Stern warning of leaking oil

Statistics suggest that one in two ships will suffer problems with sterntube seals on average every 11 years, and some in the industry feel the incidents are rising. While this may not appear too daunting, the $9m fine handed down to Royal Caribbean for discharging oily bilge water shows the attitude of some authorities to oil pollution. In some cases the definition of oil discharge is sheen on the water – regardless of the source. So sterntube leakage can not only lead to a heavy fine, but may also necessitate an unscheduled drydocking with all the expense and the inconvenience that involves.

Most ships undertake the analysis of sterntube oil to show the class society that the tailshaft bearing and seals are sound, and should not be disturbed during drydocking. Mobil says that building up a history of the system by regular oil analysis saved one of its customers around $20,000, as it could demonstrate that the port and starboard sterntube systems did not require inspection. But oil analysis is not conclusive, as oil leaking out of the sterntube does not leave any tell-tale signs for the lab to detect, and the crew just top up the header tank. Also, as Ian Weatherley, lubricant specialist within Texaco`s operations group points out, samples are tested in good faith but they could be taken from the header tank. In a gravity fed system there is no way this could be representative.

In many ways the sterntube creates unusual problems for lub oils in that the actual wear prevention is the easy part – it is the compatibility with seals and the interplay with water which can create difficulties. Most sterntubes are lubricated with engine oil that can easily provide the protection against wear and corrosion, is readily available and, as it is purchased in larger quantities, is inexpensive.

This course of action is not without potential problems. Brian Rowland, director of new product development at Deep Sea Seals, says the lub oil can affect the elastomers and polymers used in seals, causing them to swell and harden. Excessive swell can change the lip`s knife-edge contact widening it and causing the lip to collapse onto the shaft. The wider contact upsets the seal geometry, creates greater heat and shortens the seal life. If the sterntube is filled with oil which is incompatible with the seals, significant swell can occur in a matter of weeks. Should a lip harden, it is less able to follow any shaft runout, and if it loses contact with the shaft it would result in a leak. Hardened lips can also have an impact on shaft wear. While a seal may harden over a short period of time, the effects may not necessarily become evident until the shaft has become damaged.