Device for anchoring a raceway mounting of a seabed-to-surface facility

Abstract

A device and method for anchoring a raceway mounting of a seabed-to-surface facility. The apparatus having a rigid structure immersed in a subsurface by floats and anchored to the sea bottom by tension legs, the device suitable for supporting troughs in a bottom-to-surface connection installation, having a plurality of flexible lines extending to the sea bottom, the flexible lines being supported by the troughs, the trough support structured is connected to a base resting on and/or anchored to the bottom of the sea by a plurality of n tension legs tensioned in parallel, by the float, n being not less than six, each of a plurality of p tension legs from the n tension legs, where p is not less than (n2) being connected at one of its ends to a distance-varying device secured to the base or to the support structure, the distance-varying device suitable for varying the distance between the support structure and the base.

Claims

1. A device comprising a rigid support structure held immersed in a subsurface by floats and anchored to the bottom of the sea by tension legs, said support structure supporting a plurality of arch-shaped support and guide elements, referred to as troughs, in a bottom-to-surface connection installation between a single floating support and the bottom of the sea, the installation having a plurality of flexible lines comprising flexible pipes extending to the bottom of the sea where said flexible pipes are connected to wellheads, to equipment, or to the ends of undersea pipes resting on the sea bottom, said flexible lines being supported by said plurality of troughs: wherein said support structure is connected to a base resting on or anchored to the sea bottom by a total number of n tension legs tensioned substantially in parallel by said floats, where n is not less than six, each of a total number of p said tension legs from the n tension legs, where p is not less than n2, being connected at one end to a distance-varying device, said distance-varying device being connected to said base, or to said support structure, and each of the other ends of the p tension legs being fastened to an attachment element secured to said support structure or to said base, said distance-varying device being connected to or integral with said support structure or said base and suitable for being actuated to vary the distance between said attachment element and said base or said support structure to which said attachment element is fastened or connected; and wherein no end of the tension legs is directly attached to said floats and wherein a top end of said tension legs is fastened at a top attachment element secured to said support structure or to said distance-varying device, a bottom end of said tension legs being fastened at a bottom attachment element secured to said distance-varying device or to said base, wherein said distance-varying device is suitable for: varying a distance between said top attachment element and said support structure when said distance-varying device is secured to said support structure; or varying a distance between said bottom attachment element and said base when said distance-varying device is secured to said base; and wherein said support structure presents a longitudinal shape of substantially rectangular horizontal section, the device having at least six tension legs, each tension leg being connected to said distance-varying device, and said support structure having at least six top attachment elements, with four of the top attachment elements defining four corners of a rectangle, two other top attachment elements being arranged inside a circle that circumscribes said rectangle, at or in the vicinity of two long sides of the rectangle, the four corner attachment elements being arranged in a proximity of the longitudinal ends of said support structure.

2. The device according to claim 1, wherein said other two top attachment elements are arranged inside said circle that circumscribes said rectangle, at a transverse axis of said rectangle located at a middle of said rectangle.

3. The device according to claim 1, wherein said support structure is supported by and fastened to at least one incorporated bottom float.

4. The device according to claim 1, wherein said support structure comprises two portions that are hinged to pivot about a pivot axis that is substantially horizontal, suitable for allowing each of said two hinged portions to pivot relative to the other through an angle of 10 to +10 relative to horizontal said pivoting being limited by top abutments and bottom abutments of each of said two hinged support structure portions, said two hinged support structure portions being symmetrical about a vertical midplane containing said pivot axis, each of said two hinged portions being connected to said base by at least three said tension legs, an end of at least one said tension leg being connected to a distance-varying device.

5. The device according to claim 4, wherein each said hinged portion of said support structure presents a longitudinal shape of substantially rectangular horizontal section, the device having at least six tension legs, each connected to said distance-varying device, and said support structure has at least six top attachment elements, each said hinged portion of said support structure comprising at least: two top attachment elements arranged at the longitudinal ends of each said hinged portion that are further from said pivot axis; and one top attachment element arranged closer to said axis of rotation than said longitudinal end.

