EP1606159B8

EP1606159B8 - Buoyancy method and device for stabilizing and controlling lowering or raising of a structure between the surface and the sea floor

Info

Publication number: EP1606159B8
Application number: EP04742349A
Authority: EP
Inventors: Michel Baylot; Marc Bonnissel; Xavier Rocher
Original assignee: Saipem SA
Current assignee: Saipem SA
Priority date: 2003-03-26
Filing date: 2004-03-25
Publication date: 2008-07-16
Anticipated expiration: 2024-03-25
Also published as: ATE388889T1; EP1606159A2; WO2004087496A8; US20110005452A1; FR2852917A1; US8776706B2; FR2852917B1; US7882794B2; US20060225810A1; WO2004087496A3; WO2004087496A2; EP1606159B1; DE602004012398D1

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

Buoyancy device and method for stabilizing and controlling the descent or ascent of a structure between the surface and the seabed.

The present invention relates to the use of a buoyancy fluid with a density lower than that of sea water confined in a rigid or flexible waterproof envelope, to constitute an immersed 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 the 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 sea bottom, comprising or connected to at least one buoyancy element consisting of an envelope in which said buoyancy fluid according to the invention is tightly confined.

The term “structure” is understood here to mean any equipment, tool, machine and in particular risers, underwater wellhead elements on oil fields or oil processing units, which it is desired to install at sea, or at bottom of the sea, or a receptacle with a watertight compartment useful in particular for recovering polluting effluents from a wreck.

The descent and ascent of these massive structures which it is desired to descend to the bottom of the sea or to ascend from the bottom of the sea to the surface are delicate because of the mass of said structures or of said shuttle tanks. Indeed, it is known to descend packages of several hundred tonnes of apparent weight into the water, to the bottom of the sea using lifting means located on a floating support, for example a crane; but, when the depth becomes significant, 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. Synthetic cables can also be used which do not have this drawback, but their cost is very high and their implementation with winches or capstans presents extreme difficulties for heavy loads and depths of 1000m to 4000m, or even more.

To lower such packages, it is preferable to lighten them by adding to said package buoyancy elements which reduce its apparent weight in the water and consequently require lifting equipment of lower capacity.

The term "buoyancy element" means an element which has a lighter weight than sea water and which therefore makes it possible to increase the buoyancy of the assembly which it forms with the structure to which it is connected or in which it is integrated.

The term “increase the buoyancy” of an element means increasing the ratio ω between the buoyancy and its self-weight out of water, which is exerted on said element when it is immersed. Thus, if said ratio is ω <1, the element has negative buoyancy, therefore 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 ω is growing.

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 which applies to all of said structure and said buoyancy elements, is greater than or equal to the dead weight of the whole of said structure and of said buoyancy elements the result of the forces being directed upwards in the event of positive buoyancy.

This additional buoyancy is generally carried out with airtight tanks filled with air, made integral with said package. Such buoyancy elements made up of tanks filled with air must be able to withstand the maximum immersion pressure without imploding or deforming, because the buoyancy would be reduced by the same amount, or even canceled out. The tank must then have a resistance mechanical adapted to withstand the pressure corresponding to the envisaged immersion depth, which is approximately 10 MPa additional for each additional section of 1000m of water depth. Thus, in the case of very great depths, for example beyond 1000m, the envelope of the tank must be sufficiently reinforced to hold the pressure and its self-weight is consequently much greater, which then considerably reduces the performance of said buoyancy element. To limit the effects of water pressure at great depth, the tank is advantageously pressurized before lowering it, which then makes it possible to reduce the self-weight of the tank, because at the maximum immersion depth, the differential pressure between the outside and inside is weaker and the wall needs less resistance; on the other hand, the tank must be able to withstand the initial bursting pressure during pressurization.

To create this buoyancy, recourse is also made to quasi-incompressible liquids with a density lower than that of sea water, such as fresh water, diesel or methanol which make it possible to use less resistant envelopes. But these materials do not have a ratio ω (Archimedes thrust / 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 very deep water, rigid syntactic foam is also conventionally used which is composed of microspheres, generally of glass and of small diameter, mixed with a binder of the epoxy or polyurethane type. This type of foam is capable of withstanding considerable pressure and has a more interesting ratio ω (Archimedes thrust / dead weight) of between ω = 1.70 to 2.05 for foams with a density of between 0.6 and 0.5, capable of withstanding depths 1500 to 2000m. For syntactic foams capable of withstanding greater depths, their density is greater and the ratio ω then decreases rapidly. In addition, these syntactic foam-based materials are very expensive and very difficult to manufacture in large volumes, especially for extreme depths. When the package is placed on the seabed, the buoyancy should generally be removed so that it remains stable. In the case of a tank filled with air, it suffices simply to open the valves so that it fills with sea water. 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 which connect it to the parcel and to bring it up to the surface, either in a controlled manner, which represents a considerable time, or by letting it go up freely without any control, this which risks creating accidents with the various ships 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 said package is then increased by said buoyancy, as well as by the "added mass" of water, that is ie the mass of water adjacent to the package which is entrained during vertical movements, upwards or downwards. Thus, during the descent, although its apparent weight in the water may be very low, the inertial mass to be considered consists of the mass of the package itself, increased by the mass of the buoyancy elements, further increased by the " added mass "of water, which can represent an overall mass of inertia of 400 or 500 tonnes for a massive package of 100 tonnes.

In general, it is 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 package.

An object of the present invention is to provide a buoyancy material and to produce buoyancy elements to facilitate the installation of heavy packages that can weigh several hundred tonnes, or even several thousand tonnes, in water depths from 1000 to 4000m, or even more, which is inexpensive, easy to make and implement, and having an optimal ω = (Archimedes thrust / own mass) ratio, that is to say much greater than 1, in particular greater to 1, 5 and in addition 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 intake to sea currents on the package + buoyancy element.

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

Another object of the present invention is to provide a device and method making it possible to control and facilitate the descent or ascent of a heavy and, if necessary, bulky structure such as receptacles for the recovery of effluents mentioned above, but applicable any other type of structure, or even to stabilize it, between the surface and the seabed, especially at great depths.

Another object of the present invention is to provide a method and an installation making it possible to confine and recover the contents of the holds and the tanks of a ship, for example an oil tanker, resting on the seabed, in water depths important, in particular greater than 3000 meters, or even up to 4000 to 5000 meters, and which do not have the drawbacks of the 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 making it possible to confine and recover polluting effluents from the holds of a stranded ship, in particular at great depth, by means of a rigid receptacle with open base in the form hat coming to completely cover the wreck of the ship so as to channel all the effluents escaping from the ship in a single volume, or even to organize the ascent of polluting effluents from the said receptacle to the bottom of the sea in better conditions.

Another object of the present invention is therefore, more particularly, to provide a receptacle with an open base in the form of a hat, capable of completely covering a wreck at the bottom of the sea and recovering polluting effluents therefrom. escaping from it, which is technically reliable and which can be installed at the bottom of the sea according to a simple and technically reliable process.

To do this, the subject of the present invention is the use of a buoyancy fluid with a density lower than that of sea water confined in a rigid or flexible waterproof envelope, to constitute an immersed buoyancy element, characterized in that 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 ambient atmospheric temperature and pressure conditions correspond to temperatures from -10 to + 40 ° C and to a theoretical absolute atmospheric pressure of 101,325 Pa, at sea level, and whose approximate value of 100,000 Pa, i.e. 0.1 MPa, is used throughout the description of the present invention.

