Always Compliant with Seawater Lubricated Shaft Bearings

If your vessel is trading in US waters, you must not discharge oil from any oil-to-sea interface in quantities that may be harmful to aquatic life. And if your ship does leak, the lubricant must be an environmentally acceptable lubricant according to the new US Environmental Protection Agency’s Vessel General Permit (VGP) that came into effect Dec. 19, 2013. 

Seawater is a free environmentally acceptable lubricant (EAL), so one alternative that many cruise ship, ferry, bulker and tanker operators have taken is to use seawater lubricated propeller shaft bearing systems. Over 600 commercial ships are using a seawater-lubricated system that uses no oil – meaning full compliance with the VGP. Currently, the U.S. EPA recommends that all new build vessel operators endeavor to use seawater-based systems for their stern tube lubrication to eliminate the discharge of oil from these interfaces to the aquatic environment.

Existing ships can be converted to seawater-lubricated propeller shaft systems as these systems typically fit in the same space as an oil lubricated system. Several companies have converted their existing ships from oil lubricated systems to seawater lubricated shafts saving hundreds of thousands of dollars per year on stern tube lubricants and aft seal maintenance costs. The oil-to-seawater conversion is accomplished during a planned drydock where corrosion resistant shaft liners and a water lubricated forward seal were pre-ordered and the shaft and non-metallic bearings were ready for install when the ship drydocked.

Most ships use mineral oil to lubricate the propeller shaft and the oil is contained in the stern tube by the aft seal – which is the oil to sea interface.  In most cases, you cannot just replace the mineral oil with seawater or other oil-based EAL’s. Oil-based EAL’s need to be compatible with the sealing materials to ensure leakage is controlled – ship owners will need to check with their seal supplier and it may be necessary to upgrade the sealing rings or upgrade to a new seal.   

Oil-based EAL’s may impact on the seal wear life meaning increased maintenance costs.  Typically, costs of oil-based EAL’s are 3-5 times more expensive than mineral oils. If you are using a sophisticated air seal, commonly promoted as a non-polluting aft seal, you are still required to use an oil-based EAL, as it is not possible to guarantee that oil leakage will never occur.  Fishing nets and ropes can still damage the seals allowing leakage to occur.  

Still a concern for ship owners is that oil-based EAL’s are still considered a pollutant under the Oil Pollution Act of 1990 (OPA ‘90) and US Clean Water Act (if there is a sheen). Any discharges of oil-based EAL’s still require reporting of the discharge to the U.S. Coast Guard, as well as having clean-up and remediation costs. Even though biodegradable lubricants may be deemed non-toxic by OECD testing, their presence on the water surface is a threat to seabirds – the hydrophobic nature of oil causes bird plumage to absorb the oil readily, thereby decreasing a bird’s insulation, waterproofing, and buoyancy leading to death from hypothermia and starvation.  

So when it comes to discharges from a ships propeller shaft system, the ship owner that trades in US waters has two choices for their existing ships and ships they plan to build: 1) replace mineral oil with an oil-based EAL to lubricate the metal bearings and ensure the seal is compatible with the EAL or 2) convert or build a ship using a seawater-based system using non-metallic propeller shaft bearings.       

Some ship owners such as Princess Cruises, Disney Cruise Lines, Oceania Cruise Lines, US Gypsum Corporation, CSL Group, Algoma Corp., Flinter Group, ConocoPhillips, Carisbrooke Shipping and Staten Island Ferries are already compliant with the VGP, as they have chosen to use seawater as the propeller shaft lubricant and thus have reduced operating costs and no oil pollution worries.