EP0897455A1

EP0897455A1 - Oil extracting installation incorporating manifold support mounting plates, plate and method of installation

Info

Publication number: EP0897455A1
Application number: EP97921916A
Authority: EP
Inventors: Hervé Jean Albert Marcel VALLA; Pierre Henri Vasseur
Original assignee: Coflexip SA
Current assignee: Technip Energies France SAS
Priority date: 1996-05-03
Filing date: 1997-04-30
Publication date: 1999-02-24
Anticipated expiration: 2017-04-30
Also published as: FR2748293A1; BR9708969A; NO983804L; AU710019B2; OA10913A; NO983804D0; AU2780397A; WO1997042396A1; NO315531B1; EP0897455B1; FR2748293B1; US6148921A

Abstract

The invention features an offshore oil installation (1) comprising two mounting plates (6, 8) resting on the ocean bed, each mounting plate (6, 8) supporting a manifold (10; 10') connected at two ends with two respective terminal joining pieces of flexible pipe sections (5, 7; 7, 9) fixed to the manifold (10; 10') substantially in the extension of each other, each manifold (10; 10') further comprising a flange for sealed joining with a pipe (12) coming from an underwater structure such as a wellhead (2; 3), the manifolds (10; 10') of the two supporting plates (6; 8) being interconnected at one end by a flexible pipe section (7) and connected at the other end by two respective pipe sections (5, 9) to a surface extracting installation (4).

Description

Oil exploitation installation incorporating manifold support bases. base and installation method.

The present invention relates to offshore oil exploitation, and more particularly to an oil exploitation installation using flexible pipes for the transport of pressurized fluids such as hydrocarbons coming from underwater well heads.

The pipes used for transporting hydrocarbons can become clogged and clogged due to the formation of a deposit on the internal surface of the pipes, which often happens when the hydrocarbons have a high level of paraffin.

In this case, it is possible to periodically clean the pipes, also called "pigging", by exerting a mechanical action on the internal surface of the pipes. In order to carry out this cleaning operation, the submarine wellheads are isolated from the sections of flexible pipe used for transporting hydrocarbons to the surface and these sections of flexible pipe are connected together so as to form a loop allowing '' inject, from a surface operating installation, a cleaning device at one end of the loop and recover this cleaning device at the other end of the loop.

To make it easier to isolate the submarine wellheads from the flexible pipe sections used for transporting hydrocarbons to the surface, it is known to arrange connection manifolds making it possible to establish a sealed connection between the short length and small diameter from wellheads and long and larger diameter flexible pipe sections for transportation to the surface on pedestals resting on the seabed.

The use of such bases facilitates the connection of pipes of different diameters, for example the connection between a flexible pipe of 6 "internal diameter and a pipe of A" internal diameter, since it would be difficult to incorporate a manifold for this purpose on the wellhead itself given its structure. These bases can also support the valves making it possible to isolate the well heads and to make the various flexible pipe sections communicate with each other to form the loop necessary for the cleaning operation. To connect, during the cleaning operation, two sections of flexible pipe used during operation to convey the hydrocarbons from the respective well heads to the surface operating installation, it has been proposed to use a base supporting a rigid U-shaped connection, connected at its ends to two sections of flexible pipe.

The U-shaped connector is isolated by a valve during the operation of the wells so that the hydrocarbons produced by each of the wellheads travel through two separate flexible pipe sections up to the surface operating installation; to carry out the cleaning, each well head is isolated and the two flexible pipe sections are communicated by the U-connector, which makes it possible to form a loop allowing the passage, in the two flexible pipe sections, of a cleaning device. However, the use of a base supporting a U-shaped connection connected to two juxtaposed flexible pipe sections makes it difficult and costly to install the installation, insofar as one of the two flexible pipe sections is connected to a second base which must be put in the water while simultaneously managing the chain development of the two flexible pipe sections.

A second solution would be to lay the bases on the bottom and make the connection of the flexible pipes to the manifolds in situ.

However, such a solution is not easy to implement since it is difficult to horizontally move elements to be assembled on the seabed.

The object of the invention is in particular to make it easier to install an oil exploitation installation comprising manifold support bases connected to sections of flexible pipe.