The underwater ambient temperature and pressure conditions generally correspond to a temperature of 1 to 35 ° C, preferably 3 to 25 ° C, and a pressure higher than atmospheric pressure, more precisely a pressure increasing appreciably by 10 ⁵ Pa in 10 m increments.

In some arctic regions, we may encounter water at a temperature much lower than 0 ° c, for example -5 to -8 ° C, but as a general rule deep water is 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 still 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 gases and liquids, and therefore at a temperature above which said compound cannot not be in a liquid state. The present invention also provides an immersed buoyancy element conferring buoyancy on an immersed structure to which it is connected or fixed or in which it is integrated, characterized in that it comprises a said submerged envelope in which the said liquefied compound is tightly confined.

In a first variant, said envelope is made up or placed inside the walls of a compartment of an immersed 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 of hydrodynamic profile shape minimizing the forces during its vertical displacements when it is filled with said buoyancy fluid.

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

More preferably, said fluid is an almost incompressible fluid and has a density in the liquid state, from 0.3 to 0.8, preferably from 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, compounds are readily available commercially, such as: ammonia, ethane, butane, propane, ethylene, propene, butene, acetylene, methyl acetylene, propadiene and butadiene. The term “butene” is understood here to mean the various isomers such as butene-1 and the 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 characteristics of density in the liquid state and of 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 also decreases. These three compounds have densities substantially between 510 and 630 kg / m ³ and, the minimum depths with which said rigid or flexible envelopes can be filled, are included, respectively, substantially between depths of 65m to 7.5m (see table 1 below), when the ambient temperature is around 15 ° C.

Thus, if the heavy structure has, in quantity, sealed internal cavities which can play the role of rigid envelope, advantageously use butane. However, if additional flexible or rigid external envelopes are to be produced, propane will advantageously be used, so as to minimize the size of said envelopes and therefore their cost. The gain in volume of propane required being approximately 15% compared to butane, this will then result not only in a reduction in the cost of the envelope, but also in the cost of liquefied gas, since the unit prices of butane and propane are much the same. On the other hand, the transfer operations take place at a greater depth and if divers are used to supervise the operations, the necessary equipment as well as the personnel have a higher qualification, therefore with a significant additional cost compared to a simple dive of area.

The present invention also provides a method of placing a buoyancy element between the surface and the bottom of the sea. According to the invention, said fluid is stored in a tank on a surface vessel in the liquid state compressed or cooled, and it is injected in the liquid state in a pipe from the surface or it is stored up to a so-called submerged envelope at an underwater 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, picked up or folded back on itself.

Advantageously, said envelope is previously filled with sea water or another fluid preferably a liquid compound at atmospheric pressure and temperature, incompressible such as diesel, fresh water, or methanol, and the seawater or the said other fluid of the envelope as the said buoyancy fluid is filled.

In an advantageous embodiment, said envelope is previously filled with sea water and, before filling 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 sea water and a buoyancy fluid according to the invention, creates a screen avoiding direct contact between said buoyancy fluid and water and thus prevents chemical reactions leading to the formation of hydrates when said buoyancy fluid combines with water. These hydrates risk blocking the pipes and preventing the recovery of liquefied gases at the end of the installation phase.

More particularly still, the said envelope is filled at the surface with the aid of a said other fluid, and the said envelope 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 and when said other fluid is removed.

In an alternative embodiment, said buoyancy fluid is stored in the liquid state cooled in a cryogenic tank and at atmospheric pressure and is injected in the liquid state under pressure into 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 thirst brought substantially to that of seawater at the depth of said submerged envelope before filling.

The present invention also provides a device for stabilizing or controlling the descent or ascent of a structure between the surface and the sea bottom, comprising or connected to a buoyancy element according to the invention, characterized in that it comprises at least one connecting element of the cable or chain type of which: has a first end is connected to a winch on board a floating support or ship on the surface, on which winch it is wound, and H a second end is connected to a fastening element, on said structure, or on at least a first buoyancy element according to the invention, connected to said structure, and m the length of said connecting element is such that said winch is capable of winding or unwinding said first end of said connecting element, so that a lower portion of said connecting element can hang below said hooking element, that is to say below the point of at stain from said second end to said hooking element.

Said structure is therefore, if necessary, suspended from one or more said first buoyancy elements according to the invention arranged above it. Said structure may also include second buoyancy elements integrated or incorporated inside said structure, that is to say that said second buoyancy elements do not move an additional volume of water relative to the volume of water moved 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 said lower portion of the connecting element hanging below said attachment element 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 several said hooking elements and said connecting elements and said hooking elements are preferably arranged symmetrically respectively around and on the periphery of said structure.

More specifically, the present invention also provides a method of descending or ascending or stabilizing a structure between the surface and the seabed using a stabilization device, according to which steps are carried out in which one proceeds or wind up said connecting element (s) at their said first end (s) using said winch (s) and controls the speed of descent or respectively ascent by regulating the speed of unwinding or respectively winding of said connecting element (s) at the level of said winch (s), so as to adjust the length of said lower portion of said connecting element (s) during below said said attachment element (s) on said structure or said first buoyancy element, the descent, the ascent or stabilization of said structure being obtained when respectively, the sum of the weight of the part of the (or) said ( s) lower portion (s) of the connecting element (s) between, on the one hand, said point (s) of attachment to said point (s) attachment element (s) or said first buoyancy element on said structure and, on the other hand, the lowest point (s) of the said portion (s) lower (s) s), added to the weight of said structure and of said buoyancy element (s) according to the invention, is respectively greater, less than or equal to the buoyancy force exerted on all 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 consisting of a cable, said lower portion of which includes weighting blocks arranged in a chain 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 hanging below said hooking elements adopts a curved shape, two said blocks arranged side by side are capable of abutting against each 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 the level of 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 during 'a said descent or ascent of said structure.

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

In another embodiment, said connecting element comprises a chain, said lower portion of which comprises heavier links than those of the rest 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 if necessary 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 hooking 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 'exercising on all of said structure and said first buoyancy elements according to the invention, so as to ensure overall stability during the entire installation phase. By center of thrust is meant the point where the result of the Archimedes' thrust is exerted. (The center of thrust is the center of gravity of the volume of water displaced by said structure).

As mentioned previously, said heavy structure can be constituted by any package, in particular heavy package, module, tool, or base as described in the European patent application in the name of the unpublished applicant n ° 0435802.6, which one wishes to immobilize at near the bottom of the sea or anchor on a wall or element resting on the bottom of the sea.

Preferably, said structure is a rigid structure in steel, metal or composite synthetic material containing at least one, preferably a plurality of watertight 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 orifice, said sealed compartments being preferably distributed symmetrically in said walls.

The watertight compartments are cavities intended to be completely or partially filled with buoyancy fluid lighter than sea water according to the invention and therefore constitute compartments providing buoyancy to the structure, allowing it to be towed on the surface and lowered. at the bottom of the sea when it is set up under reliable technical conditions that are simple to carry out, as will be explained below.

The term “symmetrical distribution of the compartments means that these are arranged symmetrically with respect to one or more median planes of symmetry of said structure, which makes it possible, 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 profiles defining watertight compartments and forming said buoyancy elements according to the invention.

Advantageously, the tanks or balloons associated with the processing of petroleum are used in particular for carrying out the water / petroleum / gas separation, to provisionally define watertight compartments forming said buoyancy elements according to the invention.

In a particularly advantageous embodiment, said structure is a massive structure constituted by a receptacle with an open base, hat-shaped, comprising a peripheral lateral wall surmounted by a ceiling wall, capable of completely covering a wreck with a ship at the bottom of the sea to recover polluting effluents escaping therefrom, said receptacle comprising at least one orifice for discharging said effluents contained in the interior volume of said receptacle; said discharge orifice being preferably located at the level of the ceiling of the receptacle.

