Ultra low-speed engines for VLCCs make economic sense
Recent developments in low-speed diesel engines have made it possible to offer solutions which will enable significantly lower transportation costs and emissions for VLCCs, according to a paper compiled by MAN Diesel & Turbo.
One of the goals of today’s marine industry today is to reduce the impact of CO2 emissions from ships and, therefore, to reduce the fuel consumption over a wide range of conditions and different loads. This also means that the inherent design CO2 index of a new ship, the so-called energy efficiency design index (EEDI), will be reduced. Based on an average reference CO2 emission from existing tankers, the CO2 emission for new tankers in g/dwt/naut mile must be equal to or lower than the reference emission figures valid for this type of tanker.
In order to meet the stricter given reference figure in the future, the design of the ship itself and the design ship speed applied (reduced speed) have to be further evaluated by shipbuilders to achieve the reduction in the EEDI. This objective may often result in operating a vessel at lower than normal service speeds leading to reduced propulsion power utilisation. However, so far only a few, if any, tanker owners have specified lower installed power for new VLCCs.
A more technically advanced development is to optimise the aft-body of the ship, making it possible to install larger diameter propellers resulting in higher propeller efficiency. In general, the larger the propeller diameter, the higher the propeller efficiency and the lower the optimum propeller speed, taking into account the propeller pitch and propeller diameter. When increasing the propeller pitch for a given propeller diameter with optimum pitch/diameter ratio, the corresponding propeller speed may be reduced and the efficiency will also be slightly reduced, depending on the degree of change to the pitch. The same is valid for a reduced pitch, but here the propeller speed may increase.
The application of new propeller design technologies (NPT propellers) has acted as a stimulus for the use of main engines with lower rpm since the two-stroke main engine is directly coupled to the propeller. Thus, for the same propeller diameter, these propeller types are claimed to have about a 6% gain in overall efficiency at about 10% lower propeller speed.
With this in mind, MAN Diesel has developed the so-called ‘green’ ultra long-stroke G80ME-C engine with shaft speed even lower than usual. Although the super long stroke S-type engines, with relatively low engine speeds, have traditionally been used as prime movers in tankers, the new ultra-long stroke G80ME-C9.2 engine meets this perceived need for an ultra-low speed engine. Depending on the propeller diameter used, an overall efficiency increase of 4% to 9% is achievable with this engine. This would reduce the ship’s EEDI, which could be further reduced through the installation of a waste heat recovery system.
The efficiency of a two-stroke main engine depends in particular on the ratio of the maximum (firing) pressure and the mean effective pressure. The higher this ratio, the higher the engine efficiency, i.e. the lower the specific fuel oil consumption (SFOC). Furthermore, the higher the stroke/bore ratio of a two-stroke engine, the higher the engine efficiency. This means, for example, that an ultra-long stroke engine type such as the G80ME-C9 is likely to exhibit a higher efficiency compared with a shorter stroke engine type, like a K80ME-C9.
For a 320,000 dwt VLCC, MAN Diesel has carried out a case study which illustrates the potential for reducing fuel consumption by increasing the propeller diameter and introducing the G80ME-C9.2 as main engine. Based on the main engine operating for 250 days/year, and a fuel price of $460/tonne, the relative savings in operating costs in net present value is about $9.4 million for the 6G80ME-C9.2 with an MCR speed of 68 rpm and 10.4m propeller after 25 years in operation. For the de-rated 7G80ME-C9.2, with the lower MCR speed of 62 rpm and a propeller diameter of about 11.0m, the saving is about $16.3 million.
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