According to a characteristic of the invention, the oil exploitation installation comprises two bases resting on the bottom submarine, each base supporting a manifold connected at two ends to two end pieces of respective flexible pipe sections, fixed to the manifold substantially in the extension of one another, each manifold further comprising a flange for the sealed connection to a pipe coming from an underwater structure such as an underwater wellhead, the manifolds of the two bases being connected together at one end by a flexible section of pipe and connected at the other end by respective flexible pipe sections, to a surface operating installation.

It should be understood that the end pieces of flexible pipe sections are fixed to the manifold substantially in the extension of one another when the flexible pipe sections leave the base on which said manifold is fixed in substantially opposite directions.

The installation on the seabed of the oil exploitation installation can then be carried out without it being necessary to manage the simultaneous launching of two flexible pipes, because it is possible thanks to the invention to start by putting in the water a first section of flexible pipe, then to put in the water a first base, then a second section of flexible pipe, a second base and finally a third section of flexible pipe.

Advantageously, the portions of the flexible pipes close to the end fittings fixed to the manifold can each slide in a curvature limiting member secured to said base.

In a particular embodiment of the invention, said curvature limiting member is constituted by a set of vertebrae fixed at one end to said base and inside which said flexible pipe portion can slide close to the end fitting fixed on the manifold.

Advantageously, said manifolds each have a general shape of T, said flange for connection to the pipe coming from the associated underwater structure being carried by the central branch of the T. Advantageously, one of the two bases supports a valve making it possible to communicate or to isolate the manifolds situated on the two bases.

Advantageously, the installation comprises a network of remote control lines of underwater organs such as valves, consisting of umbilical type pipes, this network comprising a first umbilical connecting the surface operating installation to a first base and a second umbilical connecting the first base to the second base. The invention also relates to a method of installing an oil exploitation installation, this installation comprising two bases intended to rest on the seabed, each base supporting a manifold intended to be connected at two ends to two sections of respective flexible pipes fixed to the manifold substantially in the extension of one another, each manifold further comprising a flange for the sealed connection to a pipe coming from an underwater structure such as a wellhead, the manifolds being intended to be connected together at one end by a flexible pipe section and connected at the other end by respective flexible pipe sections to a surface operating installation, this method comprising the steps consisting in successively:

- unroll, from a starting point, on the seabed a first flexible pipe section by means of a laying vessel,

- launching a first base previously connected to the first section and to a second section while unrolling the start of the second section at the same time as the end of the first,

- unwind said second flexible pipe section on the bottom,

- launching the second base previously connected to the second section and to a third section while unrolling the start of the third section at the same time as the end of the second,

- unrolling on the bottom said third section of flexible pipe, the laying vessel making a substantially loop during the laying of the three sections of flexible pipe between said starting point and an arrival point. Advantageously, the launching of each base is carried out while the base is connected by two cables to the laying vessel, a first cable being attached to the rear of the base and a second cable being attached to a rigid articulated gantry on the base, only the cable connected to the rear of the base used to support the base at first of the descent, the tension in this cable being released when the base reaches a certain depth, the cable attached to the gantry then serving bring the base into a substantially horizontal position and then retain it until it is placed on the seabed.

Advantageously, the gantry is removably attached to the base so that it can be recovered after the installation of a base and be used for the installation of the following base.

Advantageously, the gantry comprises at least one rotary actuating member intended to be actuated by a manipulator robot, this rotary actuating member being connected by a transmission to a mechanism capable of transforming the rotation of said member into an unlocking movement of a hook for fixing the gantry on the base. Furthermore, it has been proposed to make the connection between the submarine wellhead and the manifold connection flange carried by the base by means of a rigid pipe made to measure after an in situ observation of the orientations of said flange. connection and a second connection flange provided on the well head and their relative position.

This procedure is relatively long and costly.

To facilitate the connection between an underwater structure such as an underwater wellhead and the connection flange provided on the manifold carried by the base, there is proposed, according to the invention, a new manifold support base for an oil exploitation facility.

This new base is intended to rest on the seabed and supports a manifold to establish a tight connection between a pipe coming from an underwater structure such as an underwater well head and at least one terminal part d '' at least one flexible pipe section, the manifold comprising a connection flange to said pipe and the base and / or the manifold comprising means for orienting the axis of the connection flange in a chosen direction before establishing the connection with said pipe when the base rests on the seabed .

