Collapse loads are seen in all wells. They can be caused by the environment (floating formations such as salt dome), or during drilling / production phases such as cementing operations, full evacuation, and Annular Pressure Build-up (APB).
In some cases, these external pressure cases become critical for the well integrity. The collapse resistance of the casing string can be the limiting factor for the design selection.
High shear loads on the casing string.
Non-uniform collapse pressure difficult to evaluate accurately.
High safety margin should be taken regarding collapse resistance.
Annular pressure build-up
In HP/HT offshore wells, in absence of pressure release possibility, the increase of annulus fluid temperature induces uniform external pressure on the casing. One main concern will be on the intermediate casing (usually 13 3/8"), that is intrinsically weak in collapse. Thicker pipes (13 5/8", 14", in High Collapse grades) are usually advised to overcome the APB issues.
Other collapse issues
During drilling and production phases, different cases of external pressure occur. A main example is the full evacuation scenario, where the hydrostatic pressure of the external annulus will apply collapse load on the casing, especially on the non-cemented parts.
Vallourec has developed and designed specific solutions to overcome collapse challenges: