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Gas Liquor Storage Tank Pre-stressing System

The client, a petrochemical manufacturer, had restricted the use of each of four 58 million liter gas liquor storage tanks to a maximum level of 30% because stress corrosion cracking was detected in the walls of the tanks.  These tanks are located close to plant office buildings and it was feared that a catastrophic failure would pose an unacceptable safety risk for the occupants as well as precipitate a potential ecological disaster with the release of the hazardous contents of the tanks.  Normally the usage of the tanks to a 30% level would not pose a problem, but during the yearly maintenance shutdown period, the full capacity would be needed to store the gas liquor while other parts of the plant were being worked on.

In response to a suggestion by a section manager to put “straps” around the tank to support it, a practice commonly used on farms on small water tanks, a system of cables was designed to pre-stress the tank and enable the 30% restriction to be lifted.  These cables would also act as a safety device that would hold the tank together in the event of a large break in the shell by assuming the load carrying capacity that would no longer be present in the tank.  Although straightforward in concept, the solution was nevertheless a major technical challenge.

Major technical challenges were presented by a number of factors that all required extreme care to be taken in the design of the system: 1) the thin shell had to be pre-stressed by tensioning the cables while the tank was empty, which carried a significant risk of buckling the shell, 2) As temperatures varied through the day and through the year the cable tension would change and potentially cause buckling or even become too slack to offer support, 3) the loads on the tank and cables would change substantially with the fill level that could range from empty to 15 meters, and 4) pressure vessel codes to which the tank was subject did not allow for the welding of additional structure to the tank walls nor for covering of the existing welds that had to remain open for inspection.

The problem was solved by designing a system of 26 sets of tensioning cables, spaced over 15 meters from the floor to the roof of the tank.  Each cable was split into four 90° sectors with two anchors and two tensioning posts in order to account for friction and ensure an even distribution of tension around the circumference of the tank.  The cables were lifted off the face of the tank by a number of regularly spaced support beams hanging from the top lip of the tank.  These spacers ensured that the tank structure was open for inspection and that friction effects upon tensioning were minimized.  Once assembled and tensioned the system stayed in place through friction at the contact points.

The design of this system was made possible by running a multi step non linear static analysis that included non linear loading and friction effects, which took the loading history of the process into account.  With this analysis we were able to simulate the process of cable tensioning with an empty tank and subsequent filling, all while taking into account thermal variations on cable tensions.  A sensitivity analysis was also conducted by running the load history analysis on tanks with simulated structural imperfections.  The tank was filled successfully in time for the annual maintenance shutdown and, although destined for replacement, was in use for at least 5 years after our intervention.

Click on the thumbnail to view larger image.
	One quarter finite element model of the tank (70 meter diameter, 15 meter height) and cable system.  The model includes the foundation to ensure boundary conditions were properly accounted for as well as a mesh refinement to pickup and highlight any unexpected local deformation effects at the intersection of base and wall.
	Tank wall deformations (scaled 100 times) for 33% fill level with initial buckling imperfection of 25 mm.
	Stress distribution for pre-stressed empty tank with 25 mm initial buckling imperfection.  This picture shows that the highest stress distributions, shown in red, are found further up the wall, even though cable tensions were reduced to compensate for reducing wall thickness with height.  This is where buckling failures are likely to initiate.