EP1250510B1 - Seafloor-surface linking device comprising a stabilising element - Google Patents

Abstract

A bottom-to-surface link with at least one underwater pipe having a portion with a substantially constant diameter and one or more floats in a string, the floats constituting cans surrounding the pipe and being located in a float zone around the top underwater portion of the pipe. The pipe is held at the surface by a guide device, preferably on a floating float support. The link includes at least one stabilizer situated in the top portion of the pipe. The link is arranged so that the bottom portion of the float zone, is preferably on or level with the lowest float and a transition zone is between the floats and the substantially constant diameter portion of the pipe.

Description

The present invention relates to the known domain of the bottom-surface bonds of the type having a vertical underwater line, called riser, connecting the bottom of the sea to a floating support installed in area.

As soon as the water depth becomes important the exploitation of production fields, in particular oil fields, is carried out in general from a floating support. This floating support comprises in anchoring means in order to remain in position despite the effects of currents, winds and swell. It also usually includes means storage and processing of oil and means of unloading to oil tankers. The name of these supports Floating is the Anglo-Saxon term "Floating Production Storage Offloading" (meaning "floating means of storage, production and unloading") abbreviated as "FPSO". Many variants have been developed, such as SPARS, long floating cigars held in position by anchors catenaries, or TLPs, platforms with tight anchor lines, said lines being generally vertical.

Wellheads are often spread over the entire field and production lines, as well as water injection lines and control cables, are deposited on the seabed in direction of a fixed location, on the vertical side of which the floating support is positioned on the surface.

Some wells are located vertically on the floating support and the interior of the well is then accessible directly from the surface. In this case the wellhead equipped with its "Christmas tree" can be installed in surface, on board the floating support. We can then perform, from a derrick installed on said floating support, all drilling operations, production and maintenance of the well throughout the life of the said well. This is called the dry wellhead.

To keep the riser equipped with its dry wellhead in position substantially vertical it is appropriate to exert a pull up which can be applied either by a cable tensioning system using winches or hydraulic cylinders installed on the floating support, using floats distributed along the riser and installed at various depths, again by a combination of both.

Patent No. 2,754,011 is known on behalf of IFP describing a barge and a guide system for risers, the latter being equipped with floats.

The water depth of some oilfields exceeding 1,500m and up to 3000m, the weight of the risers on such heights requires keeping them in position, efforts that can reach and exceed several hundred tonnes. We use elements of buoyancy type "can" added to submerged structures, mainly on the risers connecting the surface to ultra-deep sea (1000-3000m). The underwater pipe then consists of a riser having an underwater pipe joined to at least one float including a coaxial can surrounding said pipe and traversed by said conduct.

Preferably, means for joining said can and said conduct comprise a sealed joint assembled around the said at least one of the upper or lower openings of said can.

The floats concerned are large with, in particular a diameter greater than 5m, and a length of 10 to 20m and possess buoyancy up to 100 tonnes.

They are usually arranged in a rosary one below the other.

The floats generally extend over a corresponding length about not more than 10% of the length of the bottom-surface bond; especially over a length of 100 to 200 meters.

The riser is put in tension by these floats and is guided, from preferably at the floating support, by roller guides located in a plane for maintaining and guiding a riser with respect to floating support. Tensioning means with cables playing the role of guidance can be used.

FR 2 754 021 discloses a device for guiding a riser provided with head floats comprising wheels allowing the vertical sliding of the riser, as well as its rotation about an axis horizontally and guiding its horizontal movements, so that the Horizontal translation movements of the riser follow substantially those of the floating support. Also known in FR 99 10417 a device an improved guide system comprising wheels and skates friction arranged radially around the pipe. We finally know various guidance systems involving cable tensioning.

The entire riser then behaves like a tight rope between the bottom of the sea and the point on the axis of the guidance system at floating support level.

This riser is subject to the effects of swell, current and more horizontal movements of said floating support subject also to the same effects. Water moving in the water section creates drag effects the structure of the riser and its floats, thereby important of variable direction.

In certain configurations of water particle movements, it is occurs interaction effects between the fluid and the riser that result by alternating swirling detachments on the contour of the riser described in Figure 13.

