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Container ship speed matters when it comes to fuel

18 Oct 2008

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. Slow steaming may also be seen at present as an answer to over-capacity. But having reached historic highs of $USD 147 a barrel in July this year, the  price of oil has fallen back to below  $USD 100/barrel this week. As a result ship operators need to be prepared to manage high oil prices  and volatility.  Container ship designs have reflected the prevailing price environment at the time of  construction resulting in the  delivery of ships that are highly  unsuitable in future years when the  oil price fluctuates. Operational flexibility, enabling owners to respond to different oil prices, is an  area that Lloyd’s Register has been  addressing.  Lloyd’s Register also believes that the industry needs flexible  designs capable of operating most  effectively across different power  output bands.  Technical and economic research conducted by Lloyd’s  Register is available to ship owners  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 straight forward 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 (see graphic click here).  In addition it is a waste of engine  capacity and a capital cost penalty  to carry unused power potential.  Care is needed when running  ships at reduced speeds  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 

• loss of turbocharger efficiency  

• loss of propeller efficiency  

• fouling of hull and propeller due  to reduced ship speed 

• increased compensatory fuel  consumption of auxiliary engines  to supplement loss of heat  recovery capability 

• increased lubricating oil  consumption 

• possible increased vibration levels  and detrimental effects.