OCTG Seamless Pipe-Tubing/Casing/Line Pipe Tubing is used for extracting petroleum and natural gas. Casing serves as wall for well Certificated: API SPEC 5CT General Specification: Tubing: O.D.=2 3/8"~4 1/2" *****=4.24mm~16mm Casing: O.D.=4"~13 3/8" *****=7.8mm~13.06mm