The increasing cost of energy has imposed a revision of the propulsion system of LNG carriers from steam turbines to diesel or dual-fuel engines. The boil-off gas (BOG) available on- board at very low temperature can be reliquefied and conveyed back to the cryogenic tanks or used onboard as engine fuel in four-stroke, medium- speed or two-stroke, low-speed, dual- fuel engines. Depending upon the en- gine configuration, the fuel gas maybe fed at low or high pressure. MAN Diesel and Burckhardt Com- pression have made a strategic part- nership agreement to develop a com- pressor able to handle on one end the BOG at -256°F (-160°C) and virtu- ally at atmospheric pressure. On the other end, it can handle up to 4348 psi (300 bar) for injection into the ME-GI (Gas Injection) two-stroke, dual-fuel engines developed by MAN for LNG carrier propulsion systems. The function is accomplished with one common frame, which com- presses the cold BOG directly to the required injection pressure. The Danish subsidiary of German- based MAN Diesel has already sold 90 MAN-B&W two-stroke engines type 7S70ME and 6S70ME to partners in the Qatargas projects. Acco rdin g to MAN, these two- stroke engines — featuring a 50% thermal effi- ciency in the dual-fuel version — are sized to propel large LNG carriers with capacities of 5.124 to 95.41 MMcf (145,000 to 270,000 m 3 ) of L NG. Th ese vessels can be powered by a single main engine, with power take home system, or in the more common con fig- uration with two main engines directlycoupled to the two parallel propeller shafts. The gain in efficiency, compared to single-engine propulsion systems, can reach up to 9% depending on the vesse l siz e and its b eam/ draft rati o. The engines can burn 100% heavy fuel oil (HFO) or diesel oil or a mixture of liq- uid fuel (a minimum percentage is needed to ensure gas ignition) and natural gas. The GI system is an add-on to an ex- isting electronically controlled ME en- gine and, in principle, the system is quite simple. The gas is fed to the cylinders via high-pressure double- wall ed gas p ipes an d a set of el ectr oni- cally controlled gas valves, which allowgas injection into the combustion chamber. These valves are grouped in a block bolted to the cylinde r head. AUGUST-SEPTEMBER 2008 COMPRESSORTech Two s The 6LP250-5S_1 Burckhardt Laby-GI ver- tical compre ssor features six 9.84 in. (250 mm) cranks driving the pistons through cross-heads. LABY-GI DVD FLNG AVHigh-Pressure, Boil-off Gas Fuels Dual-Fuel Propulsion Engines By Roberto Chellini
The increasing cost of energy hasimposed a revision of the propulsionsystem of LNG carriers from steamturbines to diesel or dual-fuel engines.The boil-off gas (BOG) available on-board at very low temperature can bereliquefied and conveyed back to thecryogenic tanks or used onboard asengine fuel in four-stroke, medium-speed or two-stroke, low-speed, dual-fuel engines. Depending upon the en-gine configuration, the fuel gas may be fed at low or high pressure.
MAN Diesel and Burckhardt Com-pression have made a strategic part-nership agreement to develop a com-pressor able to handle on one endthe BOG at -256°F (-160°C) and virtu-ally at atmospheric pressure. On theother end, it can handle up to 4348psi (300 bar) for injection into the
ME-GI (Gas Injection) two-stroke,dual-fuel engines developed by MANfor LNG carrier propulsion systems.The function is accomplished withone common frame, which com-presses the cold BOG directly to therequired injection pressure.
The Danish subsidiary of German-based MAN Diesel has already sold 90MAN-B&W two-stroke engines type7S70ME and 6S70ME to partners inthe Qatargas projects.
According to MAN, these two-strokeengines — featuring a 50% thermal effi-ciency in the dual-fuel version — aresized to propel large LNG carriers withcapacities of 5.124 to 95.41 MMcf (145,000 to 270,000 m3) of LNG. These
vessels can be powered by a singlemain engine, with power take homesystem, or in the more common config-
uration with two main engines directly coupled to the two parallel propellershafts. The gain in efficiency, comparedto single-engine propulsion systems,can reach up to 9% depending on the
vessel size and its beam/draft ratio. Theengines can burn 100% heavy fuel oil(HFO) or diesel oil or a mixture of liq-uid fuel (a minimum percentage isneeded to ensure gas ignition) andnatural gas.
The GI system is an add-on to an ex-isting electronically controlled ME en-gine and, in principle, the system isquite simple. The gas is fed to thecylinders via high-pressure double-
walled gas pipes and a set of electroni-cally controlled gas valves, which allow gas injection into the combustionchamber. These valves are grouped ina block bolted to the cylinder head.
AUGUST-SEPTEMBER 2008 COMPRESSORTechTwo
s The 6LP250-5S_1 Burckhardt Laby-GI ver-tical compressor features six 9.84 in. (250mm) cranks driving the pistons throughcross-heads.
