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 symmetry sector of the storage tank was modelled as shown in the graphic to the right. Taking advantage of cyclic symmetry allowed a more detailed model to be developed and hence a more accurate solution to be calculated. 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. Half of the stiffener beam was modelled on one plane of symmetry of the 1/16th sector, whilst the other plane of symmetry was modelled as being mid-span of the stiffener beams. 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
nonlinear buckling analysis to obtain more accurate results
Phase 1: Linear Buckling Analysis (Eigen Value Approach)
A 1/16th segment of a circular tank was created and analysed with symmetry boundary conditions, hydrostatic pressure, gravity and an initial gas pressure. Using this pre-stressed model, a linear buckling analysis was carried out to obtain the lowest buckling mode shape either at the top or bottom corners of the tank. The load multiplier value was noted for the buckling mode shape of interest. The procedure above was then repeated by applying a new gas pressure each time (calculated from the equation below) until the load multiplier became approximately 1. The final pressure value calculated was taken to be the critical buckling pressure.
New Gas Pressure = Previously Applied Gas Pressure x Load Multiplier
The first buckling mode for the tank body can be seen in the graphic to the left.