EP1073823A1

EP1073823A1 - Method and device for linking surface to the seabed for a submarine pipeline installed at great depth

Info

Publication number: EP1073823A1
Application number: EP00906407A
Authority: EP
Inventors: Régis PIONETTI; Xavier Rocher
Original assignee: Bouygues Offshore SA
Current assignee: Saipem SA
Priority date: 1999-02-19
Filing date: 2000-02-17
Publication date: 2001-02-07
Anticipated expiration: 2020-02-17
Also published as: BR0004931A; OA11541A; AU2809200A; ID28051A; FR2790054B1; DE60027511D1; FR2790054A1; EP1073823B1; US6461083B1; ATE324512T1; CN1294654A; BR0004931B1; WO2000049267A1

Abstract

The present invention relates to a bottom-to-surface link system for an underwater pipe installed at great depth, the system comprising:firstly a vertical tower constituted by at least one float (5, 14) associated with an anchor system (6, 8, 16) and carrying at least one vertical riser (9, 15) suitable for going down to the sea bed (18); andsecondly at least one link pipe (4, 3) extending from said float (5, 14) to a surface support (1). According to the invention, said link pipe is a riser whose wall is constituted by a rigid steel tube, and said float (5, 14) is installed at a depth situated below the last thermocline (29).

Description

BASE-SURFACE LINKAGE METHOD AND DEVICE

BY SUBMARINE DRIVING INSTALLED

A LARGE DEPTH

The present invention relates to a method and a device for bottom-surface connection by underwater pipe installed at great depth.

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

The present invention relates to the known field of type connections comprising a vertical tower anchored on the bottom and composed of a float located at its top and connected by a pipe, taking by its own weight the shape of a chain, up to a floating support installed on the surface.

In fact, as soon as the water depth of the production fields considered in this description as being oil fields becomes significant, their exploitation is generally carried out using floating supports. The well heads are often distributed over the entire field and the production pipes, as well as the water injection lines and the control cables, are laid on the sea floor in the direction of a fixed location , vertically from which the floating support is positioned on the surface.

This floating support generally comprises anchoring means to remain in position despite the effects of currents, winds and swells. It also generally comprises means for storing and processing petroleum as well as means for unloading towards tanker-lifters, the latter being present at regular intervals to carry out the removal of production. The name of these floating supports is the English term "Floating Production Storage Offloading" (meaning "floating means of storage, production and unloading") whose abbreviated term "FPSO" will be used throughout the description next. These FPSOs are either anchored by a series of anchor lines starting from each of the angles of the floating support, in which case the FPSO keeps a substantially constant heading whatever the environmental conditions, or is connected to a reel secured to the structure of the FPSO, said reel being anchored by a series of anchor lines. In the latter case, the FPSO is free to turn around the reel, the latter keeping a constant heading; the FPSO then takes a course corresponding to the result of the forces due to wind, current and swell on the hull of the ship. In the description which follows the bottom-surface links described arrive, in the case of an FPSO anchored therefore with a substantially constant heading, generally on the edge of the ship (example of FIG. 2) and in the case of an FPSO on a reel, on the reel itself (example in Figure 6).

The bottom-surface connection pipe, called "riser", the term of which will also be used in the present description, can be produced by continuously raising the pipes laid at the bottom, directly towards the FPSO, giving a chain configuration of which the he angle with the vertical, at the level of the FPSO, is generally 3 to 15 degrees (catenary riser). These connections are imperatively made by means of flexible pipes when the water depth is less than a few hundred meters, but as soon as the depth reaches and exceeds 800 to 1000 m, the flexible pipes can be replaced by resistant and rigid pipes. made of tubular elements welded or screwed together made of rigid material, such as composite material or very thick steel. These rigid risers made of very thick resistant material, in chain configuration are commonly called by the Anglo-Saxon term "Steel Catenary Riser" (meaning "steel riser in the form of chain") of which the abbreviated term "SCR" will be used in the present description whether it is made of steel or other material such as composite.

A flexible pipe and a rigid SCR type riser, subjected only to gravitational forces, of the same height and having, at the point of attachment to the FPSO, the same angle relative to the vertical ', will have an identical curvature on their entire length in suspension. Mathematically, this curve is perfectly defined and is called chain. However, SCRs are much simpler than flexible pipes technically and much less expensive. Flexible pipes are indeed complex and costly structures made from multiple spiral metal sheaths and composite materials.

The water depth of some oil fields exceeds 1,500m and can reach 2,000 to 3,000m. The tension induced at the level of the FPSO by each of the SCRs can reach 250 to 300 tonnes and the large number of risers made necessary for the development of certain fields, leads to considerably strengthening the structure of said FPSOs and creating imbalances if the loads on port and starboard are different. In addition, during the circular movements of the FPSO around its average position, the chain formed by the SCR changes and the point of contact at ground level moves from front to back and from left to right, at the same rate q. ue the FPSO, resting or lifting a portion of the pipe. These repeated movements over long periods will create a furrow in poorly consolidated soils that are commonly encountered at great depths, which will have the effect of modifying the curvature of the chain and lead, if the phenomenon increases, to risks. damage to the pipes, either at the level of the underwater pipes, or at the level of the SCRs.

Due to the multiplicity of lines existing on this type of installation, we are led to prefer a tower type solution in which the conduits and cables converge at the foot of a tower and go up along it, that is to say at the surface, that is to a depth close to the surface, depth from which flexible pipes ensure the connection between the top of the tower and the FPSO. The tower is then provided with buoyancy means to remain in the vertical position and the risers are connected, at the foot of the tower, to the underwater pipes by flexible cuffs which absorb the angular movements of the tower. The assembly is commonly called "Hybrid Riser Tower", because it involves two technologies, on the one hand a vertical part, the tower, in which the riser consists of vertical rigid pipes, on the other hand the upper part of the riser consisting of flexible chains in a chain connecting the top of the tower with the FPSO.

