"Alternate drift" refers to alternate drift specifications listed in API 5CT.
Drifting means measuring a pipe's inner roundness.
Drifting is typically performed by passing a cylindrical mandrel through the length of the pipe to detect occlusions.
It occurs both at the pipe mill and in the field.
Drifting is performed to ensure that tools, pumps, smaller pipe, and other items can be passed through the pipe.