Casing Collapse Rating Pressure Calculations & Design
Casing collapse

The primary collapse loads are supplied by the column of fluid on the outside of the casing which acts to collapse the pipe and may cause collapsed casing sticking problem. These fluids are usually the mud and possibly the cement slurry in which the casing was set. Since the column of mud increases with depth, collapse pressure is the highest at the bottom of the hole section and is zero at the surface.
https://bit.ly/3FltOkq