We know the French patent FR 2 507 672 published on December 17, 1982 and entitled "riser for great water depths" which describes such a hybrid tower comprising a surface float connected to the FPSO via flexible pipes and carrying suspended guides through which pass only the upper portion of the vertical conduits of fluid transfer: said hybrid tower is anchored to the sea floor by a stretched cable leaving a certain flexibility of vertical movement to the assembly, the lower portion of the pipes being free and forming a bend at the bottom on which it s press. The advantage of such a hybrid tower lies in the possibility for the FPSO to be able to deviate from its normal position by inducing a minimum of stresses in the tower as well as in the portions of conduits in the form of suspended chains, both at bottom only on the surface. In fact, the FPSO is generally anchored by a multitude of lines connected to a system of anchors resting on the sea bottom. This anchoring system creates restoring forces which keep the FPSO in a neutral position. The bottom-surface connections create additional vertical and horizontal forces which have the effect of moving the axis of the FPSO relative to said neutral position. In the absence of current, wind, swell and for an average tide level, the position of the FPSO corresponds to a position PO called the reference position. Under the combined effect of the environmental conditions, on the one hand on the hull of the FPSO and on the other hand on the various constituent elements of the risers, the FPSO will move, relative to this reference position, in proportion to the value of the result of all the forces applied to the system. Thus, for efforts on the shell of the FPSO tending to move it away from the axis of the tower, the following effects are observed: - on the one hand the chain stretches and its angle with the vertical, at the point d 'attachment with the FPSO increases, which implies an increase in the vertical and horizontal effort on the FPSO; - on the other hand the angle of inclination of the tower due to said horizontal force, increases.

In order to minimize the consequences of FPSO excursions, efforts are generally made to increase the stiffness of the anchoring system and to provide flexibility at the bottom-surface connections. For this, the tower configuration associated with a chain has a great capacity to absorb the excursions of the FPSO, while minimizing the movements at the level of the tower and the deformations of the chains.

To dampen the movements of the FPSO, we seek to increase the curvature of the pipe connecting it to the top of the tower. And, flexible pipes are considered to be better suited for making connections between the FPSO and the top of the tower. In the previous achievements of "hybrid towers" described in FR 2 507 672 or in other types of structures such as those described in US 4 391 332 and EP 802 302, flexible plunging, that is to say widely descending, pipes are used below the float and then back up. This is possible because a flexible pipe is able to resist fatigue even when its curvature has a radius of curvature of only a few meters.

However, the internal structure of the hoses is very complex and their cost very high, which is why, in the previous embodiments of hybrid towers, it is sought to raise the tower as close as possible to the surface, while avoiding the zones of turbulence. at the surface, that is to say at depths generally less than 200 m, preferably of the order of 50 m. This makes it possible to implement reduced lengths of flexible pipes and therefore less costly, but also and above all, this makes it possible to make the connections of the flexible pipes at the top of the tower more accessible to divers. All the elements of these hybrid towers or of these catenary risers must be dimensioned to support the swell, the current and the movements of the surface vessel in extreme sea conditions, which leads to submerged structures of considerable scale having to support significant stresses and resist fatigue phenomena throughout their lifespan which commonly reaches and exceeds 20 years.

The problem is therefore to be able to make and install such bottom-surface connections for underwater pipes at great depths, such as beyond 1,000 meters for example, and of the type comprising a vertical tower anchored on the bottom of the sea and whose float located at its top is connected to a floating support installed on the surface, by a pipe in the form of a chain, limiting the forces on the floats and the pipes connecting it to the floating support, all of the device having to be able to withstand stresses and fatigue while accepting large displacements of the surface support and without requiring considerable and too expensive structures, and the installation of which must be able to be facilitated and be reversible to be easily maintained and replaced.

One solution to the problem posed is a bottom-surface connection device for a submarine pipe installed at great depth, comprising on the one hand a vertical tower made up of at least one float associated with an anchoring system and carrying at least one riser. vertical connecting the float to the bottom of the sea and being able to connect to underwater pipes lying at the bottom of the sea and, on the other hand at least one connection pipe from said float to any surface support such as according to the present invention said connection pipe is a riser the wall of which is a rigid resistant tube, in particular made of steel or composite material.

For a rigid pipe, the minimum tolerable radius of curvature is 10 to 100 times greater than that of a flexible pipe. To limit fatigue, it is in fact considered that the radius of curvature of a rigid steel pipe must generally be greater than approximately 100 m. To provide flexibility and provide an identical capacity to absorb the movements of the floating support and the movements of the tower, we compensate for the fact that the chain is less curved with a rigid pipe, by increasing the distance between the floating support and the float. at the top of the tower, and therefore increasing the length of the rigid pipe. However, the apparent weight in water of a more rigid pipe is greater than that of a flexible pipe, the head load at the level of the float and the forces on the float at the top of the tower are therefore increased. This could lead to oversize the float, and induce significant costs. This is why, preferably, according to the present invention, the float is installed at the top of the tower, at a greater distance from the surface of the water, in particular at a depth below the last thermocline, the latter being defined below, preferably at least 100 m below the last thermocline. In particular, the float is installed at the top of the tower at least 300 m from the surface of the water, preferably at least 500 m from the surface of the water, more preferably at a depth greater than half the depth. of water to which the tower is anchored.

By lowering the float at the top of the tower, the following advantages are cumulated:

- the length of the rigid connecting pipe between the FPSO and the top of the tower is increased, which makes it possible to further dampen the movements of the tower and of the FPSO,

- while respecting the minimum radii of curvature acceptable by rigid chain driving, regardless of the overall movements,

- while minimizing costs because the tower being lower, the submerged structure represents a less considerable structure and therefore less costly, and the floats required for its tensioning are less important and therefore less costly - and this despite the increase in the apparent weight in the water of the pipe linked to the increase in its length - because of the fact that there is little or no rise in the chain towards the float, the weight of the rigid pipe in a chain is essentially supported directly by the FPSO. However, maintaining a tower of a certain height, in particular at least 50m, preferably 100m, is advantageous because the tower, due to its mobility, contributes to damping the system under the effect of the movements of the FPSO.

