Collapse pressure:

The pressure at which a tube, or vessel, will catastrophically deform as a result of differential pressure acting from outside to inside of the vessel or tube.

The collapse-pressure rating of perfectly round tubing is relatively high.

However, when the tubing is even slightly oval, the differential pressure at which the tube will collapse may be significantly reduced.

This is an important factor in determining the operating limits of coiled tubing strings since the action of spooling the string tends to induce some ovality.