EP2758302A1

EP2758302A1 - Sea platform having external containers

Info

Publication number: EP2758302A1
Application number: EP12767065.1A
Authority: EP
Inventors: Cyrille Fargier; Philippe MONTROCHER
Original assignee: Saipem SA
Current assignee: Saipem SA
Priority date: 2011-09-19
Filing date: 2012-09-11
Publication date: 2014-07-30
Anticipated expiration: 2032-09-11
Also published as: CN103813957A; JP5727676B2; US9199700B2; FR2980164B1; KR20140050100A; SG11201400295VA; WO2013041796A1; FR2980164A1; JP2014525366A; AU2012311340B2; BR112014006396A2; CN103813957B; EP2758302B1; AU2012311340A1; US20140369765A1; KR101653892B1; BR112014006396B1

Abstract

The present invention relates to a stationary or floating sea platform (1) comprising a plant (1b) for treating a first dangerous and/or corrosive liquid (2a), preferably liquefied natural gas (LNG), on the deck (1a) of said platform, and at least one tank (2) for storing said first liquid (2a), preferably liquefied natural gas, integrated into the hull (1g) of said platform under said deck (1a), characterized in that the platform comprises at least one container (3) situated outside of said platform and situated at least in part, preferably completely, underneath the deck (1a) of said platform on which said plant is supported, said container being attached to said platform, preferably reversibly so (5), said deck comprising or supporting first transfer means (4) enabling the transfer, to said container, of leaking liquid (2a') from at least one part of said plant, especially in the event of a leak.

Description

SUPPORT INSTALLED AT SEA EQUIPPED WITH EXTERNAL TANKS

The present invention relates to a support installed at sea, either at open sea or in a protected area, such as a port, in a fixed or floating manner, that is to say resting or respectively anchored at the bottom of the sea, comprising an installation treatment of dangerous and / or corrosive liquid, preferably liquefied natural gas (LNG), on the bridge of said support and at least one storage tank of said liquid integrated within the hull of said support under said bridge.

This type of support can be in particular an FPSO or FSRU type LNG storage and handling barge or a vessel, in particular with a hull and steel or concrete storage tanks as described in WO 01/30648, such as explained below.

Methane-based natural gas is either a by-product of oil fields, produced in small or medium quantities, usually associated with crude oil, or a major product in the case of gas fields, where it is then in combination with other gases, mainly C-2 to C-4 alkanes, CO 2, nitrogen, and traces of other gases. More generally, natural gas predominantly comprises methane, preferably at least 85% methane, the other main constituents being chosen from nitrogen and C-2 to C 4 alkanes, namely ethane and propane. , butane.

When natural gas is associated with crude oil in small quantities, it is usually treated and separated and then used locally as fuel in boilers, gas turbines, or piston engines to produce electrical power and heat. calories used in separation or production processes.

When quantities of natural gas are large or considerable, we try to transport it so that it can be used in remote areas, usually on other continents. and to do this, the preferred method is to transport it in the cryogenic liquid state (-165 ° C) substantially at ambient atmospheric pressure. Specialized transport vessels called "LNG tankers" have tanks of very large dimensions and with extreme insulation so as to limit evaporation during the voyage.

On offshore oilfields, located far from the coast, petroleum products such as oil or gas are generally recovered, processed and stored on board a floating support such as the so-called FPSO (Floating -Production- Storage-Offloading "), ie floating support of production, storage and unloading. Petroleum products such as oil and / or gas are then transferred to removal vessels that regularly come, for example every week, to recover the production of the field and export it to places of consumption.

When it is a question of transfer of liquefied gas of the LNG type at -165 ° C, the transfer devices comprise at least one forward link line for the liquefied gas, and a return link line, generally of greater diameter. low, to evacuate the gas from the vessels of the removing vessel as filling LNG, especially methane gas, so reliquefier aboard the FPSO as described below with reference to Figure 1A.

Another technical area is the offshore storage of LNG near a site of use, for example to ship the gas on the continent after regasification, or to transform it on site, on board the floating support, in electrical energy for forwarding said electricity to the onshore network. In this case, the ship unloads its LNG cargo and the floating support is called "FSRU" (Floating Storage Regaseification Unit), ie floating storage unit and regasification as described below with reference to FIG. 1B. In the case of LNG, the term "treatment plant" is understood to mean any facility for liquefying natural gas in LNG, any LNG regasification facility and / or any LNG transfer facility between said support and a recovery vessel for LNG. storage of LNG of the LNG type arranged in pairs or in tandem beside said support.

