EP1606159B1 - Method and device for stabilizing and controlling lowering or raising of a structure between the surface and the sea floor - Google Patents

Abstract

A method of using a buoyancy fluid presenting density that is less than that of sea water, and that is confined in a rigid or flexible leaktight casing, so as to constitute an immersed buoyancy element, wherein the buoyancy fluid is a compound that is naturally in a gaseous state at ambient atmospheric temperature and pressure, and in a liquid state at the underwater depth to which the buoyancy element is immersed. A method is also disclosed for placing a buoyancy element in place between the surface and the bed of the sea, wherein fluid is stored in a tank on a surface ship as a liquid in the cooled or compressed liquid state, and is injected in the liquid state into a pipe from the surface where it is stored to an immersed casing at an underwater depth at which the underwater pressure is not greater than the vapor pressure of the gas corresponding to the compound at the temperature at the depth.

Description

The present invention relates to the use of a buoyancy fluid with a density lower than that of seawater confined in a rigid or flexible sealed envelope, to form a submerged buoyancy element.

The present invention also relates to a buoyancy device or buoyancy element for lightening a heavy structure, and a method of placing a said buoyancy element in a submerged position between the surface and the seabed.

The present invention also relates to a method for stabilizing and controlling the descent or ascent of a said structure between the surface and the seabed, comprising or connected to at least one buoyancy element consisting of an envelope in which said Flotation fluid according to the invention is sealed.

The term "structure" here refers to any equipment, tool, machine, and especially risers, submarine wellhead elements on oilfields.
or oil treatment units, which one wishes to install at sea, or at the bottom of the sea, or a receptacle with sealed compartment useful in particular for recovering polluting effluents from a wreck.

The descent and the ascent of these massive structures that one wishes to descend to the bottom of the sea or to go up from the bottom of the sea to the surface, are delicate because of the mass of said structures or said reservoirs-shuttles. Indeed, it is known to drop packages of several hundred tons of apparent weight in the water, to the bottom of the sea using lifting means on a floating support, for example a crane; but, when the depth becomes important, the use of conventional steel cables is problematic because, in addition to the load of said structure, it must support its own weight, which can represent up to 50% of said load capacity for a depth of 3000m. It is also possible to use synthetic cables that do not have this drawback, but their cost is very high and their use with winches or capstans presents extreme difficulties for heavy loads and depths of 1000m to 4000m or more.

To lower such packages, it is preferred to lighten them by adding to the package buoyancy elements that reduce its apparent weight in the water and consequently require lifting gear of less capacity.

'Buoyancy element' means an element which has a lighter weight than seawater and which therefore increases the buoyancy of the combination which it forms with the structure to which it is connected or in which it is integrated.

By "increase the buoyancy" of an element, increase the ratio w between the buoyancy of Archimedes and its own weight out of water, which is exerted on said element when it is immersed. Thus, if said ratio is ω <1, the element has a negative buoyancy, so it tends to sink, if ω = 1, said element is in equilibrium and, if ω> 1 said element is floating and its buoyancy increases when w increases.

The buoyancy of the structure can be made positive to facilitate the ascent of said structure. In this case of "positive buoyancy", said buoyancy elements compensate for the weight of said structure, so that the buoyancy thrust that applies to all of said structure and said buoyancy elements is greater than or equal to weight of all of said structure and said buoyancy elements the resultant forces being directed upwards in case of positive buoyancy.

This additional buoyancy is generally performed with sealed tanks filled with air, made integral with said package. Such buoyancy elements consisting of tanks filled with air must be able to withstand the maximum immersion pressure without imploding or deforming, because the buoyancy would reduce all or even cancel. The tank must then have a resistance mechanical adapted to withstand the pressure corresponding to the envisaged immersion depth, which is about 10MPa extra for each additional 1000m of water depth. Thus, in the case of very great depths, for example beyond 1000m, the shell of the tank must be sufficiently reinforced to hold the pressure and its own weight is consequently much greater, which then considerably reduces the performance of said buoyancy element. To limit the effects of the pressure of the water at great depth, the reservoir is advantageously pressurized before lowering it, which then makes it possible to reduce the self-weight of the reservoir, because at the maximum immersion depth, the differential pressure between the outside and the inside are weaker and the wall needs less resistance; however, the tank must be able to withstand the initial burst pressure during pressurization.

To create this buoyancy, it also uses almost incompressible liquids of lower density than that of seawater, such as fresh water, diesel or methanol that allow to implement less resistant envelopes. But these materials do not have a ratio w (buoyancy force / self-weight) as high as air, namely: ω = 1.026 in the case of fresh water, ω = 1.21 in the case of diesel and ω = 1.30 in the case of methanol.

To create buoyancy in the very deep sea, rigid syntactic foam is also conventionally used, which is composed of microspheres, generally of glass and of small diameter, mixed with an epoxy or polyurethane-type binder. This type of foam is able to withstand considerable pressure and has a ratio w (buoyancy thrust / self-weight) more interesting between ω = 1.70 to 2.05 for foams of density between 0.6 and 0.5, able to withstand depths 1500 to 2000m. For syntactic foams capable of withstanding greater depths, their density is greater and the ratio w then decreases rapidly. In addition, these syntactic foam materials are very expensive and very difficult to manufacture in large volumes, especially for extreme depths.

When the package is placed on the seabed, buoyancy must generally be removed so that it remains stable. In the case of a tank filled with air, it suffices to simply open the valves so that it fills with seawater. In the case of a float with a solid buoyancy material such as in syntactic foam, the only solution is to separate it by cutting the links that connect it to the package and to go back to the surface, either in a controlled manner, which represents a considerable time, or by allowing it to go up freely without any control, this which is likely to create accidents with various vessels in surface operations.

The addition of such buoyancy elements makes it possible to reduce the apparent weight in the water of the package, but the mass of the package is then increased by the so-called buoyancy, as well as by the "added mass" of water. to say the body of water adjacent to the package that is driven during vertical movements, upwards or downwards. Thus, during the descent, although its apparent weight in the water may be very small, the inertial mass to be considered consists of the mass of the package itself, increased by the mass of the buoyancy elements, increased by the " added mass of water, which may represent an overall mass of inertia of 400 or 500 tons for a massive package of 100 tons.

It is generally sought to improve the performance of the buoyancy elements, so as to minimize not only the overall mass of inertia, but also the size of said buoyancy elements, so as to limit the effects of underwater currents on the whole. of the parcel.

An object of the present invention is to provide a buoyancy material and to make buoyancy elements to facilitate the installation of heavy packages that can weigh several hundred tons, or even several thousand tons, in water depths of 1000 to 4000m, or more, which is inexpensive, easy to implement and implement, and having a ratio ω = (Archimedes thrust / mass clean) optimal, that is to say, significantly greater than 1, especially higher to 1.5 and further whose value of ω is almost independent of the depth to which it is immersed, so as to facilitate the installation of said package, in particular by limiting the lateral catch to the marine currents on the whole package + buoyancy element.

Another object of the present invention is to provide a buoyancy material that can be confined in an envelope that does not require high pressure strength properties to be placed at great depth.

Another object of the present invention is to provide a device and method for controlling and facilitating the descent or ascent of a heavy structure and, where appropriate, bulky such as effluent recovery receptacles mentioned above, but applicable to any other type of structure, or even to stabilize it, between the surface and the seabed, particularly at great depth.

Another object of the present invention is to provide a method and an installation for confining and recovering the contents of bunkers and vessels of a vessel, for example an oil tanker, resting on the seabed, in water depths. important, especially greater than 3000 meters, or even up to 4000 to 5000 meters, and which do not have the disadvantages of prior methods and devices and, in particular, which are easy and simple to implement despite their very large dimensions.

Another object of the present invention is to provide a method and an installation for confining and recovering polluting effluents from the bunkers of a stranded vessel, particularly at great depth, via a rigid receptacle with open base in form. hat entirely covering the wreckage of the ship so as to channel all of the effluents escaping from the vessel in a single volume, or even to organize the return to the surface of the polluting effluents from said receptacle to the seabed in better conditions.

Another object of the present invention is therefore, more particularly, to provide an open-base receptacle in the form of a hat, able to completely cover a wreck at the bottom of the sea and recover pollutant effluents escaping from it, which is technically reliable and can be put in place at the bottom of the sea in a simple and technically reliable way.

To this end, the subject of the present invention is the use of a buoyancy fluid of a density lower than that of seawater confined in a rigid or flexible sealed envelope, to constitute an immersed buoyancy element, characterized in that said buoyancy fluid is a compound naturally occurring in the gaseous state at ambient atmospheric temperature and pressure, and in the liquid state at the underwater depth at which said buoyancy element is immersed.

This type of compound is also commonly called (and improperly) "liquefied gas"

The conditions of ambient temperature and atmospheric pressure correspond to temperatures of -10 to + 40 ° C and to a theoretical absolute atmospheric pressure of 101325 Pa, at sea level, and whose approximate value of 100 000 Pa, ie 0.1 MPa, is used throughout the description of the present invention.

The submarine ambient temperature and pressure conditions generally correspond to a temperature of 1 to 35.degree. C., preferably 3 to 25.degree. C., and a pressure greater than atmospheric pressure, more precisely a pressure increasing substantially by 10 ⁵ Pa. for every 10 m.

In some arctic regions, water may be encountered at a temperature well below 0 ° C, for example -5 to -8 ° C, but as a rule the deep waters are around 1 to 4-5 ° C in all the seas of the world.

The compounds according to the invention have a critical temperature, preferably greater than 35 ° C, more preferably greater than 40 ° C. The term "critical temperature" is understood here to mean the temperature above which said compound is in a fluid state having properties belonging to both the gases and the liquids, and therefore to a temperature above which said compound can not not be in the liquid state.

The present invention also provides a submerged buoyancy element imparting buoyancy to an immersed structure to which it is connected or attached or in which it is integrated, characterized in that it comprises a said submerged envelope in which said liquefied compound is tightly confined.

In a first variant, said envelope is constituted or placed inside the walls of a compartment of a submerged structure.

In a second variant, said envelope is placed outside said structure to which it is connected or fixed, more particularly said submerged structure is suspended from said buoyancy element by at least one cable.

In this second variant, said buoyancy element may comprise a said flexible envelope preferably hydrodynamic profile shape minimizing the forces during its vertical movements when filled with said buoyancy fluid.

In a preferred embodiment, said buoyancy fluid is naturally in the stable liquid state when it is placed at the underwater depth of 10 to 500 m, preferably 20 to 100 m. At these depths, the temperature is between 3 ° C and 25 ° C and the pressure is respectively 0.1 MPa to 5MPa, preferably 0.2MPa to 1 MPa.

More preferably, said fluid is a substantially incompressible fluid and has a density in the liquid state of 0.3 to 0.8, preferably 0.5 to 0.7.

