Low-carbon propulsion options for smaller tankers

30 Sep 2013
Smaller tankers, such as this, can derive significant benefits in fuel consumption and emissions from optimisation of the complete propulsion system

Smaller tankers, such as this, can derive significant benefits in fuel consumption and emissions from optimisation of the complete propulsion system

Birger Jacobsen, senior two-stroke research engineer at MAN Diesel & Turbo in Copenhagen looks at how the latest propulsion technology can reduce carbon emissions, resulting in a favourable EEDI rating, for tankers in the 7,000dwt to 10,000dwt size range.

Smaller tankers, in the range 7,000dwt to 10,000dwt, are generally dimensioned at around 116m length overall, breadth 18m and scantling draught 7.0m-8.0 m. Recent developments have made it possible to offer solutions which will enable significantly lower transportation costs for small tankers. The same principles will apply to other ship types, such as bulk carriers, of similar size.

One of the goals in the marine industry today is to reduce the impact of CO2 emissions from ships and, therefore, to reduce the fuel consumption for the propulsion of ships by the widest possible amount at any engine load.

This means that the inherent design CO2 index of a new ship, the so-called Energy Efficiency Design Index (EEDI), will be reduced accordingly. Based on an average reference CO2 emission from existing tankers, the CO2 emission from new tankers must be equal to or lower than the reference emission figures valid for the specific tanker.

This drive may often result in operation at lower than normal service ship speeds compared to normal speeds under previous conditions, when fuel consumption and emissions were less critical, resulting in reduced propulsion power utilisation. The design ship speed at normal continuous rating (NCR), including a 15% sea margin, used to be as high as 14.0 knots. Today, the equivalent ship speed is expected to be slower, possibly 13 knots, or even less.

A more technically advanced development drive is to optimise the after body and hull lines of the ship – including bulbous bow. Such optimisation must consider operation in ballast condition as well as at normal cargo load. This makes it possible to install propellers of a larger diameter and, thereby, obtain higher propeller efficiency, but at a reduced optimum propeller speed. This results in use of lower power to obtain the same ship speed.

As the two-stroke main engine is directly coupled to the propeller, the introduction of the super long stroke S30ME-B9.3 engine with even lower than usual shaft speed will meet this goal.

Taking as an example a typical small tanker ship of 8,000dwt operating in compliance with IMO Tier II emission rules, it can be demonstrated that choosing the new S30ME-B engine rather than the older and normally used L35MC6 engine, will result in a significant improvement in energy efficiency.

The International Maritime Organization (IMO) based EEDI is a mandatory index required on all new ships contracted after 1 January 2012. The index is used as an instrument to fulfil international requirements regarding CO2 emissions from ships. The EEDI represents the amount of CO2 emitted by a ship in relation to the transported cargo and is measured in grams of CO2 per deadweight ton per nautical mile (g/dwt/naut mile).

The EEDI value is calculated on the basis of maximum cargo capacity (70% for container ships), propulsion power, ship speed, specific fuel oil consumption (SFOC) and fuel type. Depending on the date of keel laying, the EEDI is required to be a certain percentage lower than an IMO-defined reference value depending on the type and capacity of the ship.

As standard, the main engine’s 75% SMCR (specified maximum continuous rating) figure is applied in the calculation of the ship’s EEDI figure, which includes, in addition, the CO2 emissions from the auxiliary engines.

According to the rules finally decided on 15 July 2011, the EEDI of a new ship is reduced to a certain factor compared to a reference value. Thus, a ship bigger than 20,000dwt and built after 2025 will be required to have an EEDI 30% lower than the 2013 reference figure.

For ships smaller than 4,000dwt, there are no lower limitation demands. For the 8,000dwt small tanker in question, the EEDI reference value required after 2025 will be 7.5% lower, i.e. equal to 92.5% of the 2013 reference EEDI value.

In general, the highest possible propulsive efficiency required to provide a given ship speed is obtained with the largest possible propeller diameter d, in combination with the corresponding, optimum pitch/diameter ratio p/d.

For the example of an 8,000dwt small tanker with a service ship speed of 14 knots, the black curve in the graph shows the relationship between propeller diameter and efficiency. The required propulsion SMCR power and speed is shown for a given optimum propeller diameter d and p/d ratio.

According to the black curve, the existing propeller diameter of 3.5m may have the optimum pitch/diameter ratio of 0.72, and the lowest possible SMCR shaft power of about 3,625kW at about 219rpm. The black curve shows that if a bigger propeller diameter of 3.9m is possible, the necessary SMCR shaft power will be reduced to about 3,425kW at about 179rpm, i.e. the bigger the propeller, the lower the optimum propeller speed.

