Feature 1 | GREEN SHIPPING
At the 11th Shaſting Symposium of the
Society of Naval Architects and Marine Engineers (SNAME), a large shaft seal manufacturer, Kobelco Eagle Marine Engineering Co. Ltd, stated that: “In stern tube bearings, the radial movement of the shaſt is considerably larger than that of bearings for general industrial applications. In addition, external disturbances such as rough seas and vibration are considerable. It is practically impossible to seal the stern tube oil perfectly.” In 2005, the European Maritime Safety
Agency (EMSA) set up an agency called CleanSeaNet to provide a European wide operational system for oil slick detection based on satellite-sourced synthetic aperture radar (SAR) images. In 2007, an 18-month study by CleanSeaNet reported on ship pollution totally based on space -borne SAR remote sensing. It revealed for the first time the dramatic dimension of shipping pollution in European waters with 4027 oil slicks detected and reported, not as a result of accidents, but from routine unauthorised operational discharges. Aircraſt or vessels verified that as much as 80% of the CleanSeaNet detections were mineral oil. Ships do not operate in a closed
environment. The propeller shaft may become entangled in rope, fishing nets or monofilament
lines. The propeller
may hit the ocean floor or impact with ice or other heavy objects. Accidental oil discharges have been reported in the press over the past several years from seal repair companies. Tese companies are hired by the ship owner to fix the seals so they do not leak oil (and prevent seawater ingress so the bearing doesn’t seize) and the ship
Non-metallic sea water lubricated ropeller shaft bearings.
Sapphire Princess and Diamond Princess are both fitted with seawater bearings.
can continue to sail. Since 2008, various press articles have reported over 50 ships operating globally that required stern tube seal repair due to a damaged shaſt seal (and this is only what was reported). In 2009, Lloyd’s Register reported that:
“Defect statistics over the last 20 years indicate that the aſt stern bush represents 10% of shaſt line failures, with the forward stern bush representing 4% of total failures. Interestingly, the aſt stern gland (seal) and forward stern gland [seal] represent 43% and 24% of failures respectively.” In 2010, at IMO’s Sub-Committee on
Ship Design and Equipment (DE54), a DNV report prepared for the Norwegian Maritime Directorate stated that: “As a potentially relatively large source of operational oil discharge, however, still not effectively regulated and of unknown exact magnitude, stern tube leakage should be addressed as a particular environmental aspect in the polar environment as well as in other areas. Of particular importance under ice operation is the potential for especially high leakage rates, and the proximity to ice with regards to deposition of oil.”
Alternatives to mineral oil Two ‘conventional’ alternatives currently in use claim to reduce the impact of discharges to the environment. Seal
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manufacturers have developed more sophisticated multi-lip seals which reduce the amount of oil that escapes into the sea, but as the ship does not operate in a perfect environment, shaſt seals can still be damaged, and oil can still escape into the sea (or seawater water ingress into the stern tube). Biodegradable oils (both vegetable and synthetic oils) are also becoming more widely used. However, the research on biodegradable oils is varied and limited typically to laboratory tests. According to research, birds do not avoid oil slicks (biodegradable oils typically have a strong smell) and may be fatal if their feathers become matted. In the US, the Clean Water Act defines any substance that leaves a sheen, emulsification, or discoloration, as a pollutant and be subject to appropriate fines and regulations governing pollutants.
Can shaft oil discharges be eliminated? One alternative that has been overlooked by many ship owners is a return to seawater lubricated propeller shaſt bearings. Prior to the 1950s, all ships operated with a seawater lubricated propeller shaſt bearing system; seawater was used as the lubricant and wood (lignum vitae) was used as the bearing to support the shaft. Bearings
The Naval Architect May 2012
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