Owners retrofit water lubricated bearings

30 Sep 2010
Compac bearing for the 638mm shaft being carried to the stern tube for installation

Compac bearing for the 638mm shaft being carried to the stern tube for installation

Thordon Bearings of Canada cites a recent conversion from sealed oil lubricated bearings to open water lubricated bearings on the bulk carrier Peter R Cresswell to demonstrate a solution to oil leakage issues.

The company says that a growing number of owners of existing ships are realising that environmental and maintenance benefits of water lubricated stern tube bearings are as pertinent to their operations as to those investing in new tonnage. The zero tolerance approach pursued by many anti-pollution bodies applies equally across all ships, so any prospect of eliminating a pollutant source below the waterline can appear persuasive.

US EPA national pollutant discharge elimination system (NPDES) rules introduced in 2009 targeted oil discharges from stern tubes. Violations carry extensive civil and criminal penalties, including the threat of jail. In Europe, the 2007 EU maritime policy specifies the elimination of all vessel discharges into the marine environment by 2020.

According to Thordon, leakage from an oil lubricated stern tube does not necessarily mean negligence. The majority of merchant ships use a propeller shaft supported by oil lubricated metal bearings with oil contained in the stern tube by forward and aft shaft seals. According to seal manufacturers, the seal must exhibit slight seepage-into the sea or the ship’s bilge at the shaft/seal interfaces in order to function properly. Fishing nets or rope caught on a ship’s rotating shaft can damage the aft seal, allowing lubricant to leak into the sea. According to seal repairers, this can occur frequently.

Thordon says that a typical ocean-going ship’s stern tubes contain 1500 litres of oil. Even at the conservative stern tube leakage rate of 6 litre/day allowed for Lloyd’s Register type approval, the world fleet of around 45,000 vessels could account for over 80 million litres annually without including leaks due to damage or error.

US ship manager Seaway Marine Transport took the opportunity during the last St Lawrence Seaway freeze to convert stern tube bearings onboard bulk carrier Peter R Cresswell to Thordon’s Compac water lubricated system. The installation included, as well as the beari8ngs themselves, a water quality package to remove abrasives from the seawater using centrifugal forces; Thor-Coat shaft coating to ensure the mild steel shaft stays free of corrosion; and bronze shaft liners to operate in way of the bearings.

The Compac system pumps water from the sea through the elastomeric polymer shaft bearings and returns it to the sea. No stern tube oil is needed. The company says that its current  non-metallic bearings offer similar performance to metal shaft bearings, with a life expectancy of 15 to 20 years.

The conversion of the 1982-built ship was undertaken at Canada’s Seaway Marine & Industrial Shipyard, as part of the scheduled maintenance and survey.

According to Al Davies, SMT director, operations, Peter R Cresswell’s oil lubricated bearings were becoming problematic in view of the ship manager’s declared ‘green’ policies. With the tail shaft needing to come out for survey the close proximity of the shipyard to Thordon gave an opportunity to investigate a fresh solution.

SMT did not consider bio-degradeable oils to offer a viable solution under current regulations whichconcern any discharge, even if it leaves no sheen. Even a leak of those products would have to be notified with the risk of taking the ship out of service during investigations.

A potential disadvantage of the elastomer bearing system is that mild steel propeller shafts require corrosion protection which means higher initial cost. Thordon says that this cost is recouped through lower oil costs, lower maintenance and no risk of oil pollution fines in case of seal damage.

Conversion to water lubricated stern bearings involve a survey to ensure the existing shaft and sterntube arrangement offer sufficient space for the new system, and whether the oil system can be easily modified. With a current lead time up to eight weeks for the centrifugally cast single piece liners, the conversion needs to be planned well ahead.

In the case of the Peter R Creswell Thordon was able to produce the plans and drawings and get them approved by, LR within the envisaged time.  

In waters like the St Lawrence seaway, the conversion is particularly relevant. The shallow waters placed restriction on navigation, with high potential for vessels to run aground, damaging blade tips and oil lubricated seals. Mud, debris, obstructions, ice conditions and nets can all cause seal failures in conventional bearings. The prospect of having to be towed to port in the case of an oil leak also has to be considered, in terms of both expense and lost revenue due to downtime.

With many of the ships on the St Lawrence being old, conversion to water lubricated bearings is as much a maintenance issue as an environmental consideration. SMT is now considering conversion for the 1967-vintage gearless bulk carrier Tim S Dool among other ships in its fleet.

Thordon says that since 1998 it has equipped over 600 ships with Compac bearings, conversions providing an increasing proportion of this work.

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