One of the largest LNG carriers with dual-fuel engine propulsion, ordered by Spanish owner En Elcano, is powered by 5 x 8L 51/60 DF engines by MAN Diesel.
As the growth of LNG transport has accelerated in recent years, so has the development of the propulsion system for the LNG carriers. Traditionally, steam turbines have been used mainly because they can burn the boil-off gas from the cargo. However, they are not particularly efficient thermally; therefore, both diesel and gas turbine engine manufacturers have put forward alternative drive systems. In spite of this, Japanese tanker builders are still launching new vessels with steam turbine drives … but the market is changing.
One particularly attractive new approach is based on dual fuel, as presented in a recent paper from MAN Diesel, which aligns the merits of electric propulsion with inline dual-fuel diesel prime movers. “Dual-Fuel Diesel Electric (DFDE) propulsion is now widely accepted by the LNG industry as being more efficient than steam-based systems,” said Sokrates Tolgos, head of sales for MAN Diesel’s LNG and cruise ship applications. “Besides this and its extreme environmental friendliness, with a proper choice of multi-engine configuration, DFDE is as flexible in terms of fuel selection (HFO, MDO, MGO or gas) as steam propulsion, and in addition provides maintainability of an engine at any time during the voyage.”
Because the MAN Diesel 51/60DF can burn a selected fuel at different load levels, the DFDE installation has the capability to burn any permutation of the liquid and gaseous fuels requested by the operator.
Questions center on whether the level of total fuel flexibility can be achieved at satisfactory load settings for the individual engines throughout the different segments of the voyage, as well as whether this can be maintained when one engine is out of service. These aspects depend largely on the engine configurations chosen in four-stroke DFDE plants. The power per cylinder determines the number of cylinders required, from which a reasonable splitinto individual engines has to be found. A typical carrier will need an installed power of 40 MW split over four to five engines. Two typical layouts found in LNG carrier projects are an inline configuration with five inline eight-cylinder engines of 1000 kW/cyl and a mixed configuration with three 12-cylinderVee engines and one six-cylinder inline model with 950 kW/cyl.
Most LNG carriers follow fixed routes and run continuously on strict schedules. Harbor times are short, typicallyless than a day is used for loading and discharging, and consequently engine maintenance must be conductedonboard while under way.
For a standard carrier of 155 000 m3, maximum overall power requirements under way are around 32 MW. Thisincludes the 21% sea margin used by many designers. Another widespread guideline is that full power needs of thevessel be met by around 85% of engine load. Maneuvering in harbor may require up to 14 MW of power, whileidling and waiting periods need about 1.5 MW. When loading or unloading using the cargo pumps, around 4 to 7 MW are needed.
Typical operational profiles and charter contracts allow only the burning of evaporating cargo gas — natural boil-off gas (NBOG). While overall power needs do not vary much for laden and ballast voyages, the amount of NBOG varies considerably. When burnt as fuel in four-stroke dual-fuel engines, the amount of NBOG is around 20 MW during laden voyage. Additional power has to be generated either by liquid fuel add-up or by forced boil-off gas (FBOG).
Depending on the engine configuration and the available NBOG during laden and ballast voyage, only one or two engines will need to switch between oil and gas. The other engines can be dedicated units and stay on the same fuel for both legs.
MAN Diesel has investigated three high-load scenarios typical of LNG carrier operation. In the first of these, with a clean hull and in calm weather, the vessel needs its maximum sailing power requirement of 27 MW. In the inline configuration, four engines are enough to provide the power, with three units in gas mode and one on HFO (see figure 1). The resulting loads are 83 and 84% respectively — levels widely practiced in the marine sector. Also, conducting maintenance on the fifth engine does not impose operational restrictions.
With the mixed configuration, the worst case in terms of power availability is to have one of the large engines under maintenance, creating a serious power drop. Due to the available amount of NBOG, an obvious choice is to run the two large engines on gas and the small one on liquid to provide the required power (see figure 2). However, the remaining engine in liquid mode is not strong enough to provide the necessary power add-up.
With increased fouling, more power is needed to maintain service speed, while the quantity of NBOG remains unchanged. Thus, the loads of the gas engines remain the same, but the engine providing power add-up becomes overloaded.
There are several options to deal with this situation including reducing the vessel speed, restarting the shutdown engine and producing power by forced boil-off. The first of these does not allow schedules to be met, while the second only works if no major maintenance is due on that engine.
The optimal solution utilizes only a small amount of FBOG, where the load on the gas-burning engines can be increased slightly to reduce the load on the liquid-fueled unit. Hence, in the case of the inline configuration with less than 10% of the NBOG, smooth and favorable load factors can be achieved on all running engines, still keeping one out of service (see figure 3).
With the mixed configuration, however, the engine in liquid mode is greatly overloaded. Even adding FBOG and loading the gas-burning engines up to 100% does not bring the engine in liquid mode below its operational limit (see figure 4). To maintain vessel speed, there is no choice but to fire up the shutdown large engine. However, this may conflict with the maintenance schedule, so the mixed configuration four-engine plant reaches operational
limitations much sooner than the inline set-up with five engines of equal output.
With progressive fouling and bad weather, the 21% sea margin of the vessel may be fully used and engine power requirements will reach their maximum. In the worst-case scenario, the five engine configuration is still capable of providing enough power to maintain service speed with one unit out of service and the four working engines running at 100% load (see figure 5). Unlike conventional diesel engines, dual-fuel models in gas mode achieve their optimum efficiency at 100% load. The operator must decide whether to continue engine operation at 100% MCR (the 51/60DF engine is released for continuous operation at full load), fire up the remaining engine if possible, or reduce vessel speed slightly.
Due to the uneven power split with the mixed engine configuration, the required power for the worst-case scenariocannot be provided without all engines running. When the largest engine is out of service, the remaining engines are all well beyond their operational limits, so the large unit must be restarted (see figure 6).
- (Left) Figure 5: Engine loads for inline configuration during laden voyage under maximum design power needs (21% sea margin used) — three engines in gas mode, one in liquid mode and one out of service. (Right) Figure 6: Engine loads for mixed configuration during laden voyage under maximum design power needs (21% sea margin used) — two engines in gas mode, one in liquid mode and one large engine out of service
At low loads, when waiting to enter the terminal for instance, power requirements can be as low as 1.5 MW. In the case of the inline configuration, any of the engines will still work above 15% load, which is the minimum for continuous operation on gas. Thus, even in the extreme low-load situation, all the inline engines — regardless of their cylinder count — maintain the flexibility to burn either gas or liquid fuel. In the case of mixed configuration, however, the resulting load on the large Vee engine is below 15%, and this could mean operational restrictions if the smaller six-cylinder is not available.
The inline engine configuration shows benefits since the maximum power drop is only 20% with one engine out of service. The even distribution of installed power over five similar inline engines permits the shutdown of any unit at any time while maintaining loads on the remaining engines within limits. On the other hand, with the mixed Vee/inline configuration, a drop of around 30% of installed power results if one of the engines is out of service. Furthermore, the average maintenance time for inline engines is less than that for Vee engines.
With a well-selected multi-engine setup, the 51/60DF features fuel flexibility to an extent that matches the boilers of a steam propulsion system without compromising the vessel’s maintainability. Any engine can be serviced at any time without affecting either the ship’s sailing schedule or the level of the vessel’s fuel flexibility.
This article is based on a paper by Michael Wenninger and Sokrates Tolgos titled, “LNG Carrier Power – Total Fuel Flexibility and Maintainability with 51/60 DF Electric Propulsion,” MAN Diesel SE, 2008.