6. The device according to claim 5, wherein the top attachment elements of said first hinged support structure portion are arranged symmetrically to the three top attachment elements of the second hinged support structure portion about a substantially vertical plane containing said pivot axis.

7. The device according to claim 1, wherein said top attachment elements of said p tension legs are arranged at said support structure and said bottom attachment elements of said p tension legs are arranged at said distance-varying devices, said distance-varying devices being secured to said base, and each said distance-varying device serves to vary the distance between said bottom attachment element and said base.

8. The device according to claim 1, wherein said tension legs are cables or chains.

9. The device according to claim 1, including at least one said distance-varying device comprising an actuator.

10. The device according to claim 9, wherein said bottom attachment element is secured to a movable rod of a hydraulic actuator having an actuator cylinder secured to the base.

11. The device according to claim 9, wherein said actuator includes a pressure gauge or a pressure sensor at an orifice of an actuator cylinder, and a locking device suitable for locking a rod in position by closing an actuator chamber in a leaktight manner.

12. The device according to claim 9, wherein a hydraulic actuator is connected, or is suitable for being connected, to a pressurized fluid feeder unit on board an undersea robot.

13. The device according to claim 1, wherein said support structure supports five to twelve troughs being arch-shaped and having a radius of curvature in the range 1.5 m to 3 m, said support structure having a width lying in the range 3 m to 5 m, and a length lying in the range 10 m to 30 m, and dead weight in air lying in the range 30 t to 50 t, and said support structure has buoyancy incorporated under the troughs so that each said tension leg is subjected to tension lying in the range 0.5 t to 10 t, said tension leg being dimensioned to be suitable for supporting a tension that is two to four times said tension to which said tension leg is subjected.

14. The device according to claim 1, wherein said support structure is a metal lattice structure extending longitudinally in a horizontal direction.

15. The device according to claim 1, wherein said rigid support structure is suspended from at least one immersed top float to which said support structure is connected by flexible connection elements.

16. The device according to claim 1, wherein said support structure supports a plurality of said troughs that are laterally offset in parallel in one direction of said support structure, said troughs being arch-shaped.

17. A bottom-to-surface connection installation between a single floating support and the sea bottom, the installation comprising a plurality of flexible lines with flexible pipes extending from said floating support to the sea bottom, where the flexible lines are connected to wellheads, to equipment, or to the ends of undersea pipes resting on the sea bottom, said flexible lines being supported by a plurality of troughs, each flexible line defining two pipe portions comprising a first flexible line portion in a hanging double catenary configuration between the floating support and one trough of said plurality of troughs, and a second flexible line portion in a single catenary configuration between said one trough and a point of contact of the second flexible pipe portion with the sea bottom, said plurality of troughs being supported by a device according to claim 1.

18. A method of modifying the tensions to which said tension legs of a device according to claim 1 are subjected, wherein at least one of said distance-varying devices is actuated so as to adjust a tension of a tension leg to which said one of said distance-varying devices is connected to have a desired controlled value.

19. The method according to claim 18, wherein the tension of said tension leg to which said one of said distance-varying devices is connected is decreased by actuating said one distance-varying, and then replacing said tension leg.

20. The method according to claim 18, wherein a mechanical connection between various tension legs and said support structure is made mechanically quasi-isostatic by actuating at least one said distance-varying device.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF DRAWINGS

(1) Other characteristics and advantages of the present invention appear in the light of the following detailed description given with reference to the following figures, in which:

(2) FIG. 1 is a side view of a bottom-to-surface connection installation of the invention between a floating support 2 that is anchored at 2b, and a metal support structure 5 supporting a plurality of arch-shaped troughs 4 that are anchored to a base 8 resting on the sea bottom 3, via a plurality of tension legs 7;

(3) FIG. 2A is a side view of a prior art installation in which a single flexible pipe 1 rests on a single arch-shaped trough 4 that is anchored by a single tension leg 7 terminating at its top end in a bridle 7c so that the tension leg is attached to the trough via two top attachment points 7a, a buoyancy element being situated above the trough and connected thereto;