When the period of this alternating swirling detachment is close to the periods of proper excitation of the riser, it then occurs phenomenon called "hanging" which leads to vibrations of the riser.

The intensity of the vibrations generated by the detachments swirling alternates, during the "hanging", is all the greater that the riser length on which the detachments of whirlpools simultaneously, is important. This length is designated by those skilled in the art under the name of "correlation length".

If we look at the riser as being a thread attached to its two extremes, his overall behavior is subject to displacement several meters long with a natural frequency of excitation and harmonics corresponding to a mode called "mode guitar ", ie the riser vibrates between its two ends like a guitar string.

In the case of floatless risers, the characteristics of which section (including diameter, inertia, stiffness), are significantly uniform or continuous, only this type of vibration of type "guitar" shown in Figures 3 and 4.

Similarly, in the case of a riser with high buoyancy as in WO 99/05389 for which the buoyancy is distributed over the entire length of the riser to obtain a buoyancy of about 95 to 98%, we observe only vibrations according to the "guitar" mode.

In WO 95/27101 and WO 99/05389, there are known devices for riser stabilization subject to "guitar" type vibrations related to vortex or vortex disturbance effects around the riser.

In WO 99/05389, the floats are distributed over the entire length riser in the form of a cylindrical shell of syntactic foam. The stabilization device consists in modifying the shape of the floats in the upper part corresponding to the submissive section to the swell, to obtain a non-cylindrical surface of hexagonal section. The solution proposed in WO 99/05389 results in a reduction of volume of floats in the upper part and therefore a reduction of their buoyancy with respect to the rest of the riser, with equivalent bulk. This type of plane geometry modification has the effect of increasing the drag and a decrease in excitation induced by detachments swirling, and stabilizes the riser only by absorption of energy.

In WO 95/27101, a submissive tubular pipe is stabilized also only to a vibration according to the "guitar" mode by equipping it of a plurality of perforated envelopes at different levels around the current part of the pipe. These perforated envelopes are sliding around driving to be set up specifically at the place areas subject to vibration corresponding to the belly of the vibration.

In US 4,768,984, a device for stabilizing a riser is described. provided with a free buoy, said buoy supporting an installation including a work platform. This surface installation creates a imbalance by the elevation of the center of gravity relative to the center of thrust or center of buoyancy that it leads. We therefore seek to restore proximity, preferably coincidence between the center of gravity and the center of buoyancy of the upper part of the pipe by adding a structure in lattice or reinforcement, which constitutes a mass, below float, compensating for the weight of the structure above the float. The lattice structure or frame by its particular shape avoids create too much increase in disturbances in the area concerned.

US Pat. No. 5,771,075 discloses a device for stabilizing a riser provided with floats and a stiffening device located above floats.

The present invention relates to the stabilization of a type of connection background-surface different from those described in the prior art and proposes a solution of original stabilization.

Indeed, the present invention relates to a connecting device bottom-surface comprising at least one underwater pipe comprising at least one float, preferably a plurality of rosary floats, said floats consisting of cans surrounding said pipe, located in the submerged upper part of the pipe, said pipe being maintained at the surface by a guiding device, preferably at the level of of a floating support, and the part situated below the zone of the floats being so mostly, if not completely, without a float.

As mentioned above, the term "device for guidance "devices known to those skilled in the art allowing the vertical sliding of the riser, as well as its rotation about an axis horizontal and guiding its horizontal movements, so that these are controlled and preferably follow substantially those of the support floating where appropriate.

The inventors have discovered that in the case of this type of fitted riser floats only at the head and comprising a system of maintenance and guidance on the surface, we can observe a very different and accentuated, in which the corresponding upper portion substantially to the length of the floats, behaves like a pendulum, while the lower part below the floats shows a behavior approaching a phenomenon of the "guitar" type. The type behavior "pendulum" of the upper part of the riser is favored by the difference of structural inertia between the two sections of the riser. This movement "pendulum" very significantly influences the behavior of the riser as a whole and we then observe a coupling of the "pendulum" mode and "guitar" modes as shown in FIGS. overall behavior of the riser is particularly sensitive to any excitation which tends to generate the pendular movement of the upper part of the riser.