Such a base facilitates the laying of a pipe between the manifold provided on the base and the well head, insofar as it is no longer necessary to make a custom pipe and such a base also makes it possible to lay installation on an underwater floor with significant differences in level while using pipes fitted with fittings comprising a locking member which can come into locking position by the effect of gravity, in which case said aforementioned chosen direction corresponds to the vertical , with an angular tolerance less than or equal to 10 ° and preferably less than or equal to 5 °.

In a particular embodiment, the base comprises a fixed frame intended to rest on the seabed and a swiveling plate relative to the frame and on which said manifold is fixed.

In a particular embodiment, said plate is connected to said frame by a cardan-type joint.

Advantageously, the base includes means making it possible to orient and maintain said connection flange in the chosen position.

Advantageously, the base further comprises means allowing the removable attachment to the base of a gantry used for laying and / or recovering the base.

Other characteristics and advantages of the present invention will emerge on reading the detailed description which follows, of a non-limiting exemplary embodiment of the invention, and on examining the appended drawing in which:

FIG. 1 is a schematic view of an oil exploitation installation in accordance with an exemplary embodiment of the invention,

FIG. 2 is a schematic perspective view of a base according to an exemplary embodiment of the invention, FIG. 3 is a side elevation view of the base shown in FIG. 2, the flexible pipes not being shown,

FIG. 4 is a view similar to FIG. 3, after the flexible pipes and connection boxes for the control umbilicals have been put in place,

FIG. 5 is a top view of the base shown in FIG. 4,

- Figure 6 is a schematic top view of the manifold support plate carried by the base frame shown in Figures 2 to 5, Figure 7 shows in isolation an element of the joint to orient in a selected position the tray shown in Figure 6, - Figure 8 is a sectional view along the section line

VIII-VIII of figure 6,

FIG. 9 is a section along the section line IX-IX in FIG. 6,

FIG. 10 is a section along the section line XX of FIG. 6, FIG. 11 is a schematic view from above of the manifold carried by the base shown in FIG. 2, flexible pipes only the connection ends being represented, - Figure 12 is a schematic view showing a set of vertebrae, Figure 13 is a schematic side elevation showing a gantry used for the installation of a base, - Figure 14 is a top view of the gantry shown in the FIG. 13, FIG. 15 is a front elevation view of the gantry represented in FIG. 13, FIG. 16 represents a detail of the mechanism for detachably fixing the gantry to the base,

FIGS. 17 to 21 illustrate different stages of the installation of a base and FIG. 22 illustrates the installation of a control umbilical connection box on a connection box support fixed on a base.

There is shown schematically in Figure 1 an installation 1 according to a particular embodiment of the invention.

This installation 1 is intended for the exploitation of two submarine oil well heads 2,3 by a surface vessel 4 such as a dynamically positioned vessel. In the example described, the difference in depth between the underwater well 2 and the underwater well 3 is of the order of 150 m, the average slope between the two wells being of the order of 7 °.

The installation 1 comprises a first section 5 of flexible pipe for connecting the ship 4 to a manifold 10 supported by a first base 6, this manifold serving for the connection of the subsea wellhead 2 to said first section of flexible pipe 5.

A second flexible pipe section 7 connects the manifold 10 to a second manifold 10 'located on a second base 8. A third flexible pipe section 9 connects the manifold 10' located on the second base 8 and the ship 4.

The flexible pipe sections 5, 7 and 9 have an internal diameter of 6 "in the described embodiment and are of the" flow line "type. Each manifold 10, 10 ′ has a T-connector, this connector bearing on its central branch a flange 11 for the sealed connection to a pipe 12 coming from the well head 2, 3 associated with the base considered and constituted in the example described by a flexible pipe with an internal diameter 4 ", this pipe 12 also being called" jumper".

Each T-connector is provided on its branch carrying the flange 11 with a valve 13 making it possible to isolate the pipe 12 for a cleaning operation of the pipe sections 5, 7 and 9 as will be explained below. The manifold 10 'supported by the base 8 also comprises a valve 14 making it possible to isolate the T-connector from the flexible pipe section 7 during the operation of the well heads 2 and 3. It will be noted that the opposite connection flanges of each manifold 15 and 16 have substantially parallel axes, which allows the respective end portions 17 and 18 of the flexible pipe sections 5 and 7 to be located substantially in the extension of one of the 'other.

