In the past, because tank farms are built on site and have to withstand the elements, SMAW electrodes were used for almost all of the weld seams of fuel tanks. “Gas shielded processes were not suitable and gas is not always easy to get, particularly in Africa. So the labour-intensive stick welding process, which is associated with many stops and starts and a high potential for discontinuities or defects, was generally used,” says Bronkhorst.
“Today, we employ mechanised processes wherever possible, which are much faster, less prone to defects and offer significantly lower total project costs and greatly improved return-on-investment. Three factors govern the success of projects such as tank farm construction: time, quality and cost, and the mechanised approach improves all three,” he argues.
Fuel tanks can be sized from 6,0 m in diameter to 150 m. The ones currently under construction in the Port of Beira in Mozambique have a diameter of 37 m and a height of 22 m, each with a capacity of 20-million litres. The Beira fuel terminal with five of these tanks is being built to support needs across Africa for petrol, diesel and jet A1 fuels.
“Every tank gets erected on a base. The ground is piled and different layers of reinforcing are put in and compacted down, with a final layer of bitumen on the surface completing the civil side of the construction project,” says Bronkhorst.
“The steel tank is then built on top of the piles. The floor plates are laid down first, tapering down towards a conical drain at the centre of the base. This separates out any water that gets into the tank, because fuel floats on water,” he explains.
The floor is made up of flat plates, curved at the corners and lapped over each over in an interwoven pattern. “The joints are all lap joints, but on thicker material they can look a lot more like fillet welds,” he notes.
Before welding begins, the annular baseplate ring is placed around the tank underneath the floor, “but this ring does not get welded until the tank is completed,” Bronkhorst tells African Fusion.
Commenting on replacing the use of stick electrodes, he says: “Today we employ a submerged arc process for the base, using Lincoln LT7 tractors and Lincoln Flextec 650 power sources. But it gets very hot on the tank floor, so when welding relatively thin plates, it is critical to weld them in a pre-set sequence to avoid distortion, bowing and buckling.
“We start with the longitudinal welds and we complete diagonally opposite seams to immediately counterbalance any distortion from the previous weld pass. Only after completing the longitudinal seams do we return to complete the cross seams.
“Once the floor plate is compete, welding of the ‘strakes’ begins. These are curved plates 2.4 m high by 10 m long, which form the cylindrical walls of the tank’s shell. Each plate has a built-in curvature, depending on the diameter of the tank, and plate thicknesses vary from bottom (thicker) to top,” he says.
There are two ways of building these large tanks, according to Bronkhorst. The first and most traditional is the ‘bottom-up’ technique, which means the thickest strake sections are welded first. Then thinner sections are added until the height required is reached. Walkways, a wind girder – a reinforcing ring connected via knee braces to stiffen the top section of the tank – and a roof will then be added.
The alternative method is the ‘jack-up’ method, or the ‘top-down’ method. “The first ring of strakes is assembled – supported by temporary fishtails mounted onto the floor – and then tacked together. The vertical seams between the plates of the strake are then welded using the EGW process until the first ring of strakes is complete,” Bronkhorst explains, adding that this section will end up being the top section and is therefore constructed from the thinnest material.
The roof, which is an aluminium structure, is immediately bolted on at the height of one strake. Then the whole ring and its roof are then jacked up to allow another ring of strakes to be inserted below.
“This method has some access advantages, because all the welding work is done closer to the ground. Hooking up an EGW or AGW system and all of the peripheral equipment needed to complete a seam 22 m in the air is complex, so by keeping the bulk of the work at a height of 2.4 m, access is much easier,” says Bronkhorst.
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