(4) FIG. 2B is a front view of a prior art installation comprising a support structure with three troughs, said structure being anchored by two tension legs and being fitted with a single float;

(5) FIG. 3A is a front view in the YZ plane of an installation of the invention comprising a support structure 5 secured to a plurality of floats 6a incorporated in said structure and supporting a plurality of troughs, specifically six troughs 4a to 4f, said structure being anchored by six tension legs 7 attached at six top attachment points 7a1 to 7a6;

(6) FIGS. 3B and 3C are views of the support structure 5 as seen from beneath in horizontal section (XY plane) corresponding to the structure of FIG. 3A and showing the arrangement within a rectangle of the six top attachment points 7a1 to 7a6 of the six tension legs;

(7) FIGS. 3D and 3E are two views from beneath of the support structure 5 in horizontal section (XY plane) showing two top attachment points 7a3-7a4 situated overhanging from the support structure 5, three or four top attachment points (FIG. 3A or FIG. 3B) lying on a circle (C), while the other top attachment points are situated inside the circle C;

(8) FIG. 4A is a side view in the XZ plane of FIG. 3A showing details of buoyancy elements 6a constituted by caissons of prismatic or rectangular type section in the XZ plane that are incorporated in the support structure 5 beneath the troughs, said caissons being filled with solid, liquid, or gaseous compounds that are lighter than sea water;

(9) FIG. 4B is a side view of FIG. 3A showing details of buoyancy elements 6a constituted by caissons of cylindrical type with circular section in the XZ plane and incorporated in the trough carrier structure;

(10) FIG. 5 is a side view of an installation showing a common base 8 having two distance-varying devices 10 for individually adjusting the length of each of two tension legs 7-1, 7-2 attached to bottom attachment points 7b1, 7b2 at the rod 10b of an actuator of the invention;

(11) FIG. 6 shows a variant embodiment of a distance-varying device 10 comprising an actuator having a hydraulic pressure gauge or sensor 11a for the actuator of the invention; and

(12) FIGS. 7A to 7C show an embodiment in which said support structure 5 comprises two portions 5-1, 5-2 hinged to point about a middle axis X1X1.

DETAILED DESCRIPTION OF THE INVENTION

(13) FIG. 1 shows a bottom-to-surface connection installation comprising two flexible pipes or electric cables 1a, 1b connected at one end 2a to a floating support 2 that is held in position by anchor lines 2b, the other ends of said flexible pipes resting on the sea bottom 3 substantially at 1c. The double catenaries flexible pipes 1a, 1b are in a hanging catenary configuration going down from the floating support 2 to respective horizontal tangency points 1a, 1b, and then in a rising catenary configuration up to the entry 4a1 of respective trough 4a and 4b, of radius of curvature R greater than the minimum acceptable radius of curvature for said flexible pipes or said cables. Thus, the flexible pipe 1a enters the trough 4a at 4a1, then rests on said trough, and then leaves it at 4a2 in order to go down to the sea floor 3 substantially at 1c in a single catenary configuration 1a1. Each of the troughs 4a, 4b, etc. is positioned to be laterally offset relative to the others on the support structure 5, as shown in FIGS. 2B and 3A. In general, the troughs in a given installation all have the same radius of curvature and they are secured to one another by means of said support structure 5. Said structure has buoyancy elements 6a that may either be incorporated in said support structure 5, or else be external, generally in the form of floats 6b situated above said structure and connected thereto by means of respective single tension legs 6c1, as shown in FIG. 2A, or by means of a bridle 6c2, as shown in FIG. 2B.

(14) The support structure 5 is maintained substantially at an altitude h above the sea bottom by a plurality of tension legs 7 that are connected at their top ends via top attachment points 7a to said support structure 5, and at their bottom ends via bottom attachment points 7b to a base 8 resting on the sea bottom 3, e.g. a weight base, or indeed a suction anchor embedded in the sea bed and referred to below as a foundation.