In a preferred embodiment, the anchoring system comprises at least one vertical tendon, a lower base to which the lower end of the tendon is fixed and at least one guide through which the lower end of said vertical riser passes. More particularly, the .guide can be on the base. Advantageously, said tendon also includes guide means distributed over its entire length, through which at least said vertical riser passes. Said base can be simply placed on the bottom of the sea and remaining in place by its own weight, or can be anchored by means of batteries or any other device capable of holding it in place; the float is connected to this base via a flexible link located at the bottom, and an axial link consisting either of a cable or of a metal bar or even of a pipe. This axial link is called "tendon" in the present description.

In a preferred embodiment, the upper end of said vertical riser is suspended through at least one guide secured to said float, disposed within or at its periphery, said upper end of the vertical riser is connected by the top of said float to the the bent end of said connecting pipe, and the lower end of the vertical riser is capable of being connected to the end of a sleeve, also bent, mobile, between a high position and a low position, with respect to said base, to which this cuff is suspended and associated with a return means bringing it back to the high position in the absence of the riser, said return means being able to be a counterweight. This mobility of the bent cuff makes it possible to absorb variations in the length of the riser under the effects of temperature and pressure.

At the head of the vertical riser, a stop device integral with it comes to rest on the support guide installed at the head of the float and thus maintains the entire riser: the latter then being suspended, its apparent weight in the water is supported by part of the buoyancy of the float.

In a particular embodiment, said guide means distributed over the entire length of the tendon and through which said vertical riser passes, comprise a cylindrical cavity preferably surmounted by a conical funnel, the inside diameter of said cylindrical cavity being greater than that of the vertical riser, and said guide means comprise a flexible membrane integral with the inner wall of said cylindrical cavity, thus creating a sealed pocket between said membrane and said internal wall, a pocket which can be filled with a fluid, preferably with very high viscosity, so as to bear against the riser.

Preferably, friction pads are associated with said membrane and come to bear against the riser when said pocket is filled with fluid. The pads thus allow the sliding of the vertical riser when its length varies under the effect of temperature and pressure. The objectives of the present invention are also obtained by a linking process using, as indicated above, on the one hand, a vertical tower consisting of at least one float associated with an anchoring system and carrying at least one vertical riser capable of descend to the bottom of the sea and on the other hand at least one connecting pipe from said float to any surface support, such that, according to the present invention, said float is installed at an immersion depth situated below the last thermocline, the latter being defined and specified below, and said float is connected to the surface support by at least one rigid resistant riser constituting one of said connecting pipes. According to a preferential mode of implementation of the linking method according to the invention:

- a base is placed on the bottom of the sea which is secured to said bottom; it fixes the lower end of a tendon which is integral at its other upper end with said float, the assembly constituting said anchoring system of the vertical tower;

- Said vertical riser is gradually lowered, for example by descent from a floating support installed vertically on said float, and through one of the sets of guides thereof and until its upper end comes to bear on said float, its lower end then connecting to the upper end of a pre-installed cuff on said base.

During its descent, the vertical riser preferably passes successively in a series of guides integral with the axial link, called a tendon, and is thus maintained in a position substantially parallel to said tendon and to the other vertical risers, ie already installed in the adjacent guides, either to be installed later.

In a particular embodiment, said float is installed at an immersion depth greater than the half-depth of water to which the tower according to the invention is anchored, which then makes it possible to assemble the entire riser beforehand. vertical and transport it vertically to the vertical of the corresponding float guide to be lowered.

The result is a new process of bottom-surface connection by underwater pipe, installed at great depth and responding to the problem posed. Indeed, the study of marine currents in the various seas of the world has shown the existence of several stratifications, from the surface to the bottom of the sea. Thus, for water depths greater than 500-1000m, in an Atlantic-type ocean configuration, we observe as shown in FIG. 1: - a surface layer 18ι which can reach 50 m below the surface 19, and in which the currents are local and mainly due to winds and phenomena tide. In this area, the currents are significant, substantially uniform over the slice of water. They can reach speeds of the order of 2.5 m / s in the case of West Africa, - a transition zone 29ι, called a thermocline, of variable but small thickness (3 to 10m). In this transition zone 29ι the current decreases rapidly to reach the speed of the intermediate layer,

- An intermediate layer 18 ₂ in which the currents vary from 0.5 m / s to lm / s. This intermediate layer extends from approximately —55m to —150m and the currents are mainly thermal currents due to climatic phenomena,

- a second transition zone 29 ₂ or thermocline also of variable but small thickness (± 10 m). In this transition zone the current decreases rapidly to reach the value of the lower layer, - a lower layer 183 in which the currents are weak and do not generally not more than 0.5 m / s. These currents are due to intercontinental water movements. This layer begins at about —150 / —170 m and continues to the bottom 12 of the sea, that is to say to depths of up to 1,000 to 3,000 m depending on the location. In some seas, we can observe three thermoclines 29 on the upper part, but generally, the lower layer 183 starts around -170 / -200m.

Thus, the tower and its float according to the invention and as described below being located below this lower thermocline 29 ₂ are in the water section I 83 generating the stresses due to the weakest currents. In addition, the float is sheltered from the effects of the swell, effects which decrease rapidly with the depth and which it is customary to neglect when one exceeds 120 to 150 m deep. The forces to which the tower is then subjected are thus considerably reduced and substantially uniform throughout its height under the effect of intercontinental background currents.

The device according to the invention, made up of the tower-SCR assembly, will behave much better under the effect of environmental conditions not only usual, but also extreme such as the annual, decennial and centennial conditions. The efforts and constraints will be reduced very significantly and the fatigue life of the various critical components will be considerably increased, which will provide better service throughout the life of the field.

The float being thus at a significant depth, can be connected to the FPSO by means of at least one SCR and not of a flexible connection such as it is customary to date: these SCR connections are simple and in addition, the internal structure of the SCRs, vertical risers and pipes resting on the bottom can then be identical, which simplifies the passage of cleaning scrapers. The frequent passage of these cleaning scrapers is indeed essential in the case of solid deposits such as paraffin or hydrates and it must be possible to act very energetically and repeatedly without damaging the internal surface of the risers and of the pipes.