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

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

- attachment element allowing to hang said buoyancy elements and said cables or chains allowing the descent of said structure from the surface, and its installation and, if necessary, its anchoring at the bottom of the sea , and

- Preferably propellants, more preferably orientable propellants, allowing the movement of the receptacle in a horizontal direction to position it above said wreck.

Said attachment elements can therefore allow additional floats according to the invention to be attached to said structure.

- attachment element (s) allowing it to hang one or more buoyancy elements and said cable (s) or chain (s) allowing the descent of said structure from the surface, and its installation and, where appropriate, its anchoring at the bottom of the sea, and

Indeed, the present invention also relates to a process for 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 therefrom. escaping, characterized in that the successive stages are carried out in which:

1) 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 said sealed compartments is adjusted so as to position said structure, in particular said receptacle in equilibrium in immersion close to the surface, in particular a few meters away, for example 10 meters away, and

2) the said structure is lowered, if necessary, to its desired submerged position, in particular the said receptacle near the bottom of the sea, above the wreck, by controlling the descent using a stabilization device or for controlling the descent or ascent of a structure according to the invention, in particular using a plurality of cables preferably unwound from winches on board surface ships, said cables being connected to lengths heavy chains, the chains themselves being connected, at their other end, to said hooking elements integral with said structure, preferably distributed symmetrically over the periphery of said structure, the weight of the lengths of chains hanging below the points d 'attachments on said attachment elements allowing the descent of said structure, and the lengths of said chains hanging below said points of attachment of the elements at the attachment point hage being adapted by unwinding or winding said cables, preferably around said winches, so as to regulate the speed of lowering the receptacle and ensuring the balancing of the base of said structure, in particular the base of said substantially horizontal structure during the descent, and

3) when said structure is in place at its desired position, in particular when said receptacle is placed at the bottom of the sea so as to cover said wreck, said watertight compartments are emptied filled with a fluid lighter than water sea, and said watertight compartments are simultaneously filled with sea water.

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

In step 1) above, it is understood that the filling of said sealed compartments, with a fluid lighter than sea water, is carried out in the various 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 filling rate.

Advantageously, in step 1), additional buoyancy is provided to said structure using 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 submarine position, in particular at the bottom of the sea, said additional floats are released.

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

Heavy chains can be recovered by disconnecting them from said structure, but as explained below, to increase the stability of said structure, in particular of said receptacle, said heavy chains can be hooked at their two 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 of said receptacle after hooking of the cables connected to surface vessels, then the cables connected to the surface vessel are unhooked from said chains.

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

More particularly still, in a method according to the invention, in step

1), the said compartment (s) or envelope (s) connected to said structure are filled with seawater or a lighter first fluid as the sea water 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 at which is injected a liquefied gas under pressure lighter than sea water in the said compartment (s) or so-called envelope (s) from a surface gas vessel to form a buoyancy element according to the invention.

The use of liquefied gas as a lighter fluid than seawater makes it possible to obtain fluids with a density in the liquid state of between 0.5 and 0.7 providing two to three times greater buoyancy. than diesel (d = 0.85) and thus making it possible to use considerably reduced volumes of watertight compartments. In addition, in the event of an incident during installation, these products are much less polluting than diesel or oil, since they disperse naturally as soon as they reach the surface, returning to the gaseous state. Finally, the present invention also relates to a process for recovering pollutant effluents lighter than sea water, contained in the tanks of a shipwreck lying on the bottom of the sea in which:

1) a receptacle is put in place according to a method of stabilization and descent control according to the invention and

2) the effluents recovered are collected inside said receptacle by discharging through said upper discharge orifice.

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

Other characteristics and advantages of the present invention will emerge more clearly on reading the description which follows, given in an illustrative and nonlimiting manner, with reference to the appended drawings in which: FIG. 1 is a section in side view of a said structure consisting of a receptacle called hereinafter "sarcophagus" during descent to a wreck; Figure 2 is a sectional side view of a rigid receptacle resting at the bottom of the sea and completely enveloping the wreck; Figure 3 is a perspective view in cutaway, of the structure of the sarcophagus; Figure 4 is a sectional side view of the sarcophagus during descent, detailing the mode of regulation of the descent using heavy chains; Figures 4a and 4b detail the variable mode of implementation of said heavy chains; Figure 5 is a sectional side view of a sarcophagus composed of a rigid support structure of metal beams, associated with tanks of buoyancy filled with a low density fluid integrated between them and closed by waterproof membrane fabrics on the external face of the structure; Figure 6 is a sectional side view of a sarcophagus made of lightweight concrete, and having internal volumes forming sealed compartments filled with a low density fluid ensuring buoyancy; FIGS. 7a and 7b represent a section in side view of a sarcophagus respectively during towing, its buoyancy compartments being filled with seawater 9a, and in 9b, vertical to the wreck, during the filling phase of said buoyancy compartments with liquefied gas of low density; FIG. 8a is a side view of a shuttle tank stabilized in its ascent by a connecting cable weighed down by blocks integral with the latter and also playing the role of curvature limiter, FIGS. 8b and 8c represent states similar to that of FIG. 11a, the shuttle tank being in the ascent phase in FIG. 11b and in descent in FIG. 8c, FIG. 8d represents the detail of two blocks 31 in contact, when said connecting cable is bent, FIG. 9 shows a shuttle tank cooperating with the upper wall of a structure of the sarcophagus type to recover the oil flowing from a stranded ship and confined under the sarcophagus; Figure 10a shows in sectional side view a structure consisting of an oil processing module suspended in the sub surface by means of cables with two floating barges, the assembly being being towed to the installation site; FIG. 10b is a sectional side view of said oil processing module lowered to a depth of 20 to 40 m, a gas ship being in the process of transferring the buoyancy fluid to a flexible envelope of the balloon type; FIG. 11 represents the descent of a structure consisting of an anchoring and drilling device controlled by a stabilization chain and buoyancy elements according to the invention. In Figure 1, there is shown the hull of a wreck or a tank wall 6 resting on the sea bottom 7 filled with hydrocarbon 8 whose density is less than sea water. Said hydrocarbon is confined in the upper part of the tank or wreck 6, the lower part being filled with sea water. Since the ship 6 generally has multiple openings hermetically closed at deck level, leaks may occur. produce as soon as this tightness comes to be degraded by the deformation or the rupture of the hull during the sinking.

A rigid receptacle 1 according to the invention hereinafter called "sarcophagus" consisting of a rigid structure is lowered from the surface under the control of cables 12 connected to vessels 20 with dynamic positioning located on the surface, as shown in Figures 1 and 2.

The receptacle 1, described in 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 reversed V in cross section (YOZ), and

- a side wall 2 comprising:

Two longitudinal side walls 2a, 2b vertical or inclined with respect to said vertical axial plane of symmetry (XOZ), each being contiguous to a said longitudinal ceiling wall 3a, 3b, and

• two transverse end walls 2-ι, vertical or inclined, preferably symmetrically, with respect to a vertical transverse plane of symmetry (YOZ).