Treatment plants of this type comprise means or components such as pumps, circulation lines, compressors, heat exchangers, expansion devices, in general decompression turbines, cryogenic exchangers, reservoirs and connecting lines and connecting elements. between these different devices.

Leaks of treated and stored liquid, in particular LNG liquefied gas where applicable, may occur either at the level of valves, pumps, exchangers, tanks or pipes, or, more particularly, at the joints of the elements. connections or components, or by breaking one or more of these various components.

LNG leaks are particularly dangerous for three reasons

1. Spilled LNG gasifies rapidly in contact with air and solid surfaces, and mixing with ambient air creates a highly hazardous mixture as explosive in the presence of any spark or hot spot.

2. Equipment carrying or containing LNG (-165 ° C) is made of materials resistant to these cryogenic temperatures, generally nickel-based steels or invar. These special steels are very expensive and in general are not used for supporting elements or for the structure of the structure. FPSO for which general steel is used. But, these common steels become brittle in contact with very low temperatures and lose their mechanical strength, which may lead to the rupture of structural elements, or even the bridge of the FPSO in the event of a significant leak directly on said bridge if the critical areas are not protected by insulating materials, they are very resistant to cryogenic temperatures. 3. The contact between LNG and seawater presents a great danger, because the brutal reheating of LNG (-165 ° C) by sea water at 10-20 ° C in the presence of air, so oxygen creates very significant risks of instant explosion.

The purpose of the present invention is to remedy the consequences related to these problems of liquid leakage, especially liquefied gas, on the deck of such supports at sea.

To this end, the present invention provides a support installed at sea, in a fixed or floating manner, comprising a dangerous and / or corrosive first liquid treatment plant, preferably liquefied natural gas (LNG), on the deck of said support, and at least one storage tank of said first liquid, preferably of integrated LNG within the shell of said support under said bridge, characterized in that it comprises at least one reservoir situated outside said support and located at least partly , preferably entirely below the bridge of said support on which said installation rests, said reservoir being fixed to said support, preferably in a reversible manner, said bridge comprising or supporting first transfer means able to allow the transfer to said reservoir, of leakage liquid flowing from at least part of said installation, especially in case of leakage.

These first transfer means may comprise gutter structures and possibly pipe elements and / or liquid pumping means.

The positioning and the reversible fixing of tanks outside the support according to the present invention has the advantages of:

- Can manufacture said support in a dry dock standard size in width or length, that is to say without requiring oversized dry dock which is advantageous because the size of the dry blocks is a very limiting factor for the size of media and therefore also the storage capacities of the vats they contain, and

- be able to manufacture said tanks separately, where appropriate closer to the site at sea than said dry dock in which the support was manufactured, and to fix said tanks on said support before towing on site thereof or on site after towing and anchoring or laying on the seabed, of said support, and especially

improve the safety on board said support with regard to the risks of an incident and / or an explosion, in particular by rapidly evacuating the leakage liquid outside said support, then draining said reservoir so as to restore all the installations as quickly as possible; in maximum security.

More particularly, said first leak fluid transfer means comprise at least one collection device for said leakage liquid extending from below said at least part of the installation to above a first upper orifice of said reservoir, said collecting device being able to collect said leakage liquid flowing from said part of the installation and direct it by simple gravity to said first upper orifice of said reservoir situated below said collecting device. It is understood that the implementation of said first transfer means avoids any contact of leakage liquid with the bridge of the support among other and more generally the entire structure of the support on the one hand and on the other hand avoids prolonged contact with the atmospheric air.

The present invention therefore advantageously consists essentially in collecting the leakage flows and directing them to external reservoirs, that is to say located outside the barge, and below, so that the flows are made naturally by simple gravity and as quickly as possible and without contact with the structure of the support, especially the bridge, and without prolonged contact with the base of the treatment installations, thereby limiting the amount of LNG that may vaporize by creating a gas mixture explosive in contact with the ambient air. More particularly, said reservoir is reversibly fixed against a plating of said support.

Here, the term "bordered said support", the longitudinal side walls, as well as the transverse walls before (bow) and rear (stern). In this case, reversible attachment means of the tank on the hull may consist of simple hooks for suspending said tank eyelets against said plating. And, said first transfer means may comprise a said inclined bridge sloping downward from the longitudinal median axis XX 'of said bridge towards the edge of said support against which is fixed said tank or inclined gutter.

