Usually CT strings are removed from service for one of 3 reasons:

a) they have reached the end of their fatigue life;

b) they have too much corrosion (rust, acid corrosion, CO2 corrosion, H2S corrosion);

c) they are mechanically damaged (someone pulled too hard, hit with something, over pressured while bending).

Ovality is only an issue when working in high pressure wells, and even then it is rarely the cause of scrapping a string.

Necking (decreased diameter) is caused by mechanical damage (usually overpull) and so is included in c. above.

Ballooning (increased diameter) only happens when pumping at high pressure while bending, and is included in c.

 

High pressure operations deeper than 6000m are performed in South Texas and in South Mexico. Careful design of the tapered CT string is needed. Many things must be considered including the burst & collapse under high axial loads, fatigue life, force available at the bottom to do work, grade and strength of the CT material, length of each tapered section, etc.

Our Cerberus software is often used to perform this design. Even the CT manufacturing companies (Quality Tubing and Tenaris) sometimes use Cerberus for performing string designs. NOV CTES can also perform string designs on a consulting basis.

 

NOV CTES makes a product called Argus which uses ultrasound to measure the wall thickness at 12 locations around the diameter of the CT. It measures many hundred times a second.  The Argus is mounted at the level wind, and measures the wall thickness while the CT is run in and out of a well. This is the only product on the market that accurately measures the wall thickness. There are other products which give a less accurate wall thickness, such as the Rosen inspection system from Germany. But Rosen do not sell their system... it is used only to perform inspection services. CTES does sell the Argus inspection device.