Shaft alignment science

Bureau Veritas is drawing on past experience to offer a shaft alignment service to large-vessel owners. The service comes outside the normal scope of class. It has already been utilised on newbuildings for Hellespont, Hapag Lloyd, CMA CGM, Kuwait Oil Tanker Company and Kristen Navigation. The service has been re-launched since the re-emergence of stern tube bearing problems on a number of large containerships and VLCCs. These problems have led to some ships returning to dry-dock just weeks after delivery. Bureau Veritas says it is a problem it tackled in the 1970s when it worked on ULCCs. Its says application of a derivative to the solution it developed then can help shipyards and shipowners avoid costly penalties. The problem is a result of shipyards optimising stresses as far as possible says Claude Andreau, head of structural analysis at Tecnitas, Bureau Veritas? consultancy division. The resulting decrease in steel weight and thinner scantlings means a flexible aft, which combined to the stiffness of the shaft (because of increased power), means stern tube bearings are subjected to highly variable loads. “We are sure you have to have a scientific approach to the problem,” says Andreau. “You must have the optimum distribution of bearing reaction for each operating condition? for good compatibility between shaft and hull.” Therefore optimum alignment involves the best compromise for various operating conditions: static, low rev/min, maximum continuous rating, ballast and fully loaded. A particular problem is the aft bearing. “Many shipyards consider there is only one [aft bush] support”, explains Andreau. “We recommend two supports to give infinite bearing stiffness. To modify pressure, a slope makes the pressure distribution more even.” Ideally there is a global slope which suits all operating conditions. But commonly the different forces in static mode and at maximum continuous rating mean pressure can exceed 100 bar in static mode. Andreau says such pressure can destroy the anti-friction metal. “To overcome this you have a double slope on the bearing, one slope optimal for operations and one slope optimal for static [if there is an excess load],” he explains. This requires using white metal as, says Andreau, Railko bearings cannot be machined with two slopes. Solutions are offered at the design stage, where specific calculations are proposed. Or to ships in service, in which case only measurements can be offered.