Definition(s)
Combination Casing String
A casing string consisting of more than one type of casing is called a combination casing string. A casing string that includes a separate smaller casing string suspended from the base of the previous string (liners) will also be classified as combination casing strings.
Source: API RP 90, Annular Casing Pressure Management for Offshore Wells, Upstream Segment. First Edition, August 2006. Global Standards