Changes of course in boxship power
Efficiency optimisation trends in the containership sector, prompting a shift to lower vessel speeds, have fostered new solutions to powering arrangements and hull design based on the use of larger propellers and machinery with slower running speeds, writes David Tinsley.
Market leader MAN reports that larger container vessels are now increasingly being specified with longer-stroke S80ME-C9 and S90ME-C8/9 engines, rather than wider-bore plant, because of the opportunity these types offer to employ larger propeller diameters. In such a scenario, an S90ME-C9 diesel provides an alternative to a corresponding K98 model with the same cylinder count, for example.
A recent case in point was the decision of Vancouver-based Seaspan Ship Management to adopt the super long-stroke S90ME-C9 engine, in 10-cylinder configuration, for each of a series of 10,000TEU boxships contracted in China by parent organisation Seaspan Corporation.
The vessel design is a product of close collaboration between the owner, Yangzijiang Shipbuilding Group, the Marine Design and Research Institute of China (MARIC) and DNV, and reflects Seaspan’s three-year SAVER (Seaspan Action on Vessel Energy Reduction) programme. In relation to other ships and newbuilds in the 10,000TEU category, it promises considerable ship efficiency improvements across the board, achieving increased load capacity with reduced energy usage through design and engineering measures and operational practices.
For the new SAVER generation, the owner had initially considered the K98ME-C engine, long favoured by containership owners employing MAN power for vessels in the 8,000-10,000TEU range. However, Seaspan finally opted for the 10S90ME-C9 type, and the ship design was modified to suit.
The plant promises significant fuel savings on the basis of Seaspan’s criteria regarding optimised performance in fully-laden condition at speeds in the range of 18-22 knots, with a power reserve to make as much as 25 knots if necessary, in the event of schedules having to be recovered. In settling for the super long-stroke engine class, the ships’ aft-end form has been tailored to the lower engine revolutions and larger propeller diameter.
Aside from its technical implications, the engine deal has considerable commercial significance for the industry in China, since the Seaspan programme could extend to 25 ships. An initial commitment has been made to seven ships, with options on up to 18 further vessels. Each 10S90ME-C9 diesel will be produced by MAN licensee CSSC-MES Diesel (CMD).
Maximised energy efficiency is married with record-breaking cargo capacity in A.P.Moller-Maersk’s future Triple-E generation of line-haul vessels. At the time of writing, the make and model of propulsion machinery for the proposed two-engine/twin-propeller system had not been revealed. Ultra long-stroke engines and larger propeller diameters are central to an innovative vessel concept that also embraces optimised hull and bow forms and advanced waste heat recovery technology.
The dual engine installation for the 18,000TEU design is intended to ensure a top speed of 23 knots, two knots less than that of the largest containership to date, the 15,500TEU Emma Maersk type. Despite the very small decrease in maximum speed, and the 16% increase in payload, the Triple-E’s power requirement is in the order of 65,000kW to 70,000kW, some 19% less than the 80,000kW, single-engine installation in Emma Maersk. The Triple-E concept allows for slower engine revolutions and far greater fuel economy. The company has said that the engines will operate down to 80rpm, compared to 90rpm in Emma Maersk.
In fact, to realise the efficiency benefits of ultra long-stroke prime movers, larger propeller diameters are required. However, the size of the propeller is governed by the dimensions of the vessel and space availability beneath the keel. To mitigate these restrictions, and achieve the desired efficiency, Maersk’s own research determined that a two-engine/two-propeller layout would be superior to the classic single engine/single screw configuration.
Besides the prospective 19% reduction in power concentration for the somewhat slower ship speed, the twin-skeg configuration using two slower-running engines to turn two propellers is expected to yield further energy savings of 4% relative to a single engine/single propeller design. Furthermore, the adoption of a waste heat recovery system, capturing and re-using heat and pressure from the exhaust lines to boost engine power and reduce fuel consumption by 9%.
Thus, information released to date suggests that each of the two engines per ship will produce some 32,500-35,000kW and have a crankshaft speed of 80rpm. This combination is available from current models of super long-stroke machinery, although new designs dubbed ‘ultra’ long-stroke are attributed with slower running speeds. The Maersk fleet uses both MAN and Wärtsilä low-speed machinery, and the Emma Maersk generation alluded to above was notable for its adoption of a single 14-cylinder ‘cathedral’ engine from Wärtsilä.
Moreover, the huge power concentration of 80,080kW in this earlier record-breaker can be augmented by two 9,000kW Siemens booster electric motors, energised either by the ship’s waste heat recovery system or by the auxiliary gensets. The drive in Emma Maersk is to a fixed-pitch, six-bladed propeller of 9.6m diameter, reckoned to have been the largest when the ship was brought into service in 2006.
The Danish organisation has so far entrusted 20 newbuilds of the Triple-E type to Daewoo Shipbuilding & Marine Engineering, on the basis of deliveries starting in 2013, and another 10 such vessels are contemplated.
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