Ships – Building – Freighters
Reexamination Certificate
1999-06-14
2001-03-13
Sotelo, Jesus D. (Department: 3612)
Ships
Building
Freighters
C220S901000
Reexamination Certificate
active
06199497
ABSTRACT:
The present invention relates to a watertight and thermally insulating tank, particularly for storing a liquefied gas, such as methane, at a temperature of about −160° C., the said tank being built into the bearing structure of a ship.
French Patent 2 629 897 discloses a watertight and thermally insulating tank built into the bearing structure of a ship, the said tank comprising two successive watertightness barriers, one of them a primary one in contact with the product contained in the tank, and the other a secondary one located between the primary barrier and the bearing structure, the said bearing structure comprising, for each tank, on the one hand, walls which are substantially parallel to the axis of the ship and form the internal sides of its double hull and, on the other hand, two transverse bulkheads substantially perpendicular to the axis of the ship, these two watertightness barriers alternating with two thermally insulating barriers, the primary insulating barrier being held pressed against the secondary watertightness barrier by fastening means arranged substantially continuously in a straight line and mechanically joined to the secondary insulating barrier, the corner connection of the primary and secondary barrier elements, in the zones where the transverse bulkheads meet the internal sides of the double hull, being achieved in the form of a connecting ring, the structure of which remains substantially constant along the entire length of the solid angle of intersection between a transverse bulkhead and the internal side of the double hull. Such a tank is generally in the shape of a polyhedron, particularly an irregular octahedron, the tank corners of which generally are at angles of 90° or 135°, which involves the use of a connecting ring which can adapt to suit these different angles.
In French Patent 2 629 897, the connecting ring consists of a number of plates which have varying shapes, for example which are straight, curved or at right angles. All of these plates are welded together to define an interior volume, the cross section of which is square and one side of which corresponds to the thickness of the primary insulating barrier. In the gaps that there are inside the ring and between the ring and the solid angle of intersection at the corner of the tank, blocks of insulating material are inserted in order to ensure the continuity of the primary and secondary insulating barriers. The manufacture of this connecting ring therefore entails numerous operations of welding, forming and assembling, which make manufacture complicated and expensive.
In French Patent 2 724 623, the connecting ring is secured to the bearing structure by welding to anchoring flaps which are perpendicular to the walls. The anchoring flaps are welded to the internal wall of the double hull after the stage of applying protective paint to the double hull. The continuous welding of the anchoring flaps to the internal wall of the double hull generates high flow of heat which runs the risk of damaging the paintwork on the outer side of the internal wall of the double hull and may cause corrosion of the said internal wall of the double hull which wall is intended to be in contact with seawater when the ship is empty and the double hull is being used for ballast. To overcome this drawback, a further coat of paint is applied to those parts of the double hull which have been damaged by the continuous welding of the anchoring flaps, but such reparatory paintwork does not provide as effective a protection against corrosion and entails additional operations which have an adverse effect on the cost of manufacture.
Furthermore, it is known that when the ship is moving on the waves, the deformation of the connecting ring induces very substantial tensile stresses at the primary and secondary watertightness barriers and these stresses in fact combine with the tensile stresses induced in these watertightness barriers when the tank temperature is reduced.
In French Patent 2 709 725, the connecting ring consists in an oblique band which extends from the solid angle of intersection at the corner of the tank as far as the intersection of the primary and secondary watertightness barriers, and this makes it possible to take up the loads induced in the primary and secondary watertightness barriers in close proximity to the solid angle of intersection at a corner of the tank using the oblique band on which the resultant of the loads induced in the tank wall parallel to the double hull and in the tank wall parallel to the transverse bulkhead are exerted. However, such an anchoring band is liable to buckle and has the drawback that it passes through the primary insulating barrier, making a link between the primary watertightness barrier and the secondary watertightness barrier.
The object of the present invention is to provide a tank in which the connecting ring at the corners of the tank has a simple structure and is easy to fit, at a reduced cost. Another object of the invention is to provide a tank in which the improved connecting ring does not damage the paintwork of the double hull. A further object of the invention is to provide a tank in which the improved connecting ring provides continuity of the watertightness of the primary and secondary barriers, and continuity of the thermal insulation, while at the same time having a rigidity comparable with the bearing structure in proximity to the watertightness barriers, so as to improve the resistance of the watertightness barriers to the impacts that occur on the walls of the tank as a result of the movements of the liquid during transport, which movements are due to the rolling and pitching of the ship.
In French Patent 2 629 897 it is proposed that the thermal bridge between the primary watertightness barrier and the bearing structure be eliminated, which makes it possible to reduce the thickness and therefore the weight of the primary insulating barrier, it thus being possible for the said primary insulating barrier to be attached directly to the secondary insulating barrier, because of its lower weight. According to French Patent 2 709 725 it is known that it is advantageous, for the same tank wall thickness, to increase the thickness of the secondary insulating barrier at the expense of that of the primary insulating barrier because if there is a leak at the primary watertightness barrier, the accidental cold zone is further from the double hull, the thicker the secondary barrier. However, the thickness of the primary insulating barrier is the result of a compromise between the thermal insulation function of the primary barrier and the need for this primary insulating barrier to provide good rigidity to impacts caused by the liquid during transport.
Furthermore, as the primary insulating barrier is held pressed against the secondary watertightness barrier by the primary watertightness barrier itself, the said primary and secondary watertightness barriers being secured in watertight fashion to the secondary insulating barrier by fastening means, it is necessary to provide a double expansion joint at the attachment means so as to avoid stresses due to the differential expansion of the primary watertightness barrier and of the secondary watertightness barrier. If a single expansion joint is provided at the fastening means, then the thickness of the fastening means has to be great enough to withstand the shear generated by the absence of expansion joint between the two watertightness barriers.
The second object of the invention is to provide a tank with a simplified insulating barrier, which affords excellent rigidity to the impacts generated by the liquid during transport while at the same time eliminating the problems of differential expansion of the watertightness barriers at the fastening means.
The use of a secondary insulating barrier consisting of a thermally insulating layer of cellular plastic such as a polyurethane foam reinforced with fiberglass fabric inserted into the said foam to give it good mechanical properties, is known from French Pat
Dhellemmes Jacques
Jean Pierre
Connolly Bove & Lodge & Hutz LLP
Gaz Transport et Technigaz
Sotelo Jesus D.
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