DNV GL Study Finds Differences Between Mineral Oils and EALs

Viscosity vs. pressure for mineral oils and EALs of similar grades (courtesy DNV GL)

Published Nov 8, 2019 10:18 PM by The Maritime Executive

In response to a growing number of stern tube bearing failures in recent years, DNV GL has completed a study of the properties of environmentally acceptable lubricants (EALs) for stern tube applications. In cooperation with The Swedish Club, Norwegian Hull Club, Skuld, Gard, the University of Sheffield and INSA Lyon, DNV GL sponsored lab tests demonstrating that EALs behave differently than traditional mineral oils under transient pressure and temperature conditions. 

The majority of the observed bearing failures to date have occurred with fresh oil, according to DNV GL, so the initial portion of the study focused on lab testing non-degraded new lubricants. Traditional mineral oil lubricants are very similar with regard to their viscosity properties and load-carrying capabilities, but the study showed that EALs' properties differ in some circumstances.

“The overall conclusion we can draw from the JDP Phase 1 results is that (new and non-degraded) EALs provide safety margins equivalent to those of mineral oils in most operating conditions,” said Øystein Åsheim Alnes, Principal Engineer at DNV GL Group Technology and Research, in  a statement announcing the results. “However, there are transient conditions involving high oil film pressures and/or low oil temperatures where EALs will have a reduced load-carrying capacity.” 

To offset these liabilities, DNV GL advises applying a viscosity factor of 0.75 to non-mineral oils in order to bring the safety margins back in line. In addition, the class society recommends selecting a bearing design that requires a very low minimum speed for adequate lubrication.

DNV GL is also updating its shaft alignment design rules to differentiate between stern tubes using EALs and those using mineral oils, and it has introduced two new voluntary shaft alignment notations. 

“The DNV GL lubrication criteria provides yards and designers with a strong tool for optimizing stern tube bearing design, including both the lubricant viscosity and now the lubricant type," said Alnes.

Thordon Bearings, a leading manufacturer of seawater-lubricated bearing systems, says that it sees the results as a vindication of its skepticism about the new synthetic EALs. 

“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,” said George Morrison, regional manager EMEA – ANZ for Thordon Bearings.

Thordon says that no ship operating with a seawater-lubricated shaft bearing has been immobilized to date. As a lubricant, seawater naturally meets U.S. standards for environmental acceptability, and it is a designated EAL. 

DNV GL did not include seawater lubrication in its study, as it is already covered under a separate set of class rules, but it says that it has noticed an uptick in the number of ships adopting water lubricated shaft bearings since 2014. 

The study is still ongoing, and in its next two phases, it will examine oil film thickness, thermal properties, mid-to-long-term degradation, hydrolysis and bearing/seal wear rates.