FR2462336A1 - Tank for transporting liquefied gases by sea - incorporates fluid tightness and insulating barriers simply realised, necessitating only small sweeping nitrogen flow - Google Patents

Abstract

A fluid-tight and thermally insulating tank for the transport by sea of liquefied gases, partic. liquefied natural gases having a high methane content, incorporates two successive fluid-tightness barriers. One, the primary barrier, is in contact with the product contained in the tank. The other, the secondary barrier is disposed between the primary barrier and the bearing structure of the ship. These two fluid-tightness barriers are alternated with two thermally insulating barriers. The secondary insulating barrier is constituted of a cellular material fixed against the structure of the ship. The primary insulating barrier is held elastically against the secondary fluid-tightness barrier. The primary and secondary fluid-thighness barriers are constituted by metal plates welded edge to edge by their raised edges being welded to the two faces of a welding flange. The primary insulating barrier is constituted of rigid sheets realised e.g. in wood between which the welding flanges pass. The cellular material of the secondary insulating barrier carriers grooves in which bands are inserted allowing the fixation of the welding flanges perpendicular to the plane of the cellular material.

Description

C WE WATERPROOF AND THERMALLY INSULATING PERFECTED, INTECREE
TO THE CARRIER STRUCTURE OF A VESSEL.

 The present invention relates to the production of sealed and thermally insulating tanks intended for the transport by sea of liquefied gases and, in particular, for the transport of liquefied natural gases with a high methane content.

In French patents 1,438,330, 2,105,710 and 2,146,612, the production of a sealed and insulating tank has already been described, integrated into the carrying structure of a ship and constituted by two successive sealing barriers, a primary, in contact with the liquefied gas transported, and a secondary, disposed between the primary barrier and the ship's supporting structure, these two sealing barriers being alternated with two layers of thermal insulation called insulating barriers. the primary and secondary insulating barriers are constituted by parallelepipedic boxes filled with an insulating material and the primary and secondary sealing barriers are constituted by metal strakes, for example invar, welded with raised edges on either side of a welding wing.

 In French patent application 2,178,752, it has already been proposed to replace the boxes constituting an insulating tank barrier, with a continuous layer of insulation formed for example from a plastic foam. This replacement improves the cost price of the tank but has the disadvantage of making it fairly difficult to degas a tank when a leakage incident has occurred. Indeed, the transported liquefied gas it comes into contact with the thermally insulating plastic foam, penetrates into the cells of this foam and it is very difficult to carry out the complete extraction thereof before proceeding with a repair of the defective sealing barrier. French patent application 77-39 668, to replace the wooden boxes filled with thermally insulating cellular material, by plastic foam panels which have, on their faces facing the interior of the tank, plywood sheets constituting a protection against the penetration of liquefied gases transported in the tank, in the event that an incident occurs at the level of the sealing barriers of the tank.

This technique constitutes appreciable progress in terms of ease of degassing in the event of an incident, but has the disadvantage of being relatively difficult and costly to produce, it being understood that the presence of plywood plates on the surface of the panels plastic foam does not completely prevent the penetration of gas transport in the cells of the plastic foam constituting the composite panels of the primary thermally insulating barrier.

 The object of the present invention is to describe a sealed and insulating tank, integrated into the carrying structure of a ship, in which the primary insulating barrier is of simple construction and presents no difficulty in degassing, in the event of an incident at the level of the primary sealing barrier.

