The recent conversion from sealed oil lubricated bearings to
open water-lubricated bearings on the bulk carrier Peter R
Cresswell demonstrates the way owners of existing ships
are solving oil pollution leakage issues.
As discussed briefly on p91 of the September issue of DCI, a
growing band of owners of existing ships are coming to realize
that the environmental and maintenance benefits offered by
switching to water-lubricated stern tube bearings are as
pertinent to their operations as to those investing in new
After all, the zero tolerance increasingly being pursued by
authorities of ship source pollution applies equally across all
ships, and the prospect of eliminating a pollutant source entirely
below the waterline can only be persuasive.
Last year, new US Environmental Protection Agency National
Pollutant Discharge Elimination System (NPDES) rules
specifically targeted oil-lubrication discharges from stern tubes.
The new requirements carry extensive civil and criminal
penalties for violations, including the threat of jail. In Europe,
meanwhile, the 2007 European Union Maritime Policy specifies
the elimination of all vessel discharges into the marine
environment by 2020.
Leakage from an oil-lubricated stern tube doesn’t necessarily
mean negligence. Currently, the majority of commercial oceangoing
ships operate with a propulsion system using 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 themselves, the seal must leak
(aft-into the sea or forward-into the ship’s bilge) at the shaft/seal
interface in order to function properly. Simple fishing nets or
rope caught on a ship’s rotating shaft can damage the aft seal,
allowing stern tube oil to flow out into the sea. According to
seal repairers, this occurs on a frequent basis.
While Deepwater Horizon may be dominating the
environmental landscape, it might be worth considering that a
typical ocean-going ship’s stern tubes contain 1,500 litres of oil.
Even the conservative stern tube leakage rate of 6
litres/day set by LR Class Society Seal Type
Approvals from a world fleet of around 45,000
vessels could add up to ‘normal’ stern tube oil
pollution of over 80 million litres annually.
Owners most recently persuaded of the benefits of
water-lubricated bearings include Algoma Central
Corp, whose vessel operation management falls to
Seaway Marine Transport (a partnership with Upper
Lakes Group).
During the recent St Lawrence Seaway freeze,
SMT took the opportunity to convert stern tube
bearings on board the bulk carrier Peter R Cresswell
to the COMPAC water-lubricated system delivered
by Thordon Bearings Inc, which included a water
quality package designed to ensure that abrasives
are removed from the seawater, using centrifugal
forces and provide a consistent flow of water to
the bearings. Thor-Coat shaft coating to ensure the mild steel
shaft stays free of corrosion, bronze shaft liners to operate in
way of the bearings and Thordon COMPAC bearings.
Scott Groves, Thordon Bearings area manager for
Canada/Western USA, said that the conversion process involved
the removal of the shaft, existing oil-lubricated bearings and oil
header tank, installation of bronze shaft liners, Thor-Coat shaft
coating, fitting COMPAC bearings, Thordon Water Quality
Package and a new forward shaft seal.
Using the COMPAC system, seawater is taken from the sea,
pumped through elastomeric polymer shaft bearings and
returned to the sea. No stern tube oil is needed. New materials
and designs of non-metallic bearings now offer performance
similar to metal shaft bearings, said Groves, with a life
expectancy of 15 to 20 years.
The 30,735dwt self-unloading ship, delivered in 1982, was
converted at Canada’s Seaway Marine & Industrial Shipyard, as
part of a 12 week overhaul project involving five year survey and
hull work.
Al Davies, SMT Director, Operations, said that Peter R
Cresswell’s oil-lubricated stern tube bearings had become
increasingly problematic. “The whole environmental issue
became a source of grief with Transport Canada. As a company
we are part of a collective programme to introduce green
marine policies, extending across emissions, ballast water
management, grey and black water discharges. It’s pretty clear
that the end game will be no discharges allowed at all, and we
have bought into a process that is externally audited by Lloyd’s
Register. The initiative with water-lubricated stern tube bearings
is an example of that commitment. If anyone asks what we’ve
done for the environment, well, we can say ‘this’.
“Essentially, there came a point when the ship’s stern tube
bearing was getting tired and we knew that the tail shaft had to
come out for survey anyway. With Thordon half an hour away
and the shipyard ten minutes down the road, it seemed like a
good opportunity to take another look at this solution. We
already had experience of what Thordon could do on the ship
Quebecois where, 10 years ago, we replaced the bearing system
because of reports that Lignum Vitae might not be available
anymore. We knew that the COMPAC system worked, and the
closed season meant that we had the lead time to order the
Davies added that selecting bio-degradeable oils did not offer
a viable solution. “Regulations dictate that bio-degradeables are
considered a discharge even if they do not leave a sheen. Any
external leak, even using those products would require us to
notify Transport Canada and they could potentially stop the
vessel until an investigation was done not knowing when they
would release the vessel. The presumption is that you’re guilty
off the bat.”
“Of course, one significant difference with using elastomer
bearings is that the mild steel propeller shaft requires corrosion
protection from seawater,” Groves acknowledged. “This may
mean a higher up-front cost for the water-lubricated stern tube
bearing system, but the elimination of aft seal maintenance
means that the up-front costs are recouped in lower in-service
costs along with no aft seal damage worries, no stern tube oil
costs and no oil pollution risk [fines].”
Groves explained some of the key considerations owners
should address when contemplating stern tube bearing
conversion to water. Questions that needed answers
straightaway included whether the new arrangement would
actually fit in the space provided. This requires a review of the
existing shafting and stern tube arrangement drawings, if available.
The planning required to convert a vessel includes the
following. A review of the existing oil system to determine what
modifications are required if any to fit a water-lubricated system.
Owners may also have to accept planning for conversion well
ahead of any expected docking. “The longest lead-time items for
such projects are the shaft liners,” said Groves. “The number of
foundries that can manufacture centrifugally cast single piece
liners is limited and there is normally a six- to eight-week leadtime
to obtain these castings.”
Davies said “Thordon performed all of the measurements and
clearances on Peter R Cresswell without any issues, LR approved
all of the drawings within the envisaged timeline. Since going
back into operation in March, the ship’s performance has been
going well.” Even so, Groves said that, increasingly, owners of
older tonnage were seeing the switch to water-lubricated
bearings as a maintenance, as well as a ‘green’ issue. The shallow
waters of the St Lawrence Seaway placed restrictions on
navigation, with high potential for vessels to run aground, damage
their blade tip and their oil-lubricated shaft seals.
“The amount of manoeuvring in confined water can bring
ships close to the bottom,” said Davies. “Mud, debris,
obstructions and ice conditions, and even rogue nets, can cause
blade damage and subsequent seal failures.”
“The water-lubricated bearings available today from Thordon
offer the same lifespan as an oil-lubricating solution, but owners
avoid the prospect of having to be tugged into port in the case
of an oil leak,” said Groves. “That means both expense and lost
revenue due to downtime are avoided.”
Encouraged by the results on board Peter R Cresswell, Davies
said SMT was considering a new conversion project, this time
involving the 1967-vintage gearless bulk carrier Tim S Dool.
Other ships were also being considered for conversion.
“We will evaluate every ship due into drydock to see if the
conversion is appropriate,” he said. “Those decisions will be
based on the expected longevity of the ship, and on planning well
in advance. But this will surely be part of this year’s winter
budgetary considerations.”
There are over 600 ships equipped with COMPAC waterlubricated
stern tube bearings, with the first ship converted from
oil to water in 1998. Conversions have been on the upswing
with four completed in the past six months, including two
VLCC’s operating out of US waters, a Canadian icebreaker and
the Cresswell, said Groves.