Caution required switching to bio-oils for stern tubes

Regulators such as the US Environmental Protection Agency are looking to mandate biodegradable stern tube oil use on newbuildings and voluntary schemes such as the Green Shipping Index recognise it favourably.

There is good reason to consider pollution issues from stern tubes. Dr Dagmar Schmidt Etkin, president of Environmental Research Consulting, published a study in 2010 estimating that an estimated 4.6 to 28.6 million litres of lubricating oil from stern tubes is discharged into port waters each year. The average vessel leaks around six litres a day and no matter how stringent legislation becomes, completely stopping leaking will be near impossible, even for well-maintained newer vessels opting for designs to reduce such leakage.

The oil majors and a range of specialist manufacturers have now added biodegradable lubricants to their product range, but some in the industry are still urging caution. DNV’s Classification Support newsletter 12-77 published late 2012 states that: “For the time being, we are reluctant to believe that biodegradable oils are the sustainable solution to environmental oil spills. There are different drawbacks in the way these oils react to the challenges of lubricating machinery.” Biodegradable oils will deteriorate when mixed with water with the inevitable result that lubricating capabilities will be adversely affected.

Swelling is one example cited. All synthetic materials which are used in propeller shaft seals will swell in oil but some seem to swell more in biodegradable oils than in mineral oils. This can be positive as it increases the sealing pressure but if too much swelling occurs, the sealing pressure will make the seal shear. Current experience is based on short-term swelling, says DNV.

Additionally, DNV cites concerns over bacterial growth in the presence of water. This can result in corrosion, sludge formation, deterioration of the oil and reduction in lubricating properties. In the worst case, complete systems may need to be eptied and cleaned and have any corrosion damage repaired.

DNV’s Clean Design notation, therefore, has been revised on 1 January 2013 to include a third option for stern tube bearing lubricants in addition to water and biodegradable oil. A conventional lubricant will also be acceptable if an effective, monitored double-barrier seal system is in place. If biodegradable oil is used, an arrangement is required that will keep the water content of the oil under control and seal material must be compatible with the oil used.

However, Tim Roadnight, head of global business for oil condition monitoring and lubricants at laboratory testing and certification provider Intertek, says newer vessels will most certainly be utilising the most up to date technology including new water based lubrication systems or a lean towards more biodegradable products. “Developing emulsifiable oils with no toxic additives and that can biodegrade within the marine environment thus preventing damage to aquatic ecosystems, will minimize long term concerns. If these can be developed in partnership with the classification societies and also meet upcoming ISO, Marpol Annex and local government requirements, this will be paramount to the success of new R&D.”

He continues: “All three systems: water lubricated, mineral oil based and biodegradable have their own merits. Whichever choice is made needs to be undertaken with input from the machinery manufacturer, owner / operator and ship builder. This is to establish whether or not the vessel has been designed to use biodegradable lubricants (without redesign and upgrades) and whether the biodegradable oil is compatible with other mineral based oils either already in service or being replaced. If not, thorough flushing of the entire system will be required before the switch is made requiring dry docking.

“Biodegradable oils generally have a higher specific gravity to mineral oils resulting in higher level of solid particulate contamination and requiring higher levels of filtration. The operating temperatures will often be different, as will feed rates. Emulsifiable oils will also increase operational costs as they are inherently more expensive, up to 3 – 4 times price of mineral oil. Ship owners and architects should research all options made available and review the in-service requirements and operation environment for the vessel’s life span.”

Jean-Phillppe Roman, technical director at Total Lubmarine is seeing an increased interest for biodegradable lubricants, particularly from customers who work closer to ports and harbours and areas where strict regulations apply to safeguarding the marine environment. Total Lubmarine has created Bioneptan as a stern tube lubricant which has minimal impact on the marine environment should it come in contact with the sea. Bioneptan lubricants are synthetic ester biodegradable lubricants manufactured with synthetic base oils making them non-toxic and biodegradable to nearly 80 per cent of its ultimate state of carbon and water within 28 days. Bioneptan is also more durable than mineral lubricants and has undergone rigorously independent testing with excellent results, says Roman.

Klüber Lubrication offers Klüberbio RM 2-150 optimised for operation in moderate and warmer regions of the world and Klüberbio RM 2-100 which also copes with continuous Arctic conditions. Both are miscible with mineral oil-based stern tube oils for easy changeover.

