Oil leaks from sterntubes can be prevented if ships are built with water-lubricated shaftlines, said experts during Riviera Maritime Media’s Water-lubricated bearings: a slick choice for operators webinar

This event was held 14 February 2023, sponsored by Thordon Bearings, during Riviera’s Marine Propulsion Webinar Week.

On the panel were Thordon Bearings vice president for business development Craig Carter, ABS Europe director of engineering Chris Leontopoulos and Erik Thun technical project manager Ola Persson.

As shipowners and operators become more conscious of environmental concerns, there will be a greater need for water-lubricated stern tubes and bearings for brown and blue water vessel applications. They create far less pollution than the oil-lubricated shaftlines found on 95% of vessels.

“The time has come to eliminate pollution from ship shaftlines,” said Mr Carter. “Owners are taking steps to build more-efficient and environmentally friendly ships, but they are not yet built with water-lubricated shaftlines.”

He said shipyards should offer water-lubricant shaftlines on all newbuildings, but acknowledges this disrupts the optimised shipbuilding production streams. “This option could cost a lot more because it interrupts building processes,” said Mr Carter.

Sealed oil-lubricated sterntubes cause ocean pollution through accidental leaks or operational disposals.

“The annual global additions of oil emissions from propeller shafts of ships are estimated to range between 30M-100M litres,” said Mr Carter, quoting from a 2016 report by Chalmers University of Technology, Sweden.

These oil-lubricated systems require oil storage tanks, pumps, seals and bearings to prevent release into enginerooms and oceans. A more environmentally friendly alternative is the open seawater lubricated shaftline with no aft seal nor pollution as seawater enters through a seachest and is processed to remove silt. After this has lubricated the sterntubes, it is returned to the ocean.

Mr Carter said these are technically equivalent to oil-lubricated shaftlines, but with reduced operating costs, improved Energy Efficiency Design Index (EEDI) “reducing CO2 emissions per capacity tonne mile” and “zero oil emissions to our oceans and seas”.

One of the biggest benefits is there is no need for the maintenance of aft bearings that is needed in drydocks for oil-lubricated systems.

Water-lubricated shaftlines are proven in reliability and performance as they are on more than 2,000 ships including cruise ships and ferries, gas and oil carriers, dry cargo ships and container lines.

Erik Thun is one of the growing number of shipowners benefiting from water-lubricated shaftlines and sterntubes. It has 22 ships with water-lubricated sterntubes in use and 10 more on order for deliveries into 2026 – on dry cargo ships, product tankers and cement carriers.

Mr Persson said the issues with oil-lubricated shaftlines includes the potential need for expensive emergency dockings which take ships out of service, and maintenance costs.

“With water-lubricated sterntubes, we do not need to change seals or bearings as we see no wear on bearings,” said Mr Persson,

“We do not see any clogging issues even though we are trading in waters that are not even good to bathe in,” he said, referring to brown, silt-prone waters in rivers and estuaries.

“They are reliable and easy to maintain as there are almost no parts.”

Another benefit is preventing oil slicks from ships that could be visible by the public in coastal waters. “We do no want an oil slick as it can put a company out of business,” said Mr Persson.

ABS’ Mr Leontopoulos introduced the design concept for a Sterntubeless vessel, which it has developed with Thorden Bearings and the National Technology University of Athens.

This design removes the need for oil-lubricated sterntubes and uses bearings and a torsional vibration damper to support the shaftline.

A shortened shaftline, a short aft stern chamber, no forward sterntube bearing or cylinder casing, and an aft water-lubricated after sterntube bearing are features of the design. A temporary means of access to the stern chamber is offered for inspection and maintenance, which could be achieved without the ship needing drydocking.

“There is no tailshaft withdrawal, no propeller withdrawal and no shaftline disassembly,” said Mr Leontopoulos. “Bearing inspection and replacement is easy as there is a split bearing and tapered keyset.”

The main benefits are no risk of pollution, better go-home-capability compared with traditional oil-lubricated bearings, reduced overall operational costs due to no oil changes, minimised bearing accelerated wear, one less bearing with lower friction losses, reduced engineroom space and increased of cargo space.

“There is no barred speed range, but there is full flexibility throughout the rpm range without restrictions due to the torsional damper installation,” said Mr Leontopoulos.

“The tailshaft is continuously monitored digitally for bearing wear and temperature, and internal inspection and replacement can be done without having to disassemble the tailshaft and propeller and perform shaft re-alignment in the drydock.”

Webinar poll results

Attendees were asked to vote on a series of poll questions during the webinar. Here is a summary of the results.

Composite bearings are under-utilised on board ships

Strongly agree: 18%

Agree: 51%

Partially agree: 26%

Mostly disagree: 3%

Disagree: 2%

If the proposed sterntubeless design concept is adopted, would you consider a continuous aftmost bearing wear-down monitoring system as a necessity, or would the traditional poker gauge wear-down measurement be sufficient?

The traditional poker gauge bearing wear-down requirement is sufficient: 30%

The traditional poker gauge bearing wear-down requirement is not sufficient and an automated bearing wear-down monitoring system would be required: 70%

If the barred speed range can be negated with the use and installation of a torsional damper, would you consider installing a torsional damper?

Yes: 85%

No: 15%

All major class societies provide the same propeller shaft withdrawal notations for open seawater-lubricated propeller shaft systems as they do for sealed oil lubricated systems if monitoring criteria is met

True: 40%

False: 60%

What is the biggest single driver of uptake of seawater-lubricated propeller shaft systems for merchant fleet newbuilds?

Education about the use of a seawater-lubricated propeller shaft systems: 17%

Shipyards should offer seawater lubrication as an option: 30%

More references for my type of vessels: 16%

Similar pricing to a sealed oil system: 7%

Stronger oil pollution regulations: 30%

To what extent could the risk of the premature weardown of water-lubricated bearings be negated if the bearings can be replaced afloat dispensing with the need of drydocking?

Completely negate: 12%

Partially negate: 16%

Positive, but let’s not overstate it: 52%

Negligible impact: 12%

Zero impact: 8%