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A method of creating a watertightness barrier for a wall of a watertightened thermally insulating tank, involves steps of: arranging a repeating structure including alternately a strip of sheet metal and an elongate welding flange connected to the support surface, so that the turned-up lateral edges

A method of creating a watertightness barrier for a wall of a watertightened thermally insulating tank, involves steps of: arranging a repeating structure including alternately a strip of sheet metal and an elongate welding flange connected to the support surface, so that the turned-up lateral edges of the strip of sheet metal are positioned against the adjacent welding flanges, welding the turned-up lateral edge to the welding flange using a straight welded seam along a first longitudinal portion, continuing the straight welded seam with an end portion which is deviated in the direction of an upper edge corner, and producing a watertight edge corner welded seam along a second longitudinal portion of the strip of sheet metal such that the edge corner welded seam watertightly meets the end portion of the welded seam.

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1. A method of creating a watertightness barrier for a wall of a watertight and thermally insulating tank, comprising: arranging, on a thermally insulating barrier that has a flat support surface for the watertightness barrier, a repeating structure comprising alternately strips of sheet metal and e

1. A method of creating a watertightness barrier for a wall of a watertight and thermally insulating tank, comprising: arranging, on a thermally insulating barrier that has a flat support surface for the watertightness barrier, a repeating structure comprising alternately strips of sheet metal and elongate welding flanges connected to the support surface and projecting with respect to the latter so that the welding flanges run parallel to the strips of sheet metal over at least part of the length of the strips of sheet metal and so that lateral edges of the strips of sheet metal which are turned up with respect to the support surface are positioned against the adjacent welding flanges, for respectively a first and a second adjacent strip of sheet metal arranged on either side of a welding flange, watertightly welding the turned-up lateral edge of the strip of sheet metal to the welding flange using a straight welded seam parallel to the support surface along a first longitudinal portion of the strip of sheet metal, the straight welded seam being each time arranged beneath an upper ridge of the turned-up lateral edge of the respective strip of sheet metal, characterized by the steps of: continuing each time the straight welded seam with an end portion of the welded seam which is deviated with respect to the support surface in the direction of the upper edge corner of the turned-up lateral edge and which meets the upper edge corner of the turned-up lateral edge, and producing a watertight edge corner welded seam between the upper edge corner of the turned-up lateral edge of the first strip of sheet metal and the upper edge corner of the turned-up lateral edge of the second adjacent strip of sheet metal along a second longitudinal portion of the two strips of sheet metal so that the edge corner welded seam watertightly meets the end portion of the respective welded seam of each of the two strips of sheet metal. 2. The method according to claim 1, in which the straight welded seam is produced using a welding machine moving along resting on the two adjacent strips of sheet metal along the turned-up lateral edges, the welding machine having rotary electrode wheels and drive rolls capable of gripping the turned-up lateral edges, the drive rolls having their axes inclined towards the front of the machine in its direction of travel with respect to the direction perpendicular to the strips of sheet metal. 3. The method according to claim 2, in which the end portion of the welded seam is produced by deviating the path of the welding machine in the direction of the upper edge corner of the turned-up lateral edges. 4. The method according to claim 3, in which the welding machine is run along an inclined plane positioned on the strips of sheet metal in order to deviate its path in the direction of the upper edge corner of the turned-up lateral edges. 5. The method according to claim 2, in which a protective plate is positioned above the upper edge corner of the turned-up lateral edges in order to maintain a spacing between electrodes of the welding machine when the said electrodes reach the upper edge corner at the end portion of the welded seam. 6. The method according to claim 1, in which the end portion of the welded seam is stopped below the upper edge corner of the turned-up lateral edges, then an upper portion of the turned-up lateral edges which is situated above the uppermost part of the welded seam is eliminated. 7. The method according to claim 6, in which the upper portion of the turned-up lateral edges is eliminated by a local cut in the strip of sheet metal so that the uppermost part of the welded seam lies substantially at the bottom of the local cut. 8. The method according to claim 1, in which the end portion of the welded seam is produced before a longitudinal end of the welding flange, and in which an upper portion of the welding flange that extends beyond the upper edge corner of the turned-up lateral edges is eliminated after the end portion of the welded seam has been produced, the upper portion of the welding flange being eliminated over a length lying between the end portion of the welded seam and the longitudinal end of the welding flange. 