(15) In the prior art, as shown in FIGS. 2A and 2B, it is generally sought to minimize the number of tension legs between the support structure 5 and the base 8. Thus, for a singe flexible pipe 1, as shown in FIG. 2A, a single tension leg 7 is used, possibly connected to said support structure 5 via a bridle 7c, said tension leg then being in the same vertical plane as said flexible pipe. Likewise, for a plurality of flexible pipes or electric cables arranged on a plurality of three troughs 4a to 4c that are laterally juxtaposed, as shown in FIG. 2B, the support structure 5 is connected to its base 8 by means of two tension legs, possibly connected to said support structure 5 via respective bridles (not shown).

(16) These two means for anchoring the support structure 5 are generally preferred since they serve to minimize the number of tension legs, and thus overall costs, and furthermore each of them performs anchoring in an isostatic mode. In order to remain isostatic, it is possible in the configuration shown in FIG. 2B to envisage using three tension legs, however in order to retain this overall isostatic characteristic, it is appropriate for the attachment points of said tension legs not to be in alignment but rather to be in a triangular configuration, in any plane that is not a vertical plane.

(17) Thus, in the above-described prior art, the rupture of a tension leg either leads to destruction of the installation as in the example of FIG. 2A where there is only one tension leg, or else leads to dangerous unbalance of the support structure 5 when there are two tension legs as shown in FIG. 2B, or when there are three tension legs arranged in a triangle in a plane that is substantially vertical. This generally leads to the support structure 5 tilting to a large extent or completely, thereby running the risk of irremediably damaging the flexible pipes or electric cables and thus leading to partial or total destruction of the bottom-to-surface installation. With a single tension leg, the support structure 5 is then completely free to move upwards and in all directions without any control being possible. In addition, when such incidents involve crude oil production lines, they run the risk of leading to major pollution.

(18) Such ruptures are particularly to be feared in installations where the depth of water is not very great, i.e. a few tens to a few hundreds of meters, since at such depths, swell and current act throughout the depth of water and they are particularly dangerous for the support structure 5 with its troughs and its buoyancy elements. Furthermore, swell, wind, and current also destabilize the floating support, and the resulting movements are transferred by the flexible pipes to the support structure 5 and thus to the tension legs 7 and to their top and bottom attachment points 7a and 7b. Thus, when the depth is not very great, i.e. a depth in the range 25 m to 300 m, and when ocean and weather conditions are severe, the hoses, the trough carrier structure, and its connections with the foundation are subjected to particularly large forces that can become extreme, leading to wear and fatigue, mainly at the ends of the tension legs and at their attachment points. Such accidents have already occurred in the recent past.

(19) In order to avoid the consequences of a tension leg rupturing, or of one of its attachment points rupturing, the device of the invention advantageously anchors the support structure 5 by means of at least six tension legs that are preferably distributed symmetrically about the axis YY of said support structure 5, as seen from beneath in particular FIG. 3B. Each of the top attachment points 7a1-7a2-7a3-7a4-7a5-7a6 of the structure is connected to the corresponding respective bottom attachment point (not shown) 7b1-7b2-7b3-7b4-7b5-7b6 via a respective tension leg 7.sub.1-7.sub.2-7.sub.3-7.sub.4-7.sub.5-7.sub.6, with all of the tension legs preferably being mutually parallel and vertical.

(20) FIG. 5 shows the bottom attachment points 7b1-7b2 together with the tension legs 7.sub.1-7.sub.2, said bottom attachment points being secured, in accordance with the invention, to the foundation 8, not directly but via a respective distance-varying device 10 serving for adjusting the distance L, i.e. the respective distances L.sub.1-L.sub.2, of said bottom attachment point from said foundation 8. A rigid support structure 5 anchored via two tension legs connected to the foundation, or indeed three tension legs providing the three top attachment points 7a of said tension legs are not in alignment, nor situated in a common vertical plane, presents a mechanical configuration that is said to be isostatic, i.e. the distribution of forces in said tension leg is unique and can thus be calculated in known manner, in particular as a function of the distribution of the loads supported by the support structure 5 and the buoyancy elements incorporated in said structure. In contrast, when three tension legs have top attachment points that are in alignment, and also when there are more than three tension legs, the system becomes statically undetermined, i.e. the distribution of forces among the tension legs can no longer be calculated in unique manner. In a statically undetermined situation, as with a four-legged stool, the system can wobble and some of the tension legs might be subjected to a major fraction of the load while others are relatively lightly loaded, and indeed in certain situations completely slack, i.e. they transfer no load at all.