Indeed, in general, the bridge is itself inclined gently downward from 1 to 2% down to the longitudinal lateral edges of the support and able to allow the flow of a liquid to an evacuation at the level of and above longitudinal edges side. However, this slope may be insufficient to induce rapid flow.

Preferably, said first leakage liquid transfer means comprise a liquid collecting device comprising at least one deck element attached over said deck, said deck element comprising a central plane inclined structure preferably laterally bordered by flanges. lateral, the inclination (oc2) of said central flat structure of said deck member being greater than that (a1) of said deck, if appropriate, preferably of an inclination (oc2) of 1 to 5%, preferably of 2 to 4%.

When said reservoir is fixed against a plating of the support, it is understood that said planking element has an inclination with an uppermost point on the side of the decking closest to said median longitudinal axis XX 'of said support and the lowest point on the side. the decking closest to said plating.

More particularly, the support according to the invention comprises:

a plurality of elements of said decking, all of said decking elements preferably covering the entire surface of the bridge portion supporting a said treatment plant presenting risks of leakage of LNG, and

a plurality of reservoirs, each said decking element cooperating with at least one said reservoir.

It is understood that by "deck member cooperating with a said tank" is meant that said tank is able to collect said liquid flowing from said part of the installation located above said deck member, said deck member directing it by simple gravity toward said first upper orifice of said reservoir located below the lower end of said decking element, preferably via a first transfer conduit member.

A plurality of tanks can thus be installed on the walls of the port and starboard hull, and where appropriate on the rear and front walls, each of them collecting the liquid coming from one or more deck elements covering a limited deck area.

Advantageously, the walls or surfaces of said collecting device likely to be in contact with said leakage liquid which it collects, in particular the upper surface of the central part of said decking, are constituted or respectively covered with a layer of material resistant to cryogenic temperatures (less than or equal to -160 ° C.) of said leakage liquid such as LNG, in particular a composite material such as the Chartek®-Intertherm® 7050 sandwich marketed by the international company (UK) of the AKZO-NOBEL group, more particularly adapted to allow a cryogenic thermal insulation of LNG at -165 ° C.

More particularly, said support according to the invention comprises hooking means able to fix a plurality of said tanks along its edges and each said reservoir has a volume of not more than 300 m ³ , preferably 50 to 300 m ³ .

Tanks of this volume can be manufactured with a relatively lighter structure than that of the internal tanks, because a high level of insulation is not sought, but rather a limited heat transfer through the wall allowing rapid vaporization of the recovered LNG, without however the metal structure of the tank or its supports does not fall below -20 to -40 ° C, which could lead to a brittle fracture of the metal of said structure. More particularly, said reservoir is of elongated cylindrical shape with a vertical longitudinal axis (ΥΥ ') with a portion only said submerged tank including horizontal cross section, square and rectangular or circular.

The term "vertical axis" is understood here to mean that said reservoir axis is substantially perpendicular to the horizontal longitudinal axis XX 'of the support and substantially perpendicular to the sea level when the sea is flat.

Said tank remains partially immersed even when it is empty and part of the tank remains above sea level when said tank is full. The vertical elongated shape of the tanks is advantageous in comparison with tanks of larger horizontal dimension in that an emptying of the LNG by pumping will leave at the end of the pumping phase a balance of LNG proportional to the horizontal section of said reservoir, therefore lower than in the case of large horizontal dimension. In the case of large horizontal dimension, it will advantageously have a tank bottom slope, and the pumping device will be placed at the lowest point.

In a first variant embodiment, said reservoir comprises a bottom constituted by the upper surface of an internal float with a shaped reservoir fitted against the inner periphery of the cylindrical side wall of said reservoir, the lower end of said cylindrical lateral wall defining a lower opening so that said cylindrical side wall is filled with seawater below said float and able to move vertically relative to said float.

It is understood that said float always remains at the surface of the water and therefore that the side wall of the tank moves vertically as a function of the level of the waterline of the support which is a function of the filling rate of said tanks and the said tanks. This embodiment is advantageous in that it makes it possible to implement an even lighter tank structure because the side wall does not have to withstand the hydrostatic pressure of the surrounding sea water, nor to the Archimedes thrust exerted on a tank having a fixed and sealed bottom wall.

In a second variant embodiment, said reservoir comprises a sealed bottom fund wall at the lower end of its cylindrical side wall.