In general, the float is installed around the middle of the water, but it may be necessary to install it higher or lower to favor certain advantages that we will describe now. In all cases, the float will never be located near the last thermocline described above, but much lower, for example 100m lower, so as never to risk being subjected to the disturbances generated by the thermocline, nor to the currents existing in the upper section, in the event that disturbances of ocean currents on a planetary scale would significantly modify oceanological movements.

The SCR is connected to the vertical riser at the top of the float via a flexible joint which allows a significant variation in the angle between the axis of the tower and the axis of the chain at the level of said flexible joint. , without creating significant constraints in the SCR or in the top of the float. This flexible joint could be either a spherical ball joint with seals, or a laminated ball joint made of a sandwich of elastomer sheets and adhered sheets, capable of absorbing significant angular movements by deformation of the elastomers, while retaining a seal perfect due to the absence of friction seals, i.e. a limited length of flexible pipe capable of providing the same service.

The device according to the invention will advantageously be equipped with an automatic connector located at the flexible joint, either between the tower and the flexible joint, or between the flexible joint and the FPSO. Thus, the installation of such an SCR can be done entirely automatically, without having to call in divers. The installation sequence then consists of installing the tower, then transporting the future SCR in a vertical position, fixing it to the side of the FPSO in the final position. A cable connected to the lower end of the future SCR is then handled by a RON which is the abbreviated name of the Anglo-Saxon term "Remotely Operated Vehicle" (meaning "automatic remote controlled submarine, from the surface, and of which we will use the abbreviated term RON in the present description), to be brought to the top of the tower and to be connected to traction means integral with the float and controlled for example by the RON which then supplies the necessary power while controlling the operations at the using video cameras whose signal is brought up to the surface by operators installed on the floating intervention support, the cable is then pulled and the end of the SCR equipped with the male end, for example, with an automatic connector is brought back to the female end of the same automatic connector. At the end of the approach phase, the assembly is locked and the pulling means released to be able to intervene on the installation of the next line. The principle of automatic connectors being known to those skilled in the art in the field of hydraulics and pneumatics, will not be described here in detail. This installation mode has the advantage of being fully reversible, since the automatic connector is designed to be able to be disconnected. It is thus possible, during operation, to intervene on a single SCR to disassemble and replace it without disturbing the rest of the production and therefore without having to stop the production of neighboring risers and SCRs. In the same way, the tower and the vertical risers are advantageously installed according to the following sequence: positioning of the base and attachment to the bottom, installation of the tendon equipped with its guides and the upper float, - transport, in vertical position , from the vertical riser assembled to the vertical of its guide located in the buoy, progressive descent of the vertical riser in its guides by controlling from the surface the descent operation, at the end of the descent, the riser's head rests on the top of the float and comprises an elbow then, for example, the flexible seal on which the female part of the automatic connector described above is fixed. the lower end of the vertical riser is also advantageously equipped with an automatic connector, preferably the male part because of its smaller size, the assembly being able to be connected with the end of the underwater pipe connecting the foot of the turn to one of the well heads, said end being equipped with the female part of said automatic connector.

This method of installing vertical risers has the advantage of being completely reversible, since the automatic riser foot connector is also designed to be disconnected. It is thus possible, during operation, to intervene on a single riser to dismantle and replace it without disturbing the rest of the production and therefore without having to stop the production of neighboring risers and SCRs. Insofar as the float is installed at a depth greater than the half the water height, it will be possible to transport the riser completely finished vertically and to lower it through the float. If the float is above mid-water level, it will be necessary to position the floating installation support vertically above said float and to assemble the riser elements as and when it descends from its end lower towards and through the float and the various guides installed along the tendon, said assembly being able to be carried out either by welding, or by gluing, or even by mechanical assembly such as screwing, clamping or crimping. In a preferred version of the device, a pre-assembled length of the riser will be transported in a vertical position from an assembly location remote from the tower, said length being less than the height of water remaining between the surface and the top of the tower. Thus, the floating intervention support will be positioned vertically above the float with an optimal length of riser already assembled, equipped at the bottom with the male portion of the automatic connector and ready to be lowered to and through the float and the various guides installed along the tendon. As you descend, the missing upper part of the riser is assembled as described above.

The operating method thus described makes it possible to minimize the presence of the floating intervention support in the tower area, which minimizes the risk of accident. Thus, in order to be able to intervene later and dismantle the riser in a simple manner, preference will be given to assembly methods allowing rapid and non-destructive disassembly, such as screwing, which will make it possible to extract the riser from its support, to disassemble the screws successive sections of the only upper part necessary to release the lower part of the riser from the top of the float, the floating intervention support then leaving the position with the rest of the riser in suspension, and moving towards a location remote from sensitive installations to complete maintenance operations.

In order to minimize the presence of the floating intervention support vertically above the tower, the float is advantageously installed at a level lower than the half height of water, it is thus possible for the floating intervention support to install or extract the entire riser without having to assemble or disassemble any of its components, which further reduces the risk of accident in the area of the tower and sensitive installations. Other characteristics and advantages of the present invention will emerge better on reading the description which will follow, given in an illustrative and nonlimiting manner, with reference to the appended drawings in which: FIG. 1 is the representation of the entire slice of water in an oceanic configuration of Atlantic type, as described above, in which the indicative values of the currents in meters / second are indicated on the abscissa and the approximate depths of the different layers and of the corresponding thermoclines on the ordinate. FIG. 2 is a perspective view of an oil field development by 1,500 m of water depth, representing the surface FPSO, a central tower for recovering petroleum effluents and two lateral water injection towers, Figure 3 is a sectional view of the float associated with a side view of the central tendon and two risers, - Figure 4 is a side view of the base of the tower comprising two risers, the central tendon and two cuffs of connection to the submarine conduits, FIG. 5 is a side view of the base of a mono-riser tower, FIG. 6 is the schematic representation, illustrating the result of a static calculation, of an FPSO anchored on reel by

2,000 m of water depth and connected to a tower according to the invention located 1,000 m deep; FIG. 7 is a series of two curves representing the variations in the horizontal tension and the horizontal distance from the base for anchoring the float to the FPSO as a function of the depth of the float for a water height of 2000 m and a 8% excursion, Figure 8 is a series of two curves representing the variations in the excursion of the FPSO and the horizontal tension as a function of the depth of the float for a water height of 2000m and a distance between FPSO and 950m buoy, Figure 9 is a sectional side view of one of the riser guides relating to Figure 3, Figure 10 is a top view section along AA, relating to Figure 9. In these drawings , identical or similar elements bear, unless otherwise indicated on the contrary, the same references from one figure to another.