(21) One solution for reducing the statically undetermined nature of the system having multiple tension legs consists in designing a support structure 5 that is very flexible, i.e. that can deform to a large extent, thereby enabling all of the tension legs to contribute, but only within certain limits. The main drawback of such a configuration lies in the problems of fatigue and wear that are already expected in the tension legs and their attachment points, then become transferred to the support structure 5 where, in the event of an incident, they can lead to even greater damage.

(22) In order to restore a pseudo- or quasi-isostatic nature, i.e. in order to reduce the extent to which a system having multiple tension legs is statically undetermined, it is advantageous to install on the tension legs, and preferably on all of them, respective distance-varying devices 10 of the invention, each of which is suitable for adjusting the distance between its top attachment point 7a to the support structure 5 and its bottom attachment point to the foundation 8.

(23) FIGS. 3 to 6 show side views of a device for restoring a quasi-isostatic nature to the overall device for anchoring the support structure 5 to the base 8, regardless of the number of tension legs 7, i.e. three tension legs when there is only one row of tension legs situated in a common longitudinal vertical midplane of the support structure 5, or preferably six tension legs arranged in a rectangle, as when there are two parallel rows of three tension legs substantially in alignment in a common substantially vertical plane arranged respectively on the long sides of the rectangle 4 with four top attachment points at the four corners of the rectangle, as shown in FIGS. 3B, 3C, and 3D.

(24) In FIG. 3B, the two intermediate top attachment points 7a3 and 7a4 are arranged on the middle transverse axis XX of the support structure 5, while in FIG. 3C, the two intermediate top attachment points 7a3 and 7a4 are offset on either side of the middle transverse axis XX of the support structure 5.

(25) In FIGS. 3D and 3E, two intermediate top attachment points 7a3 and 7a4 are offset on either side of the middle transverse axis XX of the structure and they are also offset along XX to lie outside the long side of the rectangle, so that they are no longer attached to said support structure 5, but to an overhang. In FIG. 3E, a short side of the rectangle lies between two corner top attachment points 7a1, 7a2, with the opposite side of the rectangle in the longitudinal direction having only one top attachment point 7a6 in a middle position. Under all circumstances, all of the top attachment points of the tension legs then lie on or inside a circle C circumscribing the rectangle formed by the four top attachment points 7a1, 7a2, 7a5, and 7a6 that the four corners (FIG. 3D), or the circle circumscribing the triangle formed by the three top attachment points 7a1, 7a2, and 7a6 (FIG. 3E).

(26) Thus, a non-symmetrical configuration as shown in FIG. 3C, 3D, or 3E is advantageously suitable in certain situations where the flexible pipes are of different sizes, i.e. where there are differences between the vertical forces induced by those pipes and their respective locations on said support structure 5 and in the distribution of the buoyancy elements.

(27) The term quasi-isostatic is used herein to mean that all of the tension legs co-operate in taking up the tension created by the upwardly-directed result out buoyancy, and each of said tension legs takes up substantially a known and adjustable percentage of said overall tension.

(28) The quasi-isostatic adjustment and distance-varying device 10 of the invention acts on each of the tension legs of the device in individual manner to adjust the distance between the base 8 and the support structure 5, thereby distributing the unit loads in each of said tension legs in fully controlled manner, and thus making the device quasi-isostatic.

