I've actually got a novel from 1990s called "The Talisman Deception", two of the ships in that are oil tankers that have been modified with re-inforced bows, sandbag armor around the bridge and afterhouse, relocated steering, and every free space (eg not the crew accommodation) has been filled with buoyant materials (e.g. Table-Tennis Balls, Kapok, Cork, etc.) and used in a quasi Q-ship role.