Feature 1 | GREEN SHIPPING
A Seawater-lubricated bearing being installed.
were unreliable – no one knew when they would wear out, as they operated in an uncontrolled environment – wood bearings oſten had to be changed out aſter each Atlantic Ocean crossing. However, there have been significant
changes since the 1950s. First of all, the design of a seawater-lubricated system has changed. Te seawater is taken from the sea and is pumped through non-metallic bearings before being returned to the sea. Te seawater enters the forward section of the stern tube just aſt of the seal and then passes through the forward and aſt bearing prior to re-entering the sea. Seawater from the sea chest is either filtered or conditioned removing any abrasives in the water. Since the seawater is now cleaner, the non-metallic bearings will last longer. Te bearings used are non-metallic so
there is no issue of bearing corrosion. Te shaſt and inside of the stern tube, however, does require corrosion protection from the seawater. Bronze liners are typically used in way of the bearings and a flexible, anti-corrosion shaſt coating is required between the shaſt liners. Te bronze liners are typically the most expensive part of a ship owner’s decision to switch to a seawater lubricated system at the newbuild or conversion stage. New bearing designs and materials currently available have demonstrated long life performance,
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which has allowed some class societies to inspect the stern tube and not withdraw the shaft. Use of seawater lubricated bearings eliminates the aſt seal, as well as the storage, sampling and disposal of oil. Te potential impact of stern tube oil pollution is zero, as no oil used. Seawater-lubricated propeller shaft
bearings are still used by most of the world’s Navies and Coast Guards for safety reasons and non-catastrophic failure mode. Te experience gained from their continuous use has now transferred to commercial ships, as new materials and designs have shown technical equivalence to oil lubricated stern tube systems. One segment of the commercial shipping industry that has adapted seawater lubricated propeller shaft bearings on many of their ships is the cruise industry. For example, Carnival Corporation owns 15 cruise ships using seawater lubricated propeller shaſt bearings (all twin screw) with the first ship equipped since 1998 - and that ship is still running with the same bearings. Tis is quite a contrast to the original water lubricated bearing materials used in the first half of the 20th century where wood bearings lasted only a few years. Currently, there are over 750 commercial ships that use seawater lubricated propeller shaſt bearing systems including large tankers, bulk carriers, dry cargo ships and ferries.
Weighing up the issues Commercial ships have been using new design seawater lubricated propeller shaſt bearing systems since the late 1990s with very few issues. Ship owners have saved money as aſt seal maintenance costs are eliminated because there is no aſt seal. Tere are no oil storage and oil disposal costs and no emergency seal repair costs. Te bearings are acoustically quiet (in use by NOAA and other fisheries research vessels), so there is reduced noise impact on sea life. Tey pose zero risk to the ocean and sea environment as no oil-based lubricants are used. It also eliminates any risk of criminal or civil penalties and other adverse reactions such as bad public relations for the ship owner that may result from oil-based lubricant discharges into the ocean. Te cons to this type of system are related
to the corrosion protection of the shaſt and stern tube which may mean a higher upfront cost compared to an oil lubricated white metal bearing.
2020 – Zero ship discharges? Te regulation and elimination of global ship discharges are becoming the norm. Under the EU’s directive on “Ship Source Pollution” [EU/2005/35], the OSPAR Commission called to “move towards the target of cessation of discharges, emissions and losses of hazardous substances by the year 2020. In the US, under the Clean Water Act, the EPA now regulates many ship discharges, specifically oil lubrication discharges from stern tubes. Te time may be coming when discharges of oil-based lubricants may not be permitted and some ship owners are looking or have implemented other alternatives to using oil-based lubricants. A seawater-lubricated propeller shaft
bearing system offering zero pollution that operates interchangeably in the same space as an oil-lubricated stern tube system can meet the zero discharge requirement. It is not a half step towards eliminating this type of ship pollution but a realistic and existing solution to eliminate oil-based lubricant discharges from the world’s oceans and seas today. NA
The Naval Architect May 2012
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