The bases 6 and 8 also support connection boxes for umbilicals.

One must understand by the expression "umbilical control", one or more cables for transporting information, electrical energy or hydraulic energy.

More precisely, the base 6 supports two connection boxes 22, 23 and the base 8 supports a connection box 24.

The connection box 22 makes it possible to connect a control umbilical 25 coming from the vessel 4, a control umbilical 26 connected to the wellhead 2 and a control umbilical 27 connected to the connection box 23. The latter makes it possible to connect the control umbilical 27 and one end of a control umbilical 28 connected at the other end to the connection box 24. The connection box 23 is also connected by the control umbilical to the valve 13 for controlling the opening or closing the latter. The connection box 24 makes it possible to connect the control umbilical 28, a control umbilical 29 coming from the wellhead 3 and two control umbilicals respectively connected to the valves 13, 14 for controlling their opening or closing.

The control umbilical 27 is permanently fixed to the base 6, and it is placed on the seabed simultaneously with the latter.

In FIG. 1, the entire part extending above the dotted line F rests on the seabed while the part situated below extends between the seabed and the ship 4.

An arch 30 is placed on the path of the flexible pipe sections extending between the ship 4 and the seabed, the flexible pipe sections resting on this arch 30 according to a configuration known in itself and of the "lazy" type. -S ". The flexible pipe sections 5 and 9 are formed on their portion extending between the seabed and the ship 4, by flexible pipes specially adapted to the rise of hydrocarbons and of the "riser" type. During the operation of well heads 2, 3, the valves

13 are open and the valve 14 is closed.

The pressurized hydrocarbons coming from the wellhead 2 are conveyed to the ship 4 by the flexible pipe section

5 while the pressurized hydrocarbons coming from the wellhead 3 are conveyed to the ship 4 by the flexible pipe section

9.

In the case where the hydrocarbons produced are rich in paraffin, a progressive clogging of the flexible pipe sections 5 and 9 occurs, forcing periodic cleaning of the latter.

To do this, the valves 13 are closed so as to isolate the well heads 2 and 3 and the valve 14 is opened so as to join the pipe sections 5, 7 and 9 and form a loop allowing injection by one end of one of the sections 5 and 9 of a device for cleaning from the ship 4.

The cleaning device may comprise in a manner known per se a brush and / or any other scraping system.

The base 6 is shown in FIG. 2 in schematic perspective. The base 8 is substantially identical to the base 6 with the exception that it only carries a support for a control umbilical connection box and that it also carries valve 14. The base

6 has not been shown in detail in the drawing.

The base 6 comprises a frame 31, made of metal in the example described, and a plate 32 for manifold support 10, this plate 32 can advantageously be oriented in a chosen position relative to the frame 31 as will be explained below. .

We distinguish in Figure 2 two supports 33 for receiving the connection boxes 22 and 23. Each support 33 is provided with guide rails 33a for centering the connection box during its descent on the support and a guide 33b for the introduction of a spindle extending below the box connection. The guide 33b_ has in the upper part a flared opening facilitating the introduction of the spindle.

The frame 31 has a generally flattened and rectangular shape. There is shown in Figure 3, in side elevation, the base 6, the end portions of the flexible pipe sections not being illustrated in this figure.

There is shown in Figure 4, in side elevation, the base 6 when the end portions 17 and 18 of the flexible pipe sections are fixed to the T-connector of the manifold 10. Also shown very schematically in Figure 4 the connection boxes 22 and 23 and the control umbilicals 25 and 28.

As indicated above, the plate 32 is orientable with respect to the frame 31. More specifically, in the example of embodiment described, the plate 32 is connected to the frame 31 by a cardan joint type making it possible to oscillate the plate 32 around 'a first geometric axis of rotation X substantially parallel to the side of the largest dimension of the frame 31 and a second geometric axis of rotation Y, perpendicular and intersecting with the axis X. The intersection of the axes X and Y corresponds substantially to the center of gravity of the platform so that it can be adjusted in position relatively easily.

