Major findings of EAL study
To better understand how environmentally acceptable lubricants (EALs) perform compared to traditional mineral oils commonly used in stern tube bearings, DNV GL initiated a joint development project together with the marine insurers The Swedish Club, Norwegian Hull Club, Skuld and Gard. The study has so far identified two main aspects where the tested biodegradable lubricants behave differently when compared with a reference mineral oil: pressure-related and temperature-related viscosity properties. Additionally, one of the EALs are found to display shear thinning properties at high shear rates.Start Slideshow
Scope of JDP:
The aim of the JDP is to investigate properties directly affecting the load-carrying capacity of the lubricants. Differences between EALs and mineral oils relating to one or several of these properties are suspected of directly relating to the increased number of aft stern tube bearing failures experienced since the US VGP requirements came into force at the end of 2013. This is based on the fact that most of the failures had happened before any significant degradation of the lubricant could have occurred. The results of phase 1 are presented in this slide show, while project partners finish investigating the viscosity properties of various EALs available on the market. More background information are provided in our article: "Ensuring satisfactory aft stern tube bearing lubrication performance" on Maritime Impact.
Outlook on further investigations
In phase 3 this joint work aims to look into aspects related to how and why EALs in stern tubes degrade and how this affects the performance and maintenance costs. DNV GL is looking for ship owners and managers willing to share their data and experience from their operating fleet, with the aim to draw key learnings and accumulated knowledge.
Phase 1 main result: Pressure vs viscosity properties
The tested EALs show lower pressure vs viscosity coefficients than a similar-grade mineral oil throughout the relevant temperature range. This is confirmed by other published studies like Akihiko Yano et al (2015).
Phase 1 main result: Pressure vs viscosity properties - effect of increased viscosity
Consequently, the true viscosity in the aft stern tube bearing oil film at high load conditions is lower for EALs compared to mineral oils of equal grade. Given that oil film thickness and oil film load capacity are proportional to viscosity, the safety margins towards oil film breakdown and consequent bearing failure are reduced in these operating conditions. Increasing the specified viscosity of the EAL is one solution to avoid operating with a reduced viscosity in high-load conditions compared to a mineral oil.
Phase 1 main result: Temperature vs viscosity properties
EALs intended for stern tube applications have a significantly higher viscosity index than their mineral oil alternatives. Typical values range between 135 and 210 for stern tube EALs and 95–105 for mineral stern tube oils. For temperatures lower than 40°C the viscosity of an EAL will be lower than for an equal-grade mineral oil. This might be safety-critical in certain types of operation, such as cold start-up and mooring trials where the stern tube system will typically be at seawater temperature. For temperatures above 40°C the effect will be the opposite, resulting in a positive viscosity effect for EALs compared to mineral oils.
Phase 1 main result: Shear rate vs viscosity properties
One of the six EALs tested in the study was found to display shear-thinning properties due to the breakdown of long-chain polymer additives used in the lubricant formulation under high shear loads (EAL E). However, this effect is less significant than the pressure and temperature effects within the expected shear-rate range applicable for a stern tube application. Any potential detrimental effects of lubricant shear thinning will be further scrutinized in the next testing phase of the EAL study.