Also preferably, said gas is selected from ammonia, a C-2 to C-7 alkane, a C 2 to C 7 alkene, a C 2 to C 7 alkyne, and a diene. in C-4 to C-7.

More particularly, commercially available compounds such as ammonia, ethane, butane, propane, ethylene, propene, butene, acetylene, methyl acetylene, propadiene and butadiene are selected.

"Butene" is understood here to mean the different isomers such as butene-1 and cis or trans-butene-2.

In a preferred embodiment, said compound is chosen from ammonia, propane and butane.

As will be explained below, these latter compounds represent a good compromise between the values of density characteristics in the liquid state and vapor pressure. Indeed, for a gas in general, when its density in the liquid state increases, its vapor pressure at the reference temperature 15 ° C, decreases, and therefore the minimum depth of water at which the compound is intended to be placed decreases too. These three compounds have densities of substantially between 510 and 630 kg / m ³ and the minimum depths to which said rigid or flexible envelopes can be filled are respectively substantially between depths of 65 m and 7.5 m (see table). 1 below), when the ambient temperature is about 15 ° C.

Thus, if the heavy structure has, in quantity, sealed internal cavities that can act as a rigid envelope advantageously butane will be used. But if it is necessary to realize additional external flexible or rigid envelopes, it will be advantageous to use propane, so as to minimize the size of said envelopes and therefore their cost. The required volume gain of propane being about 15% compared to butane, it will then result not only a reduction in the cost of the envelope, but also the cost of liquefied gas, because the unit prices of butane and propane are substantially the same. On the other hand, the transfer operations are carried out at greater depth and in the case of the use of divers to supervise the operations, the necessary equipment and the personnel have a higher qualification, therefore with a significant additional cost compared to a simple dive. area.

The present invention also provides a method of placement between the surface and the seabed of a buoyancy member. According to the invention, said fluid is stored in a tank on a surface vessel in the liquid state compressed or cooled, and is injected in a liquid state in a conduit from the surface where it is stored in a so-called submerged envelope to a submarine depth at which the underwater pressure is greater than or equal to the vapor pressure gas corresponding to said compound at room temperature at said depth.

In the case where said envelope is a flexible envelope, it can be lowered to the desired depth, empty, collected or folded on itself.

Advantageously, said envelope is pre-filled with seawater or other fluid preferably a liquid compound at atmospheric pressure and incompressible temperature such as gas oil, fresh water, or methanol, and is evacuated. sea water or the said other fluid of the envelope as and when filling the said buoyancy fluid.

In an advantageous embodiment, said envelope is pre-filled with seawater and, before it is filled with said buoyancy fluid according to the invention, a limited quantity of methanol is injected capable of preventing the formation of hydrates. Indeed, methanol which is of intermediate density between seawater and a buoyancy fluid according to the invention, creates a screen avoiding direct contact between said buoyancy fluid and water and thus prevents the chemical reactions leading to the formation of hydrates when said buoyancy fluid combines with water. These hydrates may block the pipes and prevent the recovery of liquefied gases at the end of the installation phase.

More particularly, the said envelope is filled at the surface with the aid of a said other fluid, and said casing thus filled is lowered to a depth where the hydrostatic pressure corresponds to the pressure at which the said buoyancy fluid is then injected into said envelope as one evacuates said other fluid.

In an alternative embodiment, said buoyancy fluid is stored in the cooled liquid state in a cryogenic tank and at atmospheric pressure and is injected in the liquid state under pressure in said immersed envelope at a pressure corresponding to the hydrostatic pressure at the depth of said envelope, said buoyancy fluid passing through a heat exchanger so that the temperature of said fluid is substantially to that of seawater at the depth of said submerged envelope before filling.

• ■ une première extrémité est reliée à un treuil à bord d'un support flottant

ou navire en surface, sur lequel treuil elle est enroulée, et

• ■ une deuxième extrémité est reliée à un élément d'accrochage, sur ladite structure, ou sur au moins un premier élément de flottabilité selon l'invention, relié à ladite structure, et
• ■ la longueur dudit élément de liaison est telle que ledit treuil est apte à enrouler ou dérouler ladite première extrémité dudit élément de liaison, de sorte qu'une portion inférieure dudit élément de liaison puisse pendre au dessous ledit élément d'accrochage, c'est-à-dire dessous le point d'attache de ladite deuxième extrémité audit élément d'accrochage.

The present invention also provides a device for stabilizing or controlling the descent or ascent of a structure between the surface and the seabed, comprising or connected to a buoyancy element according to the invention, characterized in that comprises at least one connecting element of the cable or chain type, of which:

• ■ a first end is connected to a winch on board a floating support

or ship on the surface, on which winch it is rolled up, and

• A second end is connected to a fastening element, on said structure, or on at least one first buoyancy element according to the invention, connected to said structure, and
• The length of said connecting element is such that said winch is capable of winding or unrolling said first end of said connecting element, so that a lower portion of said connecting element can hang underneath said fastening element, it is that is, below the point of attachment of said second end to said fastening element.

Said structure is, if necessary, suspended from one or more so-called first buoyancy elements according to the invention arranged above it. Said structure may also comprise second buoyancy elements integrated or incorporated within said structure, that is to say that said second buoyancy elements do not move additional water volume relative to the volume of water displaced by said structure, preferably said second buoyancy elements according to the invention.

It is understood that the stabilization device makes it possible to vary the length and therefore the weight of the said lower portion of the connecting element. while below said hooking member on said structure and supported by said structure.

In the case of a massive structure, the stabilization and control device according to the invention comprises at least two said connecting elements and said structure comprises a plurality of said hooking elements and said connecting elements and said hooking elements are preferably symmetrically arranged respectively around and on the periphery of said structure.

More specifically, the present invention also provides a method for lowering or raising or stabilizing a structure between the surface and the seabed by means of a stabilization device, according to which steps are carried out in which one unwinds or winding the said link element (s) at their (their) said first end (s) with the aid of said winch (s) and controls the speed of descent or of ascent by regulating the unwinding or winding speed of said link element (s) at said winch (s), in such a way as to adjust the length of said lower portion of said link element (s) below the said hooking element (s) on said structure or said first buoyancy element, the descent, the rise or the stabilization of said structure being obtained when, respectively, the sum of the weight of the part of the said (or) s) lower portion (s) of the link element (s) between, on the one hand, the said point (s) of attachment to the said element (s) or said first buoyancy element on said structure and, on the other hand, the lowest point (s) of the (or) said lower portion (s) ( s), added to the weight of said structure and (s) said (s) first (s) buoyancy element (s) according to the invention, is respectively greater, less than or equal to the buoyancy pressure acting on the whole of said structure and said first buoyancy elements according to the invention (that is to say the weight of the total volume of water displaced).

In one embodiment the stabilization and control device comprises a said connecting element is constituted by a cable of which said lower portion comprises weighting blocks arranged in a string on a said cable, preferably metal blocks secured to said cable by crimping.

In a preferred embodiment, said blocks have a shape such that when said lower portion below said hooking elements adopts a curved shape, said two blocks arranged side by side are able to abut one against the other thus limiting the curvature of said cable.

More particularly, the curvature of said cable is limited so that the minimum radius of curvature of said cables at said lower portion makes it possible to maintain a minimum distance between said cable and said structure, sufficient to prevent any mechanical contact between them when a said descent or ascent of said structure.

More particularly and advantageously, said blocks have a central cylindrical portion framed by two frustoconical ends whose axes (that is to say the axes of said cylinder and the two frustoconical ends to cap these bases) correspond to the direction of said cable when it is arranged linearly, two adjacent blocks being in contact at said frustoconical ends along a generatrix of said frustoconical ends in the curved portions of said lower portion.

In another embodiment, said connecting element comprises a chain of which said lower portion comprises links heavier than those of the remainder of the chain, and preferably more bulky so as to limit the possible curvature of the chain.

Advantageously, said first buoyancy elements according to the invention are arranged where appropriate above said structure to which it is suspended and, where appropriate, said second buoyancy elements preferably according to the invention are integrated in the part upper of said structure, preferably integrated above said fastening elements so that the center of gravity of all of said structure and said first buoyancy elements according to the invention is located below the center of thrust s exerting on all of said structure and said first buoyancy elements according to the invention, so as to ensure overall stability throughout the installation phase.

The center of thrust is the point at which the result of the buoyancy of Archimedes is exerted. (The center of thrust is the center of gravity of the volume of water displaced by the structure).

As mentioned above, said heavy structure may be constituted by any package including heavy package, module, tool, or base as described in the European patent application in the name of the unpublished Applicant No. 0435802.6, which one wishes to immobilize at near the bottom of the sea
or anchor on a wall or element resting at the bottom of the sea.

Preferably, said structure is a rigid structure of steel, metal or composite synthetic material containing at least one, preferably a plurality of sealed buoyancy compartments capable of forming a said buoyancy element, said compartment being equipped with at least one orifice filling and preferably at least one discharge port, said sealed compartments preferably being symmetrically distributed in said walls.

Sealed compartments are cavities intended to be filled totally or partially with lighter buoyancy fluid than seawater according to the invention and therefore constitute compartments providing buoyancy to the structure, allowing its surface towing and descent at the bottom of the sea during its implementation in technical conditions reliable and simple to achieve, as will be explained later.

The term "symmetrical distribution of the compartments that they are arranged symmetrically with respect to one or more median planes of symmetry of said structure, which allows, as will be explained below, to facilitate the balancing and positioning of the base of said structure substantially horizontally.

Advantageously, the rigid structure comprises hollow tubular sections defining sealed compartments and forming said buoyancy elements according to the invention.

Advantageously, the tanks or the balloons associated with the treatment of oil are used in particular for effecting the water / oil / gas separation, to provisionally define sealed compartments forming said buoyancy elements according to the invention.

In a particularly advantageous embodiment, said structure is a massive structure constituted by an open base receptacle, in the form of a cap, comprising a peripheral side wall surmounted by a ceiling wall, able to completely cover a wreck of a ship at the bottom of the sea to recover pollutant effluents escaping from it, said receptacle comprising at least one outlet for discharging said effluents contained in the interior volume of said receptacle; said discharge port being preferably located at the ceiling of the receptacle.

In general, said receptacle has a longitudinal axis of symmetry similar to said vessels intended to be covered, and said receptacle has a longitudinal axial plane of vertical symmetry when the open base of the receptacle is in a horizontal position, and more particularly, said receptacle has a second vertical transverse plane of symmetry.

• d'élément d'accrochage permettant d'y accrocher desdits éléments de flottabilité et desdits câbles ou des chaînes permettant la descente de ladite structure depuis la surface, et sa mise en place et, le cas échéant, son ancrage au fond de la mer, et
• de préférence des propulseurs, de préférence encore des propulseurs orientables, permettant le déplacement du réceptacle dans une direction horizontale pour le positionner au-dessus de la dite épave.

