CONTAINERSHIPS Growing attention to alignment
WITH the advent of ever larger and more powerful container vessels, it has become necessary to pay greater attention to shaft alignment design and propulsion shafting installation*.
A
good shaft alignment is one in which all the supporting bearings are well loaded in the
static condition and the system is flexible enough to withstand hull deflections and imposed forces so that the bearings remain well loaded in all operating conditions. For containerships, this requirement has
historically been easier to fulfil than for other commercial vessel types such as oil tankers and bulk carriers. This has been due to the inherent flexibility of the shafting systems on container vessels as a result of the mid-position of the main engine and the subsequently long shaftlines. It is somewhat counterintuitive that long shaftlines
with many bearings are in fact easier to align than short shaftlines with few bearings. However, theory and practical experience demonstrate that the short, stiff systems found on vessels with high-powered machinery placed as far aft as practicable are the most problematic. With the advent of ever larger and more powerful
container vessels, propeller and intermediate shaft diameters are increasing, with some single-screw ULCS designs now incorporating a propeller shaft diameter in excess of 1metre. In conjunction with the larger drive shafts, main
engine designs are also incorporating shorter cylinder spacings and larger crankshaft diameters. These design trends make the coupled propulsion
shaft and engine system more sensitive to changes in bearing position. In order to prevent bearing failures, it has become
necessary to pay greater attention to shaft alignment design and propulsion shafting installation on containerships. The design stage is crucial in establishing a
suitably flexible shafting system. It is important for the designer to minimise the value of the bearing influence numbers (the measure of shaft stiffness). It is also important to keep the static bearing loads high enough to prevent them from unloading during operation. As a rule of thumb, the length/diameter ratios of
the bearing spans should be kept above 10:1. To achieve this it may be necessary to omit the sterntube forward bearing in place of an aft plummer bearing located on the propeller shaft, immediately inboard of the engineroom aft bulkhead seal. Table 1 compares the parameters of several
selected containership and oil tanker designs and shows the theoretical influence numbers. As a measure of sensitivity, the downward offset from the design position required to unload the sterntube forward bearing or, where no forward bearing is fitted, the aft-most plummer bearing is shown. Of the containership designs studied, the 4500TEU vessel has the most sensitive system,
*This article was originally published in the November issue of Container Ship Focus, a technical publication produced by Lloyd’s Register for the containership industry.
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The most common forms of propulsion shaft bearing damage are wiping or overheating at the aft end of the sterntube aft bearing
comparable to that of the oil tankers. An important aspect of the particular 4500TEU design studied is that the shaft span between the forward and aft sterntube bearing supports is only 6.5metres. This produces an L/D ratio of only 8.2:1 with a subsequently high influence number of 13.6tonne/ mm. When combined with a design bearing load
of 12tonnes, it only takes 0.88mm of downward displacement for the sterntube forward bearing to unload. A better design is the 6000TEU vessel, which has
a span of 11.0metres between sterntube bearings, producing an L/D ratio of 11.3:1. Despite the larger shaft diameter, the sterntube forward bearing’s influence number is kept down to 6.7tonne/mm. The 6000TEU design also has the advantage of a
longer shaftline with more plummer bearings (six, compared to only three in the 4500TEU design), which helps to reduce the influence number at the sterntube forward bearing. The need to achieve a good static load on the sterntube forward or aft plummer bearing
has been highlighted in recent investigations undertaken by Lloyd’s Register’s Technical Investigations. Measurements conducted on container vessels demonstrated how the bending moments imposed upon the propeller shaft change significantly during manoeuvring turns, causing the sterntube forward bearing to unload and the propeller shaft to run cross-axis in the aft bearing. This typically occurs during turns to starboard. The measurements additionally confirmed that successful sterntube aft bearing performance is dependent on achieving a static slope mismatch between the journal and bearing surfaces to within Lloyd’s Register’s limit of 0.0003radians (0.30mm/m). These results are in line with the findings
of numerous sterntube aft bearing failure investigations conducted by Lloyd’s Register’s Technical Investigations team. The most common forms of propulsion shaft
bearing damage are wiping or overheating at the aft end of the sterntube aft bearing. In a number of cases, such damage has occurred during
Vessel type
2000TEU container 4500TEU container 6000TEU container 8000TEU container 150,000dwt oil tanker 310,000dwt oil tanker
Prop. shaft Bearing Influence Downward offset diameter static load number to unload bearing (mm) 640 792 975 971 706 801
(tonne) 11 12 31 30 8
18
(tonne/mm) 3.9
13.6 6.7
12.5 16.1 27.4
(mm) 2.82 0.88 4.63 2.40 0.50 0.66
THE NAVAL ARCHITECT FEBRUARY 2007
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