Preferably, the walls of said tank are thermally insulated, preferably internally insulated, in particular with polyurethane foam. This thermal insulation aims to limit the thermal transfer due to the heating of the LNG to maintain the temperature of the steel walls of the tank, including the side walls above sea level, at a temperature above the brittle fracture temperature said steel, especially at a temperature above -10 ° C. Otherwise, said heat transfer could cool the steel structural members of the tank below a temperature where the steel is at risk of brittle fracture, that is to say below -20 to -40 ° C.

Advantageously, said reservoir comprises or cooperates with second transfer means comprising a pump and a second connecting line capable of transferring said leakage liquid contained in said reservoir to a tank, preferably a said tank within the shell of said support.

In particular, said tank has an inner pipe descending to the bottom, the latter being connected to a pump for draining said tank and return the liquefied gas to a storage tank of the floating support. Advantageously, said reservoir further comprises means for heating said liquid contained in said reservoir, preferably said heating means being joule heating means, preferably further said heating means being integrated in or against said cylindrical side wall of the tank or its thermal insulation layer. More particularly, a heating device is a device by electric heating or circulation of hot water or steam. In the case where the liquid phase is pumped out, said heating device is advantageously located in the lower part of the tank, and thus allows, after emptying, to finalize the complete purging of said tank, by vaporization of the residue and elimination of the methane gas. towards a flare, or simply in the open air.

In the case where there is no pumped emptying device, it advantageously has said heating device over all or part of the height of the wall of the tank, and optionally on the bottom of said tank.

More particularly, said tank further comprises a second upper gas evacuation orifice, at the level of the upper part of a lateral wall or upper wall forming a reservoir cover, able to allow the evacuation outside the reservoir of the liquid which it contains after evaporation thereof, preferably with the aid of a third connecting pipe to a combustion flare or to the gas sky of a said tank within the hull, or to the air free. More particularly, said reservoir comprises or is capable of cooperating with a foaming agent injection device, preferably at a third upper orifice, at the level of the upper part of a side wall or upper wall forming a lid. of the tank. This injection of foaming agent aims to create an inert medium inside the tank when the tank begins to fill with LNG. In other words, we inject said foaming agent when a leak is detected in said installation. Preferably, said reservoirs are initially filled with an inert gas such as nitrogen.

Preferably, said support according to the invention is a floating support anchored at sea, or resting at the bottom of the sea, supporting an LNG liquefaction unit and / or LNG regasification and electricity production, said liquid being LNG .

Other features and advantages of the present invention will become apparent in the light of the following detailed description of one or more particular embodiments with reference to the following figures in which:

FIG. 1A is a side view of an FPSO type floating support supporting a processing unit 1b on the bridge 1a, said processing unit 1b including LNG liquefaction equipment 1 2 and LNG transfer means between said support floating 1 and a catching vessel 15; and

FIG. 1B shows a side view of a support resting at the bottom of the sea 12 of the FSRU type comprising a processing unit 1b on its deck 1a, said processing unit comprising regasification and power generation equipment 1-3 and LNG transfer means 1-1 to a stripping vessel 15; and

FIG. 2 represents a top view of a FPSO floating support according to FIG. 1; and

FIG. 3 represents a sectional view in a vertical transverse plane perpendicular to the longitudinal direction XX 'of said floating support at a decking 4-1 and reservoir 3 according to the present invention, said reservoir comprising, according to a first variant, a bottom fully sealed 3b, and - Figures 3A, 3B and 3C show a second alternative embodiment of a tank according to the invention comprising a lower opening 3-4 and an internal float 6 whose upper surface 6a delimits a variable internal volume of said tank; and - Figure 4 shows a sectional view along AA of a decking element 4-1 of Figure 3A.

In FIG. 1A, the floating support of the FPSO type 1 is anchored to the seabed 12 by anchoring lines 10a. It receives natural gas extracted at the bottom of the seabed via the production bottom-surface connection lines 10b. It comprises LNG storage tanks 2a and transfer pipes 1-la here in unloading phase to a ship, here called LNG removal vessel 15, in a so-called tandem configuration. Said FPSO 1 has natural gas treatment and liquefaction equipment 1-2 and LNG storage tanks 2 integrated within the shell. The said FPSO is equipped with a storage and guide device 1-1 for the flexible transfer pipe 1-la for the unloading of the LNG to the removal vessel 15.