The articulation of the plate 32 on the frame 31 is effected by means of a cross 33 ', shown in isolation in FIG. 7, comprising a tube 34 of axis X and a tube 35 of axis Y fixed to the tube 34 by an assembly structure 36 located in the center of the cross.

Referring now more particularly to Figures 6 to 10, the plate 32 is secured on its underside with a pin 37 of axis X, engaged in one end 38 of the tube 34 and able to pivot in the latter around the axis geometric rotation X.

The plate 32 is secured, on the opposite side, to a bearing

39 of axis X, able to pivot around a pin 40 engaged in the end 41 of the tube 34 opposite the end 38. The pin 40 is immobilized in rotation in the tube 34, and it is integral at its opposite end to the tube 34 of a fork 42, having two parallel walls between which a nut 43 can pivot about an axis of rotation Z parallel to the axis X.

This nut 43 is engaged on a threaded rod 44 rotatably mounted on the plate 32 around a geometric axis of rotation K perpendicular and intersecting the axis Z.

The threaded rod 44 is provided at its upper end with a wheel 45 shaped to be able to be actuated in rotation by a manipulator robot. The rotation of the threaded rod 44 causes the nut 43 to go up or down and the plate to rotate around the geometric axis of rotation X since the pin 40 is fixed relative to the frame 31.

The tube 35 of the cross 33 'is mounted for rotation about the axis Y on the frame 31. More specifically, the tube 35 is mounted for free rotation at one end 46 on the frame 31 by means of a pin 90 integral of the frame 31. The tube is secured at the other end of a pin 91 supported in rotation by a bearing of the frame 31. The pin 91 is secured at its end opposite to that engaged in the tube 35 of a fork 47 comprising two branches carrying in rotation about an axis parallel to the axis Y a nut 48 engaged with a threaded rod 49 mounted in rotation on the frame 31 around an axis W substantially perpendicular to the plane of the frame 31. The threaded rod 49 is provided at its upper end with a wheel 50 shaped to be driven in rotation by a manipulator robot. The rotation of the rod 49 causes the nut 48 to go up or down and the plate 32 to pivot about the Y axis.

A spirit level 51 is fixed on the upper face of the plate 32 to allow the positioning of the plate 32 horizontally, with the axis H of the flange 11 substantially vertical, as shown in FIG. 6. Thus, after the installation of the base 6 or 8, it is possible to modify the orientation of the connection flange 11 before attaching to the latter the fitting fitted to the end of the pipe 12.

When the seabed on which the base 6 or 8 is placed is inclined, it is thus possible to position the joint plane of the flange 11 substantially horizontally, the axis H of the flange then being substantially vertical, to the tolerances close authorized for the connection to the pipe 12. The latter is advantageously equipped with a connector comprising a locking member which can come into the locking position by the effect of gravity, in which case it is particularly important that the joint plane of the flange 11 be as horizontal as possible. In the embodiment described, it is possible to rotate the plate 32 by approximately 10 ° around each axis X and Y, and ensure the verticality of the axis H of the flange 11 with an angular tolerance better than 5 °, and preferably better than 3 °. FIG. 11 shows the manifold 10 in top view.

The connecting ends of the end portions 17 and 18 of the flexible pipe sections 5 and 7 have been referenced 52 and 53.

The ends 52 and 53 and the flanges 15 and 16 are connected by means of seals 54 and 55 of a type known per se under the name "Grayloc".

In order to avoid excessive curvature of the end portions 17 and 18 of flexible pipe, the latter are each engaged in curvature limiting members constituted in the example described by a set 56 of vertebrae.

These vertebrae are not shown in FIG. 11 for the sake of clarity of the drawing.

There is shown in Figure 12 a set 56 of vertebrae consisting of the assembly, in a manner known per se, of vertebrae 57 and 58 having a symmetrical shape of revolution and constituted by tubular elements having in section in a plane containing their axis of symmetry, a U shape with edges turned inwards for the vertebrae 57 or with edges turned outwards for the vertebrae 58, the assembly of two vertebrae 58 being effected by means of a vertebra 57.