As detailed in FIG. 3, the sarcophagus 1 consists of a shell in an inverted configuration, said shell being sealed and with double walls thus constituting walls 4 of sealed compartments 4, preferably a multitude of sealed compartments in continuity with one another. other. The structure consists of transverse members 4 ₃ , perforated or solid within the same watertight compartment, and associated with longitudinal members, perforated or solid 4 ₆ . In Figure 3, we show in a section exploded transverse corresponding to the plane YOZ, a straight half of the double wall 3b of the ceiling, flat, inclined with respect to the horizontal, for example from 10 to 20 °, but which can be horizontal, and when it is inclined forming a ceiling in configuration of inverted V with the other half of double ceiling walls 3b. Each longitudinal ceiling wall 3a, 3b is connected by its lower edge to a double side wall 2a, 2b, flat, vertical or inclined relative to the vertical, in particular from 5 to 20 °, preferably at a lesser 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, 2ι ensuring the junction between the end edges of the double side walls 2a, 2b and the double walls of the ceiling 3 , 3a, 3b and said end side walls 2ι being perpendicular to the longitudinal axis XX '. The lower part is entirely free, so that the sarcophagus can come to cover, like a bell, the wreck 6 to be confined.

The volumes inside the various double walls 2ι, 2, 2a, 2b and 3, 3a, 3b and delimited by the internal and external walls and the solid members 4 ₃ , 4 ₆ form the walls 4ι of the compartments 4 tight screw with respect to the outside, which makes it possible to fill them with a fluid of density lower than sea water, said fluid then playing the role of float and coming to compensate for the self-weight of the rigid structure of the sarcophagus receptacle 1.

Said shell constituting the sarcophagus is advantageously built dry in a dock, then the sealed compartments 4 included inside the double walls 2-ι, 2, 2a, 2b and 3, 3a, 3b are sealed. After filling the dock, the sarcophagus 1 floats and greatly exceeds the water level, due to the fact that said compartments 4 are filled with air. If there is a risk of instability at this stage, a temporary ballast is advantageously added in the lower part.

The sarcophagus 1 is then towed to deep water where all 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 gas liquefied under pressure as described below. The buoyancy volume is advantageously adjusted so that the sarcophagus is in equilibrium between two waters, the overall equilibrium possibly being provided by additional floats 19 capable of withstanding the bottom pressure, that is to say approximately 350 bars for 3500 m deep. Said additional floats 19 may consist of syntactic foam, that is to say of glass microspheres 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, then, once on site, at least two, preferably four vessels 20 connect to the ends of the sarcophagus 1, in the following manner.

Each of the ships 20 comprises a winch 12 _η provided with a cable 12, preferably made of steel, the length of which is greater than the water depth, for example 130% of 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 hooking element integral with the structure and protruding from the sarcophagus 1, as explained in Figures 1-4-6.

The heavy chains 13 have a self-regulating effect during the descent of the sarcophagus towards the bottom of the sea 7 and their operation is explained in FIGS. 4, 4a and 4b.

In FIG. 4, the cable 12 is in the intermediate position and forms a double chain curve, 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 surface ship 20. Thus, the sarcophagus is kept in balance between two waters under the effect of this force F.

When the winch 12-j of the surface ship 20 rolls up the cable 12, it goes up the chain 13 as indicated in FIG. 4a, which has the effect of reducing the weight of the chain carried by the receptacle at F _m i _n , because then, the entire weight of the the chain is supported by the surface ship 20: the sarcophagus 1 then has a lower apparent weight in the water and it rises to approach an equilibrium position according to FIG. 4 and to stabilize there.

Conversely, when the winch 12- _t of the surface vessel 20 deviates from the cable 12, it lowers the chain 13 as indicated in FIG. 4b, which has the effect of increasing the weight brought by the chain up to F _max . The sarcophagus 1 thus has a greater apparent weight in water and it flows to approach its position of equilibrium according to FIG. 4 and to stabilize there.

Thus in all cases, the configuration of the chains 13 in double chain has a self-regulating effect on the position of the sarcophagus during the descent. However, it is advisable to synchronize very precisely the deflection of the cables 12 of all the winches 12ι involved in the maneuver, so that the sarcophagus 1 descends while 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 hooking elements 10 (FIG. 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 ships with dynamic positioning using a radiolocation system of the GPS type.

The descent of the sarcophagus 1 is carried out, preferably continuously up to 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 movement of all of the surface vessels 20. Said movements of the ships 20 have a delayed effect from a few minutes to a few tens of minutes, on the corresponding movements of the sarcophagus located a few thousand meters below. To facilitate the maneuver, it is advantageous to install adjustable thrusters 16, preferably at the ends of the structure, more particularly at the four corners of the ceiling, said thrusters 16 being powered by an umbilical 16ι of power and control connected to a ship 20 on the surface . In a variant illustrated in FIGS. 1 and 2, 14 winches are installed on the lateral peripheral walls of the sarcophagus, and, when said sarcophagus 1 is close to the wreck, an automatic submarine ROV 22, piloted from the surface, connects cables 14 of said winches 14ι to an anchor 15-ι, 15a pre-installed near the wreck, for example a suction anchor 15ι, or a dead body 15a-

After final placement of the sarcophagus, the heavy chains are rested on the bottom of the sea 7 as illustrated in FIG. 2, then the additional floats 19 are unhooked by means of the ROV 22, the latter then rise freely to the surface where they are recovered. If necessary, care is taken 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 in Figure 2. For this purpose, we connect from the surface, using the ROV 22, a pipe 23, preferably flexible, preferably in configuration of S, to an orifice provided with an isolation valve 4 ₄ , located in the upper part of compartment 4, having taken care to open a valve 4 beforehand ₅ located in the lower part of the same compartment 4 and allowing sea water to penetrate, as the buoyancy fluid rises to the surface.

After emptying the buoyancy compartments 4, the upper valves 4 ₄ , at least, are closed and the sarcophagus then has its maximum weight which provides it with great stability, even in the event of large leaks at the wreck. The effluents, escaping from the wreckage at the level of said leaks, come together in the upper part of the internal volume of the sarcophagus, thus creating significant buoyancy, but much lower than that of the fluid in compartments 4. Indeed, in the in the case of very viscous crude oils, the density is generally greater than 0.95 and often approaches 1.02, which creates low buoyancy and does not risk destabilizing the sarcophagus. After emptying the buoyancy compartments 4, the chains can be recovered, but if one wishes to improve the stability of the sarcophagus, the chains 13 are advantageously raised, which are suspended by their second end to the bracket already supporting the first end, or they are raised and simply placed 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 by using light metals, for example aluminum for the structure, fresh water will advantageously be replaced by a buoyancy fluid according to the invention, in particular preferably of ammonia, butane or propane as explained below.

In fact, seawater having a density of approximately 1.026 on the surface and 1.045 towards 4000 m deep and at 3 ° C, freshwater having a density of 1 in surface and from 1.016 towards 4000 m deep and at 3 ° C, the buoyancy provided by fresh water per m ³ , thus varies from 26kg at the surface to 29 kg at 4000 m deep. The overall volume of the compartments 4 of the following example makes it possible to balance the gross self-weight of the structure of the sarcophagus described below. A sarcophagus with aluminum walls 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 a mass of aluminum of 3000 tonnes, c '' is to say a weight taken off in the seawater of 1850 tons. The overall volume of the compartments is 73,125 m3, which gives a buoyancy of 1,480 tonnes when filled with 75% fresh water. An additional buoyancy of 470 tonnes is installed in the form of floats distributed along the structure and the stabilizing chains for the descent consist of four identical lengths of chain each weighing 50 tonnes, each of them being 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 then represents a mass of 7,500 tonnes, that is to say a weight taken off in seawater of 6,500 tonnes. The overall volume of the compartments is 47,550 m3, which gives a buoyancy of 6,280 tonnes when filled with 22% butane with a density of 601 kg / m3. The additional floats represent 320 tonnes, and the stabilizing chains (50T x4) remain the same as in the case of the aluminum sarcophagus.

