Repair and reconstruction of engineering structures - Assessment and Refurbishment of Steel Structures - page 273

Figure 6.112 Double-plate band
bracing of the shell—the solution
recommended when the fillet weld
joining the shell to the bottom is large
In the upper sections of the shell, is sometimes noticed plate delamination when plates
are blast cleaned. Delaminations are dangerous with respect to strength loss if they run
diagonally to the plate surface. Such delamination puts part of the plate out of operation,
and consequently the damaged area has to be cut out of the plate. The extent of the
delamination is determined by ultrasonic tests carried out at a high density of measuring
points (Figure 6.113). The cover plate prepared for welding in place of the delaminated
fragment cut out should have slightly larger curvature than that of the shell. This is
necessary to compensate for welding shrinkage following the execution of the vertical
welds. These vertical welds are always carried out first because of their significance for
primary strength. Transverse shrinkage of vertical welds will result in a partial
straightening of the welded plate, so to avoid local post-welding shell deformation, the
cover plate needs the appropriately larger curvature before welding work slabs. The
larger curvature of the cover plate can be calculated using data from a welding handbook
for transverse shrinkage of a butt joint of a given thickness, using appropriate welding
techniques. In one of the tanks under repair, it was necessary to cut a section 700 mm
wide out of a 11 mm plate strake. According to the calculations, the curvature radius of
the 'patch' plate had to be 3 mm larger than the shell's original curvature radius (Figure
6.114). On completing the welding work, it turned out that the patch fitted the tank shell
perfectly.