In order to facilitate the installation of the said structure at the bottom of the sea, it is equipped outside:

• fastening element for attaching said buoyancy elements and said cables or chains allowing the descent of said structure from the surface, and its installation and, where appropriate, anchoring to the seabed, and
• preferably thrusters, more preferably steerable thrusters, allowing the displacement of the receptacle in a horizontal direction to position it above said wreck.

Said hooking elements can thus allow to hang to said structure additional floats according to the invention.

• d'élément(s) d'accrochage permettant d'y accrocher un ou desdits éléments de flottabilité et de(s)dit(s) câble(s) ou de(s) chaîne(s) permettant la descente de ladite structure depuis la surface, et sa mise en place et le cas échéant son ancrage au fond de la mer, et
• de préférence des propulseurs, de préférence encore des propulseurs orientables, permettant le déplacement du réceptacle dans une direction horizontale pour le positionner au-dessus de ladite épave.

In order to facilitate the installation of said structure at the bottom of the sea, it is equipped outside:

• attachment element (s) for attaching one or more buoyancy elements and (s) said (s) cable (s) or (s) chain (s) allowing the descent of said structure from the surface, and its establishment and, if appropriate, anchoring to the seabed, and
• preferably thrusters, more preferably steerable thrusters, allowing the displacement of the receptacle in a horizontal direction to position it above said wreck.

Said hooking elements can thus allow to hang to said structure additional floats according to the invention.

1. 1) on remplit totalement ou partiellement desdits compartiments étanches avec un dit fluide de flottabilité selon l'invention, pour constituer un élément de flottabilité selon l'invention, et on adapte le taux de remplissage desdits compartiments étanches de manière à positionner ladite structure, notamment ledit réceptacle en équilibre en immersion à proximité de la surface, notamment à quelques mètres, par exemple à 10 mètres, et
2. 2) on descend ladite structure, le cas échéant, à sa position immergée voulue, notamment ledit réceptacle à proximité du fond de la mer, au-dessus de l'épave, en contrôlant la descente à l'aide d'un dispositif de stabilisation ou de contrôle de la descente ou de la remontée d'une structure selon l'invention, notamment à l'aide d'une pluralité de câbles déroulés de préférence depuis des treuils à bord de navires en surface, lesdits câbles étant reliés à des longueurs de chaînes lourdes, les chaînes étant elles-même reliées, à leur autre extrémité, à desdits éléments d'accrochage solidaires de ladite structure, de préférence répartis symétriquement sur la périphérie de ladite structure, le poids des longueurs de chaînes pendantes dessous les points d'attaches sur lesdits éléments d'accrochage permettant la descente de ladite structure, et les longueurs desdites chaînes pendantes dessous lesdits points d'attache des éléments au point d'accrochage étant adaptées par déroulement ou enroulement desdits câbles, de préférence autour desdits treuils, de manière à réguler la vitesse de descente du réceptacle et assurer l'équilibrage de la base de ladite structure, notamment la base de ladite structure sensiblement horizontale pendant la descente, et
3. 3) lorsque ladite structure est en place à sa position voulue, notamment lorsque ledit réceptacle est mis en place au fond de la mer de manière à recouvrir ladite épave, on vidange lesdits compartiments étanches remplis d'un fluide plus léger que l'eau de mer, et on remplit simultanément lesdits compartiments étanches avec de l'eau de mer.

Indeed, the present invention also relates to a method of setting up a structure, in particular a receptacle according to the invention, for covering a wreck of a ship at the bottom of the sea and recovering polluting effluents. escaping from it, characterized in that the following steps are carried out in which:

1. 1) the said sealed compartments are completely or partially filled with a said buoyancy fluid according to the invention, to constitute a buoyancy element according to the invention, and the filling rate of the said watertight compartments is adapted so as to position the said structure, in particular said receptacle in equilibrium immersed near the surface, in particular a few meters, for example at 10 meters, and
2. 2) said structure is lowered, if necessary, to its desired submerged position, in particular said receptacle near the seabed, above the wreck, controlling the descent using a stabilizing device or controlling the descent or ascent of a structure according to the invention, in particular by means of a plurality of cables unrolled preferably from winches on board surface ships, said cables being connected to lengths heavy chains, the chains being themselves connected at their other end to said fastening elements integral with said structure, preferably symmetrically distributed on the periphery of said structure, the weight of the chain lengths hanging below the points of fasteners on said fastening elements allowing the descent of said structure, and the lengths of said chains hanging below said points of attachment of the elements at the point of hitch hage being adapted by unfolding or winding said cables, preferably around said winches, so as to regulate the speed of descent of the receptacle and ensure the balancing of the base of said structure, in particular the base of said substantially horizontal structure during the descent, and
3. 3) when said structure is in place at its desired position, especially when said receptacle is placed at the bottom of the sea so as to cover said wreck, said sealed compartments are emptied with a lighter fluid than the water of sea, and simultaneously fill said watertight compartments with seawater.

Before and / or after step 1), but before step 2) above, it is possible to tow, using ships, said structure, in particular said surface floating receptacle, said watertight compartments being filled with air and floating between two waters flush with the surface or said sealed compartments being fully filled with a lighter fluid than seawater.

In step 1) above, it is understood that the filling of said sealed compartments, with a lighter fluid than seawater, is carried out in the different compartments according to their distribution in the walls of the receptacle, so that the open base of said structure remains substantially horizontal on the one hand and that, on the other hand, the center of thrust of the receptacle is substantially above the center of gravity of said structure. This applies to the choice of compartments to fill and their fill rate.

Advantageously, in step 1), additional buoyancy is provided to said structure by means of additional floats using said first buoyancy elements connected to said structure, in particular to said receptacle, and in step 3 ), when said structure is in the desired underwater position, especially at the bottom of the sea, it releases said additional floats.

Advantageously, after step 1) and before step 2), when said structure arrives in the desired position, in particular near the bottom of the sea, the lengths of said heavy chains hanging below said fastening elements are reduced and supported by said structure so as to stabilize said structure in suspension, and if necessary, the anchoring of said structure is carried out at the bottom of the sea, then said heavy chains are completely lowered so that their entire weight contributes to the stabilization of said structure, in particular of said structure at the bottom of the sea.

The heavy chains can be recovered by disconnecting them from said structure, but as explained hereinafter, to increase the stability of said structure, in particular of said receptacle, said heavy chains can be hooked at both ends to said hooking elements on said structure or, more simply, the free end of said heavy chains can be placed on the ceiling of said structure, in particular said receptacle after hooking of the cables connected to the surface ships, then the cables connected to the surface ship are unhooked from said chains.

Advantageously, in the method according to the invention, said structure can be positioned by actuating thrusters mounted outside said structure and distributed preferably symmetrically around its periphery.

More particularly, in a method according to the invention, in step 1), said compartment (s), or envelope (s) connected to said structure (s), are filled. using seawater or a first fluid lighter than seawater corresponding to a said buoyancy fluid according to the invention, and in step 2), said structure is lowered to a depth of 30 to 60 meters corresponding to a pressure of 3 to 6 bars to which a liquefied gas is injected under pressure lighter than seawater in the so-called compartment (s) or so-called (s) envelope (s) from a surface gas vessel to form a buoyancy element according to the invention.

The use of liquefied gas as a fluid lighter than seawater makes it possible to obtain liquid density liquids of between 0.5 and 0.7 bringing a buoyancy two to three times greater diesel fuel (d = 0.85) and thus making it possible to use volumes of sealed compartments which are considerably reduced. In addition, in the event of an incident during installation, these products are much less polluting than diesel or oil, because they disperse naturally as soon as they arrive on the surface, returning to the gaseous state.

1. 1) on met en place un réceptacle selon un procédé de stabilisation et de contrôle de descente selon l'invention et
2. 2) on recueille les effluents récupérés à l'intérieur dudit réceptacle en évacuant par ledit orifice supérieur d'évacuation.

Finally, the subject of the present invention is also a process for recovering pollutant effluents lighter than seawater contained in the tanks of a wreck of a ship resting at the bottom of the sea in which:

1. 1) a receptacle is put in place according to a method of stabilization and descent control according to the invention and
2. 2) recovering the effluents recovered inside said receptacle by evacuating through said upper discharge port.

To collect the effluents escaping from said upper discharge orifice, it is possible to use a pipe connected to a surface vessel or recovery devices as described in the patent application. FR2 804 935 of the applicant, or the shuttle tanks as described in the unpublished European application no. 03 358 003.6 of the plaintiff.

• la figure 1 est une coupe en vue de côté d'une dite structure consistant en un réceptacle appelé ci-après « sarcophage » en cours de descente vers une épave;
• la figure 2 est une coupe en vue de côté d'un réceptacle rigide reposant au fond de la mer et enveloppant intégralement l'épave ;
• la figure 3 est une perspective en vue de coupe arrachée, de la structure du sarcophage ;
• la figure 4 est une coupe en vue de côté du sarcophage en cours de descente, détaillant le mode de régulation de la descente à l'aide de chaînes lourdes ;
• les figures 4a et 4b détaillent le mode de mise en oeuvre variable desdites chaînes lourdes ;
• la figure 5 est une coupe en vue de côté d'un sarcophage composé d'une structure porteuse rigide en poutres métalliques, associée à des réservoirs de flottabilité remplis d'un fluide de faible densité intégrés entre celles-ci et fermée par des toiles membranes étanches sur la face externe de la structure ;
• la figure 6 est une coupe en vue de côté d'un sarcophage réalisé en béton allégé, et comportant des volumes internes formant des compartiments étanches remplis d'un fluide de faible densité assurant la flottabilité ;
• les figures 7a et 7b représentent une coupe en vue de côté d'un sarcophage respectivement en cours de remorquage, ses compartiments de flottabilité étant remplis d'eau de mer 9a, et en 9b, à la verticale de l'épave, lors de la phase de remplissage desdits compartiments de flottabilité par un gaz liquéfié de faible densité;
• la figure 8a est une vue de côté d'un réservoir navette stabilisé dans sa remontée par un câble de liaison alourdi par des blocs solidaires de ce dernier et jouant aussi le rôle de limiteur de courbure,
• les figures 8b et 8c représentent des états similaires à celui de la figure 11a, le réservoir navette étant en phase de remontée sur la figure 11b et en descente sur la figure 8c,
• la figure 8d représente le détail de deux blocs 31 en contact, lorsque ledit câble de liaison est courbé,
• la figure 9 représente un réservoir navette coopérant avec la paroi supérieure d'une structure du type sarcophage pour en récupérer le pétrole s'écoulant d'un navire échoué et confiné sous le sarcophage ;
• la figure 10a représente en coupe en vue de côté une structure consistant en un module de traitement de pétrole suspendu en sub surface par l'intermédiaire de câbles à deux barges flottantes, l'ensemble étant en cours de remorquage vers le site d'installation ;
• la figure 10b représente en coupe en vue de côté ledit module de traitement de pétrole descendu à une profondeur de 20 à 40 m, un navire gazier étant en train de transférer le fluide de flottabilité vers une enveloppe souple de type baudruche ;
• la figure 11 représente la descente d'une structure consistant en un dispositif d'ancrage et perçage contrôlé par une chaîne de stabilisation et des éléments de flottabilité selon l'invention.

