Abstract
To reduce environmental pollution from maritime transportation, the mandatory use of environmentally acceptable lubricants (EALs) at all seawater–oil interfaces was introduced through various legislative measures. However, the transition from mineral oils to EALs in the case of stern tube lubrication systems was accompanied by an increase in the occurrence of stern tube bearing malfunctions that required dry docking of the ship and costly repairs. As a result, today EALs suffer from a loss of trust from ship owners, as can be evidenced by the fact that only a small fraction of new ships are being commissioned with lubricating systems using EALs. Alternative solutions, such as air sealing systems with mineral oils or water lubrication systems, are often preferred solutions instead. As a result, the large number of ships operated nowadays use mineral oils, although in combination with various leak prevention solutions. To address the cause of the mentioned issues, this study focuses on the analysis of different performance characteristics between stern tube EALs and mineral oils. Their properties are discussed with regard to typical operating conditions of the stern tube bearings. The main identified differences were in characteristics such as viscosity–temperature and viscosity–pressure behavior, oxidation stability, and hydrolytic and microbial stability. Suggestions for EAL formulations tailored to the stern tube bearing lubrication are proposed, as well as the focus areas for future research to understand the EAL behavior in this application.