Shallow intermediate liner
Large diameter casing normally run as a liner and hung in a profile in the surface casing (18 in. can be called “surface extension” by regulators). Normal clearance casing OD in.: N/A. Tight clearance casing OD in.: 18 and/or 16. NOTE These are examples only. Each well can have variations in number of casing strings and sizes. Naming conventions can vary. The heavy-wall surface casing designs may allow 13 5/8 in. to be run as a liner and not tied back.
Source: API RP 96, Deepwater Well Design and Construction, First Edition, March 2013. Global Standards