Other characteristics and advantages of the present invention will emerge more clearly on reading the following description, given in an illustrative and nonlimiting manner, with reference to the appended drawings in which:

• the figure 1 is a sectional view of a side of said structure consisting of a receptacle hereinafter called "sarcophagus" being descent to a wreck;
• the figure 2 is a sectional side view of a rigid receptacle resting at the bottom of the sea and completely enveloping the wreck;
• the figure 3 is a perspective in view of cut-away, of the structure of the sarcophagus;
• the figure 4 is a sectional view of the side of the sarcophagus during descent, detailing the mode of regulation of the descent using heavy chains;
• the Figures 4a and 4b detail the mode of variable implementation of said heavy chains;
• the figure 5 is a sectional view of a sarcophagus composed of a rigid support structure made of metal beams, associated with buoyancy filled with a low density fluid integrated therebetween and closed by waterproof membrane cloths on the outer face of the structure;
• the figure 6 is a sectional view of a sarcophagus made of lightweight concrete, and having internal volumes forming sealed compartments filled with a low density fluid providing buoyancy;
• the Figures 7a and 7b represent a cross sectional view of a sarcophagus respectively being towed, its buoyancy compartments being filled with seawater 9a, and 9b, vertically of the wreckage, during the filling phase of said compartments buoyancy by a liquefied gas of low density;
• the figure 8a is a side view of a shuttle tank stabilized in its ascent by a connecting cable weighed by integral blocks of the latter and also acting as curvature limiter,
• the Figures 8b and 8c represent states similar to that of the figure 11a , the shuttle tank being in recovery phase on the figure 11b and downhill on the figure 8c ,
• the figure 8d represents the detail of two blocks 31 in contact, when said connecting cable is bent,
• the figure 9 represents a shuttle tank cooperating with the upper wall of a sarcophagus type structure to recover the oil flowing from a ship stranded and confined under the sarcophagus;
• the figure 10a is a sectional view in side view of a structure consisting of a subsurface suspended oil treatment module via two floating barge cables, the assembly being towed to the installation site;
• the figure 10b is a sectional view in side view of said petroleum processing module lowered to a depth of 20 to 40 m, a gas vessel being transferring the buoyancy fluid to a soft envelope type baudruche;
• the figure 11 represents the descent of a structure consisting of an anchoring device and drilling controlled by a stabilization chain and buoyancy elements according to the invention.

In the figure 1 , there is shown the hull of a wreck or a tank wall 6 resting on the bottom of the sea 7 filled with hydrocarbon 8 whose density is lower than seawater. Said hydrocarbon is confined in the upper part of the tank or wreck 6, the lower part being, in turn, filled with seawater. The vessel 6 having in general multiple openings closed tightly at the deck, leakage may occur as soon as this seal would be degraded by the deformation or rupture of the hull during the sinking.

A rigid receptacle 1 according to the invention, hereinafter referred to as "sarcophagus" consisting of a rigid structure, descends from the surface under the control of cables 12 connected to dynamic positioning vessels situated on the surface, as shown in FIGS. figures 1 and 2 .

• une paroi de plafond 3, 3a, 3b) comprenant deux parois longitudinales latérales 3a, 3b inclinées par rapport audit plan axial vertical de symétrie dudit réceptacle, de manière à former en section transversale (YOZ) un V reversé, et
• une paroi latérale 2 comprenant :
  • deux parois latérales longitudinales 2a, 2b verticales ou inclinées par rapport audit plan axial vertical de symétrie (XOZ), chacune étant contiguë à une dite paroi longitudinale de plafond 3a, 3b, et
  • deux parois transversales d'extrémité 2₁, verticales ou inclinées, de préférence symétriquement, par rapport à un plan transversal vertical de symétrie (YOZ).

The receptacle 1, described on Figures 1 to 3 , has a vertical and longitudinal axial plane of symmetry (XOZ) and comprises:

• a ceiling wall 3, 3a, 3b) comprising two lateral longitudinal walls 3a, 3b inclined with respect to said vertical axial plane of symmetry of said receptacle, so as to form a V in a transverse section (YOZ), and
• a side wall 2 comprising:
  • two longitudinal lateral walls 2a, 2b that are vertical or inclined relative to said vertical axial plane of symmetry (XOZ), each being contiguous with a said longitudinal ceiling wall 3a, 3b, and
  • two transverse end walls 2 ₁ , vertical or inclined, preferably symmetrically, with respect to a vertical transverse plane of symmetry (YOZ).

As detailed on the figure 3 , the sarcophagus 1 consists of a shell in reverse configuration, said shell being sealed and double-walled thus forming walls 4 ₁ of sealed compartments 4, preferably a multitude of sealed compartments in continuity with each other. The structure consists of transverse ribs 4 ₃ , perforated or solid within the same sealed compartment, and associated with longitudinal perforated or solid ribs 4 ₆ . On the figure 3 , we show in a section transverse exploded corresponding to the plane YOZ, a right double walled wall 3b of the ceiling, flat, inclined relative to the horizontal, for example 10 to 20 °, but can be horizontal, and when inclined forming a ceiling in reversed V configuration with the other half of double 3b ceiling walls. Each longitudinal ceiling wall 3a, 3b is connected by its lower edge to a double side wall 2a, 2b, flat, vertical or inclined with respect to the vertical, in particular from 5 to 20 °, preferably with a smaller inclination than said walls longitudinal inclined ceiling. The two ends of the sarcophagus 1 along the longitudinal axis XX 'are closed by double end walls 2, 2a, 2 ₁ ensuring the junction between the end edges of the double side walls 2a, 2b and double walls of the ceiling 3, 3a, 3b and said end side walls 2 ₁ being perpendicular to the longitudinal axis XX '. The lower part is completely free, so that the sarcophagus can cover, like a bell, the wreck 6 to contain.

The volumes inside the various double walls 2 ₁ , 2, 2a, 2b and 3, 3a, 3b and delimited by the inner and outer walls and the frames 4 ₃ , 4 ₆ full form the walls 4 ₁ compartments 4 impervious to the outside, which makes it possible to fill them with a fluid of less density than seawater, said fluid then acting as a float and compensating for the weight of the rigid structure of the sarcophagus receptacle 1.

Said shell constituting the sarcophagus is preferably constructed dry in a dock, then, the sealed compartments 4 included inside the double walls 2 ₁ , 2, 2a, 2b and 3, 3a, 3b are closed sealingly. After filling the dock, the sarcophagus 1 floats and greatly exceeds the water level, because said compartments 4 are filled with air. In case of risk of instability at this stage, we advantageously add a temporary ballast at the bottom.

The sarcophagus 1 is then towed to deep water where the entirety of the compartments 4 constituting the buoyancy volumes, is filled with the buoyancy fluid, for example diesel whose density is close to 0.85, but preferably a fluid consisting of ammonia, butane, or propane or a other liquefied gas under pressure as described below. The buoyancy volume is advantageously adjusted so that the sarcophagus is in equilibrium between two waters, the overall equilibrium being possibly ensured by additional floats 19 capable of withstanding the bottom pressure, that is to say about 350 bar for 3500 m deep. Said additional floats 19 may consist of syntactic foam, ie microspheres of glass trapped in a binder of the epoxy or polyurethane resin type, but advantageously consist of a liquefied gas under pressure as described below, in particular ammonia, butane, or propane.

The sarcophagus 1 is then towed to the site, and then, once there, at least two, preferably four ships 20 connect to the ends of the sarcophagus 1, as follows.

Each of the vessels 20 comprises a winch 12 ₁ provided with a cable 12, preferably made of steel, the length of which is greater than the depth of water, for example 130% of the said water depth. The end of said cable 12 is connected to a length of heavy chain 13, for example 100 m of 6 "diameter chain, the end of said chain being connected to a reinforced beam 10 constituting a fastening element integral with the structure and overflowing sarcophagus 1, as explained in the Figures 1-4-6 .

The heavy chains 13 have a self-regulating effect during the descent of the sarcophagus towards the seabed 7 and their operation is explained on the Figures 4, 4a and 4b .

On the figure 4 , the cable 12 is in the intermediate position and forms a double-chain curve, a part of the chain weight 13 (F) being supported by the sarcophagus, the other portion of the chain being supported via the cable 12 directly by the ship 20 of surface. Thus, the sarcophagus is kept in equilibrium between two waters under the effect of this force F.

When the winch 12 ₁ of the surface vessel 20 winds up the cable 12, it raises the chain 13 as indicated on the figure 4a , which has the effect of reducing the weight of the chain carried by the receptacle F _min , because then, the entire weight of the chain is supported by the surface vessel 20: the sarcophagus 1 then has an apparent weight in the lower water and it rises to approach a position of equilibrium according to the figure 4 and stabilize there.

Conversely, when the winch 12 ₁ of the surface vessel 20 deflects the cable 12, it lowers the chain 13 as indicated on the figure 4b , which has the effect of increasing the weight brought by the chain up to F _max. The sarcophagus 1 thus has an apparent weight in the larger water and sinks to approach its equilibrium position according to the figure 4 and stabilize there.

Thus in all cases, the configuration of chains 13 in double chain has a self regulating effect on the position of the sarcophagus during the descent. However, it is necessary to synchronize very precisely the deviation of the cables 12 of all winches 12 ₁ involved in the maneuver, so that the sarcophagus 1 makes its descent remaining substantially horizontal. In addition, the vessels 20 must remain at a substantially constant distance from the axis of the receptacle and preferably two vessels 20a and 20b connected to opposite attachment elements 10 ( figure 1 ) must be located substantially in a vertical plane passing through the attachment points of the chains 13 on the beams 10 of the sarcophagus 1, which implies the advantageous use of dynamically positioned vessels using a GPS-type radiolocation system.

The descent of the sarcophagus 1 is carried out, preferably continuously until a distance close to the wreck 6, for example up to 50 m from the bottom. Then, the sarcophagus is positioned at the axis of the wreck 6 and oriented in the right direction by simple overall movement of the surface ships. Said movements of the ships 20 have an effect delayed from a few minutes to a few tens of minutes, on the corresponding movements of the sarcophagus located a few thousand meters lower. To facilitate the operation, it is advantageous to install steerable thrusters 16, preferably at the ends of the structure, more particularly at the four corners of the ceiling, said thrusters 16 being fed by a umbilical 16 ₁ of power and control connected to a ship 20 area.

