9b and said flexible joint 5a.

In the present invention, the tower is able to remain vertical in the absence of the vertex tensioning float 8, as shown in FIGS. figures 2 and 3A , 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, because in the event of an incident on said head float 8, as shown in FIG. figure 2 Simply bleed the vertical lines and flexible lines, disconnect the hoses from the FPSO and keep them sub-surface thanks to a small buoy connected to a dead body, so as to reduce considerably the horizontal force, so the angle α 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.

Claims (10)

1. A tower of the multi-riser hybrid type, the tower comprising:

   a) a vertical tension leg (4) secured at its top end to a carrier structure (4a) suitable for being suspended from a float, referred to as a "top" float (8), that is immersed in the subsurface, suspension preferably being via a chain or cable (8a), said tension leg being secured at its bottom end to a bottom guide structure (5) and being suitable for being fastened to a base member resting on the sea bed or to a foundation anchor that is preferably of the suction anchor type (11) embedded in the sea bottom, the bottom end connection preferably being via a flexible joint (5a);

   b) a plurality of rigid vertical pipes (3-1, 3-2) known as risers, the top end of each pipe or riser being secured to said carrier structure (4a), the bottom end of each said rigid pipe or riser being suitable for being connected to an undersea pipe (2-1, 2-2) resting on the sea bottom, the connection preferably being via automatic connectors between said bottom ends of the risers and the ends of the undersea pipes (2-1, 2-2), and preferably via bent sleeves (2a) and/or junction pipes (2bc);

   c) a plurality of guide means (22) for guiding said risers, said guide means and said bottom guide structure (5) being suitable for maintaining said risers arranged around said tension leg, preferably being distributed regularly and symmetrically around said tension leg; and

   d) buoyancy elements (21) co-operating with said tension leg, the buoyancy elements being distributed along said tension leg, and preferably being constituted by buoyancy elements that withstand undersea hydrostatic pressure, and more preferably being syntactic foam buoyancy elements; and

   e) said tower (3) comprises a plurality of buoyancy and guide modules (20, 20-1, 20-n) constituting a plurality of independent structures suitable for sliding along said risers, said structure (20) supporting said buoyancy elements (21) and guiding said risers in a preferably regularly and symmetrically distributed position around said tension leg,

   characterized in that said independent structures constituting said buoyancy and guide modules (20, 20-1, 20-n) are suitable for sliding along said tension leg.
2. A tower according to claim 1, characterized in that, together, the buoyancy modules (20) provide accumulated buoyancy (ΣF) representing a traction force of magnitude greater than the total weight of the tower (Pt), preferably 102% to 110% of the total weight of the tower, and said top float (8) provides its own buoyancy (T1) such that T1 = T_R - (ΣF - Pt), and said foundation (11) needs to accommodate at least the resultant traction (T_R) at the top of the tower, T_R representing the upward vertical resultant tension at the top float (8) equal to 5% to 50% of the total weight of the tower (Pt), preferably 10% to 20% of the total weight of the tower.
3. A tower according to claim 1 or claim 2, characterized in that said buoyancy and guide modules (20, 20-1, 20-n) extend over a length lying in the range 2 m to 20 m and are at lest 50 in number, there preferably being 50 to 500 buoyancy modules for a tower having a height of at least 1000 m.
4. A tower according to any one of claims 1 to 3, characterized in that said plurality of buoyancy and guide modules disposed one against another occupy no more than 75%, and preferably less than 50% of the length of the tower between said top carrier structure (4a) and the bottom guide structure (5) secured to the tension leg.
5. A tower according to any one of claims 1 to 4, characterized in that the various buoyancy and guide modules (20, 20-1, 20-n) are connected to one another by links (30) suitable for preventing the first module (20-1) from moving away from said carrier structure (4a) and such that two consecutive modules (20-n, 20-n+1) do not move apart by more than a given maximum distance (d), which distance is preferably identical between the various modules, and said links (30) being of a length such that the various modules are distributed in substantially uniform manner over the entire length of the tower between said carrier structure (4a) and said bottom structure (5), the first module being linked to said carrier structure and the last module being located at said bottom structure (5) while said tower (3) is floating on the surface of the sea (13) when being towed by a ship (10-1, 10-2), with said links not preventing said modules from sliding upwards when said tower is up-ended and put into a vertical operating position in a said bottom-to-surface connection installation.
6. A bottom-to-surface connection installation (1) between a plurality of undersea pipes (2-2, 2-2) resting on the sea bottom (12) and a floating support (10) on the surface (13), the installation comprising:

   1) a tower in accordance with any one of claims 1 to 5; and

   2) a plurality of connection pipes, preferably a plurality of flexible connection pipes (6-1, 6-2) between the top ends of said risers and the floating support (10), more preferably said flexible pipes being in the form of dipping catenaries, said flexible pipes (6-1, 6-2) being connected to the top end of said risers via swan-neck type devices (7-1, 7-2).
7. A method of towing at sea a tower according to any one of claims 1 to 5, and a method of putting it into place in an installation according to claim 6, the method being characterized in that said tower floats on the surface while it is being towed by at least one surface ship (10-1, 10-2), said buoyancy and guide modules (20) being distributed along the entire length in preferably regularly distributed manner while spaced apart from one another, and after the tower has been up-ended, said buoyancy and guide modules slide upwards so as to be pressed one against another and preferably over only a fraction of the height of the tower.
8. A buoyancy and guide module (20) for a tower according to any one of claims 1 to 5, the module comprising two end plates (22) connected together by ties (24) together with said section-member buoyancy elements (21) that are blocked and held in position between and by said two end plates, preferably forming a module presenting a circular cross-section, each end plate having a central orifice and peripheral orifice (23-1, 23-4), said peripheral orifices (23-1, 23-4) and said buoyancy elements (21) being of the same shapes and arranged around said central orifice (23) in preferably regular and symmetrically-distributed manner about said central orifice (23), said orifices (23, 23-1, 23-4) forming bushings suitable for passing said risers and a said tension leg, thereby enabling said modules to be guided in sliding.
9. A buoyancy and guide module (20) according to claim 8, characterized in that each end plate comprises a plurality of end plate parts (22-1, 22-4) that are fixed to one another, preferably at least as many end plate parts as there are said peripheral orifices (23-1, 23-4), each end plate part being suitable for blocking and holding the longitudinal end of a said section-member buoyancy element (21).
10. A buoyancy and guide module (20) according to claim 8 or claim 9, characterized in that it includes first resilient elements (25a) that are interposed preferably in the form of pads between the longitudinal ends of said buoyancy elements and said end plates at at least one of said longitudinal ends of the module and preferably also second resilient elements (25b) preferably in the form of pads on the outside face of at least one of the two end plates, so as to ensure that the buoyancy forces are distributed in more isostatic manner and are better transferred between pairs of consecutive modules.

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