If the pitch for this diameter is changed, the propulsive efficiency will be reduced, i.e. the necessary SMCR shaft power will increase, see the red curve. The red curve also shows that propulsion-wise it will always be an advantage to choose the largest possible propeller diameter, even though the optimum pitch/diameter ratio would involve a too low propeller speed (in relation to the required main engine speed). Thus, when using a somewhat lower pitch/ diameter ratio, compared with the optimum ratio, the propeller/engine speed may be increased and will only cause a minor extra power increase.

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 the ratio, the higher the engine efficiency, i.e. the lower the SFOC.

Furthermore, the higher the stroke/bore ratio of a two-stroke engine, the higher the engine efficiency. This means, for example, that a super-long-stroke engine type, such as the S30ME-B9.3, may have a higher efficiency compared with a shorter stroke engine type, like the L35MC6.1.

The application of new propeller design technologies can also motivate use of main engines with lower rpm. Thus, for the same propeller diameter, these propeller types can demonstrate an improvement of up to 6% in overall efficiency gain at about 10% lower propeller speed. These figures are valid for the propellers employing Kappel technology as produced by MAN Diesel & Turbo in Frederikshavn, Denmark. High-efficiency propeller shapes like the Kappel propeller can thus be employed to take advantage of the efficiency gains available with new super-long-stroke two-stroke engines in cases where a correspondingly larger propeller cannot be accommodated.

For an 8,000 dwt small tanker, the following case study illustrates the potential for reducing fuel consumption by increasing the propeller diameter and introducing the S30ME-B9.3 as main engine. The ship particulars assumed are as shown in the accompanying table.

Based on these typical particulars, we have made a power prediction calculation (Holtrop and Mennen’s Method) for different design ship speeds and propeller diameters, and the corresponding SMCR power and speed, point M, for propulsion of the small tanker is shown in the layout diagram. The propeller diameter change corresponds approximately to the constant ship speed factor:
α = 0.28 [ref. PM2 = PM1 × (n2/n1)α.

Referring to the two ship speeds of 14.0 knots and 13.0 knots respectively, three potential main engine types, pertaining layout diagrams and SMCR points have been drawn-in and the main engine operating costs have been calculated and described.

The L35MC6.1 engine type (210 r/min) has often been used in the past as prime movers in projects for small tankers. Therefore, a comparison between the new S30ME-B9.3 and the old L35MC6.1 is of significant interest. In this connection, the existing 5S35ME-B9.3 seems to have the ideal low engine speed, however power-wise, it is too big.

It should be noted that the ship speed stated refers to NCR = 90% SMCR including 15% sea margin. If based on calm weather, i.e. without sea margin, the obtainable ship speed at NCR = 90% SMCR will be about 0.5 knots higher. If based on 75% SMCR, as applied for calculation of the EEDI, the ship speed will be about 0.2 knot lower, still based on calm weather conditions, i.e. without any sea margin.

Traditionally, long stroke L-type engines, with relatively high engine speeds, have been used as prime movers in small tankers. Following the efficiency optimisation trends in the market, the possibility of using even larger propellers has been thoroughly evaluated with a view to using engines with even lower speeds for propulsion of, in particular, small tankers and bulk carriers.

Small tankers and bulk carriers may be compatible with propellers with larger propeller diameters than the current designs, and thus high efficiencies following an adaptation of the aft hull design to accommodate the larger propeller, together with optimised hull lines and bulbous bow, considering operation in ballast conditions.

The new super-long-stroke S30ME-B9.3 small engine type meets this trend in the small tanker and bulk carrier market. Depending on the propeller diameter used, an overall efficiency increase of 3%-7% is indicated when using S30ME-B9.3, compared with the older main engine type L35MC6.1 applied so far.

The EEDI will be reduced when using S30ME-B9.3. 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) has to be further evaluated by naval architects and shipyards to further reduce the EEDI.

This text is an abridged version of an original paper that also includes calculations and graphs for main engine operating costs at 14.0 and 13.0 knots. The paper is freely available in its entirety from MAN Diesel & Turbo.

Typical dimensions – 8,000dwt tanker

Scantling draught (m) 7.5
Design draught (m 7.1
Length overall (m) 116.0
Length between pp (m) 110.0
Breadth (m) 18.0
Sea margin (%) 15
Engine margin (%) 10
Design ship speed (knot) 14.0 and 13.0
Type of propeller FPP
No. of propeller blades 4
Propeller diameter (m) target

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