In an illustrated variant on figures 1 and 2 winches 14 ₁ are installed on the lateral peripheral walls of the sarcophagus, and when said sarcophagus 1 is close to the wreck, an automatic underwater ROV 22, piloted from the surface, connects cables 14 of said winches 14 ₁ to an anchor 15 ₁ , 15 ₂ pre-installed near the wreck, for example a suction anchor 15 ₁ , or a dead body 15 ₂ .

After final placement of the sarcophagus, the heavy chains are rested on the bottom of the sea 7 as illustrated on the figure 2 then the additional floats 19 are unhooked by means of the ROV 22, they then go up freely on the surface where they are recovered. It is eventually possible to equip each of them with an acoustic beacon, which makes it possible to follow their ascent using the sonars of the ships 20 and to move the ships accordingly to avoid any collision when they surface. The sarcophagus 1 is then stable at the bottom, but its stability is further improved by recovering the cargo of buoyancy, for example diesel, as explained on the figure 2 . To this end, is connected from the surface using the ROV 22, a conduit 23, preferably flexible, preferably in the S configuration, in an orifice provided with an isolation valve 4 _4, located in the upper part of the compartment 4, having taken care to previously open a valve 4 ₅ located in the lower part of the same compartment 4 and allowing the seawater to penetrate, as the buoyancy fluid rises to the surface .

After draining the buoyancy compartments 4, the upper valves 4 ₄ , at least, are closed and the sarcophagus then has its maximum weight which ensures great stability, even in case of significant leakage from the wreckage. The effluents, escaping from the wreck at the level of said leaks, come together in the upper part of the internal volume of the sarcophagus, thus creating a significant buoyancy, but much lower than that of the fluid of the compartments 4. In fact, in the case of very viscous crude oils, the density is generally greater than 0.95 and often approaches 1.02, which creates a low buoyancy and does not risk destabilizing the sarcophagus.

After draining the buoyancy compartments 4, the chains can be recovered, but if it is desired to improve the stability of the sarcophagus, the chains 13 are advantageously raised and suspended by their second end to the bracket already supporting the first end, or they are raised and simply deposited on the roof of the sarcophagus, so that their entire weight contributes to the stabilization of said sarcophagus.

To reduce the distance between the double walls delimiting the compartments 4 and using light metals, for example aluminum for the structure, the fresh water will advantageously be replaced by a buoyancy fluid according to the invention, particularly preferably ammonia, butane
or propane as explained below.

Indeed, seawater with a density of about 1.026 at the surface and 1.045 at 4000 m bottom and at 3 ° C, the fresh water having, for its part, a density of 1 in surface and 1.016 at 4000 m depth and at 3 ° C, the buoyancy provided by fresh water per m ³ , varies from 26 kg in surface to 29 kg at 4000 m depth. The overall volume of the compartments 4 of the following example makes it possible to balance the self-weight of the structure of the sarcophagus described below. An aluminum-walled sarcophagus 180 m long, 40 m wide and 35 m high, with a distance of 3 m between the inner and outer walls of the double walls, represents an aluminum mass of 3000 tonnes, c is a weight weighed in seawater of 1850 tons. The overall volume of the compartments is 73125 m3, which gives a buoyancy of 1480 tons when filled to 75% of fresh water. A supplementary buoyancy of 470 tonnes is installed in the form of floats distributed along the structure and the stabilizer chains for the descent consist of four identical lengths of weighing chain each of 50 tonnes, each of which is installed at an angle of sarcophagus.

In the case of a sarcophagus of the same dimensions made of steel, it is advantageous to use a buoyancy fluid having a lower density than fresh water, for example diesel, but preferably a compressed liquid gas according to the invention, such as described below and the overall volume of buoyancy compartments requires a distance between internal and external walls of 2m. The sarcophagus represents a mass of 7500 tons, ie a weight plotted in seawater of 6500 tons. The overall volume of the compartments is 47550 m3, which gives a buoyancy of 6280 tonnes when filled to 22% butane density 601 kg / m3. Complementary floats represent 320 tonnes, and the stabilizer chains (50T x4) remain the same as in the case of the aluminum sarcophagus.

At the end of installation, an upper discharge orifice 9 on the ceiling of the sarcophagus is advantageously open so that the buoyancy fluid according to the invention can escape and the stability of the sarcophagus is optimal. After evacuation of the fresh water, said upper orifice 9 is closed so as to collect any leaks from the wreckage.

This same upper orifice 9 is advantageously used to recover the effluents 8 which escape from the wreck 6 in time, and come together in the upper part of the interior volume of the sarcophagus under its ceiling 3, 3a, 3b. By coming to connect to this upper orifice 9 and after having opened the isolation valve, the accumulated oil 8 is advantageously transferred since the previous intervention campaign, or by means of a pipe 23 connecting the upper orifice 9 up to to a recovery vessel located on the surface, either by using a recovery device between the sarcophagus and the surface vessel, for example a device as described in the patent application FR 2 804 935
or a shuttle-type device as described in the unpublished patent application European Commission 03 358 003.6 .

In one version of the invention illustrated on the figure 5 a carrier structure of the shed type consisting of metal or steel beams 24 joined together by welding or bolting is produced, and sealed compartments, distributed in a continuous manner or not, or on the side walls 2, are incorporated therein, 2a, 2b, either on the roof 3, 3a, 3b or in combination of the two. The entire structure is sealed against a fluid tending to escape naturally upward, by webs or membranes 25 fixed to the outside of the structure and against the latter in a sealed manner, way to collect all leaks from the wreckage and direct them to the high point where they will be stored until they are recovered, either by means of a bottom-surface link 23, or by means of a recovery device or the shuttle as explained previously.

In one version of the invention illustrated on the figure 6 , the structure of the sarcophagus is made of lightened concrete 26, armed and prestressed, and comprises compartments 4 which are filled in the same manner as previously, a fluid of density lower than that of sea water according to the 'invention. The concrete 26 is advantageously made from lightweight aggregates, such as, for example, expanded clays, combined with high-strength mortars, which gives them an excellent behavior at great depth, even at depths of 3000 to 4000 m, see more. Indeed, the expanded clays are substantially spherical and have voids filled with air or gas, which ensures a low density; taken within a matrix made of high-strength mortar, it is the actual matrix that ensures the overall strength. When the structure is subjected to a very high pressure, for example the pressure of 400 bar prevailing at about 4000 m depth, the water will migrate over time within the mass of concrete and then invade little by little the clay aggregates expanded, which will have the effect of considerably increasing the apparent weight of the sarcophagus. This migration process being relatively slow does not present any inconvenience during installation, because, after towing on site, the critical operation of descent of said sarcophagus, from the surface, to its final position resting on the bottom at above the wreck, represents a maximum duration of 12 to 24 hours. Once in place, the weight of the sarcophagus increases daily, which increases the stability, the phenomenon of water migration continues for several weeks or months. In order to retard the phenomena of migration of water towards the porous granules, the entire walls of the concrete structure in contact with the water are advantageously covered with an elastomer-type paint layer, thus creating a barrier to watertightness. effective. This layer is advantageously also applied inside the buoyancy compartments integrated in the concrete structure, to minimize the migration of the buoyancy fluid to said aggregates.

In a preferred version of the invention, use is advantageously made of a buoyancy fluid according to the invention, of very low density, which reduces the overall volume of the buoyancy compartments to be expected. For this purpose, it is advantageous to use a gas whose critical point is above ambient temperature, for example butane, propane, ammonia, or any other similar gaseous compound at ambient temperature and atmospheric pressure. Indeed, these gases have a density in the liquid state which is between 0.50 and 0.70. They are gaseous at atmospheric pressure and at a temperature of 20 ° C, but are liquefied as soon as they are compressed to a few bars. It is thus very advantageous to use them as buoyancy fluid because their efficiency w (Archimedes pressure / self weight) is much higher than the fluids commonly used, such as diesel, methanol or fresh water.

In fact, for a gas oil with a density of 0.85: ω = 1.21, for methanol: ω = 1.30, whereas for butane, propane and ammonia, the values of ω are respectively ω = 1.71, ω = 1 , 97 and ω = 1.63.

However, the filling of the compartments must be done in a special way to avoid any risk of incident and accident. Indeed, being gaseous at ambient temperature and at atmospheric pressure, they can be stored either at atmospheric pressure at cryogenic temperature or under pressure at ambient temperature.

When they are stored at atmospheric pressure, for the fluid to remain in the liquid form, the temperature of said fluid must be kept well below room temperature, for example -0 ° C to -50 ° C depending on the gases.

When they are stored at room temperature, usually around 20 to 30 ° C or more, to remain in the liquid state, they must be confined in tanks capable of withstanding high pressures, from a few bars to a few dozens of bars depending on the gas.

Storage at low temperatures is very difficult to achieve, or almost impossible, in the case of a large volume of buoyancy, because it is imperative to prevent the gas from heating up. Indeed, the fluid is warming boils and the pressure inside the tank increases. If the tank is tight, then it must be able to withstand the maximum pressure of the gas; if the tank is not tight and communicates with the outside, the bubbling gas then escapes, thereby reducing the amount of liquid gas present, therefore the buoyancy.

Storage at room temperature requires a means of confining said gas under pressure so that it remains in the liquid state. Bottles and tanks of butane gas or propane gas are able to withstand very high pressures, but their weight remains high and it would not be interesting to use them as they are, because the buoyancy w would be strongly degraded by the weight said containment means constituted by the self weight of said tank capable of withstanding the pressure. We could consider the use of composite tanks, whose density of the material is close to that of water, but they are expensive and complex to manufacture when their unit volume becomes important.

Thus, to contain the buoyancy fluid in the liquid state, it is advantageous to use a rigid or flexible envelope capable of confining said gas, the filling of said envelope being made underwater at a depth of water such that the hydrostatic pressure at said water depth corresponds to a stable liquid state of the buoyancy material whose temperature is lower than
or equal to the ambient temperature. In general, the temperature of the seawater varies from 3 ° C to 25 ° C or more, depending on the geographical area, the time of year and the depth considered, and can go down to -5 or -7 ° C in particular arctic areas.

