Two trading houses, including commodities group Glencore, have chartered at least four tankers to store oil off Singapore, taking advantage of cheap freight and crude ahead of the refinery maintenance season that typically crimps demand.

ST Shipping, Glencore’s shipping arm, booked the 300,133 deadweight tonne (dwt) Very Large Crude Carrier (VLCC) Plata Glory for a month at $22,000 per day and has the option to extend at $26,000 and $29,000 per day for the second and third month, ship brokers said.

ST Shipping has also chartered the 161,724 dwt Suezmax tanker, United Emblem, and the 112,777 dwt Aframax ship, United Grace, for 30-90 days, at undisclosed rates, a Singapore-based shipbroker said.

Clearlake, a subsidiary of the oil-trading firm Gunvor [GGL.UL], has chartered the 308,596 dwt VLCC Arenza XXVII at $33,000 per day for one to four months, the shipbroker added. VLCCs can hold up to 2 million barrels.

Both companies do not comment on commercial operations.

Traders, however, said a widening contango structure in the oil market – with forward prices trading higher than spot rates – was partly why the firms were planning to store supplies as they hope for better returns when demand perks up in winter.

After months of gains, Brent crude for August delivery fell back below $50 per barrel last week after Britain’s vote to leave the European Union. Prices for delivery towards the end of the year, however, remain above $51.

“Traders are trying to bottom-fish (for crude bargains) and store for one to two months before re-selling,” a trader with a western firm said.

At the same time, it has become cheaper to charter tankers, making it potentially attractive to store oil for a later sale.

Rates for a one-year VLCC charter have fallen by almost $20,000 since January, to between $38,000 and $42,000 a day last week, according to shipping services firm Clarkson. They were $47,500 per day a year ago.

The Plata Glory is anchored off Sungai Linggi, Malaysia, while Arenza XXVII is moored in the Johor anchorage, also Malaysia, according to shipping data on Thomson Reuters Eikon. Both tankers are currently empty.

Traders said that Russian ESPO and Abu Dhabi’s Murban crudes are the most likely fuels to be stored.

The storage play also comes as refiners plan to shut for maintenance, crimping demand, they added.

About 1 million barrels a day of processing capacity in Asia will be shut in October, according to Reuters calculations, leaving crude cargoes for loading in August unsold.

Still, the current contango structure is tight, making it risky to hold oil for long, traders said.

The number of tankers used for oil storage around Singapore fell to 30 this month from 40 in May, data from Thomson Reuters Supply Chain & Commodities Research showed, as price rises between January and June triggered sales of stored oil.