(29) To do this, the altitude L of the bottom attachment point 7b of the tension leg 7 above the foundation 8 can be adjusted by the distance-varying device 10, which is shown in this example as being a hydraulic actuator with a rod that can be blocked mechanically, as is known to the person skilled in the art. Said distance-varying device 10 of the invention is constituted by an actuator cylinder or body 10a secured to the base and by an actuator rod 10b having its top end constituting the bottom attachment point 7b of the tension leg 7. The axis of said actuator cylinder or body 10a-10b is preferably vertical. The actuator cylinder or body 10a has an orifice 11 enabling said actuator cylinder or body to be connected via a duct to a hydraulic unit (not shown) available on board an automatic undersea remotely operable vehicle (ROV) 13 that is controlled from an installation ship 14 on the surface. Thus, by pressurizing the actuator, the actuator rod is forced to retract lengthwise in a downward direction, and this serves to adjust and shorten the distance between the base 8 and the support structure 5, and thus to shorten lengths, thereby having the effect of increasing tension in the corresponding tension leg. By acting in succession on each of the tension legs, the load taken up by each of the tension legs can thus be distributed in advantageous and in fully controlled manner, thereby enabling the assembly to be made quasi-isostatic.

(30) When force is applied to the actuator by increasing the pressure P of the fluid in the actuator, its rod retracts, and the tension in the corresponding tension leg increases. Thus, the percentage of the overall force taken up by said tension leg increases, and in general the other tension legs see their tensions decrease a little.

(31) Likewise, when the pressure P of the fluid in the actuator is reduced, the rod of the actuator extends and the tension in the corresponding tension leg decreases, so that the percentage of the overall force taken up by said tension leg decreases, and in general the other tension legs see their tensions increase a little.

(32) Thus, increasing or decreasing the pressure in a said actuator serves to adjust the distance between the foundation 8 and the top attachment point 7a in each of the tension legs and in individual manner, thereby adjusting the percentage of the overall tension T that is taken up individually by said tension leg. When adjustment has been completed, a position locking device 12 for locking the position of the rod 10b of the actuator is itself actuated, and then the pressure in the actuator is released and the hydraulic feed hose is disconnected. On the right of FIG. 5, there can be seen the distance-varying device 10 acting on the tension leg 7.sub.2 in a locked position at an altitude corresponding to a distance L2, and on the left there can be seen the distance-varying device 10 relating to the tension leg 7.sub.1, which is shown while adjusting an altitude corresponding to the distance L1, the ROV 13 (an automatic submarine controlled from the surface) connected by the duct of hydraulic circuit 13a to the pressure feed orifice 11 being in the process of adjusting the pressure P in the actuator, and thus of adjusting the tension in said tension leg 7.sub.1. During this adjustment, the locking device 12 is held in the open position, and as a result the actuator is free to move in a lengthening or a shortening direction.

(33) For reasons of symmetry, it is generally preferred to fit each of the tension legs 7 with its own distance-varying device 10. Nevertheless, when there are n tension legs, it may suffice to have n2 distance-varying devices 10. The two non-adjustable tension legs, that are preferably situated at opposite longitudinal ends of the support structure 5, then define the substantially horizontal reference axis for anchoring the support structure 5 relative to the base 8, and adjusting each of the other tension legs then makes it possible to make the system quasi-isostatic, and thus enable the load on each of the tension legs to be distributed in controlled manner. likewise, it would be possible to have only n1 distance-varying devices 10 in association with n tension legs.

(34) It is even possible to install n3 distance-varying devices 10 for n tension legs, when the three non-adjustable tension legs are not in alignment so as to define a triangle that is substantially horizontal and thus a substantially horizontal reference plane for the support structure 5 relative to the base 8. Adjusting each of the other tension legs then enables the system to be quasi-isostatic, and thus enables the load on each of the tension legs to be distributed in controlled manner, however these three alternative configurations do not constitute the preferred version of the invention.

(35) Specifically, in a preferred version of the invention, where each of the tension legs has its own distance-varying device 10 enabling the length of each of the tension legs to be varied, there is no difficulty in performing maintenance operations consisting in changing any one of the tension legs without disturbing the operation of the device, and thus avoiding any need to stop oil production. It then suffices: to reconnect the duct of hydraulic circuit 13a of the ROV 13 to the orifice 11 of the actuator; then to unlock the position locking device 12 and release pressure in the actuator so as to relax said tension leg completely; then to disconnect the tension leg and replace it with a new tension leg; then to retension said tension leg to its initial value; and then to lock the position locking device 12 and disconnect the ROV 13.