FIG. 2 represents an FPSO 1 anchored on an oil field by 1500 m of water height 18, by an anchoring system not shown and comprising for example, on port side, at its plating a support system 2 for pipes SCRs of petroleum effluents 3 and of water injection pipes 4. The SCRs of petroleum effluents are connected to a tower located for example - 800m from surface 19, at the upper level of float 5 having four locations passing through it , of which only two are occupied. Said float is connected to the base 8 resting on the bottom of the sea, by means of a tendon 6 to which are fixed a multitude of guides 7 through which are installed risers 9 connected at the level of the base to cuffs connection 1 1 1 themselves connected to underwater pipes 10 at an intermediate connection block 13; other connection sleeves 1 1 ₂ are awaiting the installation of the corresponding vertical risers. Two identical water injection towers consist of a float 14 installed at 1000 m from the surface and connected to the base 16 by means of a riser 15 also ensuring the function of tendon. A connection sleeve 17 provides the connection between the riser base and the intermediate connection block 13.

The float of the tower for petroleum effluents being for example at -800m from the surface, is at a lateral distance of approximately 500m from the vertical of the plating of the FPSO for a SCR link in the form of a chain arriving at the float horizontally , which greatly facilitates installation and maintenance operations by an intervention vessel, which will not interfere with the current operations of the FPSO. In addition, said intervention vessel will be able to position itself vertical to the tower and move without risking hooking the permanent anchor lines of said FPSO. The float 14 of the tower for the injection of water being at -1000m from the surface, therefore lower than the previous tower will thus be 550m away from the shell of the FPSO. FIG. 3 represents the sectional view of the float 5 of a multi-riser tower associated with the side view of the various associated components. Said float 5 consists for example of a box filled with syntactic foam and is connected to the central tendon 6 by a connecting device 20 having at its lower end a variable inertia piece 21 ensuring the transmission of stresses between tendon and float. The float has hollow guides 22 vertical and aligned with the guide means 23 of the guides 7 installed at intervals, regular or not, over the height of the tendon 6 and secured to the latter by means of a hooking device 24. The guides 22 can be either integrated into the within the float, either installed on its periphery or in its central part. These guides receive the vertical risers 9 shown on the left side completely installed and connected to the SCR 3 and on the right side during the start of insertion phase of the male end 25 of an automatic riser connector 9.

The end of said automatic connector 25 is connected to a cable 26 passing through each of the guides 22, 23, up to the base 8 of the tower at which a return pulley 27 is installed; base 8 and pulley

27 shown in Figure 4 are shown schematically in the form of cable return

28 in Figure 3. The cable 26 rises to the surface to the intervention vessel where it is kept in tension by a winch at constant tension. Thus, the intervention vessel appears vertically from the tower with the riser 9 fully assembled, because the depth - 800 m of the float 5 in this embodiment is greater than the length - 700 m of the riser 9. A RON hooks on the end of the automatic connector 25 the cable 26, the latter having been preinstalled before the installation of the base assembly 8 - tendon 6 - float 5; the second end is raised to the surface to be connected to a winch with constant tension not shown. The operation of lowering the riser 9ι is carried out while maintaining the tension in the cable 26, which tension then requires the end of the automatic connector 25 to pass successively through each of the guides 23ι. The tension required in the cable 26 for this operation will be all the greater the higher the angle of inclination of the tower. Indeed, during the installation of the first riser on the tower, the latter will be in a substantially vertical position. After connection of the corresponding SCR connected to the FPSO, said SCR will exert on the tower a horizontal force which will generate an angular movement of the tower relative to the vertical, oriented towards the FPSO. As the successive risers are installed, this angle will increase and the tension required in the cable 26 will increase proportionally.

The left part of the same FIG. 3 represents the riser 9 ₂ installed in its guide 22: its end 30 rests on the upper part of the guide 22 and constitutes the female part of an automatic connector into which the male part 31 of said connector will be inserted, secured to an elbow 32 itself secured to a flexible joint 33 connected to the end of the SCR 3.

Due to the height of the tower in this embodiment, the length of the SCR is less than the height of the water and the latter is assembled outside the field by the intervention vessel, then transported pendant up to to the FPSO where it is transferred and connected at its upper end. Its lower end equipped with the flexible joint 33, the elbow 32 and the male part 31 of the automatic connector is connected to a cable, the second end of which is transferred by the RON to drawing means, not shown, integral with the float and the power is, for example, supplied by or through the RON. The pulling of the cable from the float puts the pipe in the form of a chain and when the male end piece 31 is near the corresponding female part 30, the two parts are assembled by means, not shown, known to the man of the art in the field of hydraulic and pneumatic connectors. After setting up the SCR3, a stop 34 is installed on the float 5 which comes to bear on a flange secured to the elbow 32 so as to take up the horizontal forces generated by the SCR and to avoid rotations of the assembly and in particular of the elbow around the axis 36 of the risers 9.

FIG. 4 is a side view of the base 8 of a multi-riser tower consisting of a weighted base plate 40, resting on the ground 12 from the sea bottom and supporting a metal structure comprising guides 41, a central flexible joint 42 capable of receiving the lower end of the tendon 6. Two risers 9 are shown, on the left the riser 9ι is connected at the male part 25ι of its automatic connector, to the female part 44ι of the same connector secured to the connection sleeve 1 11 to underwater pipes not shown. Subject to temperature variations, the riser 9 can expand by sliding in the various guides 7 distributed along the tower. In the lower part, the movement of the lower end can reach several meters in extreme variations: also the 9ι riser. associated with its cuff 11 ₂ are free to move vertically in the guides 411 and 49ι secured to the structure of the base 8.

