A 55,000 barrel-per-day (bpd) visbreaker at ExxonMobil Corp's Singapore refinery was taken down for one month of planned maintenance at the beginning of November, industry sources said yesterday.

The work is expected to cut the refinery's sales of bunker fuel this month by the equivalent of one cargo, about 20,000-30,000 tonnes, the sources said.

A visbreaker cracks straight-run fuel oil into higher value products such as gasoline and gas oil, and also yields residual fuel oil sold as marine fuel.

An ExxonMobil spokes-man was not immediately able to comment.

"One cargo is not a big volume but it does have some impact when the bunkers market is as tight as it is. Exxon is certainly offering even more conservatively than usual in the Singapore and Hong Kong bunkers market," a Singapore-based trader said.

The Asian bunkers market was tight throughout most of October, and is expected to remain so in November largely due to a low flow of imported supplies from the West.

Bunker premiums, the price differential between outright bunker prices and fuel oil cargo values, soared to highs of $15-$20 (Dh55.10-Dh73.46) a tonne in the second half of October and are currently at around $6-$7, still well above average levels of $2-$3.

ExxonMobil's marine fuels unit was the fourth largest by volume in Singapore's 2.2 million tonnes a month market last year, and one of the two largest in Hong Kong, with China Resources.

The refiner is estimated to sell an average about 500,000 tonnes of marine fuels a month in Singapore and Hong Kong.