The perennial problem of leaking stern tube bearings is one that has never been solved completely satisfactorily. With the vast majority of ships built with oil lubricated bearings, there is always the risk of pollution from a damaged seal.
Manufacturers are continually improving their products but changing entrenched attitudes of shipowners is not easy and the majority of ships are fitted with standard rather than premium products.
There are alternatives to conventional products available that have been growing in popularity; notably water lubricated seals from the likes of Wärtsilä Seals and Bearings or Thordon Bearings. However, even after the US vessel general permit changes of 2013 banned the use of most mineral oil lubricants in US waters, the strategy of most owners was to continue with oil lubricated seals but using the newly developed environmentally acceptable lubricants (EALs) although some did switch to water lubricated bearings.
Most lubricant makers have developed EALs and while some seal makers have not sanctioned their use, most have. There have been no public reports of major problems with EALs but in January, DNV GL launched a new joint development project (JDP) in cooperation with marine insurers The Swedish Club, Norwegian Hull Club, Gard and Skuld to test the potential influence of EALs on failures in stern tube bearings. DNV GL will oversee detailed laboratory testing of EALs by Leonardo Testing Services at the University of Sheffield (UoS), UK.
The JDP has been prompted by an apparent increase in stern tube bearing failures over the last few years. This coincides with the increased uptake of EALs after the 2013 VGP rules, but also with the introduction of new propulsion system designs, such as single stern tube bearing installations and larger and heavier propellers operating at lower RPM.
The test programme will investigate such aspects as hydrodynamic oil film formation, oil film thickness under varying loads and temperatures, and potential shear thinning effects at high shear rates. State-of-the-art non-invasive ultrasonic techniques developed by UoS will be utilised to examine lubricant film behaviour in real-time. The first phase of testing will be completed in the first quarter of this year, with the results scheduled for publication later in the year.
Dangers of mixing old and new
Although the JDP has EALs as its primary focus, the root cause of the problem may be in the other areas such as single stern tube bearings or propeller related issues as already mentioned or something else beside. They could for example be related to propeller shaft alignment or substandard materials.
Another factor that could be causing the increase in failures is not the EALs themselves but the manner of their being put into service in individual ships. Seal makers have advised of the necessity when switching to EALs to completely remove any traces of previously used mineral oil lubricants. This is because in some cases a mixture of EALs and mineral oil will cause a chemical reaction, resulting in a gummy mixture that is not able of performing the requirements of a lubricant. When removing old lubricant all of the tanks, piping, stern tube and the seal boxes have to be cleaned/flushed thoroughly to prevent future problems.
At a recent workshop in London organised by Wärtsilä Seals and Bearings, it was said that some shipyards carryout sea trials with mineral oil lubricated shaft bearings but deliver the ships with EALs if requested by the owner.
It could be that such a practice will leave even new vessels vulnerable to shaft failure if the oil change process did not include proper cleaning. On older ships, the problem could be attributable to changes made during drydocks or by the crew, especially if incompatible products were added at some point in the case of normal service operations.