A counterweight system consisting of a mass 48ι of a cable 46ι turned around a pulley 45ι secured to the frame of the base 8 is connected to a reinforcement 50ι of the cuff 1 11 at the point of attachment 47ι. This counterweight is dimensioned to maintain, in the absence of the riser 9ι, the cuff in the high position, the reinforcement 50ι then comes into abutment with the structure of the base 8 at the level of the guide 49ι. This high position is detailed in the right part of the figure which shows a riser 9 ₂ during descent, after passage of the male part 25 ₂ of the automatic connector through the last guide 41 ₂ . The cable 26 kept in tension from the surface and used to pull the end of the riser through the various guides was disconnected by the RON. The riser 9 ₂ is then lowered until the male part 25 ₂ enters the female part 44 ₂ . In this engagement phase, the sleeve 1 1 ₂ is always in the high position because the counterweight 48 ₂ is dimensioned to support at least the self-weight of said sleeve plus the vertical force necessary for the engagement phase. After said engagement, the riser 9 can descend until its upper part rests on the float, the cuff 11 then being in the low position and the counterweight being lifted accordingly.

Thus, in the event of future intervention requiring the removal of the riser 9 ₂ , the ROV will operate the unlocking of the automatic connector 25 ₂ -44 and during the extraction of the riser, the cuff will return to the high position thanks to the action counterweight 48 ₂ . The reinstallation of the riser 9 ₂ after repair will be carried out in the same way as the initial installation, since the device according to the invention is entirely reversible.

Figures 9 and 10 detail a guide means 7 of a riser 9, said guide means being secured, at a hooking piece 24, a tendon 6 not shown. The guide means 7 consists of a cylindrical pocket 7a surmounted by a conical funnel 7b allowing the guide, during the positioning of the riser, of the male part of an automatic connector not shown. Said connector being of a diameter greater than that of the riser 9, the guide must be of a diameter significantly greater than that of the riser 9. To limit and dampen the lateral movements of the riser in operation, the guide means 7 is advantageously provided with 'A device adjustable in diameter for adjusting the inner diameter of the cylindrical pocket 7a. During the installation or removal of the riser, the device is fully retracted, so that the cylindrical pocket 7a has a maximum diameter and it is completely expanded when the riser is in operational configuration.

The adjustable device consists of a flexible membrane 60 integral with the cylindrical guide means 7a via high and low crimping rings 61, which creates a sealed pocket 62 capable of receiving a fluid through a orifice 63 provided with an isolation valve 64. A multitude of pads 65a - 65b, for example 6 or 8 pads are integral with the membrane 60 and come to bear with the riser 9 when the pocket 62 is completely filled. In the two figures 9 & 10, in the left part of the drawing, the membrane 60 associated with the shoe 65b is shown in the retracted position, while it is represented associated with the shoe 65a in the active position in the right part, say in contact with the riser. The pocket 62 is in communication with an outer chamber limited by a membrane 66 which is itself kept sealed by two straps 67, an orifice 68 bringing the two chambers into communication. Thus, when the pocket 62 is emptied of its content by suction of the fluid through the valve 64, the membranes 60 and 66 are pressed against the cylindrical guide 7a and the multitude of pads 65 are completely retracted, thus leaving maximum passage. When the riser is in place, the filling fluid is pumped through the valve 64, until the outer membrane swells by pressure; said valve is then closed and the centralizing effect is obtained and the force can be adjusted simply by injecting an additional volume of fluid creating swelling of the outer membrane, which plays the role of a pressurized bladder, therefore of reserve under pressure. The use of a fluid with very high viscosity, such as a shooting grease, charged or not, allows the assembly to play the role of shock absorber by absorption of energy, which prevents the appearance of vibratory phenomena in the riser subject to the effects of current. The inflation, deflation or pressure adjustment phases are carried out using the manipulator arms and pumps on board the intervention RONs. The outer membrane 66 acts as a visual witness, which makes it possible, without additional measurement, to know the state of the damping guide, by simple inspection using the cameras available on the RONs.

Figure 5 is the side view of the lower part of a mono-riser tower consisting of a base 16 resting on the ground 12 and supporting the elbow connection cuff 17 at the end of which is installed a flexible joint 37 itself connected to the female part 38 of an automatic connector. The riser 15 is equipped at its base with the male part 39 with the same automatic connector. In this embodiment of a device according to the invention, the riser 15 also plays the role of tendon and the automatic connector 38-39, as well as the flexible joint 37 are dimensioned to take up the tension generated by the pressurized fluid added. of the voltage created by the float 14 and the environmental conditions on the SCR 4 - tower assembly.

FIG. 6 schematically represents two positions of an FPSO, anchored on a reel, and obtained from the results of a calculation carried out in static, without taking into account the dynamic effects, for an oil field installed by 2,000 m of bottom and with the float 5 of the tower according to the invention positioned at a depth of 1000 m: the apparent linear weight in water of the SCR 3 and of the single vertical riser 9, acting as tendon, considered full of oil, was taken into account for a value of 97.96 kg / m, and the net buoyancy at the level of float 5 at a value of 180 tonnes (buoyancy of the float-apparent weight in water of float 5, tendon and riser (s) (s) vertical 9); the SCR 3 and the vertical riser 9 are made of the same material and a configuration of the same type, such as with a diameter of 10.25 inches and a thickness of 1 inch with a longitudinal rigidity considered infinite and a given insulation; seawater is considered with a density of 1033 kg / m ³ .

The mean position of the FPSO 1 being PO, the results of the calculations detail the characteristics of a distant position Pi and of a close position P ₂ , corresponding to a maximum excursion of 8% of the water depth of 2000 m, the float 5 being positioned at a depth of water equal to approximately half of the section of water considered and connected to the bottom 12 by a riser 9 of length 1014 m:

- for the most distant position Pi: the minimum radius of curvature of the SCR3 is 506m with a head angle αl of 19 ° for a tension of 157 tonnes and an angle β l at the bottom of 15 ° for a horizontal tension of 51 tonnes; the developed length of the riser 3 is 1,322 m for an immersion of the float 5 of 1,019 m; the header angle γ 1 of the stretched riser 9 is 15 ° and the horizontal distance from the FPSO 1 to the base 8 of the riser is 1027 m.

