Icebreaking with Thordon Seawater Lubricated Prop-shaft Bearings

By George Backwell at April 17, 2011 07:08
Filed Under: Company News, General

Polar Star: USCG photograph

 

USCG icebreaker Polar Star, one of the world’s most powerful non-nuclear fuelled, will continue in service with well-proven Thordon (of Canada) seawater lubricated propellor shafts by the time she returns to service in December 2012 after a USD 62-million refit and upgrade program in the Todd Pacific Shipyard in Seattle. Why a seawater lubrication system when most ships rely on a conventional oil lubrication system to protect stern-tube bearings?

Icebreaking Stresses the Propellor Shaft

Forging a passage through polar pack-ice of necessity causes the propellor to impact with chunks of ice that cause bending moments in the propellor shaft, which in turn stress the stern tube bearings and shaft seals; when the outboard stern tube seal fails, lubricating oil leaks out to pollute pristine polar waters. (So-called ‘biodegradable lube-oil’ fails to deliver its promise in such sun-starved high-latttitudes and toxic substances in the lubricating oil may find their way into the food chain). According to those engaged in the exacting task of icebreaking, sea pollution caused by a leaking stern tube is a fairly common occurrence, demanding frequent repair work and consequent time out of service.

How Sea-water Lubrication of the Propellor Shaft Works

Polar Star and sister ship Polar Sea were both fitted with seawater lubricated bearings when they were commissioned in the late 1970's, and then Thordon bearings were installed in 1983. Briefly, Thordon’s ‘COMPAC’ bearing system draws sea-water from the ocean, pumps it through the bearings and back out again into the sea.

To achieve this, they fit non-metallic elastomeric polymer bearings to replace the white-metal shaft bearings, and to prevent salt water corrosion bronze liners are fitted to the shafts in way of the bearings. Intervening shaft surfaces are also coated with a two-part epoxy treatment specially developed by the manufacturer for its anti-corrosion properties. 

Shafting and Propulsion System of Polar Star

The propulsion systems on the 'Polar Class' vessels consist of three controllable pitch propellers each driven by either a Diesel-Electric (6,000 HP) or Gas Turbine (20,000 HP) in a CODOG configuration. The three propellor shafts have a diameter of 922 mm and each weighs in at 70 tons, which would suggest that seawater lubrication of the stern tube is also suitable for large merchant vessels. Initial outfitting costs are higher than for a conventional shaft lubrication system, but Thordon make a case for the recovery of those extra costs during the 15-year guarantee period.


Comments (1) -

As an ex MPA of the Polar Star (83-87), I can attest to the need for this retro-fit. But unless they straighten out the problems with the lignum vitae shaft bearings, they will still be spending time in drydock after every mission.

I'm glad to hear that she will be back in action soon. No non-nuclear ice breaker can come close to the 2 Polar class breakers. Relying on only the Polar Sea to make the McMurdo break-in every year was the dumbest idea I'd heard in a long time, and was sure that the Star would be back. The Polar Class were state of the art when built and consequently had a lot of problems initially (sometimes things don't work the same in the ice as on paper) but we were working through them. I'm proud to say that a lot of the solutions came on my watch.

Michael Dean |     4/19/2011 12:08:08 PM #

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