There is much uninformed theories about the risk from an LPG or LNG tanker being blown up. The link below, and the extract, tell a different story. A friend of mine was on the golf course at Aberdeen when he was called to deal with the Rosandra at Brindisi.
April 1990 .
Val Rosandra
The vessel, a 2999 m3 semi . pressurised LPG carrier with cylindrical tanks was discharging propylene at Brindisi when a fire started between the compressor house and No.3 tank. The vessel was towed out to sea with No.3 tank dome burning. This continued to burn for a further 22 days after which explosive charges were laid to breach the domes
of the four remaining tanks and allow the gas to burn off. This situation continued for a further 16 days until the vessel was scuttled.
As the gas vapourised it would torch in the atmosphere, consume the local oxygen, and extinguish. More gas would vaporised from the heat and the process continued.
http://www.sjofartsverket.se/upload/4001/77-INF2.pdf