THE TALLEST TANK ever made in the southern hemisphere will be commissioned at Sasol Secunda for its Benzene 3 per cent Reduction Project. This double walled tank has a 12 438.86 cu m capacity and stands 47.54 m tall from the ground to its roof.
Fabian Fieliciano, contracts manager at Concor Engineering, says that the company was responsible for the design, draughting and detailing, fabrication, supply and erection of this large bulk storage tank. “Designed as a feed tank to store CTN and Benzene Alkalation Slops, it could be described as one tank inside another and was engineered as a modular structure,” he says.
The inner tank, with a diameter of 18.94 m, has a height of 44.1 m to the curb and a height of 47.54 m to its roof, while the outer tank has a 22.73 m diameter and a height of 29.4 m to its curb. The outer tank does not have a roof, only a canopy.
The tank was fabricated in sections with 10 m long by 3 m wide plates which allowed for simpler erection on site. The shell courses range from 8 mm to 25 mm in thickness, while the bottom plates are 12 mm thick and the annular plates 16 mm thick. The inner tank has sixteen strakes or sections and the outer tank has ten strakes.
The empty mass of the tank is 389 ton and it is supported by anchor chairs and designed for wind loads of up to 134 km per hour. The vessel has an internal design pressure of 18 kpa gauge.
Fieliciano says the biggest challenge on this project was the erection of the roof which, despite being carried out in inclimate weather conditions, was completed within a two-hour window. “The roof was constructed adjacent to the tank and a 550 t mobile crane was used to lift the 44 ton roof into position on the inner tank. This method was chosen so as to ensure the accurate positioning of the roof on the inner tank,” he says.
“During the erection of the tank it was essential that the heavy metal sections, 10 metre plates, were kept steady during the lifting and positioning activity, and we were only able to lift when wind speeds on site were below 19 km an hour,” Fieliciano explains. “The high winds speeds did delay the project at some stages.”
Another critical factor during on-site erection activities was that the weld profiles had to be 100% so as to ensure the smooth operation of the Internal Floating Roof (IFR).
Circular shell welding was done using automated girth machines which also assisted in meeting the tight erection schedule. The plate thickness called for Post Weld Heat Treatment on all the nozzles.
Concor Engineering operated a day and night shift welding crew. At its peak, 120 people were working on the project and Concor Engineering achieved a LTIR of Zero over the 200 000 manhours spent on this project.
Fieliciano say that this can be attributed to the commitment of all the employees to the safe operation of all activities as well as the support from the client, Sasol and their contractor, ThyssenKrupp Engineering.
The contract was awarded in January 2008 and completion is scheduled for February 2010.