Safer than safe could be dubbed the term that sums up the design philosophy on four 185,000 dwt Alaskan-class tankers that will soon be carrying oil along the U.S. West Coast. Currently being built in San Diego, Calif. by NASSCO, the BP Shipping-owned vessels feature the latest innovations in safe-ship design, including Thordon COMPAC water-lubricated propeller shaft bearings.
During the design phase of the vessels, BP was determined to leave no stone unturned in its quest to minimize the risk of oil leakage. This included building in multiple redundant systems, and eliminating the use of oil wherever possible. "The prop shaft is an area where, traditionally, some lubricating oil can leak into the sea," says Stan Taylor, technical manager for BP. "So we needed to look for alternatives."
Traditionally, propeller shaft bearings on commercial tankers are made of white metal lubricated by oil. Although these systems work well, keeping the oil and water separate can be a challenge. If seawater contaminates the bearing area, prop shaft performance can be compromised. If oil escapes into the sea, that's pollution. "Even a small leakage from the prop shaft," says Taylor, "can result in a visible sheen when in port."
The Thordon COMPAC propeller shaft bearing systems now being installed on each of the BP vessels feature water lubrication (a renewed innovation pioneered for the 21st century
by Thordon), no use of oil (and therefore zero risk of pollution), and an expected minimum 10-year bearing wear life based on systems currently in operation.
To ensure a steady supply of abrasive-free seawater to the bearings, Thordon also supplied its integrated Water Quality Package. This maintenance-free system includes pumps, flow alarms and a separator capable of eliminating particles down to 50 microns from the seawater lubricating the bearings.