Engineers resurrect use of water-lubricated propeller shaft bearings on commercial shipsJun 4, 2013 12:51 PM
Photos courtesy Thordon Bearings
Thordon’s Compac water-lubricated shaft bearing reduces oil leakage.
As environmental regulations tighten, water-lubricated propeller shaft bearings are becoming a popular alternative to oil-lubricated bearings for commercial vessels.
Regulations imposed by the U.S. Environmental Protection Agency (EPA) are placing greater burdens upon shipowners to prevent environmental pollution in the event of oil leakage. Oil found in deck equipment such as cranes, windlasses, bearings and other equipment has become a growing concern as a source of ocean pollution from their oil-to-sea interface.
Carnival Corp. said three new cruise ships on order will be equipped with the Thordon Compac seawater-lubricated bearing system, including the 141,000-gross-ton Royal Princess.
“Not only does this remove the complexity of an oil filled circulation system, but more importantly it removes one of the main oil-and-seawater interfaces and risks of oil pollution of the sea,” said Chris Joly, principal manager for marine engineering for Carnival’s P&O and Princess Cruises.
Great Lakes carrier CSL Group is installing the product on Trillium-class newbuilds including Baie St. Paul.
“Wear-down of both shaft and rudder stock bearings has been minimal,” Joly said. The company’s previous experience with Thordon-fitted vessels “indicates that the bearings will outlive the planned 30-year life of the ship.”
Bearings at the stern tube and the strut of the vessel — used to support the propeller shaft — generally take on one of two configurations. Either they are water-lubricated or oil-lubricated. The choice of specific bearing lubrication type has changed over time.
Approximately 50 years ago, most large ship propeller shaft bearings were water-lubricated and had a hard material called lignum vitae as their bearing material. Then, commercial shipowners shifted to oil-lubricated because they allowed shafting to stay in service longer without being removed.
Oil-lubricated shaft bearings are typically made from white metal — an alloy, such as babbitt, a material typically used for bearings. They use two seals, one forward and one aft. Both are designed to contain the oil in the stern tube bearings as well as the strut bearings, preventing seawater intrusion into the bearing.
“The reason most ships use oil-lubricated bearings is because they offer a controlled maintenance environment,” said Craig Carter, director of marketing for Thordon Bearings, based in Burlington, Ontario.
An oil-lubricated system is a sealed system that offers some advantages. Its temperature may be carefully monitored and controlled to ensure there is no overheating from undue friction or faulty performance of the bearing. It allows shipboard engineers to sample the oil regularly to ensure its chemistry, and its particulate contents provide adequate lubrication performance. Of the 48,000 or so larger commercial oceangoing ships in the world, a mere 3 percent to 4 percent use water-lubricated bearings, Carter said. The rest use oil-lubricated ones.
A growing number of shipowners are turning to water-lubricated bearings for their environmental benefits. In an oil-lubricated bearing, the entire amount of oil contained in oil-lubricated shaft bearings is approximately between 400 and 800 gallons. This oil is a mineral oil and there is always some leakage.
“It is inevitable that some of the greases used will drop off and ultimately be washed into the sea,” said Dirk Fabry, market manager for the maritime and offshore industry at Klüber Lubrication, who makes biodegradable lubricants for such applications. “The use of eco-compatible lubricants can help prevent excessive impact on the environment and marine organisms.”
According to a study conducted by Dagmar Schmidt Etkin of Environmental Research Consulting, each shipboard stern tube leaks 1 to 7 gallons of oil per year. If all stern tubes in all vessels fitted with them leaked, between 34 and 64 million gallons of stern tube lubricating oil would enter the ocean each year. Leaks come about from seal wear, propeller contact, ropes or fishing line wound around shaft, as well as marine growth and debris.
According to Carter, the appeal of water-lubricated bearings is growing. The U.S. Navy, as well as U.S. and Canadian coast guards have used water-lubricated bearings. CSL Group, a Canadian shipowner that operates bulk carriers on the Great Lakes, said it plans to include the water-lubricated systems on three of its vessels.
Water-lubricated bearings avoid oils and grease lubricants altogether. Seawater is pumped into the bearing and it simply discharges to the sea. It lubricates and dissipates heat from shaft friction and provides equal performance. These bearings are a viable alternative to the environmental risk of oil-lubricated ones.
The new requirement in draft is the 2013 EPA’s Vessel General Permit. Its rules would address the environmental threat of oil, lubricant and fluid leakage from propeller shaft bearings and other equipment and machinery with an oil-to-sea interface and mandate replacement with a biodegradable substitute. Under the proposed regulation, oil used in oil-to-sea interface equipment or systems, such as oil-lubricated bearings, must utilize biodegradable oil instead of the mineral oil. The same holds true for other lubricants and fluids such as grease and hydraulic oil. It applies to non-recreational, non-military vessels greater than 79 feet. It will cover approximately 70,000 existing Vessel General Permit vessels, according to the EPA.
Eco-compatible or biodegradable lubricants are significantly more expensive than mineral-based oils and lubricants currently in use. Specifically for the propeller shaft bearings, whenever filament from fishing nets or lines find their way into the seal and wrap around it, and potentially damage the seal, the shipowner has to take immediate action, notify the Coast Guard and contract for in-water seal repair, which costs upwards of $250,000.
The cost of replacement oil, coupled with the noncompliance and fines shipowners could face from oil leakage, give shipowners new points to consider in choosing between oil-lubricated or water-lubricated propeller shaft bearings.