AMEC requested that IDAC carry out a finite element analysis (FEA) on a newly constructed tank to establish the stress distribution for normal operational conditions, and to evaluate the critical load to cause buckling. Most importantly, the analysis was required to determine the impact of an internal overpressure in the tank, and whether failure to the roof / curb joint occurred before failure at the wall / floor joint. Expectation from AMEC was to confirm that any failure of this nature would result in condensate remaining within the storage tank, and not result in a spillage.
The conclusion of the analysis would provide AMEC with guidance as to whether further works to the frangibility of the roof joint needed to take place. This case study describes the analysis procedure and the conclusions that were reached.
Analysis
IDAC carried out structural FE analyses, using ANSYS, of the condensate storage tank design incorporating the frangible roof joints. The objective of the analysis was to evaluate whether, or not, the roof/curb joint would fail before the floor / wall joint. The analysis conditions were provided by AMEC. A 1/16th segment was considered to be the minimum size that could be modelled to allow for challenging geometry of the storage tank (10m diameter span of the tank and 6mm thickness of the roof and wall), whilst maintaining reasonable computational times. The model, (including the welds and stiffener beams) was meshed entirely with solid elements; this allowed for stresses and strains to be evaluated through the welds and the thickness of the roof and floor.
The analysis was carried out in two phases:
- Linear buckling analysis to evaluate the buckling modes and critical buckling pressure
- Non-linear buckling analysis to obtain more accurate results
Conclusions
From the non-linear analysis a more accurate critical buckling pressure was calculated, as well as a safe pressure where the plastic strain remained below 10%. The analysis confirmed that the tank could withstand the critical buckling pressure safely. The pressure at which 10% strain was reported was also calculated and was found to be 17% higher than the critical pressure load.
It was found that the top and bottom welds were the critical welds in the tank. The analysis also showed that the first region to experience plastic strain greater than 10% was in the roof weld. From the results it was concluded that the roof would fail first at the side which was welded to the curb bar, and that the failure would then proceed through the thickness of the weld. This result satisfied the customer as the condensate storage tank was required to fail first at the roof/curb joint to allow gases to escape rather than any of the condensate.
Design benefit
IDAC were able to evaluate the internal pressure and location at which the local buckling occurred. AMEC having worked closely with IDAC, were able to present the recommendations to their client with confidence having gained an insight into the behaviour of the various components of the Condensate tank.
Alan Hopgood of AMEC said: “The report was well received by both AMEC and our client. The FEA analysis provided assurance to both the client and AMEC that any unexpected overpressure within the tank would not result in the loss of condensate inventory. Based on the report findings, a decision was reached within the project that no further construction work was required on the tank roof joints to improve the roof frangibility.”