At the end of the installation, an upper evacuation orifice 9 at 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 wreck.

This same upper orifice 9 is advantageously used to recover the effluents 8 which escape from the wreck 6 over 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 oil 8 advantageously accumulated since the previous intervention campaign is advantageously transferred, either by means of a pipe 23 connecting the upper orifice 9 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 patent application FR 2 804 935 or else a device of the shuttle type such as described in the unpublished European patent application No. 03 358 003.6.

In a version of the invention illustrated in FIG. 5, a hanger-type supporting structure is produced, consisting of metal or steel beams 24 assembled together by welding or bolting, and there are incorporated watertight compartments, distributed from whether continuous or not, either on the side walls 2, 2a, 2b, or on the roof 3, 3a, 3b or in combination of the two. The entire structure is sealed against a fluid tending to escape naturally upwards, by cloths or membranes 25 fixed to the outside of the structure and against it in a sealed manner, way to collect all the leaks from the wreck and direct them to the top point where they will be stored while waiting to be recovered, either by means of a bottom-surface connection 23, either by means of recovery device or the shuttle as explained previously.

In a version of the invention illustrated in FIG. 6, the structure of the sarcophagus is made of lightened concrete 26, reinforced and prestressed, and comprises compartments 4 which are filled in the same manner as before, with a fluid of density lower than that of sea water according to the invention. Concrete 26 is advantageously made from light aggregates, such as, for example, expanded clays, associated with high resistance mortars, which gives them excellent behavior at great depth, even at depths of 3000 to 4000 m, see more. In fact, expanded clays are substantially spherical in shape and have voids filled with air or gas, which gives them a low density; taken within a matrix made of high resistance mortar, it is the matrix itself which ensures the overall resistance. When the structure is subjected to a very high pressure, for example the pressure of 400 bars prevailing at about 4000 m deep, the water will migrate over time within the mass of concrete and then gradually invade the clay aggregates expanded, which will significantly increase the apparent weight of the sarcophagus. This migration process being relatively slow does not present any drawback during installation, because, after towing on site, the critical operation of lowering said sarcophagus, from the surface, to its final position resting on the bottom at - above the wreckage, represents a maximum duration of 12 to 24 hours. Once in place, the self-weight of the sarcophagus increases day by day, which increases its stability, the phenomenon of water migration continuing over several weeks, even months. To delay the phenomena of water migration towards porous aggregates, the entire walls of the concrete structure in contact with water are advantageously covered with a layer of paint of elastomer type, thus creating a sealing barrier. effective. This layer is advantageously also applied inside the buoyancy compartments integrated into the concrete structure, to minimize the migration of the buoyancy fluid to said aggregates. In a preferred version of the invention, it is advantageous to use a buoyancy fluid according to the invention, of very low density, which correspondingly reduces the overall volume of the buoyancy compartments to be provided. To this end, a gas is advantageously used, the critical point of which is above ambient temperature, for example butane, propane, ammonia, or any other gaseous similar 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 liquefy when they are compressed to a few bars. It is thus very advantageous to use them as buoyancy fluid because their yield ω (buoyancy / self-weight) is much higher than the fluids commonly used, such as diesel, methanol or even fresh water.

Indeed, for a diesel fuel with a density of 0.85: ω = 1.21, for methanol: ω = 1.30, while for butane, propane and ammonia, the values of ω are respectively ω = 1, 71, ω = 1 , 97 and ω = 1.63.

However, the compartments must be filled in a special way to avoid any risk of incident and accident. In fact, 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, so that the fluid remains in the liquid form, the temperature of said fluid must be kept much lower than room temperature, for example -0 ° C to -50 ° C depending on the gas.

When they are stored at room temperature, generally 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 temperature is very difficult to achieve, even almost impossible, in the case of a large buoyancy volume, because it is imperative to prevent the gas from heating up. Indeed, the fluid heating up starts to boil and the pressure inside the tank increases. If the tank is tight, then it must be able to withstand the maximum gas pressure; if the tank is not sealed 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. Commercial butane or propane gas cylinders and tanks are capable of withstanding very high pressures, but their weight remains high and it would not be interesting to use them as they are, because the buoyancy efficiency ω would be greatly degraded by the weight of said containment means constituted by the self-weight of said tank capable of withstanding the pressure. One could consider the use of composite material tanks, the density of which is close to that of water, but they are expensive and complex to manufacture as soon as their unit volume becomes large.

Thus, to contain the buoyancy fluid in the liquid state, an envelope is used, rigid or flexible, capable of confining said gas, the filling of said envelope being carried out in a submarine at a depth of water such that the hydrostatic pressure at said depth of water corresponds to a stable liquid state of the buoyancy material whose temperature is less than or equal to ambient temperature. In general the sea water temperature varies from 3 ° C to 25 ° C, or more, depending on the geographic area, the time of year and the depth considered, and can drop to -5 or even -7 ° C in specific arctic areas.

For this purpose, for a heavy structure such as a sarcophagus or other, we proceed as follows:

- after construction on land or in a dock, we put in the water, the heavy structure or the sarcophagus 1, then,

- Supporting said heavy structure or said sarcophagus 1 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 the figure 7a, 10a and 10b, the sarcophagus being connected to each of said barges 27 by a cable 28 connected to a winch 28ι, in association with a heave compensator 29 aimed at avoiding cable breaks 28. The compartments 4 or rigid envelopes 19ι ( on the left figures 10a-10b) are filled with water and the flexible envelope 19-t, of the balloon type being empty of air and water is picked up on itself as illustrated on figure 10a (on the right), and

- transported into the open sea to the installation site, then, as explained in Figures 7b and 11b, we descend the structure or the sarcophagus 1 to a depth of 20m to 60m, corresponding substantially to a pressure of 2 to 6 bars, pressure at which the butane gas, which will be injected into the compartments 4, and tanks 19ι is liquid. We then descend, then connect, a line 23 at the high point 4 ₄ of the buoyancy compartments and valves 19 ₂ , 19 ₄ of the buoyancy elements 19, and the liquid gas stored on board is injected under pressure. a specialized gas ship 61, known to those skilled in the art. The lower orifice 4 ₅ of compartment 4 being open, the liquefied gas drives out the sea water which is there, and gradually fills the entire compartment 4. At the end of filling, the upper valve 4 ₄ is tightly closed. One fills the flexible balloon 19ι 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 retaining 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 play the role of stabilizer during the whole descent to the bottom of the sea.

In Figure 7b, the four 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 _{of 5,} wherein is in the open position. In FIG. 10b, the compartments 4 constituted by the tubular profiles of the support structure, as well as the rigid buoyancy element 4-19, 19ι on the left, are filled with buoyancy fluid in the liquid state, the element of right buoyancy with flexible envelope of the balloon type being in the course of filling of said fluid.

At the end of installation, one can be satisfied with slightly opening the upper orifice 4 situated at the top of each of the buoyancy compartments 4, which lets the gas escape in liquid form. It then rises naturally towards the surface, first in liquid form, to finally gasify on the surface and dilute in the atmosphere. These gases are harmless for the environment and personnel, as long as the instantaneous quantities are reasonable, i.e. represent a few tens, even a few hundred kilograms per hour, but it is preferable for ecological reasons to recover the cargo. liquefied gas. To this end, a bottom-surface connection 23 is installed, as already explained in FIG. 2, connection which connects the upper orifice 4 ₄ , 19 ₂ of the compartments and buoyancy elements to the gas ship 61 situated on the surface. This connection makes it possible to recover almost all of the gas cargo in a very short time, because the gas in liquid form has an extremely low viscosity. And, due to the very great depth of water, the differential pressure between the interior of said pipe and the exterior is considerable, because the pressure differential increases by approximately 4 MPa for every 1000m of additional depth for a fluid butane-type buoyancy, the density of which is approximately 0.6 compared to seawater.