The end portion of each flexible pipe section engaged in a set of vertebrae 56 is free to slide inside thereof. Each set of vertebrae 56 is fixed, at its end adjacent to the manifold 10, to the plate 32 by means of a collar 59. Thus, the resumption of bending forces is carried out by the sets 56 of vertebrae fixed to the plate and the connector in T is not subject to significant constraints which would otherwise damage the "grayloc" type seals or the fitting itself.

There is shown in Figures 13 to 15 a gantry 60 used when installing a base 6 or 8, as described below. This gantry 60 comprises a rigid U-shaped frame, the uprights 61 of which are pivotally fixed to the frame 31, around a geometric axis of rotation V parallel to the geometric axis Y. More precisely, each upright 61 is fixed at its lower end removably on the frame 31 by means of a locking mechanism 62 comprising a rotary member 63 rotatably mounted on the upright 61 and connected by a universal joint transmission 64 to a mechanism 65. This mechanism 65 actuates a hook 66 engaging on the end 67 of an axis fixed to the frame 31, to retain the gantry 60 on the frame 31 while allowing its rotation. The angular movement of the frame is limited for the base

8 by stops 61a_ arranged on one side of the Y axis and by cables 61b connecting the crosspiece of the gantry 60 and the frame 31, these cables 61b being attached near the free edge of the frame 31 on the same side as the stops 61a_. The cables 61b are tensioned when the gantry is arranged substantially perpendicular to the plane of the frame 31. The cables 61b make it possible to keep the gantry substantially perpendicular to the base even in the case where the center of gravity of the latter is not in the vertical half-plane delimited above by the geometric axis of rotation of the gantry. The base 6 which comprises two supports 33 of the umbilical connection box for control is better balanced than the base 8 and the angular movement of the frame is limited by stops 61a _, arranged on either side of the Y axis.

The mechanism 65 comprises a nut 68 engaged on a threaded rod 69 driven in rotation by the transmission 64. The nut 68 is articulated, around an axis parallel to the axis V, at one end 70 of a connecting rod hook 66, itself articulated at 69 around an axis of rotation parallel to the axis V. The rotation of the member 63 causes via the transmission 64 the rotation of the threaded rod 69 and the ascent of the nut 68, which causes the hook 66 to pivot downwards around the articulation axis 69. The pivoting of the hook 66 allows the end 67 to be released as illustrated in Figure 16. There is shown in dotted lines in this figure the position of the hook 66 at the end of pivoting after the ascent of the nut 68. Conversely, the gantry 60 can allow the recovery of a base placed on the seabed. We will now describe with reference to FIGS. 17 to 21, the manner of carrying out the installation of the installation 1 previously described.

We start by unwinding a first section of flexible pipe, for example section 9. When we arrive near the end of this section, we resume the tension due to the weight of the submerged part of the flexible pipe by means of a sleeve tightened on the flexible pipe and also called "chinese finger" or "sock", fixed to a cable 72 wound on a winch.

The base 8 can then be brought to the rear of the laying vessel, on a tilting ramp 73. The respective end portions 21 and 20 of the flexible pipe sections 9 and 7 are fixed to the manifold 10 'mounted on the base 8. The gantry 60 rests on the rear stops 61a .. The frame 31 of the base 8 is fixed at the rear to a first cable 74 wound on a winch and the gantry 60 is fixed by its crossmember to two slings connected to a second cable 75 also wound on a winch.

Before tilting the ramp 73, a slack is ensured in the flexible pipe portion 9 situated above the sock 71. The ramp 73 is then rocked as illustrated in FIG. 18. The sock 71 supports the pipe 9 avoids the appearance of bending movements which would otherwise damage the connection between the pipe 9 and the manifold 10 '. The end portion 20 of flexible pipe is held by means of a gutter 76 suspended by a crane to minimize the bending moments exerted on the ends of the flexible pipes. The ramp 73 is provided at the top with a deflection pulley 77 on which passes the cable 74, which supports the base 8 when the latter is gradually lowered into the water in a substantially vertical position. The cable 74 takes up almost all of the weight of the base 8 when it is launched. The base 8 is then lowered into the water and when it reaches a depth of a few tens of meters, the sock 71 is detached using a manipulator robot (ROV) as illustrated in FIG. 19. The cable 75 connected to the gantry 60 is not taut. The weight of the base 8 and of the flexible pipe section situated under the latter is taken up by the cable 74.

