By Sarah Kent and Georgi Kantchev 

The supertanker TI Oceania was built to ferry vast quantities of oil across oceans, but for the next year it is expected to remain anchored off the coast of Singapore, storing millions of barrels of oil for Vitol SA, a giant trading house.

According to shipbrokers and analysts, the 3-million-barrel megaship--one of the largest in the world--is just one example of efforts by traders to turn a profit in the slumping global oil market. The strategy is simple: buy and store oil at cheap prices now, selling futures contracts to lock in the higher oil prices expected later.

"It is one of the easy ways to make money and that's one of the interesting things about it from a trading perspective: It's a counter cyclical source of profit for the Vitols and Glencores and Trafiguras," said Craig Pirrong, a finance professor at the University of Houston, referring to a handful of the biggest oil traders in the world.

According to shipbrokers and analysts, major traders including Vitol SA, Gunvor SA, Trafigura Beheer BV and Koch Supply & Trading Co. Ltd have chartered supertankers capable of storing a combined total of more than 30 million barrels of oil--many of them in the past few weeks. Vitol, Gunvor and Trafigura declined to comment. Koch didn't respond to requests for comment.

The opportunity to stockpile oil in such large quantities has come from the dramatic shift in the market for the commodity in recent months. Since June, prices have collapsed, tumbling by more than 50% amid soaring production from the U.S. and unwavering output from the Organization of the Petroleum Exporting Countries, at a time when global economic growth--the main determinant of demand--is slowing.

The oversupply has given rise to a so-called contango in the market, when the current price of a commodity is lower than prices for delivery in the future. That makes it attractive for buyers to purchase oil now at the cheaper rates, store it and strike sales agreements at a higher price in the future, locking in profits.

The price difference between the March and August contracts for Brent crude oil, the international benchmark grade, is currently $6 a barrel. That is the steepest premium since an oil-price slump in 2008 and 2009.

For years, oil trading houses have contended with high prices and low volatility, which have squeezed margins. Firms have responded by investing in infrastructure like oil-storage tanks, terminals and refineries to gain flexibility in trading, as well as better information about what is happening in the market.

Combined with the companies' access to the physical oil market, these investments have made the trading firms uniquely well-positioned to exploit the shift in the market and store oil for a profit.

Glencore PLC, a Swiss commodity-trading giant, and Trafigura Beheer, one of the world's largest independent oil traders, have both already highlighted to investors that the market's dramatic change since June is expected to bolster their profits.

Onshore storage tanks are filling up fast. According to Citigroup Inc., China's coastline storage facilities ran out of space as the country filled up strategic oil reserves last year. Stocks at the U.S. storage hub at Cushing, Okla., have risen more than 20% since December, according to Genscape Inc., a data provider based in Kentucky.

That means more unusual storage options, such as the ships, are becoming increasingly popular.

"Because so much oil doesn't have a home right now, there is a frenzy of traders and companies looking to hire supertankers," said Halvor Ellefsen, chief executive officer of Galbraiths, a London-based shipbroker .

The last time there was a similar situation, in spring 2009, more than 70 million barrels of oil were stored in tankers, according to shipbrokers.

The current tanker craze may not quite reach those levels, as the disparity between current and futures prices isn't as steep at the moment. Bank of America Merril Lynch predicts the volume of oil stored on tankers could rise to 55 million barrels by the end of the second quarter.

However, the potentially lucrative storage trade isn't open to everyone, nor is it risk free. It requires detailed knowledge of the way oil is moved around the world that few outside a tightknit group of oil traders possess. Making a profit depends on numerous factors, including rates for freight and storage and, ultimately, finding a buyer for the crude.

"If people think the contango is some kind of magical way to make money they are incorrect," said Benoit Lioud, senior research analyst at Mercuria Energy Group, a Swiss-based trading house. "Storing big quantities of crude oil is not an easy game. It's not a game at all."

Costas Paris in London and Christian Berthelsen and Nicole Friedman in New York contributed to this story.

Write to Sarah Kent at sarah.kent@wsj.com and Georgi Kantchev at georgi.kantchev@wsj.com

Subscribe to WSJ: http://online.wsj.com?mod=djnwires