Speed reduction is a popular method of reducing fuel consumption, but engines may need reserve power for safe navigation in choppy seas
2020 may turn out to be a benchmark year for environment regulations. Along with the sulphur cap, phase 2 of the Energy Efficiency Design Index (EEDI) mandate prescribed by IMO kicks in next year. This means newbuilds will have to scale up their carbon emission reduction plans, becoming much more efficient and emitting far fewer CO2 grams per tonne of deadweight times nautical mile travelled.
There are various pathways to lowering the EEDI: more efficient ship designs and engines; using natural gas or alternative fuels; and less resistant hulls and better performance coatings. But the quickest and technically easiest solution may well be to run the ships slower.
Some critics are, however, say owners are more concerned about freight rates than environmental concerns, adding that slower ships can mean more money.
The benefits of speed reduction are proven, though, according to Torsten Mundt, principal research engineer for emissions to air at DNV GL. He says speed and power share a cubic relationship; any reduction in speed will reduce power and, as a consequence, fuel consumption to the power of three. Mr Mundt recalls the US$100 per barrel surge in oil prices in 2008 and how shipowners reduced speed and reaped the benefits of far lower fuel consumption then.
Common sense would seem to indicate that running slower will mean voyaging longer, which would offset any benefit. But Mr Mundt notes the cubic relation is such that when owners reduced container ship speeds from 21-22 knots to 17-18 knots after the 2008 surge, the fuel consumed reduced from 250 tonnes to 100 tonnes. This meant owners could put another ship out to take care of delays and still save fuel.
And there are other, well-founded fears. Ships need a certain amount of power for safe passage during rough weather. Any major reduction in ship speed would affect the safety of the ship. Mr Mundt proposes that the shaft power should be limited at a certain level to keep EEDI down, but the engines can be designed to have a higher power rating, which can take care of rough weather requirements. The control could be in the hands of the master who can flip a switch in a true rough weather situation and allow the engine to take up more load. Such changes would have to be rare, genuine and recorded, adds Mr Mundt.
While EEDI applies only to newbuilds, proposals exist to reduce the speed of the existing fleet thanks to compelling economics. This would mean derating the engine, which would require tweaks to auxiliary equipment such as turbochargers. In the case of newbuilds, however, Mr Mundt argues that the entire ship design can be optimised for the shaft power limitation, which would be the operating norm, and engine-makers can focus on part-load operation.
Some observers point out that hull performance coatings are typically designed for higher speeds and hull resistance would be greater at reduced speeds. If fuel savings from speed reduction are significant and can help meet environmental goals, it may well be that other technologies like coatings have to adapt.
The IMO has left it to the industry to find the pathways towards reduced EEDI. It is likely to take up shaft power limitation early next year.
Notes for graph:
Engine power is determined by the yard and owner pre-build. If the design requires a limiting of the shaft power to meet EEDI requirements, power is limited. In an emergency, the master must have the power to override these limitations, so the ship can continue to comply with stringent EEDI requirements and the safety requirements set out in Marpol Reg. 21.5 (image: Torsten Mundt DNV GL)