• après construction à terre ou dans une darse, on met à l'eau, la structure lourde ou le sarcophage 1, puis,
• on supporte ladite structure lourde ou ledit sarcophage 1 proche de la surface au moyen de câbles reliés à des treuils installés sur des barges 27, de préférence deux ou quatre barges, flottant en surface, comme illustré sur la figure 7a, 10a et 10b, le sarcophage étant relié à chacune desdites barges 27 par un câble 28 relié à un treuil 28₁, en association avec un compensateur de pilonnement 29 visant à éviter les ruptures de câble 28. Les compartiments 4 ou enveloppes rigides 19₁ (à gauche figures 10a-10b) sont remplis d'eau et l'enveloppe souple 19₁, du type baudruche étant vide d'air et d'eau est ramassée sur elle-même comme illustré sur la figure 10a (à droite), et
• on transporte en pleine mer sur le site d'installation, puis, comme explicité sur les figures 7b et 11b, on descend la structure ou le sarcophage 1 jusqu'à une profondeur de 20m à 60 m, correspondant sensiblement à une pression de 2 à 6 bars, pression à laquelle le gaz butane, que l'on va injecter dans les compartiments 4, et réservoirs 19₁ est liquide. On descend alors, puis l'on connecte, une conduite 23 au point haut 4₄ des compartiments de flottabilité et vannes 19₂, 19₄ des éléments de flottabilité 19, et l'on injecte sous pression le gaz liquide stocké à bord d'un navire gazier spécialisé 61, connus de l'homme de l'art. L'orifice inférieur 4₅ du compartiment 4 étant ouvert, le gaz liquéfié chasse l'eau de mer qui s'y trouve, et remplit peu à peu l'intégralité du compartiment 4. En fin de remplissage, la vanne supérieure 4₄ est fermée de manière étanche. On remplit la baudruche souple 19₁ par l'orifice unique 19₄, en contrôlant son remplissage de manière à éviter qu'elle n'éclate. Une fois remplie, on ferme la vanne 19₄ et l'on déconnecte la conduite de remplissage 23. Lorsque tous les compartiments et enveloppes sont pleins, les barges 27 utilisées lors du remorquage peuvent être libérées après déconnexion des câbles de retenue 28, et
• ladite structure lourde ou ledit sarcophage est alors prêt à être descendu comme explicité précédemment, après avoir connecté les chaînes lourdes 12, 13 qui jouent alors le rôle de stabilisateur pendant toute la descente jusqu'au fond de la mer.

For this purpose, for a heavy structure such as a sarcophagus or other, one proceeds as follows:

• after construction on land or in a dock, the heavy structure or sarcophagus 1 is launched, then
• said heavy structure or said sarcophagus 1 is supported close to the surface by means of cables connected to winches installed on barges 27, preferably two or four barges, floating on the surface, as illustrated in FIG. figure 7a , 10a and 10b , the sarcophagus being connected to each of said barges 27 by a cable 28 connected to a winch 28 ₁ , in association with a heave compensator 29 to prevent cable breaks 28. The compartments 4 or rigid envelopes 19 ₁ (left Figures 10a-10b ) are filled with water and the flexible envelope 19 ₁ of the balloon type being empty of air and water is collected on itself as illustrated on the figure 10a (right), and
• it is transported at sea on the installation site, then, as explained on the figures 7b and 11b the structure or the sarcophagus 1 is lowered to a depth of 20 to 60 m, corresponding substantially to a pressure of 2 to 6 bar, at which pressure the butane gas, which will be injected into the compartments 4, and tanks 19 ₁ is liquid. Then down, then connected, a pipe 23 at the high point 4 ₄ of the buoyancy compartments and valves 19 ₂ , 19 ₄ of the buoyancy elements 19, and is injected under pressure the liquid gas stored on board. a specialized gas ship 61, known to those skilled in the art. The lower orifice 4 ₅ of the compartment 4 being open, the liquefied gas flushes seawater therein, and gradually fills the entire compartment 4. At the end of filling, the upper valve 4 ₄ is tightly closed. The flexible pouch 19 ₁ is filled by the single orifice 19 ₄ , controlling its filling so as to prevent it from bursting. Once filled, the valve 19 ₄ is closed and the filling line 23 is disconnected. When all the compartments and envelopes are full, the barges 27 used during towing can be released after disconnection of the holding cables 28, and
• said heavy structure or said sarcophagus is then ready to be lowered as explained above, after having connected the heavy chains 12, 13 which then act as a stabilizer throughout the descent to the seabed.

On the figure 7b , 4 right compartment is full of buoyancy fluid in the liquid state, while the left-hand compartment is being filled, sea water escaping through the bottom valve 4 _5, which is in the open position . On the figure 10b , the compartments 4 constituted by the tubular sections of the supporting structure, as well as the rigid buoyancy element 4-19, 19 ₁ on the left, are filled with buoyancy fluid in the liquid state, the element of buoyancy of right flexible envelope of the balloon type being filling said fluid.

At the end of the installation, it is sufficient to slightly open the upper orifice 4 ₄ located at the top of each of the compartments 4 of buoyancy, which allows the gas to escape in liquid form. It then naturally rises to the surface, first in liquid form, to finally gasify on the surface and dilute in the atmosphere. These gases are safe for the environment and personnel, since the instantaneous quantities are reasonable, ie they represent a few tens, or even a few hundred kilograms per hour, but we prefer for ecological reasons to recover the cargo. of liquefied gas. For this purpose, a bottom-surface connection 23 is installed, as already explained on the figure 2 a connection which connects the upper port 4 ₄ , 19 ₂ compartments and buoyancy elements to the gas ship 61 located on the surface. This connection makes it possible to recover almost all the cargo of gas in a very short time, since the gas in liquid form has an extremely low viscosity. And, because of the very great depth of water, the differential pressure between the inside of said pipe and the outside is considerable, since the pressure differential increases by about 4 MPa per 1000-meter increment for a fluid. buoyancy type butane, whose density is about 0.6 compared to seawater.

In the case of a heavy structure, for example wellhead elements, or oil treatment or pumping units that must be lowered on the seabed, the support structure of the equipment to be advantageously achieved is advantageously carried out. using tubular sections, rather than U-shaped, U-shaped or H-shaped, as is commonly practiced. Said tubular sections are sealed, then are filled with liquefied gas in the same manner as previously explained with respect to the figure 7b , through orifices provided with valves provided for this purpose.

The tanks or balloons 19 ₆ of the petroleum treatment unit are also advantageously used as rigid shells capable of receiving liquefied gas and that is purged after installation and before the start of the oil treatment unit installed on the bottom of the sea.

The additional buoyancy elements 19 are advantageously made from a flexible envelope constituting a balloon functioning as an airship, as shown in FIG. figure 10b . The envelope is flexible and waterproof, preferably in the form of an inverted drop of water, or spherical when it is full. It is connected to said heavy structure by a bundle of cables 59, preferably surrounding said flexible and watertight envelope, said cable bundle 59 being integral with the heavy structure and being capable of transferring the Archimedes thrust which is exerted on said envelope filled with said liquefied gas, said heavy structure 1. The filling of said balloon is performed in the same manner as explained on the figure 7b and draining at the end of installation and carried out by simply opening the valve 19.4 connected to a pipe 23.

The flexible envelope of the balloon is advantageously made using resistant fabrics coated with neoprene-type rubber, or polyurethane-type compounds, such as those used for inflatable boats of the ZODIAC ^® brand, or for the manufacturing of flexible tanks sold by PRONAL ^® France.

The preferred gases that can be used as buoyancy fluid are listed in Table 1 below in order of increasing density in the liquid state at a temperature of 15 ° C.

The vapor pressures given in Tables 1 and 2 are absolute pressures, and thus relative to vacuum.

The corresponding depth is indicative and corresponds substantially to an atmospheric pressure of 0.1 MPa and a seawater of 1.026 density relative to fresh water. <b> TABLE 1 </ b> density in the liquid state vapor pressure at 15 ° C water depth (at sea) fluid kg / m3 at 15 ° C MPa absolute pressure (x 10 ⁶ Pa) m ethylene 322 4.9 468 ethane 401 3.38 320 acetylene 465 4.09 389 propane 519 0.77 65 propene 547 0.9 78 butane 601 0176 7.5 propadiene 609 0.62 51 butene 619 0.22 11.7 trans-butene 627 0.46 35 ammonia 629 0.77 65 methyl acetylene 644 0.44 33 butadiene 645 0203 10 cis-butene 645 0132 3.1

The gases are classified in Table 2 below in order of vapor pressure at the temperature of 15 ° C. <b> TABLE 2 </ b> density in the liquid state vapor pressure at 15 ° C water depth (at sea) fluid kg / m ³ at 15 ° C MPa absolute pressure ( x10 ⁶ Pa) m cis-butene 645 0132 3.1 butane 601 0176 7.5 butadiene 645 0203 10 butene 619 0.22 11.7 methyl acetylene 644 0.44 33 trans-butene 627 0.46 35 propadiene 609 0.62 51 propane 519 0.77 65 ammonia 629 0.77 65 propene 547 0.9 78 ethane 401 3.38 320 acetylene 465 4.09 389 ethylene 322 4.9 468

In the case where the vessel 61 for storing the fluid is of the cryogenic type, that is to say that the fluid is stored substantially at atmospheric pressure, at a temperature well below 0 ° C., for example -42 ° C. in the case of propane, to carry out the transfer of said fluid to the balloon or the reservoir, the procedure is slightly different from that explained above. The fluid is extracted from the cryogenic tanks by a pump, then passing through a seawater heat exchanger, goes heat to a temperature close to said seawater, for example 15 ° C at the heater outlet. It will then descend to the bladder or to the reservoir through the pipe 23 and, since from the pump to the bladder, the pressure in the pipe is greater than the vapor pressure at 15 ° C (0.77MPa in the case of propane), the fluid remains in the liquid state.

Recovery of the gas at the end of the installation of the heavy structure then requires the implementation of a liquefaction unit, because the fluid from the ultra-deep bottom is at a temperature of about 4 ° C and must be cooled, in the case of propane, at a temperature below -42 ° C to remain in the liquid state in the tanks of said cryogenic vessel, the latter being substantially at atmospheric pressure.

At low temperatures, butane and propane tend to combine with water to form hydrates that can block the pipes or prevent recovery of liquefied gases at the end of the installation phase. When the envelope is previously filled with water, to avoid the formation of these hydrates, at the beginning of filling of a said rigid envelope
or flexible, one injects a volume of methanol, for example 100 or 200 liters, so that the methanol, of intermediate density between the sea water and the liquefied gas, creates a screen avoiding the direct contact between the butane- propane and water. In addition, methanol, mixed in small proportion with water prevents chemical reactions leading to the formation of hydrates.

In each of the variants of the invention described above, the watertight compartments are positioned and dimensioned so as to respect the rules of the art of shipbuilding, and in particular the so-called pa rule, which consists in maintaining the center of vertical thrust due to buoyancy, above the center of gravity of the structure. It is customary to consider that for a value pa> 1 m, the structure is considered stable and therefore does not risk to overturn by pivoting about its axis XX '. For this purpose, it is advantageous to add external floats 19 preferably located above the structure of the sarcophagus and, possibly, weights at the bottom.

