High or volatile oil prices and environmental concerns, point to the need for new designs capable of operating efficiently at different speeds. Lloyd's Register warns that care needs to be taken when running at reduced power outputs. Most container ships trading today, and on order, were designed for a world of relatively low energy prices. With oil at recent high levels many owners have been implementing or considering slow steaming strategies.
Technical and economic research conducted by Lloyd's Register is available to shipowners and shipyards to provide guidance when investigating solutions. Altering operational speed has substantial implications for both hull forms (slower ships can be boxier and, therefore, roomier) and for propulsion systems.
There are diminishing returns in slow steaming with machinery designed to operate at higher outputs - particularly when reducing speeds to below 20 knots. There are technical considerations involved in running at reduced power outputs: to ensure reliable operation from engines designed to run optimally at higher outputs, closer surveillance of engine performance and operating parameters, fuel quality, lube oil consumption and power-speed conditions will be required.
By way of example a relatively straightforward calculation demonstrates that for a large container ship designed for 25 knots at 70,000kW main engine power, speed reduction to 20 knots would require just 50% power. Given that voyage time will increase as a consequence of the reduced speed, the fuel saving will be somewhat less, about 40%. So slow steaming can offer a large saving in fuel consumption; however, it can be calculated that total NOx emissions increase - by up to 40 tonnes - when steaming between 20 and 25 knots. In addition it is a waste of engine capacity and a capital cost penalty to carry unused power potential.
From a technical classification perspective, owners looking to operate ships outside the recommended continuous operating design envelope should initiate careful analysis to ensure their engine operates reliably and it will not lead to consequences detrimental to safe, reliable ship operations. Factors to be taken into account:
- Possible loss of effectiveness of heat recovery systems
- Increased compensatory fuel consumption of auxiliary engines to supplement loss of heat recovery capability
- Loss of turbocharger efficiency
- Loss of propeller efficiency
- Fouling of hull and propeller due to reduced ship speed
- Increased lubricating oil consumption
- Possible increased vibration levels and detrimental effects.