Environmental Technology

Stern tube leaks, seals and deck runoffs


Malcolm Latarche
Malcolm Latarche
ShipInsight

09 November 2018

Stern tube leaks, seals and deck runoffs

Illegal discharges as a result of deliberate acts undoubtedly cause most of the pollution from shipping but lubricant leak from propeller shaft and rudder bearings are also a common cause of pollution and have attracted attention in recent years. Until around 50 years ago, many ships were fitted with propeller shaft bearings made from lignum vitae an extremely dense timber with a high degree of natural lubricity but these were abandoned in favour of metal bearings and mineral oil lubricants.

Now, as environmental regulations tighten, water-lubricated propeller shaft bearings are becoming a popular alternative to oil-lubricated bearings for commercial vessels. This was already happening before the US EPA revised the VGP in 2013 but that action is likely to accelerate take up of water lubricated bearings and new seal types and also a greater use of new approved lubricants.

Conventional seals inevitably leak over time due to wear and damage but water-lubricated bearings avoid oils and grease lubricants altogether. Seawater is pumped into the bearing and it simply discharges to the sea. It lubricates and dissipates heat from shaft friction and most manufacturers of water lubricated seals say their products provide equal performance to conventional seals.

The need for more environmentally friendly seals is not limited to conventional propulsion systems with a tail shaft and propeller but is also a necessary consideration for makers of podded propulsion systems and thrusters. Each new generation of these brings improvements in seals and designs that limit the possibilities of lubricant leakage.

Environmentally friendly alternatives

Under the new VGP introduced in the US in 2013 the list of permitted substances and the quantity each ship above 300gt will be allowed to discharge was reduced – quite dramatically in some cases. One of the changes under the VGP affects lubricants in any equipment or system that has an oil-to sea interface. In essence, that affects all propulsion systems and also deck machinery where run-off over the ship’s side could occur.

Previously under the earlier 2008 VGP, operators were free to use any lubricant they wished but from December 2013 the rules require environmentally acceptable lubricants (EALs) unless doing so would be ‘technically unfeasible’. EALs are defined as biodegradable, which rules out all mineral-based lubricants and even some synthetic alternatives.

The exact definition of an EAL is contained in an EPA document, EPA 800-R-11-002 November 2011, which can be accessed via the organisation’s website. Operators have to apply for a VGP before a vessel enters US waters and to do so they need to identify all oil-to-sea interfaces and lubricants involved. Although ship operators can expect product makers to advertise compliant products there are some labelling schemes that may give added assurance.

EPA does not consider on-deck equipment that comes into contact with rain, splashed with waves, wave-generated spray, or subject to icing to be a form of immersion, and therefore not an oil-to-sea interface. Vessel operators are not required to use EALs in on-deck machinery that is not subject to immersion. However, discharges from deck machinery are subject to other discharge requirements, such as those for Deck Washdown and Runoff (Section 2.2.1 of the VGP), which recommends the use of EALs.

Among the most obvious systems that are considered oil-to-sea interfaces are the stern tube, rudder bearings, CP propellers, thrusters, and fin stabilisers. In addition, winches, cranes, hatch covers, and even crane wires and the like must be considered. The ship will be required to document all lubricants and any reason why the use of an EAL would be technically unfeasible.

Most major oil companies and some specialist suppliers have formulated compliant products that are readily available although with a premium price tag. However, these products are not necessarily compatible with some makes of seals, especially conventional rubber seals. This is a known problem and most combinations of lubricants and seals have been tested for compatibility over normal drydocking cycles of two to three years.

In selecting an EAL, operators must therefore seek advice from the seal manufacturer and great care must be exercised if the vessel makes use of enhanced or extended dry docking strategies. Inspections with regard to EALs would involve visual sheen tests and inspections of deck runoff.