DNV GL Study Ties Stern Tube Bearing Failures To The Use Of Synthetic EALs
Maritime classification society DNV GL has published the results of an investigation into a surge in propeller shaft bearing failures over the past five to six years. DNV GL concluded the uptick in propeller shaft bearing failures is due to the use of a number of environmentally acceptable lubricants (EALs).
Since late 2013, the Environmental Protection Agency (EPA) has required certain commercial vessels to use biodegradable EALs rather than traditional mineral oil-based lubricants in “oil to sea interfaces” in order to reduce waterway pollution. However, since that time, DNV GL said bearing failures have been on the rise.
“The introduction of the Vessel General Permit (VGP) requirements in 2013 resulted in a sudden and massive uptake of environmentally acceptable lubricants (EALs) as stern tube lubricants for ships,” DNV GL stated in the report. “This trend coincided with a reported widespread increase in the number of stern tube bearing failures, and it was inevitable that the maritime industry started to question the actual lubrication performance of the EALs.”
DNV GL initiated the study in late 2017, with the results released October 11. The study found that most bearing failures occurred “during hard maneuvering at high ship speeds, during mooring trials, and when operating with a partly submerged propeller.” In all those conditions, EALs were found to have “a reduced load-carrying capacity,” which could result in increased friction and bearing failure.
In a November 7 letter to customers, Thordon Bearings, which manufactures, among other things, propeller shaft bearings, said the DNV GL study left no doubt the safest EAL available to operators—simple seawater.
“While no ship operating a propeller shaft bearing lubricated by water—an EAL designated by the U.S. Environmental Protection Agency—has been immobilized to date, there has been conjecture concerning the poor operational performance of some synthetic lubricants,” said Thordon Bearings Regional Manager George Morrison. “It is now unequivocal: the performance of these EALs is less than satisfactory.”
Morrison is regional manager for Thordon Bearings’ European, Middle Eastern, African and Australasian markets.
Morrison pointed out that the primary cause of failure identified was bearing wiping, which occurs when oil film is lost. DNV GL found the synthetic EALs behave differently than mineral oils under varying operating conditions, which affects oil viscosity and load-bearing capacity.
“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,” Morrison said.
Morrison pointed out the DNV GL study did not include seawater-lubricated propeller shafts. While seawater-lubricated propeller shafts fall under different class rules, they’re nonetheless perfectly acceptable on DNV GL-classed vessels, according to the Thorndon statement.
Thordon Director of Marketing & Customer Service Craig Carter added that he expects the study’s findings to cause operators to seriously reconsider the use of synthetic lubricants in propeller shaft bearing applications.
“There is no doubt that lubricating white metal bearings with biodegradable oil can be technically and commercially risky,” Carter said. “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.”
As part of the company’s letter to shaft bearing customers, Thordon touted its COMPAC open seawater-lubricated propeller shaft bearing system.