On the Figures 8a to 8d and 9 is represented a shuttle tank 32 of the type used to recover effluent from a wreck at the bottom of the sea by descent and ascent of said shuttle tank respectively empty and full between the surface and the bottom of the sea. The shuttle tank 32 is constituted a flexible and waterproof side wall 34, for example made of strong reinforced plasticized fabrics, integral in the upper part of a dome 3 with a circular horizontal section and with a vertical section profile in the form of a shell made of a resistant material and rigid, preferably of composite material, and integral in the lower part of a flat, solid, resistant and rigid bottom, preferably circular, also preferably of composite material, so as to represent an apparent weight in the minimal water, while guaranteeing rigidity and extreme resistance. Said bottom 5 is pierced at its center with a main opening 35 ₁ and is equipped with a valve, preferably full-bore, for example of guillotine type, the latter being equipped with a flange. A lateral complementary orifice of smaller diameter is provided with a valve ₂ , thus permitting the exchange of seawater between the inside of the shuttle tank and the marine environment, and in particular when the tank is filled with oil, to sea water to escape.

The dome 33 and the bottom 35 may have a diameter of 5 to 10 m, the dome 3 a height of 2 to 5 m and the side wall 4, once unfolded, a height of 10 to 50 m.

Is advantageously adjusts the apparent weight in water of the shuttle tank 32 by integrating into the uppermost portion of the dome 3, the buoyancy, e.g., syntactic foam 3 ₁ consisting of glass microspheres embedded in epoxy resins, polyurethane or others.

Thus, the shuttle tank 32 is lowered towards the wreck or tank 6, or towards a sarcophagus 1 placed above a said wreck or tank, in the picked up position, and has an apparent weight in very low water and which can be adjusted in positive or negative, which facilitates its installation directly by an ROV (automatic submarine piloted from the surface and equipped with manipulator arms).

The figure 8 illustrates the rise of the shuttle reservoir 32 is controlled by a connecting cable 12, a portion of its lower portion 13 is increased, for example, by metal blocks 31 secured to said cable 30 by a crimping 31 ₁ in a string like beads on a cable.

As represented figure 8d these beads 31 have a cylindrical central body which is prismatic or of revolution and, frustoconical ends such as when the cable is bent, said frustoconical ends of the two adjacent beads then abut against each other at 31 ₂ , thus limiting the local radius of curvature to a value greater than R ₀ . Thus, the connecting cable 12 being hooked to the shuttle reservoir 2 on the said first attachment point 36 in the lower part of the tank, descends downwards and then deviates in an arc of radius R ₀ , to finally rise vertically or in a chain configuration at a distance of at least 2R ₀ from the side wall 4 of said shuttle tank, thus avoiding any mechanical contact during the ascent, which makes it possible to avoid damage by friction.

On the figure 8a , the buoyancy of the hydrocarbon-filled shuttle tank F _v which corresponds to the buoyancy force acting on the tank and its cargo, is compensated by the weight of the cable to the point of horizontal tangency corresponding to the bead 31 _i , added the weight of the beads 31 g between the reservoir and the pearl 31i the lowest, that is to say 8.5 beads on the figure 11a , the weight of the set P _e then corresponding to a system equilibrium.

As an example to illustrate the figure 8a , the 250-cubic meter shuttle tank of a 1011 kg / m ³ density oil in seawater at 3 ° C density 1045 kg / m ³ has a buoyancy of approximately 8.5 tonnes .

Each of the balancing device beads 30-31 then has a weight in water of about 1 ton.

On the figure 8b , the upper end of the connecting cable 12, connected to a winch installed aboard a surface vessel (not shown) is raised, which has the effect of bringing the bead 31 _g in a low horizontal position, reducing thereby the number of beads weighing under the 6.5-pearl reservoir, the overall weight opposing the thrust Fv being reduced to P-. The resultant F _v + P- is then positive upwards and the shuttle reservoir can go up until the balance of forces of the figure 8a be reached.

Similarly, in figure 8c , the upper end of the connecting cable 12 is deviated, which has the effect of bringing the bead 31 _k in a low horizontal position, thereby increasing the number of beads weighing under the tank to 10.5 beads, the weight of set being then equal to P +. The resultant F _v + P + is then positive downwards and the shuttle reservoir can go down again until the balance of forces of the figure 8a be reached.

Thus, the stabilization device according to the invention has a stabilizing effect for the recovery of the shuttle tank. When the surface vessel moves excessively under the effect of the swell or deviates from the vertical position of the shuttle tank, the movements have instant effect only on the area of the beads surrounding the beads 31 _g at 31 _k , the bead 31 _i corresponding to the average value of the oscillations.

Thus, to control the ascent of the shuttle reservoir 32, it is sufficient to wind the connecting cable on the winch located aboard the surface vessel 20 at a speed compatible with the natural ascent of said shuttle, said shuttle still naturally seeking to resume its equilibrium position illustrated on the figure 8a . In case of difficulties, it is sufficient to slow down or stop the winding on the winch, the shuttle tank then finding almost immediately its equilibrium position, pending a new movement of the winch.

The figure 9 represents a shuttle tank 32 installed vertically of a discharge device 9 equipped with a valve provided on the upper wall of a sarcophagus 1 to which it is connected by a link 50. When the valve is in the open position, it passes crude oil accumulated in said sarcophagus after having passed tanks of the vessel 6. Thus, it can be collected in the shuttle tank, which can return to the surface once filled and rupture of the link 50, the rise to the surface is doing under the control of a device for stabilization and control of the ascent and descent according to the invention. The sarcophagus 1 is equipped with a stabilization and control device with connecting elements 12 consisting of cables whose lower portion comprises metal blocks 31 in strand.

The device for controlling the descent or ascent of a heavy or massive structure has been described as consisting of either a cable provided with blocks
or beads crimped onto said cable, or chain link modified to create by simple stop between links, the minimum radius of curvature R ₀ . But, it remains in the spirit of the invention if said weighted portion of said connecting elements is constituted by a string of weighted bars hinged together, so that the deformation of the string of articulated bars creates load unbalance, P + or P- with respect to the equilibrium load Pe, as described previously with respect to Figures 8a, 8b and 8c , said bars advantageously having, at the level of the joints, mechanical stops which make it possible to limit the curvature to a minimum value R ₀ .

• un corps de forage 54₁ comprenant des moyens permettant d'actionner en translation et en rotation une scie cloche 55 qui, à travers une ouverture correspondante prévue dans ladite embase, permet de percer un grand orifice dans ladite paroi 6 de manière à permettre l'évacuation d'un fluide contenu dans ladite cuve, et
• des chariots latéraux 56 comprenant des moyens permettant d'actionner en translation et en rotation des scies cloches 57 aptes à percer des trous dans ladite paroi 6 pour ancrer l'embase 52 sur ladite paroi, les scies cloches 57 se déplaçant à travers des orifices 58 de ladite embase.

On the figure 11 , there is shown a heavy structure consisting of a device 1 for laying and anchoring a base 52 on the wall 54 of a tank and / or a wreck at the bottom of the sea. This device 1 comprises a structure support 54 consisting of a parallelepiped welded mechanical frame supporting itself:

• a drilling body 54 ₁ comprising means for actuating in translation and in rotation a hole saw 55 which, through a corresponding opening provided in said base, makes it possible to pierce a large orifice in said wall 6 so as to allow the evacuation of a fluid contained in said tank, and
• lateral carriages 56 comprising means for operating in translation and in rotation of the bellsaws 57 adapted to pierce holes in said wall 6 to anchor the base 52 on said wall, the bellsaws 57 moving through the orifices 58 of said base.

The figure 11 represents the descent of a structure 1 consisting of an anchoring device and drilling controlled by a stabilization chain 12,13 according to the invention and a buoyancy element 19 according to the invention. The part left bottom of the base 52 is shown in section to view the cutting means 57 within an orifice 58 provided in said base.

The device 1 is suspended by a link 59 to a buoyancy element 19. A connecting element 12 of the cable type with a lower portion 13 comprising weighting blocks 31 arranged in a string as mentioned above, which extends from a support floating on the surface up to the level of a fastening element 36 at the base of the buoyancy element 19, makes it possible to control the speed of descent and ascent of the device 1 and to stabilize it, if necessary, close to the wall 6, according to the present invention.

The buoyancy fluid according to the invention has been described in order to facilitate the installation of packages or heavy structures in extreme depths, but it is also advantageously used to play the role of permanent float on underwater structures, such as oil or gas production towers, or water injection installed on oil fields in deep water depths, from 1000 to 3000m or more, as described in particular in WO 00/49267 and WO 03/65788 in the name of the plaintiff.

The buoyancy fluid according to the invention can be used at any depth but, because of its particular implementation, has the most interest at large depths. It is particularly advantageous for the abyssal depths, for example 10,000 or 11,000m, or beyond, because it is almost incompressible, that is to say that its volume does not vary substantially when the depth of water, therefore the pressure increases. In fact, for the very great depths (4000-5000m and more), its volume is reduced by a few%, but the sea water, also quasi-incompressible, also sees its density increase significantly. As the volume of the buoyancy fluid decreases and the density of the seawater increases, this then results in a slight variation of buoyancy pressure, and therefore buoyancy, which is automatically compensated by the link (s). 12, 13 as described above, and whose equilibrium point will vary slightly as a function of said buoyancy variation.

Claims (37)