Thus, the problem of the invention is to prevent or reduce the appearance of vibrations corresponding to the combined modes of type "guitar-pendulum" on the risers stretched by floats located at the head, when they are excited by the swell and the current or the displacements the barge, reducing or preferably eliminating the pendulum behavior of the upper part of the riser.

In particular, the inventors have shown that the problem is to avoid or reduce hydrodynamic phenomena at the origin of the excitation of vibratory modes of the coupled system "riser-floats", and avoid or reduce the response of the coupled system "riser-floats" to excitations.

As this is typically a "fluid-structure coupling" problem, the interaction between the two aspects, namely, the excitement and the system response, is strong. Hydrodynamic phenomena as well as the movements of the barge act on the structure which, itself, feedback on the excitation of hydrodynamic origin. The invention relates to thus the establishment of devices providing a solution to one or the other, or both aspects of the problem.

The problem of the present invention therefore relates to a first aspect, the elimination or reduction of the excitation of the modes vibratory system coupled "riser-floats". The inventors evidence that according to this first aspect, the problem concerns only the implementation of devices influencing the original excitation hydrodynamics of vibratory modes due to detachments vortices appearing on the contour of floats or risers.

The problem of the present invention also relates, according to a second aspect, the elimination or reduction of the system response coupled "riser-float" to excitations due to phenomena hydrodynamic or horizontal movements of the barge.

The inventors have shown that according to this second aspect, the The problem concerns the implementation of devices aimed at modifying frequencies of the vibration modes of the coupled system "riser-float" so to move away from the vortex detachment frequencies and avoid the "hooking phenomenon".

To do this, the present invention provides a connection device bottom-surface comprising at least one underwater pipe comprising at least one float, preferably a plurality of rosary floats, said floats consisting of cans surrounding said pipe, located in the submerged upper part of the pipe, said pipe being maintained at the surface by a guiding device, preferably at the level of a floating support. The part below the float zone is therefore mostly, if not completely, without a float.

• la partie basse de la zone des flotteurs, de préférence sur ou au niveau du flotteur le plus bas, et
• la zone de transition entre les flotteurs et la partie courante à diamètre sensiblement constant de ladite conduite.

According to the present invention, said device comprises at least one stabilizing device located in the upper part of the pipe constituted by:

• the lower part of the float zone, preferably on or at the level of the lowest float, and
• the transition zone between the floats and the substantially constant diameter portion of said pipe.

Said "riser-float" transition zone corresponds to an area where the mechanical characteristics of the pipe (the diameter, the section or the inertia of the pipe) gradually decrease down to reach the current part of the pipe, which corresponds to the part substantially constant diameter pipe below said pipe transition zone.

The invention takes advantage of the very particular pendulum behavior of this type of bottom-surface connection, to contribute to the stabilization of the system.

Indeed, the stabilization devices according to the invention are advantageously located at the lower end of the float zone or below the rosary of floats because the inventors have highlighted that this location corresponds to the area where the excitement of detachments swirling is predominant. Indeed, this excitement is strongly related to the amplitude of the horizontal movement of the riser; and at this location the lever arm is the largest relative to the axis of rotation of the pendulum type movement. This location increases the effect stabilizer of the stabilization device. In addition, said location constitutes a singular point of the said conduct because the tension is there Max.

The stabilizing devices of the invention thus make it possible to reduce, or even eliminate, the excitation and response modes vibratory system coupled "riser-float".

• un dispositif d'absorption d'énergie,
• un dispositif augmentant la masse d'eau entraínée au cours de son mouvement, et
• un dispositif abaissant le centre de gravité de ladite partie supérieure de la conduite.

More particularly, said stabilizing device comprises at least one device chosen from:

• a device for absorbing energy,
• a device increasing the mass of water driven during its movement, and
• a device lowering the center of gravity of said upper part of the pipe.

• l'absorption d'énergie,
• l'augmentation de la masse d'eau entraínée au cours de son mouvement, et
• l'abaissement du centre de gravité de ladite partie supérieure de la conduite.

Preferably, the stabilizing device combines at least two, preferably all three, effects chosen from:

• energy absorption,
• the increase in the mass of water driven during its movement, and
• lowering the center of gravity of said upper part of the pipe.

In one embodiment the energy absorption is obtained by a structural element increasing the area of contact with water and / or creating a contact surface with non-cylindrical water with respect to the axis of said pipe.

