Interestingly, invulnerable tankers are a historical fact. These guys were the world's greatest fireworks display when they were full, but when they were empty, think about it!
You have a double hull with the hulls quite a distance apart--a ship within a ship. It weighs nothing and floats so high you have to dangle an anchor to keep it from rising off the water.
Wait! Here come a half dozen torpedoes. They can blow the hell out of the outer hull and never put a pinprick in the inner one. There are lots of accounts of empty tankers taking six or more torpedoes and just steaming over the horizon like nothing ever happened. Drove more than one skipper to drink.
