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.
Shipowners are realizing that there are major environmental
and maintenance benefits to water-lubricated stern tube bearings.
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. A typical oceangoing
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. For more details on
this conversion, please look out for the October issue of Dry
Cargo International, where the story will be featured at greater