Less than five percent of the global fleet of supertankers is currently used for storing oil as the cost of renting the ships, known as very large crude carriers or VLCCs, is too high to make this profitable, leading operator Frontline said.

Around 20-30 VLCCs, or between 3.1-4.7 percent of the global fleet of 645 ships, is now used for storage, Frontline Management Chief Executive Robert Hvide Macleod told Reuters.

The vessels now used for storage could typically store a combined 40-60 million barrels of oil.

Oil traders who expect crude prices to rise will often buy cargoes and store them on ships, but the cost of renting the vessels has risen sharply this year thanks to rising output from leading producers.