Consider seawater propeller shaft lubrication

Nov 30 2013

Seawater is a free environmentally acceptable lubricant (EAL) under the forthcoming US Environment Protection Agency Vessel General Permit (VGP).*

If a vessel is trading in US waters, the owner/operator need to be aware of the new oil to sea interface law that comes into effect on 19th December, 2013. According to the new VGP, all vessels built on or after this date, must use an EAL in all oil to sea interfaces.

For all vessels built before this date, unless technically not feasible, shipowners must use an EAL in all oil to sea interfaces.

Most vessels 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 EALs.

Oil-based EALs need to be compatible with the sealing materials to ensure leakage is controlled – shipowners will need to check with their seal supplier. It may be necessary to upgrade the sealing rings, or upgrade to a new seal. Oil-based EALs may impact on the seal wear life meaning increased maintenance costs.

Typically, costs of oil-based EALs are three to five times more expensive than mineral oils. If 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.

A concern for shipowners is that oil-based EALs 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 EALs still require reporting of the discharge to the US 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.

According to Canada’s Migratory Birds Convention Act ‘No person, or vessel shall deposit a substance that is harmful to migratory birds, or permit such a substance to be deposited, in waters, or an area frequented by migratory birds, or in a place from which the substance may enter such waters, or such an area.’


Seawater is free

For newbuildings, many shipowners are now specifying seawater lubricated propeller shaft bearing systems, illustrated by over 600 commercial ships using a seawater-lubricated system that uses no oil – meaning full compliance with the VGP.

Currently, the EPA recommends that all newbuilding vessel operators endeavour 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 seawaterlubricated 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 can be accomplished during a planned drydocking where corrosion resistant shaft liners and a water lubricated forward seal are pre-ordered and the shaft and non-metallic bearings are ready for install when the ship arrives at the shipyard.


Two choices

So when it comes to discharges from a ship’s propeller shaft system, the shipowner 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

2) Convert, or build a ship with a seawaterbased system using non-metallic prop shaft bearings.

Some shipowners, such as CSL, Algoma, ConocoPhillips and others are already compliant with the VGP, as they have chosen to use seawater as the propeller shaft lubricant thus have reduced operating costs with no oil pollution worries.


*This article was taken from a presentation by Craig Carter, director of marketing and customer service, Thordon Bearings.

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