When world shipping lines were enjoying one of their most profitable years last year, oil prices were among the few clouds on the industry's horizon. As booming container trade and strong demand for bulk cargo and oil boosted the industry, shipowners faced mounting bills for the thick fuel oil known as bunkers.
This year, the cloud over bunkers -- whose name derives from the coal bunkers used to store steamships' fuel -- has grown still darker. Prices for IFO380 -- one of the cheapest blends -- stood at $313-$316 per metric tonne last month in Singapore, the busiest bunkering port. A year ago the price was $190 per tonne. Prices were about $50 in 1999.
The new rise has coincided with falling freight rates. Lines shipping steel, coal, wheat or other bulk cargoes can charge only a third of the prices at last year's peak. Rates for the largest crude oil tankers are less than a quarter of those last autumn. Only container shipping demand has remained steadier.
Yet the price rise is having widely differing effects. Some lines are having to pay higher fuel prices out of income sharply reduced by falling freight rates. Others have established mechanisms to recover higher costs from customers.
Oil tanker operators earn so much from strong oil demand that bunker costs are a side issue.
At the worst-affected end of the scale, according to Mark Page, research director of London-based Drewry Shipping Consultants, are dry bulk carriers. Most work in highly-competitive markets arranging deals on a per-trip basis.
"In a weak market you can't really get an increase in your rate to compensate for the costs going up," Mr Page says.
Many bulk operators are small and privately owned, but some large shipping lines -- including Japan's NYK and K Line -- have bulk divisions.
Carriers committed to long-term contracts also face problems.
Ray Fitzgerald, chief operating officer of Wallenius Wilhelmsen, a Swedish-Norwegian car carrier operator, says his fuel costs are 45 per cent higher than envisaged in this year's business plan.
For environmental reasons Wallenius Wilhelmsen uses expensive bunkers with a lower proportion of harmful sulphur -- a choice which alone costs an extra $12m annually.
Some of the risk is hedged and customers can be made to pay a surcharge to cover fuel price rises. However, surcharge clauses in customers' contracts generally take effect only some time into the one-to-five-year deal. Much of the cost remains uncovered.
"Our recovery is not at all where it needs to be," Mr Fitzgerald says.
The position is much easier for container shipping lines, which levy a straightforward, per-container fuel surcharge. Some customers even allege they are being overcharged.
Philip Green, chief executive of P&O Nedlloyd, a major container line, says the oil price's potentially depressing effect on world trade is his main concern.
"I would be much more concerned about [world trade] than a direct impact on profit and loss," he says.
Many container lines have even maintained the higher speeds at sea which have become the norm in the past year, despite the increased fuel consumption.
The faster speeds follow increased container port congestion and the introduction of new, bigger ships, both of which increased the time ships spent in port. Lines faced a choice between running faster at sea or buying expensive new ships to maintain schedules.
"There's a cost in speeding the ship up," says Michael Parker, chairman of the UK arm of CMA-CGM, a major French container line. "But if it saves you putting another ship in the service, the economics tend to balance out again."
New, more efficient marine engines have helped to contain the cost impact. Such technology is likely now to get more attention.
Some lines may even in future try avoiding oil altogether. Wallenius Wilhelmsen last year exhibited a conceptual design for a car-carrier which would depend solely on solar panels, wind and wave power for propulsion.
Yet, for one of the biggest sectors of the shipping industry, the oil price effect works very differently.
Rates for oil tankers are still well above their average for the past two decades, although rates for very large crude carriers -- one of the largest classes -- have fallen from more than $225,000 a day last autumn to about $30,000 a day now.
"The more demand, the higher the oil price -- the more demand, the higher the need for tankers." says Evangelos Pistiolis, president and chief executive of Top Tankers, an Athensbased tanker operator.
High bunker prices are far less significant for tanker operators, he says, than the big increases in freight rates at times of high demand.
"Oil tankers are the only part of the market that smiles when the oil price is high." Mr Pistiolis says.