• une modification de la forme de la surface du flotteur ou de la conduite, celle-ci présentant alors une forme non cylindrique, par rapport à l'axe de ladite conduite, ou
• un élément de structure additionnel associé à la surface du flotteur ou de la conduite.

Thus, this structural element can be constituted by:

• a change in the shape of the surface of the float or the pipe, the latter then having a non-cylindrical shape, with respect to the axis of said pipe, or
• an additional structural element associated with the surface of the float or pipe.

Said additional structural element increasing the surface of contact of the float or the pipe with the water may have a surface to three-dimensional or flat geometry. Preferably, this surface to three-dimensional geometry is a surface not parallel to the surface cylindrical of the pipe.

• la forme non cylindrique de la surface extérieure d'un flotteur ou d'une partie de conduite, et
• une rampe hélicoïdale entourant un dit flotteur ou ladite conduite.

Advantageously, said stabilizing device comprises, one of the following embodiments, taken separately or in combination:

• the non-cylindrical shape of the outer surface of a float or part of a pipe, and
• a helical ramp surrounding a said float or said pipe.

As said device lowering the center of gravity of the party superior of the conduct can be mentioned a stabilizing device comprising an additional mass located in or around a float or surrounding said pipe.

In one embodiment, said stabilizer device comprises a box, preferably coaxial surrounding said pipe whose surface outer includes perforations. By "perforation" we mean everything space or any opening allowing the water to pass inside.

In some embodiments, particularly when the device stabilizer is a perforated box, this one combines different aspects, know that it consists of a device lowering the center of gravity since is located in the lower part of the float area, and a device increasing the mass of water driven during its movement in the as it contains water, and an energy absorbing device of the increase of the drag related to the perforations of the surface.

As a device providing additional mass, there may be mentioned a unperforated float partially or completely filled with water.

The invention therefore consists in using in particular either devices additional, such as propellers or perforated caissons, or floats not cylindrical in various forms, or the combination of these two types solutions.

In one embodiment, the float area includes several floats, preferably at least four.

Preferably, the stabilizer device is located on or at the level of float, or in the transition zone, just below the lowest float.

Said structural elements with three-dimensional geometry absorb energy, so as to limit the appearance of detachments swirling and / or reducing their lengths of correlation, while increasing the mass of water driven.

Stabilizer devices by energy absorption and increase of the driven water body also contribute to modify advantageously, preferably to reduce, the natural frequencies of vibration of the system "riser-float" and therefore reduce its response to the excitement due to hydrodynamic phenomena or horizontal movements of the barge.

The invention also comprises the implementation of a device stabilizer to increase the inertia of the string of floats in lowering its center of gravity, and secondly, in a device allowing to increase the mass of water driven during its movement, mass water known to those skilled in the art as "added mass". These two types of stabilizers can be concomitant in the same device as mentioned above.

These lowering devices of the center of gravity and increase the "added mass", apart from their favorable effect on the frequencies vibration of the coupled system "riser-float", allow to stabilize the upper part of the riser by increasing its inertia and thus contributes to the damping of the pendulum movement. These devices are also installed preferably on the lower part of the chain of floats or in below where they are particularly effective because the dynamics of the system coupled is mainly governed by the pendular movement of floats, and thus significantly influences the natural frequency oscillation of the moving riser, taken as a whole.

In the case of bundled risers, the excitation of the modes vibratory is no longer just due to vortex detachments on the riser considered but also to the interactions of the flow with the other beam risers. Setting up stabilizing devices considered in the context of this invention thus makes it possible to disrupt the wakes around each riser and advantageously contributes to reducing excitation of vibratory modes on neighboring risers.

The present invention also relates to a connection device bottom-surface comprising a plurality of subsea pipes according to the invention, that is to say comprising at least one stabilizing device and grouped into bundles.