The thermal insulation characteristics of this primary insulation barrier are lower than those of the primary insulating barriers constituted by wooden boxes filled with heat-insulating material or constituted by plastic foam panels, but this reduction in thermal insulation is compensated dbe to the primary barrier by an increase in the thermal insulation characteristics of the secondary insulating barrier. According to the invention, the secondary insulating barrier consists of a large thickness of plastic foam and this use of the plastic foam is not a problem. even in the event of damage, due to the fact that the secondary insulation barrier is protected on the one hand, by the primary sealing barrier and on the other hand by the secondary sealing barrier; however, when there is a leak in the tank, it comes from a rupture in the sealing of the primary sealing barrier but generally does not reach the secondary sealing barrier. , between the primary sealing barrier and the secondary sealing barrier, a primary insulating barrier is interposed consisting of rigid plates, made for example of plywood, these plates having the advantage of perfectly supporting the primary sealing barrier and to distribute the localized forces that the primary sealing barrier can undergo over a large area of the secondary barrier. It can therefore be seen that, in the embodiment according to the invention, the risks of rupture of the primary sealing barrier are minimized, the difficulties in degassing the primary insulating barrier are completely eliminated and the insulation characteristics of the element are maintained. by simply increasing the thickness of the secondary insulation barrier. In addition, since it is thus possible, without particular difficulties, to use a secondary insulation barrier made of plastic foam, the cost price can be limited. tanks, which constitutes an important economic advantage for the production according to the invention. In addition, the material constituting the primary insulating barrier being a solid non-cellular material, it is possible to reduce the flow rate of the blowing in of blown nitrogen for safety reasons between the primary and secondary sealing barriers, which constitutes an additional advantage. .In terms of mechanical resistance, the structure according to the invention has, as has been said above, advantages, since the insulating layer, which supports the primary sealing barrier, allows better screw resistance. vis-à-vis the shocks produced on the walls of the tank by the movements of the liquid during transport, movements which are due to the rolling and pitching of the vessel during transport. If, however, damage occurs at the level of the primary sealing barrier, it is relatively easy to carry out a repair weld since the metal strakes of the primary sealing barrier bear on wooden plates which support relatively The temperature rise is quite good: on the contrary in previous embodiments where the primary insulation barrier included a layer of plastic foam, there was a difficulty due to the low thermal resistance of the foam during the repair weld.

 The present invention therefore relates to a sealed and insulating tank integrated into the carrying structure of a ship, said tank comprising two successive sealing barriers, one primary in contact with the product contained in the tank and the other secondary disposed between the primary barrier and the ship's supporting structure, these two sealing barriers being alternated with two thermally insulating barriers, the secondary insulating barrier consisting of a cellular material fixed against the carrying structure of the ship, the barrier primary insulation being resiliently supported on the secondary sealing barrier, the primary and secondary sealing barriers being constituted by metal strakes with raised edges towards the interior of the tank, said strakes being welded edge to edge by their raised edges on both sides of a welding wing, characterized in that the foam material of the insulation barrier secondary comprises parallel rectilinear grooves, where band inserts are fixed, each insert band allowing the fixing of a weld wing substantially perpendicular to the boundary plane of the cellular material, the weld wing being used both for welding the edges readings of the two adjacent strakes of the secondary sealing barrier and of the two adjacent strakes of the primary sealing barrier, the primary insulation barrier interposed between the primary sealing barrier and the secondary sealing barrier consisting of plates rigid substantially parallelepipedic, between which pass the aforementioned welding wings.

In a preferred embodiment, the strip inserts are retained in the grooves, which contain them, by mechanical engagement with the edges of the grooves and / or by gluing in the grooves; each strip insert has an L-shaped cross section, one of the branches of the L coming to bear on the bottom of the groove, which contains it, and having the width of said groove the other branch of the L coming to cooperate by its outer edge with a longitudinal edge of the groove and retaining on its inner edge a welding wing; each weld wing is fixed to a band insert by means of a tongue, the two cooperation zones of the tongue and of the weld wing being folded in a U to form a sliding assembly joint; between the sliding joint and the edge of the groove on which the strip insert does not rest, is disposed a strip wedge held in place by mechanical engagement with the edge of the groove, which is adjacent to it, and / or by bonding to the corresponding strip insert; the constitutive plates of the primary insulation barrier comprise, from place to place, along the welding wings which border them, recesses in which are arranged retaining strips fixed on said plates and welded with raised edges on the wing of welding, which is adjacent to them, to maintain the primary insulation barrier on the secondary sealing barrier, before the installation of the primary sealing barrier; the tank angles are produced by means of a plurality of identical crosspieces, arranged side by side and each consisting of two hooking half-wings and two tank support half-wings, each hooking half-wing being in the extension of a support half-wing to constitute a wing of the cross-piece, the two wings of all the cross-pieces of the same angle connecting at the same edge substantially parallel to that of the tank angle considered, the half-wings for attaching the braces being fixed to the carrying structure of the ship and the braces supporting the braces being secured to the metal vires of the secondary sealing barrier; the cross-pieces of the same tank angle are covered, in the angle of the support half-wings which is oriented on the side of the tank, by an angle iron having the same cross-sectional shape as the faces of the cross-pieces which carry it, this angle iron being made of the same metal as the strakes of the secondary sealing barrier which come to be welded onto it, said angle bar upoan & su each of the dci-ìles supporting the cross-pieces, shims having the thickness of the plates of the barrier primary insulation, these shims being held by bolts; the above-mentioned shims are covered, in the area of the angle of the angle iron on which they are fixed and on the side where the tank is located, with a sheet of the same metal as that of the strakes of the primary sealing barrier, which come to be welded on it; a connection angle is welded to the sheets, which cover two adjacent wedges with a tank angle, to form the primary seal of the angle; the metal strakes of the primary sealing barrier and the secondary sealing barrier, as well as the weld wings and their tongues, the connection angles and the retaining strips are made of thin invar sheet; the rigid plates constituting the primary insulation barrier, as well as the wedges and the insert bands, which cooperate with the secondary insulation barrier, are made of plywood; the braces of the tank angles are made of stainless steel.