1. The use of a buoyancy fluid presenting density that is less than that of sea water, and that is confined in a rigid or flexible leaktight casing (4₁, 19₁), so as to constitute an immersed buoyancy element (4, 19), said use being characterised in that said buoyancy fluid is a compound that is naturally in the gaseous state at ambient atmospheric temperature and pressure, and in the liquid state at the underwater depth to which said buoyancy element is immersed.
2. A use according to claim 1, characterised in that said buoyancy fluid is naturally in the stable liquid state when it is placed at an underwater depth of 10 m to 500 m, and preferably of 20 m to 100 m.
3. A use according to claim 1 or claim 2, characterised in that said buoyancy fluid is a fluid that is quasi-incompressible, and that presents a density in the liquid state of 0.3 to 0.8, and preferably of 0.5 to 0.7.
4. A use according to one of claims 1 to 3, characterised in that said gas is selected from ammonia, a C-2 to C-7 alkane, a C-2 to C-7 alkene, a C-2 to C-7 alkyne, and a C-4 to C-7 diene.
5. A use according to claim 4, characterised in that said compound is selected from the list: ammonia, ethane, butane, propane, ethylene, propylene, butene, acetylene, methyl acetylene, propadiene, and butadiene.
6. A use according to claim 5, characterised in that said compound is selected from ammonia, propane, and butane.
7. A use according to one of claims 1 to 6, characterised in that said casing is constituted by, or is placed inside, the walls (4₁) of a compartment (4) of an immersed structure (1).
8. A use according to one of claims 1 to 6, characterised in that said casing (19₁) is placed outside an immersed structure (1) to which it is connected or secured.
9. A use according to claim 8, characterised in that said immersed structure (1) is suspended from said buoyancy element (19) by at least one cable (59).
10. A use according to one of claims 1 to 9, characterised in that said immersed buoyancy element (4, 19) imparts buoyancy to an immersed structure (1) to which it is connected or secured, or in which it is integrated, said buoyancy element comprising a said immersed casing (4₁, 19₁) connected to said structure, and in which said compound is liquefied and confined in leaktight manner.
11. A use according to claim 10, characterised in that said buoyancy element comprises a said flexible casing (19₁), preferably having a hydrodynamic profile, minimizing forces during its vertical movements when it is full of said buoyancy fluid.
12. A method of putting a buoyancy element according to claim 10 or claim 11 into place between the surface and the bed of the sea, said method being characterised in that said fluid is stored in a tank on a surface ship (61) as a liquid in the cooled or compressed state, and it is injected in the liquid state into a pipe (23) from the surface (61) where it is stored to a said immersed casing (4₁, 19₁) at an underwater depth at which the underwater pressure is not less than the vapour pressure of the gas corresponding to said compound at the ambient temperature at said depth.
13. A method according to claim 12, characterised in that said casing (19₁) is a flexible casing that is lowered to the desired depth empty, in a folded state.
14. A method according to claim 12 or claim 13, characterised in that said casing (19₁) is prefilled, at atmospheric pressure and temperature, with sea water or with another fluid, preferably an incompressible liquid compound such as gas oil, fresh water, or methanol, and the sea water or said other liquid is discharged from the casing as it fills with said buoyancy fluid as defined in claims 1 to 6.
15. A method according to claim 14, characterised in that said casing is prefilled with sea water, and before it is filled with a said buoyancy fluid, a limited quantity of methanol is injected, since methanol is suitable for preventing the formation of hydrates.
16. A method according to claim 14 or claim 15, characterised in that said casing is filled at the surface with a said other fluid, and said casing filled in this way is lowered to a depth at which the hydrostatic pressure corresponds to the pressure at which said buoyancy fluid is subsequently injected into said casing with said other fluid being discharged.
17. A method according to one of claims 12 to 16, characterised in that said buoyancy fluid is stored as a liquid in the cooled state in a cryogenic tank and at atmospheric pressure, and it is injected in the pressurized liquid state into said immersed casing at a pressure corresponding to the hydrostatic pressure at the depth of said casing, said buoyancy fluid passing through a heat exchanger so that the temperature of said fluid is brought substantially to that of the sea water at the depth of said immersed casing prior to filling said casing.
18. A device for stabilizing or controlling the lowering or raising of a structure (1, 32) between the surface (15) and the bed (7) of the sea, said structure including or being connected to a buoyancy element (4, 19) according to a use of claim 10 or claim 11, said device being characterised in that it includes at least one connection element of the cable (12) or chain type, having:

    ■ a first end that is connected to a winch (12₁) on board a floating support or ship (20a, 20b) on the surface, and on which winch it is wound; and

    ■ a second end that is connected to a fastener element (10, 36) on said structure (1, 32), or on at least a first buoyancy element (19) that is connected to said structure; and

    ■ the length of said connection element (12) is such that said winch (12₁) is capable of winding or unwinding said first end of said connection element (12), so that a bottom portion (13) of said connection element (12) can hang beneath said fastener element (10, 36).
19. A device according to claim 18, characterised in that it includes at least two of said connection elements (12), said fastener elements (10, 36) preferably being disposed symmetrically, respectively around and on the periphery of said structure (1, 32).
20. A device according to claim 18 or claim 19, characterised in that said connection element (12) is constituted by a cable having a bottom portion (13) that comprises weighting blocks (31) disposed in a string on a said cable, said weighting blocks preferably being metal blocks secured to said cable by clamping.
21. A device according to claim 20, characterised in that said blocks (31) present a shape such that when said bottom portion (13) hanging beneath said fastener elements curves, two of said blocks (30) disposed side by side are capable of coming into abutment against each other, thereby limiting the curvature of said cable.
22. A device according to claim 21, characterised in that the curvature of said cable is limited so that the minimum radius of curvature (R₀) of said cables at said bottom portion (13) enables a minimum distance (2R₀) to be maintained between said cable (12) and said structure (1, 32) that is sufficient to prevent any mechanical contact between them while said structure is being lowered or raised.
23. A device according to one of claims 20 to 22, characterised in that each of said blocks (31) presents a cylindrical central portion (31) between two frustoconical ends (31₂) having axes that correspond to the direction of said cable (12) when said cable is disposed linearly, two adjacent blocks being in contact at said frustoconical ends along a generator line (31₂) of said frustoconical ends in the curved parts of said bottom portion (13).
24. A device according to claim 18 or claim 19, characterised in that said connection element comprises a chain having a bottom portion (13) that comprises links that are heavier than the links of the rest of the chain, and that are preferably larger so as to limit any curvature of the chain.
25. A device according to one of claims 18 to 24, characterised in that said first buoyancy elements (19) are disposed above said structure.
26. A device according to one of claims 18 to 25, characterised in that said structure includes second buoyancy elements (4, 33), preferably according to claim 10 or claim 11, that are integrated in said structure (1, 32), and more preferably integrated above said fastener element(s) (10, 36) so that the center of gravity of said structure together with said first buoyancy elements according to claim 10 or claim 11 is situated below the center of thrust that is exerted both on said structure (1) and on said first buoyancy elements (19) according to claim 10 or claim 11.
27. A method of lowering, raising, or stabilizing a structure (1, 32) between the surface (15) and the bed (7) of the sea by means of a device according to one of claims 18 to 26, said method being characterised in that it comprises the following steps: unwinding or winding each connection element at its first end by means of a said winch (12₁); and controlling the speed at which each connection element is lowered or raised by regulating the speed at which each connection element (12) is respectively wound off or on said winch (12₁), so as to adjust the length of said bottom portion (13) of said connection element (12) hanging beneath said fastener element (10, 36), the lowering, raising, or stabilizing of said structure being obtained when the sum of the weight of the fraction of said bottom portion(s) (13) of the connection element(s) (12) between firstly said fastener point(s) for fastening to said fastener element(s) (10, 36) and secondly the lowest point of said bottom portion(s) (13), plus the weight of said structure (1, 32) as a whole and of said first buoyancy element(s) (19) according to claim 10 or claim 11, is respectively greater than, less than, or equal to the buoyancy thrust that is exerted on said structure (1, 32) and on said first buoyancy element(s) (19) according to claim 10 or claim 11.
28. A method according to claim 27, characterised in that said structure is a rigid structure of steel, metal, or composite synthetic material containing at least one and preferably a plurality of leaktight buoyancy compartments (4) that are suitable for forming a said buoyancy element according to claim 10 or claim 11, with each of said compartments being fitted with at least one filling orifice (4₁) and preferably with at least one emptying orifice (4₅), said leaktight compartments (4) preferably being distributed symmetrically in said structure.
29. A method according to claim 27 or claim 28, characterised in that said structure is a massive structure constituted by an open-based receptacle (1) in the form of a cap, the receptacle comprising a peripheral side wall (2, 2a, 2b, 2₁) surmounted by a roof wall (3, 3a, 3b) and being suitable for completely covering a wreck (6) of a ship on the sea bed (7) in order to recover polluting effluent (8) escaping therefrom, said receptacle having at least one emptying orifice (9) for discharging said effluent contained in the inside volume of said receptacle; said emptying orifice (9) preferably being situated in the roof (3, 3a, 3b) of the receptacle.
30. A method according to claim 28 or claim 29, characterised in that said receptacle is constituted as an upside-down double-walled ship hull, said leaktight compartments (4) being defined by spaces between said double walls and by structural elements (4₃, 4₆) interconnecting the double walls (2, 2a, 2b, 2₁, 3, 3a, 3b).
31. A method according to one of claims 27 to 30, characterised in that the rigid structure includes hollow tubular bars defining leaktight compartments (4) and forming said buoyancy elements according to claim 10 or claim 11.
32. A method according to one of claims 27 to 31, characterised in that said structure is fitted on the outside:

    with fastener elements (10, 14₁) enabling said buoyancy elements and said cables (12, 14) or said chains (13) to be secured thereto for lowering said structure from the surface (15), and for putting it into place, and, where appropriate, anchoring it (15₁, 15₂) to the sea bed (7); and

    preferably with thrusters (16), more preferably steerable thrusters enabling said structure to be moved in a horizontal direction in order to position it.
33. A method according to one of claims 27 to 32, characterised in that it comprises the following steps:

    1) filling said leaktight compartments (4) completely or partially with a said buoyancy fluid, so as to constitute a buoyancy element according to claim 10 or claim 11, with the extent to which said leaktight compartments (4) are filled being adjusted so as to cause said structure (1) to occupy an equilibrium position when immersed close to the surface;

    2) lowering said structure (1) to the desired position by means of a device according to one of claims 16 to 24 for controlling lowering, so as to regulate the speed at which the receptacle is lowered, and so as to provide equilibrium to the base of said substantially horizontal structure while it is being lowered; and

    3) once said structure (1) is immersed to the desired depth, emptying said leaktight compartments (4) filled with fluid lighter than sea water that is recovered at the surface, and simultaneously filling said leaktight compartments with sea water.
34. A method according to claim 33, characterised in that

    in step 1), additional buoyancy is provided to said structure by means of said first buoyancy elements (19) consisting of additional floats (19) connected to said receptacle; and

    in step 3), once said structure is in the underwater position at the desired depth, said additional floats (19) are detached.
35. A method according to claim 33 or claim 34, characterised in that after step 1) and before step 2), once said structure (1) has reached the desired position, preferably in the vicinity of the sea bed, the lengths of said heavy stabilizing cables (or chains) (12) hanging beneath said fastening elements (10, 10a, 10b) are reduced so as to stabilize said structure (1) in suspension, and

    where appropriate, said structure (1) is anchored (14, 15₁-15₂) to the sea bed (7), and then

    · said heavy stabilizing cables (or chains) (12) are fully lowered so that their entire weight contributes to stabilizing said structure.
36. A method according to claim 35, characterised in that

    in step 1), said compartment(s) (4) or casing(s) (19₁) connected to said structure are filled with sea water or with a first fluid that is lighter than sea water; and

    in step 2), said structure (1) is lowered to a depth of 30 m to 60 m corresponding to a pressure of 3 bars to 6 bars, at which depth a buoyancy fluid, as defined in one of claims 1 to 6, consisting of a liquefied gas that is lighter than sea water is injected under pressure into said compartment(s) (4) or casing(s) (19₁) from a gas tanker ship (61) on the surface, so as to constitute a buoyancy element according to claim 10 or claim 11.
37. A method of recovering polluting effluent that is lighter than sea water, as contained in the tanks of a shipwreck (6) lying on the sea bed (7), in which method:

    1) a said receptacle is put into place in accordance with the method of one of claims 29 to 36; and

    2) the effluent recovered inside said receptacle (1) is collected by being emptied out through said top emptying orifice (9).

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