• la figure 1 est une vue de côté d'un riser équipé de flotteurs associés à une coupe en vue de côté d'une barge ancrée supportant des équipements de traitement,
• les figures 2, 3 et 4 sont des vues de côté d'un riser tensionné en tête par des moyens extérieurs non représentés, en situation respectivement de repos (figure 2), et de vibration de type "guitare" respectivement en mode 1 (figure 3) et en mode 2 (figure 4),
• les figures 5, 6 et 7 sont des vues de côté d'un riser tensionné par des flotteurs, en situation respectivement de repos (figure 5), et de vibration de type "guitare-pendule" respectivement en mode 1 (figure 6) et en mode 2 (figure 8),
• les figures 8 et 9 sont des vues de côté d'un riser comportant des flotteurs, l'ensemble étant en position inclinée, ce qui permet de détailler les forces F de flottabilité et P de masse liée au centre de gravité, respectivement sans masse additionnelle et avec masse additionnelle,
• la figure 10 est une vue de côté d'un riser comportant des flotteurs, des absorbeurs d'énergie de type "hélice" étant installés sur le flotteur le plus bas,
• la figure 11 est une vue de côté d'un riser comportant des flotteurs, équipé d'un dispositif absorbeur d'énergie de type caisson perforé à turbulences, situé en dessous et en prolongement du flotteur le plus bas,
• la figure 12 est une vue de côté d'un riser équipé de flotteurs de formes extérieures variées dont le but est d'empêcher la formation de décrochements tourbillonnaires.
• la figure 13 est une coupe en vue de dessus d'une conduite soumise à un courant provoquant des tourbillons.

Other features and advantages of the present invention will become apparent in the light of the detailed description which follows, with reference to the following figures in which:

• FIG. 1 is a side view of a riser equipped with floats associated with a sectional view of an anchored barge supporting processing equipment,
• FIGS. 2, 3 and 4 are side views of a riser tensioned at the head by unrepresented external means, respectively in resting position (FIG. 2), and of "guitar" type vibration respectively in mode 1 (FIG. 3) and in mode 2 (Figure 4),
• FIGS. 5, 6 and 7 are side views of a riser tensioned by floats, respectively in a resting position (FIG. 5), and of a "guitar-pendulum" type vibration respectively in mode 1 (FIG. 6) and in mode 2 (FIG. 8),
• FIGS. 8 and 9 are side views of a riser comprising floats, the assembly being in an inclined position, which makes it possible to detail the forces F of buoyancy and P of mass linked to the center of gravity, respectively without additional mass and with additional mass,
• FIG. 10 is a side view of a riser comprising floats, with "helix" type energy absorbers being installed on the lowest float,
• FIG. 11 is a side view of a riser comprising floats, equipped with a turbulence perforated box-type energy absorber device situated below and in extension of the lowest float,
• Figure 12 is a side view of a riser with floats of various external shapes whose purpose is to prevent the formation of vortex strokes.
• Figure 13 is a sectional view of a pipe subjected to a current causing eddies.

FIG. 1 is a side view of a bottom-surface connection device comprising a riser 1 equipped with four floats 2. The riser opens in a barge 15 drill bay supporting processing equipment 17. The four floats constitute said zone 4 of the floats. The float superior is of greater diameter and buoyancy especially because it is fully located in the drill bay, that is, an area of the hull of the floating support, sheltered from the effects of the swell and the current.

The connecting device comprises in its lower part a pipe 1 of substantially constant diameter, as well as a transition zone 5 with a slight taper just below the float area. The top of the riser is guided by guiding system comprising a device roller 16 secured to the structure of said barge. These means of guiding the sliding of said riser along its longitudinal axis and guide lateral movements in a perpendicular horizontal plane to the longitudinal axis of the riser. In Figures 1 to 12 the guidance system is shown schematically, the structure making it integral with the barge not being represented.

FIG. 13 is a sectional view from above of a pipe 1 a current causing vortices 26 downstream of said conduit.

The vorticity phenomenon is represented in position A. The whirlpool originates in this position, develops and then detaches itself of the structure and is then trained in the current, downstream of the conduct. At the same time as a vortex develops in position A, a Another whirlpool is born in position B, which will evolve thereafter. This position B is substantially symmetrical with the position A with respect to the axis of the current. The vortices are thus generated alternately in the positions A and B. This instability is called "swirling detachment" The appearance of a vortex is accompanied by a lack of pressure, and when it occurs on the same side simultaneously on a height the side forces generated tend to move the pipe to the left, ie to the C position. generating a new vortex in position B will then result in a pressure to the right, that is to the position D. Efforts lateral pressure will thus succeed each other on both sides of the pipe. When the excitation frequency related to this instability is close to eigen frequencies of the pipe, it causes vibrations of this conduct. The initiation of this phenomenon of vibration is called "Hooking".

