Storage tanks are containers for liquids and compressed gases. They usually have a flat bottom and a spherical or cylindrical shape. Some have fixed roofs others have roofs floating on top of the liquid contained in the tank. They are used to store oil and fuels, chemicals, pharmaceuticals, food, beverages and water. The walls of the tank may be single or double depending on its use. Compressed gases and liquefied natural gas (LNG) require double-walled tanks to maintain the internal pressure and to provide an extra safety factor.
The simplest and cheapest method of storage tank construction is to build the walls from the bottom and work upwards. This method needs scaffolding to enable work at higher elevations.
A more efficient tank construction method involves the use of a hydraulic jacking system. The jacks lift the tank walls as they are assembled on the ground. This is a safer method that does not require work at height and reduces accident risk.
Tank construction begins by assembling the base of the container at the construction site. Prefabricated steel plates are welded to each other to form the base. This stage is identical for both bottom-up and jacking operations.
If the tank has a permanent roof, this is prefabricated elsewhere and transported to the construction site. The hydraulic jacks are assembled around the perimeter of the roof. The jacking system lifts the roof.
In the bottom-up system, the tank course, or strake, is assembled from steel plates up to 10 feet wide by welding the vertical sides to each other and by horizontal welding to the base. In the jacking method for a floating roof tank, a system of hydraulic lifting jacks is set up around the whole perimeter of the tank base. The plates are welded together vertically.
In the bottom-up system, the next level of steel plates is welded horizontally on to the previous course all the way to the top of the tank. In a jack up system, the hydraulic jacks lift the first course of plates. The second course is welded horizontally to the base of the first course. If the tank has a fixed roof, the hydraulic jacks lift the roof and the plates are welded to the roof’s perimeter.
This stage applies only to the jack up method. Further steel plate courses are assembled by welding the plates vertically to each other and horizontally to the base of the previous course. The hydraulic jacks lift the assembled course to enable welding below.