 To better understand the object of the invention, we will now describe, by way of purely illustrative and nonlimiting example, an embodiment shown in the accompanying drawing.

In this drawing - Figure 1 shows a section of the primary and secondary barriers of the tank perpendicular to a welding wing; - Figure 2 shows, in perspective with cutaway, the assembly of the various teeth of the primary and secondary barriers to form a tank face - Figure 3 shows, in perspective with cutouts, the assembly of the various elements of the primary and secondary barriers in a tank angle - Figure 4 shows a section of a tank angle, perpendicular to the edge of said angle.

 Referring to the drawing, it can be seen that the carrier structure of the ship is designated by 1, consisting of the double hull or a transverse partition which delimits a sealed and insulating tank intended for the transport of liquefied natural gases. On the wall 1, a secondary insulating barrier 2, made of a thickness of about 20 cm of expanded cellular material, such as polyurethane foam reinforced in three perpendicular directions, has been fixed by any suitable known means. fiberglass yarn. The secondary insulating barrier 2 can be produced, for example, as indicated in French patent application 77-39,668.

The secondary insulating barrier 2 has on the surface, that is to say towards the inside of the tank, parallel grooves 3, inside which are inserted strip-inserts 4 made of plywood. The insert strips 4 have a cross section in L, the branch 4a of the L being arranged flat on the bottom of the groove and occupying the entire width of the latter, while the branch 4b of the L, perpendicular to the first, has a thickness equal to the depth of the groove 3 and a width equal to half the width of the groove 3. The longitudinal edges of the strip 4 cooperate with the longitudinal edges of the groove 3 by mechanical engagement of sawtooth profiles, which maintains the band insert 4 in the groove 3, when exerting on the band insert 4 a force perpendicular to the support structure 1. On the internal edge of the branch 4b of the insert 4, a tongue 5 made of invar sheet 0.5 mm thick is fixed, for example by stapling. The end of the tongue 5, which faces the inside of the tank, is folded in a U shape and it cooperates with the end folded in a U shape with a welding wing 6, to form a sliding joint. allowing translational movements perpendicular to the plane of Figure 1.The weld wing 6 is made of the same material as the tongue 5 and it extends perpendicular to the support structure 1. To avoid that, under the effect of a traction exerted on the welding wing 6 towards the inside of the tank, the sliding joint 5-6 cannot open, we have set up, in the space of the groove 3, which n is not occupied by the band insert 4, a band wedge 7 made of plywood, the edge of this band wedge 7, which comes in the vicinity of a longitudinal edge of the groove 3 cooperating with this edge by mechanical engagement of sawtooth profiles. Maintaining in place the strip insert 4 and the calabash 7 in the groove 3 is also carried out by gluing on the faces parallel to the bottom of the groove 3. This type of mounting makes it possible to secure the the welding wing 6 relative to the secondary insulating barrier 2, which is itself secured to the support structure 1.

 On this assembly, there is the secondary sealing barrier 8 constituted by metal strakes with raised edges made in total of invar of 0.5 mm thick. These strakes 8 have a width between edges of approximately 500 mm and are placed between two consecutive weld wings 6.

The raised edges of the strakes 8 are welded on either side of the welding wings 6 and the secondary sealing barrier of the tank is thus formed.

 On the secondary sealing barrier 8, plywood plates 9 are put in place constituting the primary insulation barrier. The plates 9 have a thickness of 25 mm.

The plates 9 have a width of approximately 495mm and are placed between the two raised edges of a metal strake 8. The plates 9 have a length measured perpendicular to the plane of FIG. 1 of approximately l200inm, this length corresponding only to ease of handling for placement; two consecutive plates 9 arranged between the raised edges of the same strake 8 are put in place with contiguous edges. The plates 9 have, on their longitudinal edge, in the median zone and at the ends of this edge, recesses 10 of approximately 5mm in depth, piece of width and 100mm in length. On the bottom of these recesses lO, the by means of staples 11, retaining strips 12 having a raised edge which comes to bear against a weld wing 6.