The characteristics of this instability change with parameters related to the flow (speed) and the geometry of the pipe (diameter). he It should be noted that, in some very special cases, the vibration occurs in the direction of the current and not perpendicularly, the detachment vortex is no longer then "alternated" but "simultaneous", in the positions A and B of Figure 13.

Figures 2, 3 and 4 are side views of a pipe 1 recessed in the foot and tensioned at the head by external means not shown, and guided by a roller device 16, respectively in resting position (Figure 2), and vibration type "guitar" respectively in mode 1 (Figure 3) and in mode 2 (Figure 4).

In Figures 3 and 4, the bellies 12 and nodes 11 are formed in planes 10 located substantially at L / 2 or L / 4.

Figures 5, 6 and 7 are side views of a riser 1 tensioned by floats 2 and guided at the head by a roller device 16, in situation respectively of rest (figure 5), and of vibration type "guitar-pendulum", respectively in mode 1 (Figure 6) and mode 2 (Figure 7).

The deformations were greatly exaggerated for clarity description, which shows that the planes 10 in which the nodes 11 and the bellies 12 are strongly shifted upwards. On the 6, the riser oscillates between the two extreme positions 13a-13b represented, while Figure 7 represents the riser only in one of its extreme positions.

FIG. 5 shows to the right of the riser the variation substantially linear of the riser voltage along its longitudinal axis. The voltage increases from the guide point. The voltage is maximum (Tmax) at the junction of the float zone with the transition zone 5. Then, the tension decreases with the depth in the current part and gives to the riser a mode of deformation of the type "vibrating rope" (figures 6 and 7) also called "guitar mode". In the float zone 4, the voltage decreases when we go back to the surface, to reverse and become compression in the emergent part of the riser above the float higher and achieve maximum compression over the system of guidance at the wellhead. As a result, the upper part of the riser features as a pendulum swiveling around a fixed point at the head of riser, which is the particularity of this type of riser which constitutes a system coupled "pendulum-vibrating rope" under the effect of the current and the swell on the riser and on the barge.

Figures 9 to 12 show stabilization devices which aim to reduce or eliminate the pendular behavior of the party high of the riser. These stabilization devices are located in the area floats 4 or in the transition zone 5 "riser-float" which corresponds to an area where the diameter of the pipe decreases gradually down until reaching the current part of the pipe, which corresponds to the part of the pipe with a diameter substantially constant below said transition zone.

In FIGS. 8 and 9 representing the zone of the floats, the forces F buoyancy and mass P linked to the center of gravity of the whole, respectively without additional mass (Figure 8) and with mass additional 7 installed in the lower part of the lower float (Figure 9). The additional mass 7 is obtained by an enclosure surrounding the float containing a weighty compound such as iron ore. He could be a float itself filled with water.

In FIG. 8, the mass of the zone of the floats corresponds to P1.

The additional mass 7 increases the mass value from P1 to P2 and lowers the center of gravity. The recall moment of the P2 force is thus increased, which has the effect of stabilizing driving.

FIG. 10 is a side view of a riser 1 comprising floats 2, "propeller" type stabilizers 8 comprising helical ramps are installed on the lowest float, preferably in its low. The propellers 8 could advantageously be installed on the part constituting the transition zone 5, located just below the last float. The maximum efficiency zone is in one of these two positions. The propellers are installed very punctually, by example on a height of three or four meters. Each helical ramp 8 only realizes a partial angular section of the float contour.

These structures with three-dimensional geometry related to the surface of the float confer a double stabilizing effect by absorption of energy and increase of the mass of water, or even a third stabilizing effect by lowering the center of gravity.

FIG. 11 is a side view of a riser 1 comprising floats 2, equipped with a box-type stabilization device perforated to turbulence 9, located below and in continuation of the lowest float, for example in the transition zone, that is to say at a depth approximately 50 to 100 m away from the effects of the swell. This box is consisting of a simple envelope pierced with holes of variable section or no, supportive of the riser. The mass of water trapped inside the caisson can represent 20 to 50 tons will be put in motion during Lateral movements of the riser and opposes by its inertia to said trips. The openings made in the box allow limited movements of the trapped water outwards, then inwards, this which creates additional damping by energy absorption and thus considerably increases the stabilizing effect.

