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Conclusion

MAN B&W ME-C series for LNG Carriers. Conclusion

The benefit of diesel engine propulsion of LNG carriers is calculated to be up to approx. US$ 3.0 million per vessel per year. Especially the LNG selling price has a positive impact on the advantage of diesel engine propulsion. The benefit gained in operating costs and the additional income from the sale of LNG by diesel engine propulsion and reliquefaction will, in all cases, be sufficient to justify even large differences in investment costs, if such are called for at all.
Basically, diesel propulsion offers a CO2 emission reduction of about 30% compared to the steam plant.

Exhaust emissions

MAN B&W ME-C series for LNG Carriers. Exhaust emissions

The relative energy consumption for the two concepts is shown in Fig. 10, which also shows the expected annual exhaust emissions. The CO2 emission is obviously largest for the steam plant due to its low efficiency.
The SOx from the fuel sulphur is about the same, as the same amount of fuel is used. This can be reduced by using fuel with low sulphur content.
The proposed diesel solution complies with the IMO limits for NOx emissions and is therefore without any NOx abatement.

Insestment costs

MAN B&W ME-C series for LNG Carriers. Insestment costs

A system comprising the traditional steam plant is estimated to cost around US$ 20 million.
The direct-coupled diesel solution requires lower investment cost than the steam plant, as far as equipment is concerned.
To this come installation costs, which are not considered.
Most shipyards (all) that today build LNG carriers have much more experience of installing diesel engines than steam turbines and boilers, which adds to the advantage of diesels.

Economical evaluation

MAN B&W ME-C series for LNG Carriers. Economical evaluation

The operating costs and the additional income from the transport and sale of LNG for a 150,000 m3 LNG carrier is analysed, including an analysis of the fuel oil, lubricating oil and maintenance costs for both propulsion and electricity production under various operating conditions, comparing steam turbines with proposed configuration.
The analysis is based on state-of-the-art insulation of tanks, and thus BOG rate, and a traditional service speed of the vessel. 

Propulsion redundancy and gas handling

MAN B&W ME-C series for LNG Carriers. Propulsion redundancy and gas handling

LNG carriers, like oil tankers, are not permitted to immobilize their propulsion machinery while in port and port areas. Hence, redundancy is required.
For the steam ship, redundancy is considered fulfilled by having two boilers, whereas no redundancy is required for the single steam turbine, propeller shaft and propeller. The two boilers will have a steam-dumping condenser to be used for surplus steam when the turbine is not operating.

Diesel engine technology

MAN B&W ME-C series for LNG Carriers. Diesel engine technology

MAN B&W offers a full programme of marine diesel engines for every conceivable application.
The low speed engine programme is developed in Denmark and manufactured by a family of licensees at major ship-building centres in the world. Single unit powers range from 2,000 hp to well over 100,000 hp, all for direct coupled installation at propeller speeds from 250 rpm down to 60 rpm for the largest propellers.
The power requirement for an LNG carrier calls for some 40,000 ph, typically two off 60 or 70 cm bore units. 
MAN B&W low speed engines hold a world-wide market share of about 65% in their segment.

Demonstration plants

MAN B&W ME-C series for LNG Carriers. Demonstration plants

One test plant for the Moss Reliquefaction System (Moss RS) is located at the Ukrainian company Sumy Frunze located in Sumy, Ukraine. The Moss RS patent holder, Moss Maritime of Norway, is responsible for this plant. It comprises a pre-treatment plant and a liquefaction plant. Gas is supplied from the local gas grid, and a large pre-treatment plant is used for delivering gas over a wide specification range.

Control systems

MAN B&W ME-C series for LNG Carriers. Control systems

Generally, the temperature in the nitrogen loop decides the quality of N2 in the coolant circuit.
Increasing or decreasing the amount of nitrogen in the loop changes the cooling capacity. The amount is changed by injecting or withdrawing nitrogen from the receiver. If the cooling capacity is too high, the inlet expander temperature will decrease. The control valve to the receiver at  the compressors discharge will open to withdraw the nitrogen from the main loop. Correspondingly, if the cooling capacity is too low, the inlet expander temperature will increase. The control valve from the receiver to the compressor suction side will open to inject nitrogen into the main loop.

Nitrogen cycle

MAN B&W ME-C series for LNG Carriers. Nitrogen cycle

The cryogenic temperature inside the cold box is produced by means of a nitrogen compression-expansion cycle, shown in Fig. 3. Nitrogen gas at a pressure of 13.5 bar is compressed to 57 bar in a 3-stage centrifugal compressor. The gas is cooled by water (seawater or indirect) after each stage. After the last cooler, the gas is led to “warm” part of the cryogenic heat exchanger where it is pre-cooled to about -110oC and then expanded to a pressure of 14.5 bar in the expander. The gas leaves the expander at about -163oC and is then introduced into the “cold” part of the cryogenic heat exchanger where it cools and reliquefies the boil-off gas to LNG.

Boil-off cycle

MAN B&W ME-C series for LNG Carriers. Boil-off cycle

The cargo cycle consists of an LD compressor, a plate-fin cryogenic exchanger, a separator and an LNG return pump. Boil-off is evacuated from the LNG tanks by means of a conventional centrifugal low duty compressor. The vapour is compressed to 4.5 bar and cooled at this pressure to approximately -160oC  in a plate-fin cryogenic heat exchanger. This ensures condensation of hydrocarbons to LNG. The fraction of nitrogen present in the boil-off that cannot be condensed at this condition remains as gas bubbles in the LNG. Phase separation takes place in the liquid separator. From the separator, the LNG is dumped back to the storage tanks, while the nitrogen-rich gas phase is discharged (to atmosphere or burnt in an oxidizer).

Reliquefaction technology

MAN B&W ME-C series for LNG Carriers. Reliquefaction technology

While reliquefaction is widely used in gas handling on land, it has been used on board ship so far only on LNG carriers.
Recently, the technology for reliquefying LNG on board ship has been matured and commercialised. The present analysis is based on the Moss Reliquefaction, sold worldwide by Hamworthy KSE.
The patented system (Moss RS) for reliquefying boil-off gas, establishes a solution for pumping LNG back to the tanks and selling more LNG to the buyers of gas.

LNG carrier propulsion by ME engines

MAN B&W ME-C series for LNG Carriers. LNG carrier propulsion by ME engines

LNG carriers represent the last stand for the – in all other markets – practically extinct marine steam turbines. With efficiencies of only about 30%, versus the diesel engines’ more than 50%, and in combined systems even higher, diesel engines are the propulsion system of choice in the marine industry.
This reason for the dominance of the diesel engines is clearly demonstrated in Fig. 1, showing the thermal efficiency of the various prime movers.

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