In FIGS. 1 and 2, there is shown a floating support of the FPSO type comprising 4 cylindrical tanks 3 of vertical axis YY 'with a square horizontal cross-section on a first lateral edge of the shell, each of which is fixed in a reversible manner with means reversible hooking respectively 5 against each said bordered the. 3 tanks 3 on each plating collect leakage liquid 2a 'from the treatment facility lb resting on the bridge of the floating support. More specifically each of these 6 tanks 3 receives leakage liquid 2a 'collected at a decking element 4-1 collecting the leakage liquid from a portion of said installation 1b. The portion of the bridge supporting the liquefaction unit 1-2 is here covered by a set of 6 decking elements 4-1 covering all said bridge surface portion capable of receiving leakage liquid from said unit 1-2 liquefaction with 3 decking elements 4-1 pouring the liquid to each of said lined the opposite so as to pour the leakage liquid 2a 'respectively in a tank 3 for each decking element 4-1. A seventh decking element 4-1 covers the rear portion of the bridge surface supporting the storage and guide device 1-1 of transfer conduit 1-la disposed near the rear wall ld of the hull lg, apt discharging the leakage liquid from the installation 1-1 to one of the bordered the equipped with a fourth tank of similar shape.

In FIG. 1B, the support 1 of the FSRU type rests on the bottom of the sea 12. It comprises a plant 1b comprising an LNG regasification unit and an electricity generating unit 1-3 including a transformer station for sending the electric current. to the earth. The support 1 comprises storage tanks 2 of LNG. They are here in the loading phase from a LNG-type vessel 15, referred to herein as a supply vessel, in a so-called tandem configuration, said support 1 being also equipped with a storage and guide device 1-1 for flexible driving. 1-la transfer for loading LNG from a supply vessel 15.

In FIGS. 1A and 1B, the support 1 comprises 3 substantially parallelepipedal storage tanks 2 arranged side by side successively in the longitudinal direction XX 'and extending over the entire width of the floating support inside its shell in the cross direction perpendicular to the direction XX '.

In FIG. 3, the steel walls of the tanks 2 are covered with an inner thermal insulation layer 2a-1 made of polyurethane foam and an internal membrane in contact with the LNG in thin stainless steel, resistant at cryogenic temperatures, so that these tanks can be considered as cryogenic tanks capable of maintaining the LNG 2a they contain in the liquid state. Likewise, the external reservoirs 3 have steel walls equipped on their inside with the same thermal insulation material 3-5 so that said tanks can also be considered as cryogenic tanks capable of containing LNG in the liquid state. at -165 ° C with however a much lower level of insulation, so as to promote the vaporization of LNG, without however the structural elements of said tanks do not reach temperatures lower than -20 to -40 ° C, in order to avoid fragile breaks in said structural elements. Thus, in this same figure, the lower portion of the tank (walls and bottom) in permanent contact with seawater may have a low level of insulation, because there will be no risk that the temperature of the structure lower does not fall below -20 ° C, due to direct and permanent contact with seawater at 10-20 ° C. On the other hand, the upper portion emerged permanently or not (variations of draft depending on the load) is in contact with the ambient air, also at 10-20 ° C. Since heat exchange with air is lower than with contact with seawater, it will be necessary to have a more efficient insulation system so that the heat exchange through said water system. insulation do not lead to a cooling of the structural elements such that their temperature falls below -20 to -40 ° C, thereby avoiding the fragile fractures of the steel of said structures.

The outer side walls 1a and 1d and the bottom bottom wall 1c and the bridge defining the shell 1g form the "ship beam", ie the overall resistant structure 1f of the floating support. In FIG. 3, the plane of the bridge is shown as being inclined downwardly from the horizontal median axis XX 'of the support and the bridge towards the plating constituting the side walls. longitudinal dimensions of the shell at an angle of about 1 degree. The upper wall or lid 3c of the tanks is slightly against the bottom of the upper end of the plating. The vertical cylindrical side walls 3a of the reservoirs 3 have a lateral face 3a-1 comprising 2 hooks 5-1 arranged in the upper part and in the lower part able to be suspended on parts having a hollow shape complementary to the said hooks or hinges 5-2 applied against the outer face of said planks 1a so that said reservoir can be suspended and thus hung reversibly when the hooks 5-1 cooperate with said parts 5-2. Collection and transfer of the leakage liquid 2a 'from the treatment plant 1b to the interior of the tank 3 is done with the aid of a collection device consisting of a plurality of deck members 4-1. . Each decking element 4-1 comprises a steel or grating bearing structure 4a covered with a layer of resistant and insulating composite material 4a-1, for example a Charterk®-Intertherm® 7050 sandwich from International (UK) AKZO-NOBEL group. The central portion 4a-1 is inclined at an angle α 2 corresponding to a slope of 1 to 5%, preferably 2 to 4% relative to the horizontal. It is fixed over the bridge la, downhill from the end of the decking closest to the longitudinal median axis XX 'towards the end of the lowest deck reaching beyond the plating. that is, towards the outside of the floating support 1 to above a first upper opening 3-1 through the cover 3c of the reservoir 3. The channeling of the leakage liquid flowing on the deck 4-1 to its lower end towards said first opening 3-1 of the tank is made using a device 4-2 comprising a small upper chamber 4-2a and a lower pipe element 4-2b allowing the leakage liquid 2a 'of flow inside the tank 3.