Finally adding this box below the floats confers also a third effect of lowering the center of gravity.

FIG. 12 is a side view of a riser 1 equipped with floats 2 of various external forms whose purpose is to absorb energy for prevent the formation of swirling detachments leading to snapping effects.

Indeed, the vortex detachments occur on the outer surfaces of the floats or pipes as explained in the description of FIG. 13. When the generatrices are cylindrical (2 _a ), the vortices 26 tend to be formed simultaneously. the same side on varying lengths. As soon as the length becomes sufficient, for example 5 or 10 meters or more, the generated forces accumulate and are directed in the same direction, which causes the phenomenon "hooking" leading to the vibration of the riser.

By removing the cylindrical generators of the float 2 and by replacing by various forms such as cones 2b or forms of convex revolution 2d or concave 2c, the risks of collision are reduced or even eliminated. These particular shapes can also be external screens added to conventional cylindrical drums.

These particular shapes are advantageously combined with other stabilizing devices such as the float water filling the lowest or the association with a perforated box in the zone of transition 5.

Claims (14)

1. A bottom-to-surface link comprising at least one underwater pipe (1) having at least one float (2), preferably a plurality of floats in a string, said floats constituting cans surrounding said pipe (1) and being located in the top underwater portion of the pipe, said pipe being held at the surface by a guide device, preferably on a floating float support (15), the link being characterised in that it comprises at least one stabilizer (8) situated in the top portion of the pipe and constituted by:

   the bottom portion of the float zone (4), preferably on or level with the lowest float; and

   the transition zone (5) between the floats (4) and the ordinary portion (6) of substantially constant diameter of said pipe (1).
2. A bottom-to-surface link according to claim 1, characterised in that the stabilizer comprises at least one device selected from:

   an energy absorber device (7, 8, 9);

   a device (7, 9) increasing the mass of water entrained during movement thereof; and

   a device (7, 9) lowering the centre of gravity of said top portion of the pipe.
3. A bottom-to-surface link according to claim 2, characterised in that said stabilizer combines at least two and preferably three effects selected from:

   energy absorption (7, 8, 9);

   increasing the mass of water entrained during movements thereof (7, 9); and

   lowering the centre of gravity (7, 9) of said top portion of the pipe.
4. A bottom-to-surface link according to one of claims 1 to 3, characterised in that said link comprises a structural element increasing its contact area with the water (8) or creating a contact surface with the water that is not cylindrical about the axis of said pipe (2_b, 2_c, 2_d).
5. A link according to one of claims 1 to 4, characterised in that said stabilizer comprises the non-cylindrical shape (2_b, 2_c, 2_d) of the outside surface of a float (2) or of a portion of pipe (5).
6. A link according to one of claims 1 to 5, characterised in that said stabilizer comprises at least one helical ramp (8) surrounding one of said floats (2) or said pipe (1).
7. A link according to one of claims 1 to 6, characterised in that said stabilizer comprises a coaxial caisson surrounding said pipe and having an outside surface with perforations (9).
8. A link according to one of claims 1 to 7, characterised in that the stabilizer comprises additional mass (7) situated in or around a float (2) or surrounding said pipe (1).
9. A link according to claim 8, characterised in that it comprises a float that is partially filled with water, preferably the lowest float.
10. A link according to one of claims 1 to 9, characterised in that the float zone (4) comprises a plurality of floats (2), preferably at least four floats.
11. A link according to one of claims 1 to 10, characterised in that said stabilizer is situated in the bottom portion of the float zone (4), preferably on or level with the lowest float.
12. A link according to one of claims 1 to 10, characterised in that said stabilizer is situated in said transition zone (5), preferably immediately below the lowest float.
13. A link according to claim 12, characterised in that the float zone comprises a plurality of floats whose outside surfaces have non-cylindrical shapes that are different with generator lines that can be straight (2_b) or curved (2_c, 2_d).
14. A bottom-to-surface link according to one of claims 1 to 13, characterised in that it comprises a plurality of said underwater pipes grouped together in bundles.

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