- for the closest position P ₂ : the minimum radius of curvature of the SCR3 is 300 m with a head angle α2 of 13 ° for a tension of 133 tonnes and a base angle β2 of - 10 ° and a horizontal tension of 30 tonnes; the developed length of SCR 3 is of course the same as in the above position, namely 1,322 m and the immersion of float 5 is 1,000 m; the header angle γ2 of the stretched riser 9 is 9.6 ° and the horizontal distance from the FPSO 1 to the base 8 is 868 m, the distance to the average position PO being L = 947 m.

FIG. 7 represents on the basis of the hypotheses detailed in FIG. 6 the variations in horizontal tension and in the distance L from the base 8 to the FPSO 1 as a function of the depth of the float 5. It is thus observed that for an increase in the depth of float 5, the horizontal tension decreases and presents a minimum for - 1400 m. In addition, for a depth between - 1,000 and - 1,800 m, the tension is between 52 and 53 tonnes, therefore substantially constant. Likewise, the distance L to FPSO 1 represents a maximum value for - 1,400 m and remains substantially constant around - 950 / - 960 m for a depth between - 1,000 and - 1,800 M. Thus, if we install two towers at substantially the same distance from the FPSO with floats located at very different depths, their performance will be similar, but the SCRs being radically different will not risk interfering with each other. FIG. 8 represents on the basis of the detailed hypotheses of FIG. 6 the variations of the excursion of the FPSO and of the horizontal tension as a function of the depth of the float 5 and for a distance of the FPSO 1 and base 8 of 950 m (position PO). The calculation was made on the basis of an 8% excursion corresponding to a float depth of 1000 m. In the design phases of oil fields, it is customary to consider a maximum excursion corresponding precisely to 8% of the water height, which corresponds to 160 m for a water depth of 2000 m. We thus observe that for a reduction in the depth of the float 5, the maximum excursion and the horizontal tension tend to increase whereas for an increase in this depth, the excursion remains stable around 8% and the tension remains stable around 50 tonnes. It thus appears that for depths greater than 1000 m, the maximum excursion and the tension remain stable in static. This set therefore constitutes an invariant of the system, which invariant will have a stabilizing effect for this system subjected to dynamic effects. Thus, according to the invention, the location of the float 5 at a depth greater than the half height of water has a great advantage for the stability of the system and therefore for its resistance to fatigue throughout the life of the field. It thus appears that for the development of fields requiring a multitude of turns, by locating the floats in the lower half slice of water, there will be a great latitude of choice as to the position of the floats, leading to small variations in the horizontal forces and of the tower-FPSO distance. By proceeding in this way, a multiplicity of tower-SCRs assemblies can be positioned in space, avoiding interference between the floats and the SCRs between them, which increases the safety and performance of the installations during the life of the field.

In all the descriptions of the devices according to the invention, male parts and female parts of the automatic connectors have been described in a given position, but they can, without changing the character of the invention, be reversed. In the same way, the position of the automatic connector and of the adjacent flexible joint can be reversed without changing the character of the invention.

In general, a tower increases the excursion capacity of the FPSO around its average position, while a large SCR improves the damping of the system. Indeed, the mathematical curve represented by the chain constituted by a line of linear mass and constant inertia presents, from the FPSO towards the float, a constant variation of its curvature, which has a minimum value (maximum radius of curvature) at the level of the FPSO, then increases towards a maximum value (minimum radius of curvature) at the level of the float. The FPSO, subject to environmental conditions, will transmit its movements to the assembly made up of the SCR (s) and the tower. Excitation of the SCR will lead to overall movements of said SCR generating localized variations in radius of curvature which will generate transverse movements which will have the effect of absorbing part of the energy. Thus, large amplitude SCRs will absorb maximum energy over their entire length and the transfer of excitation energy to the float will be minimized. The SCR thus plays, vis-à-vis the tower, the filter role for the excitation movements generated by the FPSO. The tower, favorable for improving excursion capacity for low angular variations, is a poor shock absorber and moreover it is subject to vibrations generated by vortex phenomena (vortex), this is why the device according to the invention consists in installing the tower and its float at great depth, in an area where the currents are stable and the vortex effects are weak.

Thus, on an oil field installed for example by 1,500m of water height, in the case of a tower of low height, for example located 100m above the bottom, the SCR, with a height of about 1 400m, behaves with respect to the FPSO like a conventional SCR, without however presenting the drawbacks existing in the prior art and linked to the formation of a stain at the point of contact and the risks of damage to the SCR in this zoned. The presence of articulated joints at the level of the FPSO and at the level of the tower float facilitates the excitations of the chain, which will lead to energy absorption, therefore to overall damping, while minimizing the transmission of forces at the level of the ends, both at the FPSO and at the tower float, by removing the embedding.

A tall tower will be preferred if you are looking for a high-performance insulation system such as a pipe-in-pipe. The pipe-in-pipe concept consists of two concentric pipes between which an insulation system is installed. This insulation system can be polyurethane foam, syntactic foam or even a gas at absolute pressure which can vary from the pressure prevailing at the bottom, for example, to absolute vacuum, the latter having the best level of performance in terms of insulation. We recall in this regard that the syntactic foam consists of microspheres, generally glass coated in a matrix of crosslinkable materials of epoxy or polyurethane type. Such a pipe-in-pipe system is expensive and has a certain complexity of implementation because it generally consists of elements 12 or 24 m in length assembled by welding or by screwing. If it is particularly well suited to tower risers, its use for SCRs is more delicate and it is preferable to use more resistant but less efficient and less expensive insulation systems, such as syntactic foam shells for medium depths. Thus, with a tall tower we implement, within the tower only, an expensive but efficient pipe-in-pipe technology and with a maximum of lifetime guarantees, because the tower is in the part the calmer the slice of water. In the upper part, SCRs are used, associated with insulation systems that are less efficient thermally, but more able to withstand environmental conditions during the lifetime of the installations, at considerably lower costs. Thus, the fluid arriving at the bottom of the tower at a temperature, for example 55 ° C., it will lose during its journey in the tower a few degrees, for example 4-5 ° C., essentially due to the depressurization of the effluent on a path representing for example 45% of the water height and, on the course of the SCR representing the complement, i.e. 55% of the water height, it will still lose a few degrees, for example 7-9 ° C due in part to insulation less efficient and partly due to the depressurization of the effluent. In the example cited, the fluid will thus have lost a total of 11 to 14 ° C using two insulation systems having very different performance levels, because the objective sought is an optimization of the overall insulation assembly based on lifetime and cost criteria. A tall tower will also be preferred in the event that gas plugs tend to form in the riser. Indeed, such plugs are followed by a liquid front which can move at very high speeds and inherently causing internal phenomena of the type water hammer. These phenomena are reflected on the SCR and go back to the FPSO by creating internal pressure fronts within the fluid. Such water hammer within vertical risers can generate loads of several tonnes at the ends. These efforts will then occur at the level of the float, the overall mass of which can reach 100 to 200 tonnes, which makes the consequences of such phenomena on the riser system insignificant. We thus consider that the effects of such water hammer are second order when they occur on the vertical tower whereas they are first order when they occur within a SCR of the same height.