The tanks 3 are advantageously installed on the hull lg after launching the hull on the shipyard, but before towing on site at sea. In the case of towing on very long distances such as several thousand km, they can be advantageously installed only after arrival on site and be manufactured in a site closer to the site.

In top view as shown in Figure 2 each decking member cooperating with at least one reservoir 3 has a trapezoidal shape with a narrowing of its lateral flanges 4b towards the portion 4-2 of the collection device and transfer to the reservoir 3. It is understood that the flanges 4b are intended to prevent the leakage liquid leaving the central part of the deck 4a and channel the flow of the leakage liquid to the tank 3 by narrowing the width of the central portion 4a towards the bordered.

FIG. 4 is a sectional view along the plane AA of FIG. 3A of a decking element 4-1 comprising a steel support structure 4a, a resistant and insulating composite material 4a-1 and raised edges 4b for channeling the LNG. to the reservoir below.

The deck elements 4-1 thus form gutters collecting and channeling the leakage liquids from the treatment plant ld to and up to the upper orifice 3-1 of the tanks 3. Once the leak or leaks have been controlled, the tanks 3 are filled with leakage liquid 2a 'at variable levels, it is then sought to empty them as quickly as possible, so as to put all the facilities to a maximum level of security. Several variants are possible. According to a first variant embodiment, second transfer means 8 comprising a pump 8-1 for circulating the leakage liquid 2a 'inside a second connecting pipe 8-2 extending through from the proximity of the bottom 3b of the tank to and through the cover 3c and extending beyond for example in the direction and to the gas sky 2a-1 of a tank 2 of LNG storage.

In a second variant embodiment, regasify if the liquid 2a 'is required by means of a heating device 9 which can be immersed in the leakage liquid 2a' inside the tank 3 or incorporated against the side wall of the tank 3 and / or in or against the internal thermal insulation layer 3-5 of the tank 3. Once regasified, the LNG 2a 'can evacuate through a third pipe element 3-6 passing through a second upper opening 3-2 of the tank cover 3c 3. The third driving element 3-6 thus allow the evacuation of the gas, either simply in the atmosphere or towards a combustion flare 14 installed at one end of the floating support as represented in FIG. 2, by way of pipes not shown. Even in the absence of implementation of the heating device 9, the second opening 3- 2 and second pipe element 3-6 can evacuate the gas either in the open air or to the flare, as previously explained. . In the embodiment of FIG. 3, the reservoir 3 comprises a portion which remains always immersed, representing a height of ¼ to ¾ of the height H of the reservoir 3 from its bottom 3b, more particularly from 1/3 to Vi of the height H of the tank 3 from its bottom 3b, that is to say below the sea level 11. It is understood that the height H varies according to the height of the waterline of the hull which varies according to whether the tanks 2 are empty (water line at approximately ¼ of the height of the tank above its bottom wall 3b) or that the tanks 2 are full of liquid 2a (water line at approximately ¼ of the height the tank below its upper wall 3c or about ¾ above the bottom wall 3b). In this embodiment, the buoyancy thrust is exerted on the entire immersed tank volume, and moreover, the structure of said tank must withstand the pressure, especially in its lower part. Thus, the attachment points must withstand efforts mainly directed downwards in the case where the reservoir is full of leakage liquid and where the FPSO is partially or completely empty, and efforts directed upwards when the reservoir is empty and the FPSO is completely full.