Then proceed to the progressive tilting of the base 8 to bring it into a horizontal position. For this, the tension of the cable 74 is gradually reduced by increasing that of the cable 75 and when the latter is stretched, the cable 74 is detached by means of a manipulator robot. We are then in the configuration of Figure 20, the base 8 being located about ten meters high from the bottom. This figure shows by hatching the location on which the base 8 must be deposited. After laying the base 8 on the seabed, slack is given to the cable 75 before detaching the gantry 60 from the frame 31 of the base 8 as shown in FIG. 21.

To detach the gantry 60, the rotary members 63 are rotated by means of a robot manipulator so as to release the hooks 66 from the ends 67. Thanks to the distance separating the rotary members 63 from the seabed, the rotary members 63 are located at the time of their actuation at a sufficient height from the seabed so that any clouds of particles raised by the propellers of the manipulator robot do not interfere with the visibility of the latter.

After detachment from the gantry 60, the latter is recovered and placed on the base 6 which is placed in a similar manner.

To carry out the laying of sections 5, 7 and 9 and bases 6 and 8, the laying ship has described a loop without having to reverse. It is not going beyond the scope of the invention to proceed in the opposite direction, that is to say by first laying the section 5.

To install the connection boxes for the control umbilicals, the connection box is brought to the end of a cable above the corresponding support and a chain extending below the spindle is engaged in the guide 33b_ of the latter. associated centering, as illustrated in figure 22. After fitting the connection boxes, the operations for connecting the umbilicals to each other are carried out using a robot manipulator.

Before connecting the flange 11 with the associated pipe 12, the orientation of the axis H of the flange 11 is corrected if necessary by turning the knobs 45 and 50 using a robot manipulator. observes thanks to a camera placed on the manipulator robot, the spirit level 51 which provides information on the horizontality of the plate 32. It will be noted on examining FIG. 2 in particular that the flange 11 is located at the bottom of a structure of guide comprising two panels lla_, llb_ used to position the connector fitted to the pipe 12 before it descends on the flange 11. A guide ll £ allows the passage of a cable used to press if necessary said connector against the panels 11a and 11b before its lowering on the flange 11. It will be noted in FIG. 5 that the panels 11a and 11b are connected by a hoop llcl which does not hinder the access of the robot manipulator to the flange 11, to the knobs 45 and 50 and allows the robot manipulator to see the spirit level 51. Of course, the invention is not limited to the embodiment which has just been described.

One could in particular, in a variant embodiment not shown, mount the plate 32 on three feet arranged like the vertices of a triangle on the frame 31, and the height of which would be adjustable, so as to be able to position the plate with a chosen orientation compared to the frame.

It would also be possible, in another variant not shown, to fix the manifold on the frame for the connection of the flexible pipes and the pipe coming from the well head, by simply mounting the flange for the connection of the pipe coming from the sub well. sailor on a swivel joint of the spherical joint type, which would be positioned with the desired orientation using a manipulator robot before making the connection.

Claims

1 - Offshore petroleum exploitation installation (1) comprising two bases (6.8) resting on the seabed, each base (6.8) supporting a manifold (10; 10 ') connected at two ends to two end pieces of respective flexible pipe sections (5, 7; 7.9) fixed to the manifold (10; 10 *) substantially in the extension of one another, each manifold (10; 10 ') further comprising a flange (11) for the tight connection to a pipe (12) coming from an underwater structure such as a well head (2; 3), the manifolds (10; 10 ') of the two bases (6; 8) being connected to each other at one end by a flexible pipe section (7) and connected, at the other end, by respective flexible pipe sections (5,9), to a surface operating installation (4 ).

2 - Installation according to claim 1, characterized in that the portions of the flexible pipes close to the end pieces fixed on the manifold can each slide in a curvature limiting member (56) integral with said base (6; 8).

3 - Installation according to claim 2, characterized in that said curvature limiting member (56) is constituted by an assembly (56) of vertebrae (57,58) fixed at one end to said base (6,8) and to the 'inside which can slide said portion of pipe close to the nozzle attached to the manifold.

