Learning Forum

[Zoltan Torok]

Hi,

I want to make a static strutural analysis on a buried HDPE water tank for my thesis.

For material I used the built in Plastic, HDPE one from the engineering data

I made the model, it is a midsurface and halfed, as seen below.
The top cylindrical face is flush with the surface.

Mesh looks good. Mainly Quad4, element quality avarage is 0,928 and there are 59000 elements

I applied the following boundary conditions:

A: Displacement 0mm in X,Y,Z
B: Displacement 0mm in X
C: Displacement 0mm in Y
D: Variable pressure normal to surface - this is for the soil pressure, see picture. Would be better apply only in the Y direction, but can't figure it out how to do that.
E: Pressure for the soil from below - An elastic support should be better? Please help with this.

I would be happy if i can loose the feets from the tank bottom with other boundary conditions.

Analysis settings:

• Large Deflection ON
• Quasy-Static Solution ON
• Initial substeps 500
• Minimum substeps 200
• Maximum substeps 10000

I always got "The solver engine was unable to converge on a solution for the nonlinear problem as contrained."

How can I solve this problem?
Thanks in advance

[peteroznewman]

Hello Zoltan,

I found this YouTube video on an analysis of a buried water tank. The first few minutes show all the loads that need to be considered. The video shows a load called surcharge loads, which was unfamiliar to me, but I found this article that explains it.

You need loads from both the soil and the water in the tank. The analysis needs to consider at least three load cases.

1. Tank full before being buried (leak test).
2. Tank empty and buried.
3. Tank full and buried.

When the buried tank is empty, there is only pressure on the outer wall, but when the tank is full of water, the hydrostatic pressure of the water in the tank cancels out some of the pressure on the outside of the tank.

One way to apply soil pressure is to mesh a block of soil that has a tank-shaped cavity. Ansys includes soil models such as Mohr-Coulomb. This will complicate your model so it is easier to apply loads instead.

Here are 3 videos that cover the loads on a tank buried in soil.

1. Part 1
2. Part 2
3. Part 3