The customers are being alerted by Thordon Bearings about the publication of the study by DNV GL that concludes that the recent increase in the failures in propeller shaft bearing in actually related to the usage of some lubricants that are environmentally accepted.
In conclusion to the first phase of an ongoing investigation, DNV GL reported that “there are transient conditions involving high oil film pressures and/or low oil temperatures where [synthetic] EALs will have a reduced load-carrying capacity. The vast majority of stern tube bearing failures in recent years occurred in the same type of transient conditions – during hard manoeuvring at high ship speeds, during mooring trials, and when operating with a partly submerged propeller”.
George Morrison, Regional Manager, EMEA – ANZ, Thordon Bearings, said: “While no ship operating a propeller shaft bearing lubricated by water – an EAL designated by the US Environmental Protection Agency – has been immobilized to date, there has been conjecture concerning the poor operational performance of some synthetic lubricants. It is now unequivocal: the performance of these EALs is less than satisfactory.”
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A report by DNV GL and a subsequent webinar, titled Environmentally Acceptable Sterntube Lubricants – How to Avoid Costly Failures revealed that a large increase in sterntube damage that has an impact on all types and sizes of ships after the introduction of rules in 2014 that prohibits the usage of mineral oil in oil-to-sea interfaces of vessels that are trading in the US waters.
“This investigation verifies our long-held view that these EALs can impede shaft bearing and seal performance, damage critical components and compromise oil-tight integrity, resulting in emergency remedial repairs at significant cost to the shipowner,” added Morrison.
Seawater lubricated propeller shaft bearings was not included in the investigation because this arrangement fell in a different class rules which are related to sterntube bearings that re lubricated by mineral or synthetic oils.
“We have different rules covering seawater-lubricated bearings, and there is no problem to use those kind of designs for DNV GL classed ships [sic]. We did not see a necessity to include those in the same study,” said DNV GL in response to a written question put forward during the webinar.
The industry is expected to be rippled by the finding of the report because of the investment in EAL oils which was broadly seen as a technically viable solution for the reduction of marine pollution from mineral oil based lubricants.
Craig Carter, Director of Marketing & Customer Service, Thordon Bearings, said: “I would not be surprised if the classification societies start to revise their shaft alignment and withdrawal rules for bearings using this kind of lubricant. There is no doubt that lubricating white metal bearings with biodegradable oil can be technically and commercially risky. With mineral oil-based lubricants now rightly regarded as environmentally unacceptable, the only proven EAL option available for lubricating the propeller shaft bearing is seawater.”
DNV GL says that it has been seeing a rise in the number of ships that have taken to water lubricated propeller shaft bearing arrangements since 2014.
The classification society aims to continue the EAL study further investigating about the thickness and thermal properties of oil film. Mid-to long-term degradation, hydrolysis and wear rates of those seals and bearings using these types of lubricants will be seen in the third phase of the investigation.
Reference: DNV GL