(36) During such a maintenance operation on a tension leg, which is generally performed after said tension leg has ruptured, the overall force T is temporarily distributed over the n1 active tension legs, with tensions generally increasing in all of said n1 tension legs, and subsequently returning to their initial values once a new tension leg has been reinstalled and its own tension readjusted by using the device 10, as explained above. These operations of changing a tension leg may advantageously be performed in preventative manner, e.g. once every five years, so as to avoid problems of fatigue and rupture with the severe consequences that are to be feared.

(37) For simplicity and clarity of explanation, the adjustment and distance-varying device 10 is described above on the basis of a single-acting hydraulic actuator, since measuring the pressure P in the actuator gives very accurate information about the tension T applied in the corresponding tension leg. Nevertheless, it is entirely possible to use a distance-varying device 10 that is constituted by a mechanical actuator using a screw or a rack. However, under such circumstances, it is appropriate for said device also to incorporate a cell for measuring the load or tension applied to said tension leg, such as a dynamometer for reading directly by an ROV, or for transmitting data to the surface for a control station situated on board the floating support so as to be able to adjust the distribution of all of the loads in the various tension legs correctly.

(38) FIG. 6 shows a preferred version of the invention in which the actuator does not have a device 12 for blocking the rod but is blocked hydraulically by leaktight closure 11b of the internal chamber of said actuator. The actuator is thus permanently under pressure and a pressure gauge or sensor 11a is advantageously installed on a permanent basis on the orifice 11 of said actuator. A permanent display is thus made available showing the tension in each of the tension legs, which tension is correlated with said actuator pressure, and this display can be consulted very simply during routine inspections performed at regular intervals, e.g. by means of an ROV 13. By fitting said orifice with a pressure gauge or sensor 11a, information can be transmitted in automatic and permanent manner to the FPSO, either via an electric cable, or acoustically, thus providing a command center with accurate information about all of the hose arches and their anchor systems. In the event of any one of the tension legs rupturing, the command center is immediately informed, and is capable, where applicable, of determining which tension leg has failed. Likewise, in the event of damage to a buoyancy element 6, be that complete rupture or partial invasion, the overall vertical tension will decrease and some of the tension legs 7 will have their tensions drop. The command center of the FPSO is then rapidly informed and can thus launch corrective action.

(39) By way of example, a support structure 5 for supporting troughs 4 may have five to 12 troughs with a radius of curvature lying in the range 1.5 m to 3 m, having a width lying in the range 3 m to 5 m, a length lying in the range 5 m to 30 m, and dead weight in air that may reach or exceed 30 t to 50 t, or even more. The buoyancy incorporated in the support structure 5 or in the form of a float 6b situated above said structure is dimensioned so as to compensate for the dead weight of said support structure 5 when fitted with its trough and various accessories that are not shown, together with the dead weight of all of the flexible pipes 1 in catenary configuration. Additional buoyancy is incorporated in the assembly so as to create permanent upward tension lying in the range 3 t to 60 t, and preferably in the range 6 t to 30 t. The device of the invention thus enables the tension to be adjusted in the various tension legs, substantially dividing the above-specified forces by six, so that each of the tension legs is subjected to a permanent tension in the range 0.5 t to 10 t, and preferably in the range 1 t to 5 t. In order to ensure that the device does not fail in the event of a tension leg or an attachment point 7a-7b failing, or even in certain circumstances in the event of two tension legs or their attachment points failing, the tension legs and their respective attachment points are advantageously dimensioned to have a safety factor of 2 to 4, for example; for a nominal force of 2 t, the tension leg and its attachment points are dimensioned for forces in the range 5 t to 10 t. This avoids problems of fatigue and wear and also the risk of rupture, which even if it occurs, does not in any event put the entire bottom-to-surface connection into danger.

