Propulsion trends in bulk carriers
Influence of propeller diameter and pitch on SMCR for a 205,000dwt Large Capesize bulk carrier operating at 14.7 knots
As ship operators continue their quest for higher efficiency and lower costs, it is maybe time to think again about the best engines for ships carrying bulk cargoes over long distances; Birger Jacobsen, senior researcher at MAN Diesel & Turbo in Copenhagen, gives an engine designer’s view.
The demand for raw materials like coal, steel and copper has increased considerably since the turn of the millennium, especially in consequence of globalisation and the great demand for raw materials in China, owing to the economic growth in this large country. This means that the Chinese industry, among others, is absorbing large quantities of iron ore and other bulk cargoes.
The bulk carrier market, therefore, is very attractive, which caused a boost in newbuildings until the latest economic crisis in 2008. Since then, bulk carrier orders declined over a short period, but are now picking up again. The optimum propeller speed is changing as well, steadily becoming lower, because the larger the propeller diameter that can be used for a ship, the actual propeller power and pertaining speed requirement will be correspondingly lower, and the lower the propulsion power demand per ton bulk transported.
These factors have an influence on which main engine type should be selected and installed as the prime mover, and also on the size of the bulk carrier to be built. Recent development steps have made it possible to offer solutions which will enable significantly lower transportation costs for bulk carriers.
One of the goals in today’s marine industry is to reduce the impact of CO2 emissions from ships and, therefore, to reduce the fuel consumption for the propulsion of ships to the widest possible extent at any load. This means that the inherent design CO2 index of a new ship, the so-called energy efficiency design index (EEDI), will be reduced. In the future, this drive may probably result in operation at lower than normal service ship speeds compared to earlier, resulting in reduced propulsion power utilisation. However, it still seems to be unchanged.
A more technically advanced development drive is to optimise the afterbody and hull lines of the ship, including the bulbous bow, also considering operation in ballast condition. This makes it possible to install propellers of larger diameter, thereby obtaining higher propeller efficiency, but at a reduced optimum propeller speed, i.e. using less power for the same ship speed. As the two-stroke main engine is directly coupled to the propeller, the introduction of the latest MAN B&W ultra long stroke G engine types meets this trend of installing large propellers in the bulk carriers which may reduce the ship’s fuel consumption. Therefore, today bulk carriers are often ordered with a G engine type as prime mover.
A bulk cargo is defined as loose cargo that is loaded directly into a ship’s hold, rather than in barrels, bags, containers, etc., and is usually homogeneous and capable of being loaded by gravity. Ships designed for such cargoes, normally known as bulk carriers or bulkers, were first developed in the 1950s and are one of today’s three dominant merchant ship types, along with tankers and container vessels. Today, bulk carriers comprise about 43% of the world fleet in tonnage terms.
Bulk carrier sizes and classes are:
- Small < 10,000dwt;
- Handysize 10,000-35,000dwt;
- Handymax 35,000-55,000dwt;
- Panamax 55,000-80,000dw;
- Capesize 80,000-200,000dw;
- Large Capesize 200,000-300,000dwt; and
- VLBC (very large bulk carrier) >300,000 dwt.
The average ship speed Vdes, used for design of the propulsion system and valid for the design draught Ddes of the ship, is shown as a function of the ship size. Today, the average design ship speed – except for Small and Handysize bulk carriers – is generally higher than or equal to 14.5 knots. The trend shown for large Capesize and VLBC shows an even higher selected design ship speed. In general, the selected design ship speed today seems not to be lower than before the economic crisis in 2008-2009. The reason is probably that shipowners still wish to operate the ships at a high ship speed, if needed, but in normal service on reduced ship speeds. Thus, many ships are today installed with main engines prepared for efficient low load operation at reduced ship speeds.
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.
An example is illustrated for a 205,000dwt Large Capesize bulk carrier with a service ship speed of 14.7 knots. The graph shows the needed propulsion SMCR (specified maximum continuous rating) power and speed for a given optimum propeller diameter d and p/d ratio. According to the black curve, the existing propeller diameter of 8.3m may have the optimum pitch/diameter ratio of 0.71, and the lowest possible SMCR shaft power of about 17,700kW at about 88rpm. The black curve shows that if a bigger propeller diameter of, for example, 9.3m is possible, the necessary SMCR shaft power will be reduced to about 16,700kW at about 70rpm, i.e. the bigger the propeller, the lower the optimum propeller speed.
