Repair and reconstruction of engineering structures - Assessment and Refurbishment of Steel Structures - page 265

6.2.1.4 Methods of tank repair using welding techniques
Repair of the bottom
Assessment of what repairs need to be carried out on a tank can be made after a thorough
removal of oil residues that have settled on the bottom and the lower sections of the shell.
Any residue left on the bottom evaporates and produces dangerous concentrations of
hydrocarbon vapours in the air, making it impossible to do any welding or cutting jobs
inside the tank that involve open fire.
In a great number of tanks the bottoms are not flat, which is a result of using
inappropriate welding techniques in constructing the tank. If the bottoms have bulges that
have not been filled up with the sand, they are likely to deform when the quantity of fuel
stored in the tank changes. Under such circumstances the plates in the bottom are subject
to bending in the bulging zone. If, in addition, the plates are weakened by corrosion pits
particularly continuous ones, the pitting can result in fatigue cracks. In the case of large
bottom deformation, prior to painting or laminating the bottom with a plastic material it is
necessary to carry out one of the following repair procedures:
• Cut through the bottom and stretch the plates using rigging screws to eliminate the
bulging, then weld the cuts together
• Stabilize the deformed bottom by filling the bulge with bedding sand or con crete. The
filling material is let into the bulging space through holes cut in the bottom. It is also
necessary to make some air vents. Each of the openings is sealed with welded cover
plates as soon as the bulging has been filled with bedding sand.
The first of these procedures was adopted in the repair of a tank of 32000 m 3 capacity. Its
diameter was 52.21 m and its steel plates had the thickness characteristics given in Table
6.1. In the bottom there were two large bulges (Figure 6.104).