The raised edges of the retaining strips 12 are welded on either side of the weld wings 6, so as to maintain the plates 9 in position against the secondary sealing barrier 8.

 There are then shims 13 in the recesses 10, these shims 13 having a thickness of approximately 4.5 mm, so as to constitute, at the level of the face of the plates 9 which is opposite the interior of the tank, a surface substantially continuous.

The primary sealing barrier is then placed on this surface, which is constituted by metal strakes 14 identical to the metal strakes 8. The metal strakes 14 are fixed by welding with raised edges on either side of the welding wings 6. All the welds & raised edges carried out on the welding wing 6 are carried out by means of an automatic welding machine, which ensures, at the time of welding, traction on the welding wing 6, so that after the welding, an elastic force applies the welded element on the underlying elements in the direction of the support structure 1.

The realization, which has just been described and which corresponds to the drawing in FIGS. 1 and 2, makes it possible to produce the flat panels of the tanks: FIG. 3 shows the construction in a tank angle. In the corner of the tank, stainless steel crosspieces designated by 15 as a whole are put in place, these crosspieces each consisting of two perpendicular wings, each wing comprising a support half-wing 16 and a half-wing of attachment 17. The two wings (16,17) of a cross cut at right angles along an edge, which is arranged parallel to the edge of the corresponding angle of the supporting structure 1, which supports the tank. attachment 17 comprise, at their end, cleats l7a, which are fixed on waiting studs carried by the support structure 1.

The area of space between the two attachment half-wings 17 of a spider 15 and the support structure 1 is filled with the plastic foam constituting the barrier of secondary insulation 2. Similarly, the area of space included, of the two sides of the tank angle, between the support half-wings 16 and the support structure 1 is filled with the plastic foam constituting the secondary insulation barrier 2. The width of a cross 15, measured parallel to its edge d intersection, is about 25 cm and the braces 15 are arranged side by side along an edge of the tank. The set of braces is is covered, in the angle which is opposite the inside the tank, by an angle 18 having a square section, said angle being welded by its edges on the two support half-wings 16. The angles 18 are made of invar sheet 0.5 mm thick. secondary insulation barrier 2 has recesses 36 in line with the support half-wings 1 6, so that the plane which limits the secondary insulation barrier 2 towards the inside of the tank is in the extension of the face, which has the angle 18 towards the inside of the tank. The strakes 8 of the secondary sealing barrier can therefore be welded, by their end edge 8a to the angle iron 18. The plates 9, which constitute the primary insulation barrier, cover the strakes 8 and come in support on the wings of the angle iron 18, despite the thickness of said strakes 8 thanks to a step 9a of 0.5mm of the plate 9 provided beyond the edge 8a of the strake 8. The plates 9, which converge towards the edge of the cross-pieces 15 and of the angle iron 18, stop at about 15 cm from the said edge and they are connected to shims 19, which are fixed by bolts on the support half-wings 16 of the cross-pieces 15 and which are produced resin impregnated wood known under the trade name "Permali". The bolts 20 pass through the support half-wings 16, the angle iron 18 and the shims 19 and their nuts are arranged in recesses 19b provided for this purpose in the shims 19, so as not to exceed the plane defined towards the inside of the tank by the shims 19. The bolts 20 are welded by a peripheral flange 20a to the angle iron 18, so as to maintain the seal. The plane defined by the shims 19 towards the inside of the tank is an extension of the plane defined towards the inside of the tank by the plates 9 which are connected with it. In their zone closest to the edge of the tank, the shims 19 comprise a rabbet 19a 0.5 mm thick, in which is housed a covering sheet 21 fixed on the shim 19 by screwing. The sheet 21 is a 0.5 mm thick invar t61e, which is disposed in the rebate 19a and comes back on the edge of the wedge 19, which is closest to the edge of the tank. strake 14 of the primary sealing barrier, which is supported on the plate 9, overlaps the sheet 21 and can be welded by its edge 14a on said sheet 2L and can be welded by its edge 14a on said sheet 21 The sealing of the primary sealing barrier is closed off by means of an angle 22 which is welded to the two sheets 21 of two shims 19 arranged at right angles, in the angle formed by this. The angle 22 is made of invar sheet of 0.5mm of passer.

