OCTG comes in three forms, drill pipe, casing and oil well tubing. It does not include pipe for conveying oil/gas from the point of production to the refinery or customer; this is linepipe. Drill pipe connects the drill bit with the drilling motor during well drilling and is usually about 2-6.5in (50-165mm) outside dia. Drilling mud is pumped down the pipe to cool the drill bit, while drilled material travels up it to the surface. The very demanding operational conditions mean drill pipe is always seamless.
Casing acts as the liner and structural wall of oil/gas wells, preventing contamination of the well and of the surrounding water table, and can be up to 26in dia. Casing accounts for about three quarters of all OCTG shipments. The third type is oil well tubing. This is used to bring oil/gas out of a well and is usually around 2-4.5in dia. And for more information click and see this article.