In the case of a heavy structure, for example wellhead elements, or petroleum treatment or pumping units which have to be lowered to the bottom of the sea, the load-bearing structure of the equipment is advantageously produced. using tubular profiles, rather than I, U or H profiles, as is commonly practiced. Said tubular sections are sealed, then are filled with liquefied gas in the same manner as explained above with reference to FIG. 7b, through orifices provided with valves provided for this purpose.

The tanks or balloons 19β of the oil processing unit are also advantageously used as a rigid envelope capable of receiving liquefied gas and purge after installation and before starting the oil processing unit installed on the seabed.

The additional buoyancy elements 19 are advantageously made from a flexible envelope constituting a balloon functioning as an airship, as shown in FIG. 10b. The envelope is flexible and waterproof, preferably in the form of an inverted drop of water, or even of spherical shape when it is full. It is connected to said heavy structure by a bundle of cables 59, preferably surrounding said flexible and waterproof envelope, said bundle of cables 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, at said heavy structure 1. The filling of said bladder is carried out in the same manner as explained in FIG. 7b and the emptying at the end of installation and carried out by simple opening of the valve 19.4 connected to a pipe 23.

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

The preferred gases which can be used as buoyancy fluid are classified in table 1 below in order of increasing density, in the liquid state, at the temperature of 15 ° C.

The vapor pressures indicated in Tables 1 and 2 are absolute pressures, therefore relative to the vacuum.

The corresponding depth is indicative and corresponds roughly to an atmospheric pressure of 0.1 MPa and to sea water of density 1.026 compared to fresh water. TABLE 1

The gases are classified in Table 2 below in order of vapor pressure at a temperature of 15 ° C.

TABLE 2

In the case where the fluid storage vessel 61 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 effect the transfer of said fluid to the balloon or the tank, the procedure is slightly different from that which has been explained above. The fluid is extracted from the cryogenic tanks by a pump, then passing through a sea water heat exchanger, will reheat to a temperature close to said seawater, for example 15 ° C. at the outlet of the heater. It will then descend to the bladder or to the tank through line 23 and, since from the pump to the balloon, the pressure in the line is greater than vapor pressure at 15 ° C (0.77 MPa in in the case of propane), the fluid remains in the liquid state.

The recovery of the gas at the end of installation of the heavy structure then requires the use of a liquefaction unit, since the fluid coming from the ultra deep seabed is at a temperature of around 4 ° C. and must be cooled, in the case of propane, to 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 which risk blocking the pipes or preventing the 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 start of filling of a said rigid or flexible envelope, a volume of methanol is injected, for example 100 or 200 liters, so that that methanol, of intermediate density between seawater and liquefied gas, creates a screen avoiding direct contact between 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 sealed compartments are positioned and sized so as to comply with 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 to be stable and therefore does not risk overturning by pivoting around its axis XX '. To this end, it will be advantageous to add external floats 19 preferably located above the structure of the sarcophagus and, possibly, ballast at the bottom. In FIGS. 8a to 8d and 9 is shown a shuttle tank 32 of the type used to recover effluents 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 consists of a flexible and watertight side wall 34, for example of high strength reinforced plasticized fabric, secured in the upper part of a dome 3 with circular horizontal section and with shaped vertical section profile. shell made of a resistant and rigid material, preferably of composite material, and integral in the lower part of a bottom 35, flat, solid, resistant and rigid, preferably circular, also preferably of composite material, so as to represent a minimum apparent weight in water, while guaranteeing rigidity and extreme resistance. Said bottom 5 is pierced in its center with a main orifice 35ι and is equipped with a valve, preferably with full passage, for example of guillotine type, the latter being equipped with a flange. A lateral complementary orifice of smaller diameter is provided with a valve 35 ₂ , thus allowing the exchange of sea water between the interior of the shuttle tank and the marine environment, and in particular when filling the tank with petroleum, to the 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.

The apparent weight in the water of the shuttle tank 32 is advantageously adjusted by incorporating buoyancy, for example syntactic foam 3ι, made of glass microspheres coated in epoxy, polyurethane resins, in the highest part of the dome 3. or others.

Thus, the shuttle tank 32 is lowered towards the wreck or tank 6, or even towards a sarcophagus 1 placed above a said wreck or tank, in the picked up position, and has a very low apparent weight in water and which can be adjusted in positive as in negative, which facilitates its installation directly by an ROV (automatic submarine piloted from the surface and equipped with manipulator arms). FIG. 8 illustrates the ascent of the shuttle tank 32 is controlled by a connecting cable 12 of which a portion of its lower portion 13 is weighed down, for example, by metal blocks 31 secured to said cable 30 by a crimping in 311 in a chain like beads on a cable.

As shown in FIG. 8d, these pearls 31 have a cylindrical prismatic or revolutionary central body and, frustoconical ends such that when the cable is bent, said frustoconical ends of the two adjacent pearls then come into abutment one against the another in 31 ₂ , thus limiting the local radius of curvature to a value greater than Ro. Thus, the connecting cable 12 being hooked to the shuttle tank 2 on said first attachment point 36 in the lower part of the tank, descends downwards then deviates in an arc of circle of radius Ro, to finally go up vertically or in chain configuration at a distance of at least about 2R ₀ from the side wall 4 of said shuttle tank, thus avoiding any mechanical contact during the ascent, which avoids damaging it by friction.

In FIG. 8a, the buoyancy of the shuttle tank filled with hydrocarbons F _v which corresponds to the buoyancy exerted on the tank and its cargo, is compensated by the weight of the cable up to the point of horizontal tangency corresponding to the pearl 31 j, added to the weight of the pearls 31g between the reservoir and the lowest pearl 31 i, that is to say 8.5 pearls in FIG. 11a, the weight of the assembly P _e then corresponding to an equilibrium of the system .

By way of example to illustrate FIG. 8a, the shuttle tank with a volume of 250 m3 of oil with a density 1011 kg / m ³ , in seawater at 3 ° C. with a density of 1045 kg / m ³ , has a buoyancy of approximately 8.5 tonnes.

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

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

Similarly, in FIG. 8c, the upper end of the connecting cable 12 is deflected, which has the effect of bringing the pearl 31 _k into the low horizontal position, thereby increasing the number of pearls weighing under the reservoir at 10.5 pearls, the overall weight then being equal to P +. The resultant F _v + P + is then positive downwards and the shuttle reservoir can descend again until the balance of forces in FIG. 8a is reached.

Thus, the stabilization device according to the invention has a stabilizing effect for raising the shuttle tank. When the surface vessel moves excessively under the effect of the swell or deviates from the vertical of the position of the shuttle tank, the movements have an instantaneous effect only on the zone of the pearls surrounding the pearls 31 _g at 31 _k , the pearl 31 i corresponding to the average value of the oscillations.

Thus, to control the ascent of the shuttle tank 32, it is sufficient to wind the connecting cable on the winch located on board the surface ship 20 at a speed compatible with the natural ascent of said shuttle, said shuttle always seeking naturally to resume its equilibrium position illustrated in FIG. 8a. In the event of difficulties, it suffices to slow down or stop the winding on the winch, the shuttle tank then finding its position of equilibrium almost immediately, pending a new movement of the winch.