 It is clear that the embodiment, which has just been described, ensures, including in the tank angles, continuity of the primary and secondary sealing barriers.

The primary insulation barrier is thin but has the advantage, because of its constitution by solid plates, of requiring only a small flow of purging nitrogen and being able to be easily degassed, the characteristics of thermal insulation being compensated at the secondary insulation barrier.

Claims (12)

1 - Sealed and insulating tank integrated into the carrying structure of a ship, said tank comprising two successive sealing barriers, one primary in contact with the product contained in the tank and the other secondary disposed between the primary barrier and the supporting structure of the ship, these two sealing barriers being alternated with two thermally insulating barriers, the secondary insulating barrier consisting of a cellular material fixed against the carrying structure of the ship, the primary insulating barrier being resiliently supported on the secondary sealing barrier, the primary and secondary sealing barriers being constituted by metal strakes with raised edges towards the inside of the tank, said strakes being welded edge to edge by their raised edges on the two sides of a wing of welding, characterized in that the cellular material of the secondary insulation barrier comprises rectilinear grooves pa strips where strip inserts are fixed, each strip insert allowing the attachment of a weld wing substantially perpendicular to the boundary plane of the cellular material, the weld wing being used both for welding the raised edges of two strakes adjacent to the secondary sealing barrier and two adjacent strakes of the primary sealing barrier, the primary insulation barrier interposed between the primary sealing barrier and the secondary sealing barrier consisting of rigid plates substantially parallelepiped between which pass the aforementioned welding wings.  2 - Tank according to claim 1, characterized in that the insert-strips are retained in the grooves, which contain them, by mechanical engagement with the groove edges and / or by gluing in the grooves.  3 - Tank according to one of claims 1 or 2, characterized in that each band insert has a t-shaped cross section, one of the branches of the L coming to bear on the bottom of the groove, which continent and having the width of said groove, the other branch of L coming to cooperate by its outer edge with a longitudinal edge of the groove and retaining on its inner edge a welding wing. U to form a sliding assembly joint. 4 - Tank according to claim 3, characterized in that each weld wing is fixed to a strip insert by means of a tongue, the two zones of cooperation of the tongue and the weld wing being folded in  5 - Tank according to claim 4, characterized in that between the sliding joint and the edge of the groove, on which the strip insert does not rest, e8t disposed a wedge band held in place by mechanical engagement with the groove edge, which is adjacent to it and / or by bonding to the corresponding strip holder.  6 - tank according to one of claims 1 to 5, characterized in that the plates of the primary insulating barrier comprise, from place to place, along the welding wings, which border them, recesses where are arranged laselles holding fixed on said plates and welded with raised edges on the welding wing, which is adjacent to them. 7 - Tank according to claim 6, characterized in that the tank angles are produced by means of a plurality of identical cross-pieces, arranged side to side and each consisting of two half-attachment wings and two half -vessel support wings, a hooking half-wing being in the extension of a support half-wing to constitute a wing of the cross-piece, the two wings of all the cross-pieces of the same angle being connected according to the same substantially parallel to that of the tank angle considered, the half-wings for attachment of the cross-pieces being fixed to the support structure of the ship and the half-wings supporting the cross-pieces being secured to the metal strakes of the secondary sealing barrier.  8 - Tank according to claim 7, characterized in that the crossbars of the same tank angle are covered, in the angle of the support half-wings which is oriented from the side of the tank, by an angle having a cross section adapted to that of the cross, this angle being made of the same metal as the strakes of the secondary sealing barrier which come to weld on it, said angle supporting, on each of the half-wings supporting the cross, shims having the thickness plates of the primary insulation barrier, these shims being held by bolts.  9 - Tank according to claim 8, characterized in that the shims are covered, in the area of ------- the angle of the angle iron on which they are fixed and the 8- tee where the tank is located , of a t81e of the same metal as that of the strakes of the primary sealing barrier> which come to be welded on it.  10 - Tank according to claim 9, characterized in that a connection angle is welded to the sheets, which reopen two adjacent wedges at a tank angle.  11 - Tank according to claims 4 and 10 taken if multandment, characterized in that the metal strakes of the primary and secondary sealing barriers as well as the welding wings and their tongues, the connection angles and the retaining strips are formed of thin invar ttle.  12 - Tank according to claim 5, characterized in that the rigid plates constituting the primary insulation barrier as well as the insert-strips and the wedge strips inserted in the grooves of the insulation insulation are bonded are made of wood plywood.