Potential and risk of using environmentally acceptable lubricants (EAL) in stern tube bearings of marine propulsion systems – an experimental and numerical study

Approximately 80 million liters of mineral oil-based lubricants are discharged annually from merchant ship stern tube systems. Therefore, the environmental compatibility of lubricants used in the shipping industry is of high importance. Apart from water lubrication, which requires extensive design and material modifications of the systems, another effective approach is the use of environmentally acceptable lubricants (EALs). However, the properties of mineral oils and EALs differ, e.g., in terms of viscosity-temperature and viscosity-pressure relationships. These relationships can affect lubricant film formation and friction, which can further influence the safe operation of the ship. To examine their influence on lubricant film formation and friction, both experimental and numerical approaches have been carried out. The combined influence of temperature and pressure on viscosity was considered for the first time in numerical simulations of stern tube bearings. Compared to mineral oil, the EAL showed an increase of 9.4% in friction and an increase of 7.3% in minimum lubricant film thickness under normal conditions. Under severe operating conditions, a similar or slightly better performance of the EAL in terms of lubricant film formation and friction was demonstrated. These results show that EALs can be used in stern tube systems under normal and severe operating conditions. This is important for ship operators from a regulatory-compliance perspective, as it can aid the transition to EALs. It also showed that the combined effects of temperature and pressure on viscosity should be considered when performing numerical simulations for the design and durability assessment of stern tube systems.