Conversation so far:
Describe the Afra tanker scale.
AFRA uses a scale that classifies tanker vessels according to deadweight tons, a measure of a ship's capacity to carry cargo
Why was the Afra tanker scale developed?
The term is based on the Average Freight Rate Assessment (AFRA), a tanker rate system created in 1954 by Shell Oil to standardise shipping contract terms. Due to their favorable size, Aframax tankers can serve most ports in the world.
What are supertankers?
Supertankers are the largest tankers, and the largest man-made mobile structures. They include very large crude carriers (VLCC) and ULCCs with capacities over 250,000 DWT.

Question: Describe their invention
Self-contained rewrite: Describe supertankers' invention.