FIG. 9 represents a shuttle tank 32 ° installed vertically with an evacuation device 9 equipped with a valve provided on the upper wall of a sarcophagus 1 to which it is connected by a connection 50. When the valve is in open position, it lets through the crude oil accumulated in said sarcophagus after having drained from the tanks of the ship 6. Thus, it can be collected in the shuttle tank, which can rise to the surface once filled and rupture of the link 50, the surface ascent under the control of a stabilization and control system for ascent and descent according to the invention. The sarcophagus 1 is equipped with a stabilization and control device with connecting elements 12 made up of cables, the lower portion of which comprises metal blocks 31 in a chain.

The device for controlling the descent or ascent of a heavy or massive structure has been described as consisting either of a cable provided with blocks or pearls crimped on said cable, or of chain with links modified so as to create by simple stop between links, the minimum radius of curvature R ₀ . However, it remains in the spirit of the invention if said heavier portion of said connecting elements consists of a chain of heavier bars hinged together, so that the deformation of the chain of hinged bars creates the load imbalance, P + or P- with respect to the equilibrium load Pe, as described above with reference to FIGS. 8a, 8b and 8c, said bars advantageously having, at the joints, mechanical stops which make it possible to limit the curvature to a value minimum Ro.

FIG. 11 shows a heavy structure consisting of a device 1 for placing 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 support structure 54 consisting of a mechanically welded parallelepiped frame supporting itself:

- a 54ι drilling body comprising means for actuating in translation and in rotation a hole saw 55 which, through a corresponding opening provided in said base, allows to drill a large orifice in said wall 6 so as to allow the evacuation of a fluid contained in said tank, and

- side carriages 56 comprising means for actuating in translation and in rotation hole saws 57 capable of drilling holes in said wall 6 to anchor the base 52 on said wall, the hole saws 57 moving through orifices 58 of said base.

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

The device 1 is suspended by a link 59 from a buoyancy element 19. A connecting element 12 of the cable type with a lower portion 13 comprising weighting blocks 31 arranged in a chain as mentioned above, which extends from a support floating on the surface up to the level of a hooking 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 near the wall 6, in accordance with 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 towers installed on oil fields in significant water depths, from 1000 to 3000m or even 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, is of most interest at significant depths. It is particularly advantageous for abyssal depths, for example 10 000 or 11 000 m, or beyond, because it is almost incompressible, that is to say that its volume does not vary appreciably when the water depth, therefore the pressure increases. In fact, for very great depths (4000-5000m and more), its volume is reduced by a few%, but seawater, also almost incompressible, also sees its density increase significantly. As the volume of buoyancy fluid decreases and the density of seawater increases, this results in a slight variation in buoyancy, and therefore buoyancy, which is automatically compensated for by the link (s) 12, 13 as described above, and the equilibrium point of which will vary slightly as a function of said variation in buoyancy.

Claims

1. Use of a buoyancy fluid with a density lower than that of sea water confined in an envelope (4-ι, 19ι) rigid or flexible tight, to constitute an immersed buoyancy element (4, 19), 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 submarine depth at which said buoyancy element is immersed.

2. Use according to claim 1, characterized in that said buoyancy fluid is naturally in the stable liquid state when it is placed at an underwater depth of 10 to 500 m, preferably from 20 to 100 m

3. Use according to one of claims 1 or 2, characterized in that said buoyancy fluid is an almost incompressible fluid and has a density in the liquid state, from 0.3 to 0.8, preferably from 0 , 5 to 0.7.

4. Use according to one of claims 1 to 3, characterized in that the said gas is chosen 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. Use according to claim 4 characterized in that said compound is chosen from the list: ammonia, ethane, butane, propane, ethylene, propene, butene, acetylene, methyl acetylene, propadiene and butadiene.

6. Use according to claim 5 characterized in that said compound is chosen from ammonia, propane and butane.

7. Use according to one of claims 1 to 6, characterized in that said envelope is constituted by, or placed inside the walls (4ι) of a compartment (4) of a structure (1) immersed .

8. Use according to one of claims 1 to 6, characterized in that said envelope (19ι) is placed outside a submerged structure (1) to which it is connected or fixed.

9. Use according to claim 8, characterized in that said immersed structure (1) is suspended from said buoyancy element (19) by at least one cable (59).

10. submerged buoyancy element (4, 19) giving buoyancy to an immersed structure (1) to which it is connected or fixed or in which it is integrated, characterized in that it comprises a said envelope (4ι, 19ι ) submerged in which said liquefied compound is sealed in accordance with the use of one of claims 1 to 9.

11. buoyancy element according to claim 10 characterized in that it comprises a said flexible envelope (19ι), preferably of hydrodynamic profile shape minimizing the forces during its vertical movements when it is filled with said buoyancy fluid such as defined in claims 1 to 6.

12. A method of setting up between the surface and the seabed a buoyancy element according to claim 10 or 11 characterized in that said fluid is stored in a tank on a vessel (61) on the surface at compressed or cooled liquid state, and it is injected in the liquid state into a pipe (23) from the surface or it is stored (61) as far as a so-called submerged envelope (4-ι, 19-ι) at a depth underwater at which the underwater pressure is greater than or equal to the vapor pressure of the gas corresponding to said compound at the temperature at said depth.

13. Method according to claim 12 characterized in that said envelope (19-ι) is a flexible envelope which is lowered to the desired depth when empty folded back on itself.

14. Method according to claim 12 or 13 characterized in that said envelope (19ι) is previously filled with sea water or another fluid preferably a liquid compound at atmospheric pressure and temperature, incompressible such as diesel, fresh water, or methanol, and seawater or said other fluid is removed from the envelope as and when filling said buoyancy fluid as defined in claims 1 to 6.

15. The method of claim 14 characterized in that said envelope is previously filled with sea water and before filling with a said buoyancy fluid, a limited amount of methanol is injected capable of preventing the formation of hydrates.

16. Method according to one of claim 14 or 15 characterized in that the said envelope is filled on the surface with the aid of said other fluid, and the said envelope thus filled is lowered to a depth where the hydrostatic pressure corresponds to the pressure at which said buoyancy fluid is then injected into said envelope as and when said other fluid is removed.

17. Method according to one of claims 12 to 16 characterized in that said buoyancy fluid is stored in the liquid state cooled in a cryogenic tank and at atmospheric pressure and is injected in the liquid state under pressure into said envelope immersed 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 brought substantially to that of sea water at the depth of said submerged envelope before filling.

18. Device for stabilizing or controlling the descent or ascent of a structure (1, 32) between the surface (15) and the bottom (7) of the sea, comprising or connected to a buoyancy element (4, 19 ) according to one of claims 10 or 11, characterized in that it comprises at least one connecting element of the cable or chain type (12) of which: a first end is connected to a winch (12-ι) on board a floating support or vessel (20a, 20b) on the surface, on which winch it is wound, and π a second end is connected to a hooking element (10, 36) on said structure (1, 32), or on at least one first buoyancy element (19) according to one of claims 10 or 11 connected to said structure, and

H the length of said connecting element (12) is such that said winch (12-ι) is capable of winding or unwinding said first end of said connecting element (12), so that a lower portion (13) of said element link (12) can hang below said hooking element (10,

36).

19. Device according to claim 18, characterized in that it comprises at least two said connecting elements (12), said hooking elements (10,36) being preferably arranged symmetrically respectively around and on the periphery of said structure (1, 32).

