Demands for increased engine power and greater energy efficiency are putting greater stress on propulsion components, particularly the aft stern-tube bearing. The result is heavier propeller loads at very low shaft revolutions. The primary source of problems for the stern-tube bearing is its improper interaction with the propulsion shaft; their relationship is a complicated matter which continually changes during operations, depending upon such factors as the initial shaft alignment, the loading conditions, draft, hull deflection, rudder angle and speed.
The problem becomes even more complicated with ultra-large and/or twin-screw vessels and any design that optimizes energy efficiency by reducing steel weight, resulting in increased flexibility of the hull structure.
In answer to the need to address this problem, ABS has worked with industry partners to develop the Digital Shaft Alignment Monitor (D-SAM), a system that significantly enhances the ship operators’ ability to detect and correct these failures.
D-SAM uses a series of proximity sensors and custom-built software to monitor the clearance between the surfaces of the stern-tube bearing and the propulsion shaft. By monitoring the operating health of these components, the system helps prevent failures that can render ships inoperable, potentially endangering their crews and the environment.
The new operating conditions require not only an improvement in the approach to alignment designs and installation, they also demand a better way to verify that alignment while operations are ongoing. Other than temperature probes, which may detect a problem too late to prevent damage to, or a failure of, the stern-tube bearing, I am not aware of another system that allows owners and operators to monitor events inside the stern tube between the propulsion shaft and this key bearing.
Traditionally, the condition of the stern-tube bearing has been validated solely by monitoring its temperature, with sensors imbedded below the surface of the bearing. An alarm sounds when temperatures have exceeded established thresholds. Unfortunately, the warning sometimes comes too late to save the bearing.
D-SAM is designed to continuously monitor temperature as well as the clearance between the shaft and the stern-tube bearing. A number of proximity probes are imbedded inside the bearing to measure the distance between the shaft and the bearing at several locations.
When clearance information is combined with the temperature readings, remedial action can be taken if the shaft comes into proximity to the bearing, causing the temperature to rise. This level of dashboard monitoring, enabled by customized software, provides a real-time report of the shaft’s position inside the bearing.
Because twin-screw designs are increasingly popular for the propulsion of large LNG and container vessels, D-SAM was first trialed on a Maersk Triple E class containership, where the system has been active for several months, and it is currently being installed in a new Atlantic-max LNG carrier.
ABS is developing an optional classification notation for vessels and units that incorporate D-SAM, as it is expected to provide an enhanced level of safety and operating efficiency.
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