SECTOR FOCUS: MARINE
Ensuring satisfactory aft stern tube bearing lubrication performance
Erlend Hanssen Nervold, Principal Engineer, DNV GL Maritime Advisory
The maritime classification society DNV GL has recently updated its oil lubrication acceptance criteria to differentiate between environmentally acceptable lubricants (EALs) and mineral oils in terms of their ability to form a reliable lubricant film for aft stern bearing performance.
In recent years an increasing number of costly stern tube bearing failures have been reported across multiple ship segments and sizes. These failures have coincided with the increased uptake of environmentally acceptable lubricants (EALs) after the December 2013 introduction of regulations prohibiting the use of mineral oils in all oil-to-sea interfaces on board commercial vessels trading in U.S. waters. While the majority of the breakdowns were caused by a combination of factors, the performance of the lubricants themselves has been drawn into question by the industry.
New eco-lubricants raise questions To better understand how 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 (1). The aim of the project was to map out potential differences between the lubricants’ physical properties that determine their oil film load-carrying capacities, with a focus on how the lubricant viscosity is influenced by parameters such as temperature, pressure and shear rate. Operational issues and potential long-term degradation issues have not been covered by the study as the majority of the observed failures occurred
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with fresh oil. Laboratory testing has been carried out by and under the guidance of leading tribology experts at the University of Sheffield in the UK and at INSA Lyon in France.
Understanding the physics In order to decide which influence parameters to focus on when investigating EALs, it is essential to understand the major factors that affect the performance of an aft stern tube bearing.
“The physical forces acting on propeller shafts under highly variable operating conditions result in a complex array of challenges that must be overcome to achieve a satisfactory shaft alignment and ensure adequate lubrication of the aft stern tube bearing,” explains Øystein Åsheim Alnes, Principal Engineer, at DNV GL Group Technology and Research. Over the past years several EEDI-driven vessel design features have helped improve overall vessel energy efficiency but have had an unwanted secondary impact: there is no doubt that bigger and slower-rotating propellers tend to reduce the safety margins of the aft stern tube bearing. Likewise, various hull and propeller wake optimisation features have been found to do the same for certain operating conditions. As the maritime industry works towards achieving IMO’s ambitious goals for reducing its environmental footprint, this trend towards more energy-efficient ships needs to continue. The industry-standard approach of only evaluating the bearing projected pressure and the simplified shaft vs bearing relative slope for static conditions is no longer adequate.
EEDI: The Energy Efficiency Design Index (EEDI) is a measure provided by the International Maritime Organization (IMO) promoting more energy efficient ships.
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