Slow steaming

Slow steaming refers to the practice of operating transoceanic cargo ships, especially container ships, at significantly less than their maximum speed.^[1] An analyst at National Ports and Waterways Institute stated in 2010 that nearly all global shipping lines were using slow steaming to save money on fuel.^[2][3]

Rationale & History

Slow steaming was adopted in 2007 in the face of rapidly rising fuel oil costs (July 2007 to July 2008: 350 to 700 USD/tonne).^[4] According to Maersk Line, who introduced the practice in 2009–2010,^[5][6] slow steaming is conducted at 18 knots (33 km/h; 21 mph).^[1] Speeds of 14 to 16 kn (26 to 30 km/h; 16 to 18 mph) were used on Asia-Europe backhaul routes in 2010.^[7] Speeds under 18 kn (33 km/h; 21 mph) are called super slow steaming.^[1] Marine engine manufacturer Wärtsilä calculates that fuel consumption can be reduced by 59% by reducing cargo ship speed from 27 knots to 18 kn (33 km/h; 21 mph), at the cost of an additional week's sailing time on Asia-Europe routes.^[8] It adds a comparable 4-7 days to trans-Pacific voyages.^[7]

The large container ship Emma Maersk can save 4,000 metric tons of fuel oil on a Europe-Singapore voyage by slow steaming.^[5] At a typical 2008 price of USD 600-700 per tonne,^[4] this works out to USD 2.4-2.8 million fuel savings on a typical one-way voyage. Maersk's Triple E class of ships was designed for slow steaming, with hulls optimized for lower speeds. Because of this, it has less powerful engines than its predecessors.^[5]

Cost Benefits & Operational Issues - A Trade Off

Lowering speed reduces fuel consumption because the drag imparted by a fluid increases quadratically with increase in speed. Thus traveling twice as fast requires four times as much energy (and therefore fuel) for a given distance, which is why driving an automobile at 80 km/h (50 mph) requires less than 85% of the power required by the same automobile driving at 100 km/h (60 mph).

Although lowering speeds lowers the power requirements, the total practical amount of speed reduction is limited by other factors, such as economically viable total voyage time, and the fact that a ships engine and propeller are designed to operate within a certain range of RPMs. Attempting to steam too slowly will place the engine and propeller outside their most efficient range, and will therefore begin to counteract the benefits. Although some ships are being placed in service that are designed to steam most efficiently at slower speeds, the great cost of building a ship and need to remain competitive means that radical changes are unlikely until conditions merit such a risk.

"Smart" Steaming

The trade off between fuel cost savings vs increased cost of personnel, insurance and inventory due to the longer voyage duration is a non-trivial optimization problem. A further level of complexity relates to schedule adherence and reliability - if a ship is planned to move at less than full speed it has the possibility of speeding up if it encounters a delay en route (e.g. weather) to recover its original scheduled arrival time. The balance between cost, duration, emissions and risk is the subject of the EC-funded research project SYNCHRO-NET^[9]. "Smart" steaming is a strategy by which the vessel speed is dynamically optimised based on the real-time state of the sea, weather AND the destination port - for example if there is congestion in the port there is no point in rushing to get there at full speed simply to wait for a berth for two days - instead the ship can go more slowly to conserve fuel and still berth at the same time.

References