4 - Installation according to any one of claims 1 to 3, characterized in that said manifolds each have a general shape of T, said flange (11) for connection to the pipe (12) coming from the associated underwater structure being carried by the central branch of the T.

5 - Installation according to any one of claims 1 to 4, characterized in that one (8) of the two bases supports a valve (14) for communicating or isolating the manifolds (10,10 ') located on the two bases (6,8).

6 - Installation according to any one of claims 1 to 5, comprising a network of remote control lines of underwater organs such as valves, consisting of pipes of the umbilical type, characterized in that this network comprises a first umbilical connecting the surface mining installation and a first base (6) and a second umbilical connecting the first base to the second base (8).

7 - Manifold support base (6; 8) for an oil exploitation installation, this base being intended to rest on the seabed and supporting a manifold (10; 10 *) to establish a tight connection between a pipe (12) coming from an underwater structure (2; 3) such as an underwater wellhead and at least one terminal part (17; 18; 20; 21) of at least one section (5, 7, 9) of flexible pipe, the manifold (10; 10 ') further comprising a connection flange (11) to said pipe (12) coming from said underwater structure (2; 3) and the base and / or the manifold comprising means for orienting the axis (H) of said connection flange (11) in a chosen direction before establishing the connection with said pipe (12). 8 - manifold support base according to claim

7, said pipe (12) being fitted with a connector comprising a locking member which can come into the locking position by the effect of gravity, characterized in that said chosen direction corresponds to the vertical, with a lower angular tolerance or equal to 10 ° and preferably less than or equal to 5 °.

9 - manifold support base according to one of claims 7 or 8, characterized in that it comprises a fixed frame (31) intended to rest on the seabed and a plate (32) orientable relative to the frame and on which said manifold (10; 10 ') is fixed.

10 - manifold support base according to claim 9, characterized in that said plate (32) is connected to said frame (31) by a cardan joint.

11 - manifold support base according to any one of claims 7 to 10, characterized in that it comprises means for positioning and maintaining in the selected position said connecting flange (11).

12 - manifold support base according to any one of claims 7 to 11, characterized in that it comprises means for the removable fixing (67) of a gantry (60) used for the installation and / or the recovery of the base placed on the seabed. 13 - Method for installing an oil installation, this installation comprising two bases (6.8) intended to rest on the seabed, each base (6.8) supporting a manifold (10; 10 ') intended to be connected at two ends to two respective flexible pipe sections (5, 7; 7, 9) fixed to the manifold (10; 10 *) substantially in the extension of one another, each manifold further comprising a flange ( 11) for the watertight connection to a pipe (12) coming from an underwater structure (2; 3) such as a wellhead, said manifolds being intended to be connected to each other at one end by a pipe section flexible (7) and at the other end by respective flexible pipe sections (5, 9) to a surface operating installation (4), this laying process comprising the steps consisting in successively:

- unroll on the bottom by means of a laying vessel a first section of flexible pipe (9), launch a first base (8) previously connected to the first section and to a second section, while unrolling the start of the second section and the end of the first,

- unwind said second flexible pipe section (7) on the bottom,

- launching the second base (16) previously connected to the second section and to a third flexible pipe section, while unwinding the start of the third section and the end of the second, - unwinding said third section of pipe on the bottom flexible, the laying vessel making a substantial loop to lay the three pipe sections (9,7,5).

14 - Method according to claim 13, characterized in that the launching of each base (6.8) is carried out while the base is connected by two cables (74.75) to the laying vessel, a first cable (74) being attached to the rear of the base and a second cable (75) being attached to a rigid gantry (60) articulated on the base, only the cable (74) connected to the rear of the base used to support the base first of the descent, the tension in this cable being released when the base reaches a certain depth, the cable (75) attached to the gantry (60) then serving to bring the base in a substantially horizontal position and then to retain it until it is placed on the seabed.

15 - Method according to claim 14, characterized in that the gantry (60) is removably attached to the base so as to be able to be recovered after the installation of the base and serve for the installation of the following base.

16 - Method according to claim 15, characterized in that the gantry (60) comprises at least one rotary actuator (63) intended to be actuated by a manipulator robot, this rotary actuator being connected by a transmission (64) to a mechanism able to transform the rotation of said member (63) into an unlocking movement of a hook (66) for fixing the gantry (60) on the base.