In FIGS. 3A, 3B and 3C, there is shown a second embodiment of reservoir 3 in which the lower end of the cylindrical side wall 3a defines a lower opening 3-4 of the reservoir. In this second embodiment, the tank 3 encloses a float 6 whose upper surface 6a delimits the bottom wall of the tank, it being understood however that the float 6 has a shape, in particular substantially parallelepipedal, and an outline, in particular substantially of square section, adjusted to that of the inner surface of the cylindrical side wall 3a so as to allow a vertical sliding of the tank 3 around the float 6 which float has a buoyancy so as to remain permanently substantially at the upper surface 11 of the sea Thus, the height H of the immersed portion of the tank 3 varies as a function of the height of the floating line of the floating support, between a maximum value Hi corresponding to a FPSO whose tanks are full (FIG. 3B) and a minimum value H ₂ corresponding to an FPSO whose tanks are empty (Figure 3C). The portion of the tank between the underside 6b of the float 6 and the lower opening 3-4 of the tank 3 is filled with a varying seawater height 13. This second embodiment has the advantage that the possible empty volume of the tank is always located above the sea level 11 so that the structure of the side walls of the tank may have a relatively low mechanical strength not having to to resist the hydrostatic pressure of the seawater contrary to the first embodiment of Figure 3 in which it may be that a portion of the empty internal volume of the tank 3 is immersed and must withstand the hydrostatic pressure. Likewise, the forces on the fastening elements are no longer alternated as a function of the filling of the FPSO and it can be considered that the forces in the structure of the tank and its supports are substantially constant. In practice, whatever the shape of the internal horizontal section of the reservoir, the outer contour of the float corresponds to this form of the internal section of the tank, it being understood that a game, preferably regular, for example a few centimeters, exists between said float and said inner wall of said tank, over the entire periphery of said float. The disadvantage of this second embodiment of the figures

3A to 3C is that, where appropriate, the available part of the internal volume of the tank 3 tends to decrease in the measurement of the emptying of the storage tanks 2. In this case, the implementation of the second transfer means 8 or means of evacuation 3-6 is more particularly advantageous.

The float 6 is advantageously constituted in known manner of syntactic foams in view of their high mechanical strengths and their excellent behavior at cryogenic temperatures (-165 ° C). Because LNG has a negative temperature of about

-165 ° C, when it falls on the upper surface 6a of the float, the ring of seawater located between the periphery of the float 6 and the tank wall, freezes almost instantly and turns into ice and then block said float 6 against the cylindrical side wall 3a of the tank and thereby seals the bottom of the tank, thereby preventing said float from sinking below the sea level 11 under the effect of the weight of leakage liquid 2a he supports.

Advantageously, the tanks 3 are provided with a third opening 3-3 in the upper part of their cylindrical side walls 3a allowing the introduction by a pipe element 3-7 inside the tank of inert foaming agent from of a foam generator not shown. Thus, in the event of a leak in the installation lb, as soon as the tank begins to fill, the foam generator is actuated so as to confine the LNG and limit the air introductions, the oxygen of the air having risks explosion or fire mixed with natural gas. The presence of this foam does not interfere with the vaporization of the LNG or the evacuation of gas to the outside of the tank as described above. Suitable foaming agents are of the fire-fighting foam type known to those skilled in the art (in English "fire fighting foam") marketed by ANGUS FIRE (UK).

It is understood that said first, second, third openings 3-1, 3-2 and 3-3 are advantageously equipped with an optional closure device such as a valve. Devices for transferring and collecting leaking liquids

2a 'described above allow to collect and quickly eliminate the leakage liquid 2a', disposal can be done in a controlled manner and at will in the various means of transfer and evacuation described above in liquid form or gaseous so as to avoid any risk of explosion or fire, and to restore the installations as quickly as possible in maximum security configuration.

The volume of each of said tanks is sized according to the volumes of LNG for the area covered by the collection devices connected to said tank. We will consider:

- on the one hand the volume of the containers concerned (pipes, tanks, pumps, ...) located between upstream valves and downstream valves, and

- On the other hand the current production volume during a period of time corresponding to the beginning of the leak incident and the effective closure of all valves upstream and downstream concerned, ie in general several minutes.

Thus, the volume of each of the tanks will depend on its location relative to the installation, and may vary in significant proportions, for example from 50 to 300m ³ . Tanks having a lid at the top have been disclosed through which LNG or foam feed lines, and gas discharge lines, pass; but, in a simplified version, said tank does not have a lid. It is then imperative, as soon as a leak occurs, to fill said tank with foam, so as to confine the LNG, the evaporation of said LNG being then directly in the open air, through the thickness of a layer of said foam.

Claims

1. Support (1) installed at sea, fixed or floating, comprising a treatment plant (1b) of dangerous and / or corrosive first liquid (2a), preferably liquefied natural gas (LNG), on deck ( la) of said support, and at least one tank (2) for storing said first liquid (2a), preferably integrated LNG within the shell (1g) of said support under said bridge (1a), characterized in that comprises at least one reservoir (3) located outside said support and located at least partly, preferably entirely, below the bridge (1a) of said support on which said installation rests, said reservoir being fixed to said support, preferably in a reversible manner (5), said bridge comprising or supporting first transfer means (4) able to allow the transfer to said reservoir of leakage liquid (2a ') flowing from at least part of said installation, especially in case of leakage.