Thus, in general, in effluent production configurations and especially those requiring insulation, high towers will be preferred.

In the case of water injection, which is carried out with great stability of the fluid stream and therefore does not generate phenomena of water hammer, preferably install a low tower to approach the configuration of '' a simple SCR resting on the bottom of the sea, without however meet the drawbacks of the prior art described above.

In this same case, the central tendon will advantageously be replaced by a pipe through which the injection water will circulate. In fact, water injection risers are generally very limited in number and are connected at sea level to multiple branches from which submarine pipes reach the water injection wells. This tendon pipe will perform a double function, an option which although possible in the case of the production of petroleum effluents is not desirable since maintenance operations then require disassembly of the float-pipe-tendon assembly.

Oil field developments are often carried out in sequence over several years, as wells are completed and wellheads are installed. The device according to the invention advantageously makes it possible to install around the FPSO a multiplicity of turns independent of each other and located at different depths, which has the advantage of locating the foot of each of them at horizontal distances of the FPSO the larger the deeper the float. This arrangement allows a large number of underwater pipes to converge towards each of the tower feet, without interfering with the neighboring tower feet or their associated underwater pipes.

Claims

1. Bottom-surface connection device for underwater pipe installed at great depth, comprising on the one hand a vertical tower made up of at least one float (5, 14) associated with an anchoring system (6, 8, 16 ) and carrying at least one vertical riser (9, 15) connecting said float to the bottom of the sea (18) and being able to connect to a said underwater pipe resting on the bottom of the sea, and on the other hand at least one connecting pipe (4, 3) from said float (5, 14) to any surface support (1) characterized in that said connecting pipe (4, 3) is a riser whose wall is a resistant tube rigid.

2. Pipe according to claim 1, characterized in that said float (5, 14) is installed at an immersion depth located below the last thermocline, preferably at an immersion depth greater than 300 m, preferably still more than 500 m.

3. Connecting device according to claim 2, characterized in that said float is installed at a depth greater than the half depth of water to which the tower is anchored.

4. Connection device according to one of claims 1 to 3, characterized in that the anchoring system comprises at least one vertical tendon (6), a lower base (8) to which the lower end of the tendon is fixed, and at least one guide (41) through which the lower end (25) of the vertical riser (9) passes.

5. Connection device according to claim 4 characterized in that the lower end (25) of the vertical riser (9) is capable of being connected to the end (44) of a movable bent cuff, between a high position and a low position, relative to said base (8), to which this cuff is suspended and associated with a return means bringing it back to the high position in the absence of riser (9).

6. Device according to any one of claims 4 or 5 characterized in that said tendon (6) comprises guide means (7) distributed over its entire length and through which pass at least said riser (9) vertical.

7. Device according to any one of claims 1 to 6 characterized in that the upper end (30) of said vertical riser (9, 15) is suspended through at least one guide (22) integral with said float (5, 14 ) and connected by the top thereof to the bent end (32) of said connecting pipe (4, 3).

8. Device according to one of claims 6 or 7, characterized in that said guide means (7) comprising a cylindrical cavity (7a) preferably surmounted by a conical funnel (7b), the inside diameter of said cylindrical cavity (7a) being greater than that of the vertical riser (9), and said guide means comprise a flexible membrane (60) integral with the internal wall of said cylindrical cavity (7a), thus creating a sealed pocket (62) between said membrane (60) and said internal wall, pocket which can be filled with a fluid, preferably with very high viscosity, so as to bear against the riser.

9. Device according to claim 8, characterized in that friction pads (65) are associated with said membrane (60) and come to bear against the riser (9) when said pocket (62) is filled with fluid.

10. Method of bottom-surface connection by submarine pipe installed at great depth using on the one hand a vertical tower made up of at least one float (5, 14) associated with an anchoring system (6, 8, 16 ) and carrying at least one vertical riser (9, 15) able to descend to the bottom of the sea (18) and on the other hand at least one connecting pipe (4, 3) from said float (5, 14) to any surface support (1) characterized in that said float (5, 14) is installed at an immersion depth located below the last thermocline (29).

11. A connection method according to claim 10 characterized in that said float (5, 14) is connected to the surface support (1) by at least one rigid resistant riser constituting said connection pipe (4, 3).

12. A linking method according to any one of claims 10 or 11 characterized in that:

- a base (8) is placed on the bottom of the sea (12) which is secured to said bottom (12), and to which the lower end of a tendon (6) which is secured is attached, at its other upper end, of said float (5), the assembly constituting said anchoring system of the vertical tower;

- Said vertical riser (9) is gradually lowered from the surface (19) and through a guide assembly (22) of said float (5) until its upper end (30) comes to bear on said float (5 ), its lower end (25) coming to connect to the upper end of a cuff (1 1) pre-installed on said base (8).

13. A connection method according to any one of claims 10 to 12 characterized in that said float (5, 14) is installed at an immersion depth greater than the half-depth of water to which the tower is anchored. .

14. A connection method according to claims 12 and 13 characterized in that the entire riser (9) is assembled together in a vertical position and transported in a vertical position to the vertical of the guide (22) corresponding to the float (5 ).