The compatibility between seal and lubricant is important and shipowners must evaluate this carefully when considering changing oil types. Klüber Lubrication cooperated closely with its affiliated companies SIMRIT and Merkel Freudenberg Fluidtechnik in the development of the lubricants. Both companies are suppliers of shaft seals and profiled seals, and like Klüber Lubrication, these two companies belong to the Freudenberg Group. Together, the three companies are therefore in a unique position to match elastomers and lubricating oils for the taxing requirements of propeller manufacturers and shipowners, says Dirk Fabry marketing and application engineering manager at Klüber Lubrication.

“The use of biodegradable stern tube lubricants is just beginning and everybody in the industry needs to get experience with the new oils,” says Fabry. “All the seals developed over past decades have been optimised for compatibility with mineral oils not synthetic biodegradable oils. Although it is not necessarily the oil type itself but also the additives that can influence performance and reliability. These additives may improve the properties in bearings such as extreme pressure or wear protection but have varying compatibility with the elastomers used in seal manufacturing.” The Freudenberg Group has an on-going research project called ‘Lube & Seal’ further evaluating oil and additive combinations under dynamic load.

Due to their good seal compatibility, Klüberbio RM 2 oils are approved by leading propeller shaft seal manufacturers and are in service in several ships. Klüber’s biodegradable lubricants already on the market are deemed non-toxic to marine organisms according to OECD 201, 202 and 203 tests. They have also been tested to OECD 301 F and achieved biodegradability of over 60 per cent in 28 days. Although the test results were actually higher than 60 per cent, Klüber prefers not to publish one time laboratory results as some other manufacturers do. This could create a misleading impression as actual biodegradability rates in the ocean can vary greatly in response to environmental parameters such as temperature and bacteria abundance.

Peter Vickers, chairman and managing director of Vickers Oils, also cautions the industry on performance statistics. He believes that the adoption of what he calls environmentally acceptable lubricants has been held back by the absence of internationally agreed standard definitions which has led to a situation of ‘buyer beware’. In a paper published in 2012 he states that some suppliers rely on misleading statements and meaningless performance claims for products that are not as environmentally friendly as the claims suggest. He recommends suppliers should be asked for details of the specific test standards used and the laboratory test results.

“Some products have been promoted as non-sheen but actually have limited biodegradability and may be toxic,” he says. “Other products, such as some food-grade lubricants, are assumed to be environmentally acceptable lubricants but are actually based on mineral oil and are neither biodegradable nor renewable.”

The toxicity of a lubricant is at least as important as its biodegradability, says Vickers. Algae have been shown to have high sensitivity towards toxic substances so Vickers believes it is essential to include algae in any toxicity tests. Test results are expressed in terms of the concentration of a substance required to kill or inhibit the growth of 50 per cent of the test population. “Terminology varies, with GESAMP for example regarding >1,000mg/l as non-toxic whilst others refer to that as relatively harmless. Some suppliers quote a result of ‘pass’ which is completely meaningless without any reference to actual levels.”

More research is required into bioaccumulation – the build up of organic chemicals in the fatty tissues of organisms which is harmful and can pass up the food chain. The availability of non-bioaccumulating additives is improving but their technical performance today is limited. However, the concentration of non-biodegradable additives is generally lower in environmentally acceptable lubricants than mineral oil-based ones, he says.

The use of renewable bases such as natural oils and some synthetic esters is also important to Vickers. “Non-renewable base materials should be avoided where possible. However, the performance requirements of the final lubricant may at present require the inclusion of some non-renewable biodegradable material. This may change in the future.”

According to Vickers, over 1,100 vessels have now been supplied with environmentally acceptable lubricants with for distinct benefits: superior performance, evidence of continuous improvement for compliance with programs such as ISO 14001 and TMSA, enhanced corporate reputation and as a mitigating factor in some cases of accidental leakage. He sees a greater rate of adoption in the future, not least as a result of regulatory activity.

This includes

IMO Guidelines for Ships Operating in Arctic Ice-Covered Waters which states: “Sterntube bearings, seals and main propulsion components located outside the hull should not leak pollutants. Non-toxic, biodegradable lubricants are not considered to be pollutants.”