2. Support according to claim 1, characterized in that said first leakage liquid transfer means comprise at least one collecting device (4-1) of said leakage liquid extending from below said at least part of the installation (1b) to above a first upper orifice (3-1) of said tank, said collecting device being able to collect said leakage liquid (2a ') flowing from said part of the installation and directing it by gravity towards said first upper orifice of said reservoir situated below said collecting device.

3. Support according to one of claims 1 or 2, characterized in that said tank is fixed reversibly against a plating (the, ld) of said support.

4. Support according to one of claims 1 to 3, characterized in that said first means for transferring (4) leakage liquid comprise a liquid collecting device comprising at least one deck element (4-1) reported by- above said bridge, said decking element comprising an inclined central plane structure (4a) preferably laterally bordered by lateral flanges (4b), the inclination (oc2) of said central flat portion (4a) of said deck member being greater than that (a1) of said bridge if necessary, preferably an inclination (oc2) of 1 to 5%, preferably 2 to 4%.

5. Support according to claim 2 and one of claims 2 to 4, characterized in that the walls or surfaces of said collecting device likely to be in contact with said leakage liquid that it collects are constituted or covered respectively with a layer of material resistant to cryogenic temperatures (4a-1), in particular a composite material.

6. Support according to claim 4 or 5, characterized in that it comprises:

a plurality of elements of said decking (4-1), all of said decking elements covering at least the entire surface of the bridge portion supporting a said treatment plant (1b), and

a plurality of tanks (3), each said deck member (4-1) cooperating with a said tank.

7. Support according to one of claims 1 to 6, characterized in that it comprises hooking means (5,51 - 52) adapted to fix a plurality of said tanks along its edges (le) and said tanks have a volume of not more than 300 m ³ , preferably 50 to 300 m ³ .

8. Support according to one of claims 1 to 7, characterized in that said tank is elongated cylindrical shape of vertical longitudinal axis (ΥΥ ') with only a portion of said reservoir (3) immersed (13).

9. Support according to claims 1 to 8, characterized in that said reservoir comprises a fund (3b) constituted by the surface upper (6a) a float (6) internal to the shaped reservoir fitted against the inner periphery of the cylindrical side wall (3a) of said tank, the lower end of said cylindrical side wall (3a) defining a lower opening (3); -4) such that said cylindrical side wall (3a) is filled with seawater (13) below said float and able to move vertically with respect to said float.

10. Support according to one of claims 1 to 8, characterized in that said reservoir comprises a bottom wall (3b) fixed tight at the lower end of its cylindrical side wall (3a).

11. Support according to one of claims 1 to 10, characterized in that the walls of said tank are thermally insulated (3-5), preferably insulated internally.

12. Support according to one of claims 1 to 11, characterized in that said reservoir comprises or cooperates with second transfer means (8) comprising a pump (8-1) and a second connecting pipe (8-2) adapted to transfer said leakage liquid contained in said tank to a tank, preferably a tank (2) within the shell of said support.

13. Support according to one of claims 1 to 12, characterized in that said reservoir further comprises heating means (9) of said liquid contained in said reservoir, preferably said heating means being joule effect heating means more preferably, said heating means being integrated in or against said cylindrical side wall (3a) of the tank or its thermal insulation layer (3-5).

14. Support according to one of claims 1 to 13, characterized in that said tank further comprises a second upper gas discharge port (3-2), at the upper part of a side wall (3a). ) or upper lid wall (3c) of the tank, adapted to allow the evacuation outside the tank of the liquid which it contains after evaporation thereof, preferably by means of a third connecting line (3-6) towards a combustion flare (14) or towards the gas sky (2-1) d 'a said tank within the hull or in the open air.

15. Support according to one of claims 1 to 14, characterized in that said reservoir comprises or is adapted to cooperate with a foaming agent injection device, preferably at a third upper orifice (3-3 ) at the top of a side wall (3a) or top wall lid (3c) of the tank.

16. Support according to one of claims 1 to 7, characterized in that it is a floating support anchored (10a) at sea or resting at the seabed (12) supporting a liquefaction unit (1). -2) LNG and / or regasification (1-3) of LNG and electricity production, said liquid (2a) being LNG.