Thordon Bearings Retrofitting Ocean-Going Vessels With Seawater-Lubricated Propeller Shaft Bearings To Reduce Stern-Tube Oil Pollution; Annual Leaks Estimated at About 2x Exxon Valdez Spill
by Jeff Curtis and Jack Rosebro
|Cross-section of COMPAC seawater-lubricated stern tube bearing system for ocean-going vessels. Source: Thordon Bearings Click to enlarge.|
At last week’s Ocean Innovation 2009 conference in Victoria, British Columbia, Craig Carter, Director of Marketing and Customer Service at Thordon Bearings detailed the company’s progress in the retrofitting of large ocean-going vessels with seawater-lubricated propeller shaft bearings to reduce waterborne pollution.
At present, most commercial ocean-going ships use a propeller shaft that is supported by oil-lubricated bearings in the ship’s stern tube. Although the stern tube is typically fitted with forward and aft shaft seals, the seals are designed to expel contaminants by leaking aft into the sea or forward into the bilge. Aft seals are also commonly damaged by fishing net or rope that becomes entangled on the propeller shaft.
A 2005-2007 study by CleanSeaNet, a European Maritime Safety Agency project that uses satellite-based synthetic aperture radar (SAR) images to identify oil slicks in real time, identified 4,027 oil slicks attributed to routine operational discharges from ocean-going vessels, rather than accidents, in an 18-month period.
Such discharges are unavoidable under even the best of conditions; Wärtsilä Propulsion, which is one of the largest manufacturers of stern tube shaft seals, has estimated that even the rate of oil leakage consistent with propeller shafts operated in an ideal, controlled laboratory environment would exceed 10 million liters (2.6 million gallons US), per year worldwide.
A typical commercial (>1,000 DWT) ocean-going ship’s stern tube contains about 1,500 liters (396 gallons US) of oil. Assuming a conservative leakage rate of six liters per day per ship (Lloyd’s Register Class Society Seal Type Approvals), stern tube oil pollution is estimated to be more than 80 million liters per year worldwide, Carter noted, not counting leakage from damaged seals. By comparison, the Exxon Valdez oil tanker discharged 41.6 million liters of crude oil following its 1989 accident.
Rather than attempt to limit stern tube leakage through improved seals or develop biodegradable oils, as other companies have done, Thordon Bearings has developed a stern tube design, called COMPAC, which uses seawater to lubricate non-metallic propeller shaft bearings. The Thordon COMPAC Stern Tube Bearing System includes:
COMPAC elastomeric polymer stern tube bearings (with bearing carriers when appropriate). To reduce start-up friction and eliminate stick-slip, COMPAC’s formulation includes special lubricants to provide a low coefficient of friction. To promote early formation of a hydrodynamic film between the shaft and bearing, the lower (loaded) portion of the bearing is smooth, while the upper half of the bearing incorporates grooves for flow of the water lubricant/coolant.
Thordon Water Quality Package. A steady supply of seawater with the significant abrasives removed is an important element in ensuring long, predictable, bearing wear life. The Thordon Water Quality Package is designed to supply seawater to the stern tube bearings for lubrication and cooling at a minimum flow rate of 0.15 liters/minute/mm (1 US gallon/ minute/inch) of shaft diameter and to condition seawater from the water supply by removing suspended solids greater than 50 microns (0.002”) with a specific gravity of 1.2 or higher. Seawater is sourced directly from the ship’s sea bay, or alternately, from a suitable existing onboard supply.
Shaft liners. Since bearing “clearance” is the sum of both bearing wear and the wear of the shaft liner, a high quality shaft liner is an important component of a COMPAC bearing system. Thordon Bearings recommends shaft liners made from a good quality centrifugally cast bronze, stainless steel or Inconel alloys.
Thor-Coat shaft coating. Where the shaft is exposed to salt water, the shaft must be protected by a corrosion-resistant coating. Thor-Coat is a modified epoxy coating having enhanced flexibility and no woven fabric component.
Seawater lubricated forward seal. Thordon recommends a face seal or lip seal for use with the COMPAC seawater lubricated stern tube bearing system. Thordon’s new Sea Thigor mechanical face seal is a recommended option for shaft diameters up to 400mm (15.75”).
The only seal required is the forward seal, and no lubricating oil is used. A 15-year bearing guarantee is offered on the COMPAC system; in-use data suggests that the system may last 30 years or more.
Water-lubricated propeller shaft bearings may also contribute to greater propulsion efficiency. Such systems can use stern tubes with smaller diameters, reducing wake peaks that reach the ship’s propeller.
Research conducted by Fincantieri, a shipyard in Italy, indicates that a 2% gain in efficiency can be realized—a significant improvement for large commercial vessels. The coefficient of friction for water, while higher than oil at low propeller shaft speeds, is actually lower at higher shaft speeds.
Over the past seven years, COMPAQ seawater-lubricated stern tube system has been fitted to several cruise ships operated by Carnival Corporation, as well as BP oil tankers, US Navy TAK-E replenishment ships, Swedish Coast Guard patrol ships, and the Glomar Explorer which currently operates in the Black Sea as a deep-water drilling rig.