Field of invention

The present invention relates to tanks for storage and transportation of fluids such as hydrocarbons, including low temperature liquefied natural gas. This includes tanks for ships, for gravity base and floating offshore structures, and for land-based installations.

Background of the invention

Tanks for storage and transportation of liquefied gas may be designed with a single containment shell or with two containing shells to achieve the desired level of safety. Each of these liquid containing shells may also be partial. The internal shell may not include a roof so that the inside shell provides liquid containment, but will not contain the gas. Alternatively an external containment may be reduced to being what is termed a drip tray which serves to retain limited accidental leakage due to the development of fatigue cracks or due to penetration by dropped objects, projectiles such as windblown debris and similar cases.

Such double shell structures must at least have some common means of support. It is convenient to connect the two shells, whether partial or full, by ribs or stiffeners to form a sandwich structure. In the event that fabrication is unsuccessful locally in maintaining the required quality of base material or weld material, or in the event of accidental occurrences as described above, repair of a section will be required. For aluminium structures, which is a common cryogenic material, construction is by MIG welding. Such MIG welding requires backing by plates or by ceramics.

However, backing plates produce sharp stress concentrations which will cause fatigue in enclosed spaces where grinding cannot be performed. Similarly ceramic backing plates need to be removed to allow inspection of the welds by radiography.

Said sandwich panels do not allow access for placing radiographic film on the inside of the plate being inspected. The object of the invention

The object of the present invention is to provide a method of performing a welded repair to a sandwich panel, for example like those disclosed in WO 2006/001711 A2, the content of which is hereby incorporated by reference.

Summary of the invention

The present invention provides a method of repairing sections of a sandwich panel by welding and documenting it by radiography by the specified procedure.

The method of construction of sandwich panels is by joining the flanges of a plurality of beams of an H-shape by welding the flanges of adjacent beams together to form panels and to assemble these into complete prismatic tanks. After such tanks have been assembled there is no access to internal voids of the sandwich structure to allow placing of backing material to weld and the subsequent removal to allow radiographic inspection. Neither is it possible to place film in the voids of the sandwich panel and to remove it later for development.

For aluminium it is possible to make welds by using TIG without backing, but it is too time-consuming and expensive for use in large tank structures.

Thus, the present invention provides a method of performing repairs of sandwich panels in aluminium with at least one weld bead using tungsten inert gas welding and making up the remainder of the weld with metal inert gas welding. Inspection of the welds is performed by using a radioactive isotope placed at an inclined angle such that welds directly opposite each other can be discerned separately on a film placed on the opposite side of the sandwich panel.

The invention may further be defined as providing:

A method of repairing welds in aluminium sandwich panels, comprising the steps of making the root bead of the repaired welds by tungsten inert gas welding, and making up the remainder of the welds by MIG welding.

The method may further comprise the step of radiographically inspecting the welds located opposite each other in the sandwich panels by applying a radioactive source eccentric to a perpendicular plane through the two welds and obtaining an image where each weld is shown independently on a film placed on the opposite side of the aluminium sandwich panel.

Brief description of the drawings

Figure 1 shows a cross section of the welded panel. Detailed description of the drawings

Figure 1 shows a cross section 1 of the H shaped extrusion that makes up the sandwich panels. The weld 3 is shown with a TIG root bead 2. The remaining part of the weld is made up by MIG. The location of the radiographic source 4 is shown on one side of the panel and the imaging film 5 is shown on the opposite side.