Class society DNV GL is updating its design rules based on a comparison of the load-carrying capacities of biodegradable lubricants and mineral oils
A joint development project (JDP) comparing efficacies and safety tolerances of the two types of lubricants showed that, under certain operating conditions, environmentally acceptable lubricants (EALs) have a reduced load-carrying capacity compared with mineral oils, which are typically petroleum-based and less biodegradable.
As DNV GL principal engineer Øystein Åsheim Alnes put it, "Phase one has looked into how pressure, temperature and shear rate influence the viscosity of the lubricants, thereby affecting the oil film thickness and the load-carrying capacity. The findings show that while EALs provide safety margins that are equal to mineral oils in most operating modes, there are transient conditions where the EALs can have a reduced load carrying capacity."
In essence, Mr Alnes said the results show that for operating conditions involving high oil film pressures or low temperatures or both factors combined, EAL safety margins are less than those of similar grade mineral oils.
Mr Alnes said the JDP results "fit with the damage picture" that owners and shipyards have reported in recent years, leading DNV GL to differentiate between the two types of lubricants in its classification design rules.
Results from phase one of the JDP led DNV GL to add a viscosity influence parameter to the current lubrication rule criteria for most aft propeller shaft bearings. Ultimately, Mr Alnes said, limitations on the peak oil film pressure in design are “very important”.
The new requirements come into play on newbuilding designs from 1 July 2019, however Mr Alnes also offered recommendations regarding aft lubrication in existing vessels.
"If you want to use an EAL in stern tube bearings, you will need to do further optimisation or use a higher-viscosity mineral oil," he said.
"If you are converting between mineral oils and EALs, DNV GL recommends a switch to the next grade higher viscosity (of EALs)."
Mr Alnes said the JDP is advancing to its next stage, where "oil film forming capabilities, mixed/boundary lubrication behaviour and lubricant degradation will be further scrutinised".
DNV GL is partnering in the JDP with some of the largest insurers in the maritime industry – Swedish Club, Norwegian Hull Club, Gard and Skuld.
The project was launched in January 2018 in response to an increase in stern tube bearing failures and consequent shaft alignment problems following 2013 US regulations mandating the use of EALs in its waters.