9. A watertight and thermally insulating tank arranged inside a bearing structure comprising a bearing wall, the tank comprising a tank wall positioned along the bearing wall, the tank wall comprising a watertightness barrier and a thermally insulating barrier positioned between the watertightness barrier and the bearing wall, the thermally insulating barrier having a flat support surface for the watertightness barrier, the watertightness barrier having a repeated structure comprising alternately strips of sheet metal arranged on the support surface and elongate welding flanges connected to the support surface, projecting with respect to the latter and running parallel to the strips of sheet metal over at least part of the length of the strips of sheet metal, the strips of sheet metal comprising lateral edges which are turned up with respect to the support surface, arranged against the adjacent welding flanges,each of the two strips of sheet metal arranged on either side of a welding flange comprising a first longitudinal portion in which the turned-up lateral edge of the respective strip of sheet metal is watertightly welded to the welding flange by a straight welded seam parallel to the support surface, the straight welded seam being each time arranged beneath an upper ridge of the turned-up lateral edge of the respective strip of sheet metal,characterized in that each of the two strips of sheet metal arranged in either side of a welding flange furthermore comprises:a second longitudinal portion in which the turned-up lateral edge is watertightly welded to the turned-up lateral edge of the other adjacent strip of sheet metal by an edge corner weld, andan intermediate portion in which the edge corner weld of the second portion and the straight welded seam of the first portion are connected watertightly, the welded seam each time having an end portion which is deviated with respect to the support surface in the direction of the upper edge corner and meeting the upper edge corner of the turned-up edge in the intermediate portion of the respective strip of sheet metal, the edge corner weld being continued in the intermediate portion of each of the two strips of sheet metal in such a way as to create a watertight joint with the end portion of the respective welded seam of each of the two strips of sheet metal. 10. The tank according to claim 9, in which the upper edge corner of the turned-up lateral edge of the strip of sheet metal is straight and parallel to the support surface in the intermediate portion of the strip of sheet metal. 11. The tank according to claim 9, in which a nominal height of the turned-up lateral edge in the second longitudinal portion is equal to a height of the turned-up lateral edge in the first longitudinal portion. 12. The tank according to claim 9, in which the nominal height of the turned-up lateral edge in the second longitudinal portion is less than the height of the turned-up lateral edge in the first portion. 13. The tank according to claim 9, in which the turned-up lateral edge has a cut in the intermediate portion of the strip of sheet metal in order to lower the upper edge corner of the turned-up edge, the edge corner weld meeting the deviated end portion of the welded seam at the bottom of the cut. 14. The tank according to claim 9, in which the tank further comprises a retaining reinforcement positioned along one edge of the bearing wall and connected to the bearing structure in order to react tensile load, the retaining reinforcement comprising a flat connecting element arranged in a border region of the wall of the tank some distance from the bearing wall in order to allow the thermally insulating barrier to pass between the flat connecting element and the bearing wall, the strip of sheet metal comprising a longitudinal end edge welded to the connecting element in the border region of the wall of the tank, the second longitudinal portion of the strip of sheet metal comprising a border portion extending between the longitudinal end edge and one end of the welding flange, the upper edge corner of the turned-up lateral edge in the border portion of the strip of sheet metal rising up progressively from the longitudinal end edge up to a nominal height of the second longitudinal portion. 15. A ship for transporting a cold liquid product, the ship having a double hull and a tank according to claim 9 arranged in the double hull. 16. The use of a ship according to claim 15, in which use a cold liquid product is carried through insulated pipes from or to a floating or shore-based storage installation to or from the ship's tank in order to load or offload the strip. 17. A system for transferring a cold liquid product, the system comprising a ship according to claim 15, insulated pipes arranged in such a way as to connect the tank installed in the hull of the ship to a floating or shore-based storage installation, and a pump for driving a flow of cold liquid product through the insulated pipes from or to the floating or shore-based storage installation to or from the ship's tank. 18. The method according to claim 3, in which a protective plate is positioned above the upper edge corner of the turned-up lateral edges in order to maintain a spacing between electrodes of the welding machine when the said electrodes reach the upper edge corner at the end portion of the welded seam. 19. The method according to claim 4, in which a protective plate is positioned above the upper edge corner of the turned-up lateral edges in order to maintain a spacing between electrodes of the welding machine when the said electrodes reach the upper edge corner at the end portion of the welded seam. 20. The method according to claim 2, in which the end portion of the welded seam is stopped below the upper edge corner of the turned-up lateral edges, then an upper portion of the turned-up lateral edges which is situated above the uppermost part of the welded seam is eliminated.