20. Device according to claim 18 or 19, characterized in that said connecting element (12) consists of a cable of which said lower portion (13) comprises weighting blocks (31) arranged in a chain on a said cable, preferably metal blocks secured to said cable by crimping.

21. Device according to claim 20, characterized in that said blocks (31) have a shape such that when said lower portion (13) hanging below said hooking elements adopts a curved shape, two said blocks (30) arranged side by side. side are capable of abutting against each other thus limiting the curvature of said cable

22. Device according to claim 21, characterized in that the curvature of said cable is limited so that the minimum radius of curvature (Ro) of said cables at said lower portion (13) makes it possible to maintain a minimum distance (2xRo ) between said cable (12) and said structure (1, 32) sufficient to prevent any mechanical contact between them during a said descent or ascent of said structure.

23. Device according to one of claims 20 to 22, characterized in that said blocks (31) have a cylindrical central part (31) framed by two frustoconical ends (31 ₂ ) whose axes correspond to the direction of said cable (12 ) when the latter is arranged linearly, two adjacent blocks being in contact at said frustoconical ends along a generator (31 ₂ ) desdiles frustoconical ends in the curved parts of said lower portion (13).

24. Device according to claim 18 or 19, characterized in that said connecting element comprises a chain, said lower portion (13) of which comprises heavier links than those of the rest of the chain, and preferably more bulky so as to limit the possible curvature of the chain.

25. Device according to one of claims 18 to 24, characterized in that said first buoyancy elements (19) are arranged above said structure.

26. Device according to one of claims 18 to 25, characterized in that said structure comprises second buoyancy elements (4, 33), preferably according to claim 10 or 11, integrated in said structure (1, 32), preferably still integrated above said hooking element (s) (10, 36) so that the center of gravity of said structure and said first buoyancy elements according to claim 10 or 11 is located below the center of thrust acting on all of said structure (1) and said .premas buoyancy elements (19) according to claim 10 or 11.

27. A method of lowering or raising or stabilizing a structure (1, 32) between the surface (15) and the sea floor (7) using a device according to one of claims 18 to 26 , characterized in that steps are carried out in which the said connecting element (s) is unwound or wound up at their said first end (s) using said winch (s) ( s) (12-ι) and the speed of descent or respectively ascent is controlled by regulating the speed of unwinding or respectively winding of (s) dif (s) connecting element (s) (12) at ( s) said (s) winch (s) (12ι), so as to adjust the length of said lower portion (13) of (s) said (s) connecting element (s) (12) during below the (s) said attachment element (s) (10, 36), the descent, ascent or stabilization of said structure being obtained when, respectively, the sum of the weight of the part of the portion (s) said portion (s) ) lower (s) (13) of the connecting element (s) (12) between on the one hand, the said point (s) of attachment to the said element (s) (s) attachment (10, 36) and, on the other hand, the lowest point of the (or) said (s) portion (s) lower (s) (13), added to the weight of the whole of said structure (1, 32) and of said buoyancy element (s), according to claim 10 or 11, is respectively greater than, less than or equal to the Archimedes thrust exerted on said structure (1, 32) and of (s) said (s) first (s) buoyancy element (s) (19) according to claim 10 or 11.

28. The method of claim 27, characterized in that said structure is a rigid structure of steel, metal or composite synthetic material containing at least one, preferably a plurality of sealed buoyancy compartments (4) capable of forming a said element of buoyancy according to one of claims 10 or 11, said compartment being equipped with at least one filling orifice (4ι) and preferably with at least one evacuation orifice (4s), said sealed compartments (4) being of preferably distributed symmetrically in said structure.

29. Method according to one of claims 27 or 28, characterized in that said structure is a massive structure constituted by a receptacle (1) with an open base, hat-shaped, comprising a peripheral side wall (2, 2a, 2b , 2 ^ surmounted by a ceiling wall (3, 3a, 3b), capable of completely covering a wreck (6) of a ship at the bottom of the sea (7) to recover polluting effluents (8) s' by escaping, said receptacle comprising at least one discharge opening (9) of said effluents contained in the interior volume of said receptacle; said discharge orifice (9) being preferably located at the level of the ceiling (3, 3a, 3b) of the receptacle.

30. Method according to one of claims 28 or 29, characterized in that said receptacle is constituted as an inverted ship hull with double walls, said watertight compartments (4) being defined by spaces delimited by said double walls and elements structure (4 ₃ , 4 ₆ ) bringing together the double walls (2, 2a, 2b, 2-ι, 3, 3a, 3b).

31. Method according to one of claims 27 to 30, characterized in that the rigid structure comprises hollow tubular profiles defining watertight compartments (4) and forming said buoyancy elements according to claim 10 or 11.

32. Method according to one of claims 27 to 31, characterized in that said structure is equipped externally: - hooking elements (10, 14ι) allowing to hang said buoyancy elements and said cables ( 12, 14) or said chains (13) allowing the descent of said structure from the surface (15), and its installation and, if necessary, its anchoring (15-ι, 15 ₂ ) at the bottom of the sea ( 7), and

- Preferably propellants (16), more preferably orientable propellants, allowing the movement of said structure in a horizontal direction to position it.

33. Method according to one of claims 27 to 32, characterized in that the successive stages are carried out in which:

1) said watertight compartments (4) are completely or partially filled with a said buoyancy fluid to constitute a buoyancy element according to claim 10 or 11, and the filling rate of said watertight compartments (4) is adjusted so as to position said structure (1) in equilibrium in immersion near the surface, and

2) the said structure (1) is lowered into the desired position using a descent control device according to one of claims 16 to 24, so as to regulate the speed of descent of the receptacle and ensure the balance of the base of said substantially horizontal structure during descent, and

3) when said structure (1) is submerged to the desired depth, said sealed compartments (4) are filled with a said fluid lighter than sea water, which is recovered at the surface, and simultaneously filled said watertight compartments with sea water.

34. Method according to claim 33, characterized in that

in step 1), additional buoyancy is brought to said structure using said first buoyancy elements (19) consisting of additional floats (19) connected to said receptacle, and

- In step 3), when said structure is in the underwater position at the desired depth, said additional floats (19) are lifted.

35. Method according to claim 33 or 34, characterized in that after step 1) and before step 2), when said structure (1) arrives in the desired position, preferably near the bottom of the sea, reducing the lengths of said heavy cables (or chains) (12) for stabilization during (e) s below said hooking elements (10, 10a, 10b) so as to stabilize said structure (1 ) suspended, and

- If necessary, the anchor (14, 15ι-15) of said structure (1) is made at the bottom of the sea (7), then

- We descend completely said (e) s cables (or chains) heavy (e) s (12) stabilization so that all of their weight participates in the stabilization of said structure.

36. Method according to claim 35, characterized in that - in step 1), the said compartment (s) (4) or envelope (s) are filled

(19-0 connected to said structure using seawater or a first fluid lighter than seawater, and

- in step 2), said structure (1) is lowered to a depth of 30 to 60 meters corresponding to a pressure of 3 to 6 bars at which a buoyancy fluid is injected consisting of a liquefied gas under pressure more light than seawater, as defined in one of claims 1 to 6, in the said compartment (s) (4) or envelope (s) (19) from a surface gas vessel (61) to constitute a buoyancy element according to claim 10 or 11.

37. Process for recovering pollutant effluents lighter than sea water, contained in the tanks of a shipwreck (6) lying at the bottom of the sea (7) in which:

1) a said receptacle is put in place according to a method of one of claims 29 to 36, and

2) the effluents recovered are collected inside said receptacle (1) by discharging through said upper discharge orifice (9).