(40) FIGS. 7A to 7C show a said support structure 5 comprising two portions 5-1, 5-2 that are hinged to pivot about a substantially horizontal middle pivot axis X1X1 so as to allow each of said two hinged-together portions to pivot through an angle alpha lying in the range 10 to +10, and preferably in the range 5 to +5, said pivoting being limited by top and bottom abutments 5a and 5b on each of said two hinged support structure portions. Said two hinged support structure portions are symmetrical in shape and arranged about a vertical midplane containing said pivot axis X1X1. In practice, the top plane of said first hinged portion 5-1 varies through an angle alpha relative to the top plane of said second hinged portion 5-2. Each of said two hinged portions 5-1, 5-2 is connected to said base via three said tension legs, each connected at one of its ends to a respective distance-varying device (not shown). Each said hinged portion 5-1, 5-2 of said support structure presents a longitudinal shape of substantially rectangular horizontal section that is substantially symmetrical about a longitudinal vertical midplane (YZ), each said hinged portion 5-1, 5-2 of said support structure 5 comprising: two top attachment points 7a1-7a2, 7a5-7a6 arranged at the longitudinal ends close to the corners of each of said hinged portions that are furthest from said pivot axis (X1-X1); and one top attachment point 7a3, 7a4 arranged closer to said pivot axis than a said longitudinal end.

(41) The three top attachment points 7a1-7a2-7a3 of said first hinged support structure portion 5-1 are arranged in an isosceles triangle that is symmetrical to the three top attachment points 7a4-7a5-7a6 of the second hinged support structure portion 5-2 about the substantially vertical plane containing said pivot axis.

(42) The term floating support is used herein to cover equally well a barge or a ship or a semi-submersible platform of the above-described type.

(43) It should be understood that said top portion of the support structure, supporting or having fastened thereto said troughs of the present invention, is a rigid structure other than a float.

(44) In a particular embodiment, a said flexible pipe is held in a said trough by retaining and/or attachment means. This characteristic seeks to stabilize the set of flexible pipes and to facilitate stresses and movements in said first portions of said flexible pipes.

(45) The trough support structure 5 is a rigid structure, however stresses and movements, in particular at the points of contact between the pipes and the sea floor, are nevertheless considerably reduced as a result of said support structure being tensioned by said float.

(46) In order to make it easier to lay flexible pipes from a laying ship, as explained below in the description, the ends of the troughs include deflectors of profile suitable for avoiding damage to the portion of flexible pipe that might come into contact with said deflector during laying of the flexible pipe on a said bottom trough.

(47) The high point of the bottom of the trough is the point situated halfway along the curvilinear length of the trough.

(48) Said support structure may also support troughs for guiding and supporting flexible lines other than said flexible pipes, and thus of smaller diameter.

(49) For clarity in the figures, the troughs are described as being portions of a truncated torus presenting a circular cross-section of diameter slightly greater than the diameter of the flexible pipe, while the form of the arch in the XZ plane may equally well be of the type comprising an ellipse, a parabola, or any other curve of varying curvature, with its maximum curvature being less than the limiting critical curvature of said flexible pipe. Likewise, the cross-section of the trough may be of any shape, e.g. it may be U-shaped, it being understood that the inside width of the U-shape in the trough portion is slightly greater than the diameter of the flexible pipe. A locking device (not shown) secures each of the pipes to its respective trough so as to avoid any axial sliding of said hose relative to its own trough.

(50) The various troughs are shown in the figures as having identical radii of curvature, however it is advantageous to adopt radii of curvature that are adapted to each of the pipes, thus enabling overall weight to be minimized and thereby reducing the buoyancy that is needed.

(51) The double catenaries flexible pipes 1a, 1b deform significantly when the floating support 2 moves as a result of swell, wind, and current. In contrast, the single catenary portions 1a1 and 1b1 deform very little and thus remain substantially stationary regardless of the movements of the floating support.

(52) The adjustment and distance-varying device 10 is described as being secured at one of its ends either to the base 8 or to the support structure 5, and at its other end to the tension leg, however said device 10 could also be secured at one end to said tension leg and at its other end via a hinge connection, e.g. to a second tension leg, which second tension leg has its other end secured either to the base 8 or the support structure 5. Said device 10 is then arranged between first and second tension legs, however this particular configuration does not constitute the preferred version of the invention.