If the pitch, for example for the 8.8m propeller 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 particularly depends 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 specific fuel oil consumption (SFOC). Therefore, today the main engine may often be de-rated. 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 G70ME-C9, may have a higher efficiency compared with a shorter stroke engine type, like a super long stroke S70ME-C8.
The application of new propeller design technologies may also motivate use of main engines with lower rpm. Thus, for the same propeller diameter, these propeller types can demonstrate an up to 4% improved overall efficiency gain at the same or a slightly lower propeller speed. This is valid for the Kappel technology propellers supplied by MAN Diesel & Turbo, of Frederikshavn, Denmark. Furthermore, due to lower emitted pressure impulses, the Kappel propeller requires less tip clearance that can be utilised for installing an even larger propeller diameter, resulting in a further increase of propeller efficiency. Hence, with such a propeller type, the advantage of the new low speed G engine types can also be utilised even though a larger propeller cannot be accommodated.
Based on the already-described average ship particulars and ship speeds for bulk carriers built or contracted during 2000–2013, with due consideration of the latest ships contracted, MAN Diesel has made a power prediction calculation (Holtrop and Mennen’s method) for such bulk carriers in various sizes from 5,000dwt up to 400,000dwt. For all cases, a sea margin of 15% and an engine margin of 10% is assumed, i.e. a service rating of 90% SMCR, including 15% sea margin.
The average ship particulars used refer basically to standard single-side bulk carriers, but the SMCR power demand found may, as a good guidance, also be used for double-side bulk carriers, by referring to a slightly higher deadweight tonnage than valid for the single-side hull design. For example, a 54,000dwt double-side hull design could correspond to a single-side hull design of about 55,000dwt.
The graph shows the above-mentioned table figures of the specified engine MCR (SMCR) power needed for propulsion of an average bulk carrier. The SMCR power curves valid for the future -1.0 knot lower compared to the average design ship speed are also shown.
When the required ship speed is changed, the required SMCR power will change too, as mentioned above, and other main engine options could be selected.
If for a required ship speed, the needed nominal MCR power for a given main engine is too high, it is possible to de-rate the engine, i.e. using an SMCR power lower than the nominal MCR power, which involves a lower SFOC for the engine. Considering the high fuel price and the EEDI demands, current normal practice to select a de-rated main engine in order to get the lowest possible SFOC.
For Small and Handysize bulk carriers, the selection of main engines is not so distinct as for the large bulk carrier classes. Some owners and yards might prefer four-stroke engines, while others prefer and specify two-stroke engines. For the larger bulk carrier classes, the selection of main engine is, as mentioned, more uniform.
The main engines most often selected for Handymax bulk carriers are the five and six cylinder S50MEC8/ ME-B9, with the 6/7S50MEB9 and 6/7G50ME-C9 types being the optimum choice for meeting the power demand of Handymax bulk carriers sailing up to 15.0 knots in service. The main engines used for Panamax bulk carriers are mainly the 5/6G60ME-C9, 6/7G50ME-C9, with the 7S50MEB9 and 7G50ME-C9 types being the optimum choice for meeting the power demand for nearly all Panamax bulk carriers sailing up to 15 knots in service.
Today, the 6S60MEC8, 6G60ME-C9, as well as the 5/6S70MEC8 and 5/6G70ME-C9, engines are used for propulsion of Capesize bulk carriers. For large Capesize, the 6G70ME-C9 is of particular interest. For VLBCs, the 7S80ME-C9 and 7G80ME-C9 engine types are almost exclusively used as the main engine.
The ship speed influences the EEDI as well as the propeller diameter and corresponding main engine type. Taking, once again, a 205,000dwt Large Capesize bulk carrier as an example, for a design ship speed of 14.7 knots, the 6G70ME-C9 with either 8.2m or 8.7m propeller diameter is the only case able to meet the 2015 reference EEDI. But at an even slightly reduced design ship speed of 14.0 knots, with the G70ME-C9 engines, it will be possible to meet the 2020 reference EEDI figure without further optimisation of hull and/or propeller.
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