DNV GL, together with marine insurers The Swedish Club, Norwegian Hull Club, Skuld and Gard have completed a study of the properties of environmentally acceptable lubricants (EALs) for stern tube applications, in response to the growing number of stern tube bearing failures recently. In fact, DNV GL sponsored lab tests demonstrating that EALs behave differently than traditional mineral oils under transient pressure and temperature conditions.
According to DNV GL, the majority of the observed bearing failures to date have occurred with fresh oil, thus the initial portion of the study focused on lab testing non-degraded new lubricants. Traditional mineral oil lubricants may be similar in regards to their viscosity properties and load-carrying capabilities, but the study showed that EALs’ properties can differ sometimes.
Øystein Åsheim Alnes, Principal Engineer at DNV GL Group Technology and Research, stated that “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.”
Whatsoever, the Principal Engineer added that there may be transient conditions involving high oil film pressures and/or low oil temperatures where EALs have a reduced load-carrying capacity.
To balance out these liabilities, the class society proposes applying a viscosity factor of 0.75 to non-mineral oils in order to bring the safety margins back in line and further recommends selecting a bearing design that requires a very low minimum speed for adequate lubrication.
Moreover, DNV GL is updating its shaft alignment design rules aiming to differentiate between stern tubes using EALs and those using mineral oils, and it has introduced two new voluntary shaft alignment notations.
In fact, Øystein Åsheim Alnes, added that “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.”
The still ongoing study further aims to examine oil film thickness, thermal properties, mid-to-long-term degradation, hydrolysis and bearing/seal wear rates.
The classification society launched the joint development project (JDP) with marine insurers The Swedish Club, Norwegian Hull Club, Gard and Skuld in January 2018, aiming to test the potential influence of Environmentally Acceptable Lubricants (EALs) on failures in stern tube bearings.
Moreover, During Nor-Shipping 2019, DNV GL informed that it will update its shaft alignment design rules to differentiate between Environmentally Acceptable Lubricants (EALs) and mineral oils in stern tubes, based on the phase 1 findings from the joint development project (JDP).