LABY-GI DVD
F LNG A VHigh-Pressure, Boil-off Gas Fuels Dual-Fuel Propulsion Engines
The control system, developed in co-operation with Burckhardt andKongsberg, adjusts the amount of gasinjected into the ongoing combustion.The gas injection pressure is modulatedbetween 2174 and 4348 psi (150 and300 bar) according to the engine load.The slightly modified cylinder headalso features an internal gas accumu-
lator. These are the only modificationsmade on the original ME engines totransform them into the ME-GI ver-sion, which has obtained approval forship propulsion by all major classifica-tion societies.
An ME-GI land -bas ed un it in apower generation plant has already performed over 20,000 operatinghours, demonstrating the high relia-bility of the dual-fuel diesel engine.This dual-fuel engine is capable of burning HFO and natural gas in al-most any ratio according to the diesel
cycle (up to 92% natural gas). Thehigh-surplus air volume and thehigh-pressure gas injection systemcan sustain combustion when the en-gine is burning a variety of gases,other than natural gas, even with low methane content.
The BOG, naturally available fromtank evaporation, is estimated to beabout 80 to 90% of the fuel requiredfor laden voyages and 40 to 50% onballast voyages — with the enginesdriven at full power to maintain acruising speed of 19 to 21 knots. Thefuel-gas supply system placed inside ahouse on the top deck can be pro-
vided with different compressor com-binations depending upon productavailability. With two 100% capacity Laby-GI compressors, one of which iskept in standby, a theoretical avail-ability of 99.25% can be reached.
However, this solution entails apoor part-load performance of thecompressor in service. Two 50% ca-pacity Laby-GI compressors couldalso be installed to increase their part-load efficiency and integrate HFO
with gas injection in case of an outage
of one unit. However, two 75% unitsare estimated to be the right size for areliable and efficient system. In fact,BOG is very clean, and compressormaintenance — including the valves(the most delicate components of acompressor) — can be performed atlong intervals.
The 6LP250-5S_1 Burckhardt Laby-
GI compressor, of the vertical type,features six 9.84 in. (250 mm) cranksdriving the pistons through cross-heads. Four cylinders of the Laby type serve as the first three compres-sion stages (the first stage compres-sion is carried out by two separatecylinders working in parallel). Twohigh-pressure cylinders serve processrequirements. For application onsmaller-sized LNG carriers, with re-duced BOG to be handled, Burck-hardt has developed the 4LP250-5S_1Laby-GI compressor featuring four
compressor cylinders instead of sixfor mass balancing.
The low-pressure section of thecompressor is similar to thoseBurckhardt has supplied to many pro-ducing and receiving LNG terminals
for compression of BOG to dischargepressures in the range of 145 to 725psi (10 to 50 bar). These double-act-ing, nonlubricated cylinders featurecontactless labyrinth seals on the pis-tons and piston rod packings to avoidmechanical friction of the slidingparts, thus ensuring extremely longlife to the sealing components andhigh availability and reliability.
The two first-stage cylinders, which
are exposed to very low temperatures,are cast in nodular cast-iron GGGNi35(Ni-Resist D5) with 35% nickel, whichexhibits a remarkable ductility at low temperatures and a very low thermalexpansion coefficient. The pistons areof cast iron with laminar graphite.This piston-cylinder material combina-tion enables starting the compressorat ambient temperature 86°F (30°C)and cooling it down to BOG tempera-ture -256°F (-160°C) without the needfor special precautions or pre-coolingof the compressor.
The second- and third-stage labyrinth
cylinders operate at higher tempera-tures and are provided with coolingjackets. Nodular cast iron is used forthe second-stage cylinder block andgrey cast iron for the third stage.
The oil-lubricated, high-pressure,forged steel cylinders are provided
with water jackets to remove the heatof compression. They are designed toperform the fourth compression stageon the top and the fifth on the lowerend of the same cylinder block. Pistonand piston rod are machined from asingle steel forging. Any gas leaking
from the fifth stage piston rod packingis returned to the fourth-stage suction.The two identical four and five com-pression stage cylinders are placedside by side on the upper part of thecompressor frame. The compressor
AUGUST-SEPTEMBER 2008 COMPRESSORTechTwo
s Diagram showing two Burckhardt Laby-GI compressors feeding two MAN ME-GI dual-fuel en-gines or a GCU in parallel.
s Elevated drawing of a MAN ME-GI, dual-fuel engine for LNG carrier propulsion. Inset showsnew components for conversion to dual-fuel from straight diesel fuel.
frame is of conventional vertical,slow-speed crosshead design.
The forged steel crankshaft andconnecting rods are supported by heavy tri-metal, forced lubricatedmain bearings. A single distance piecein the upper frame section providesseparation between the lubricatedcrank mechanism and the nonlubri-cated compressor cylinders. Pistonrods, actuated by the crossheads attheir lower end, are guided in theirrectilinear reciprocating motion by alubricated bearing placed at the dis-tance piece lower section, which pre-
vents metal contact of labyrinth seals.
The passage of the crankshaftthrough the wall of the crankcase issealed off by a rotating double-sidedring seal immersed in oil. Thus, theentire inside of the frame is integratedinto the gas-containing system withno gas leakage to the environment.
The crankshaft is directly connectedto the driving electric motor. Size is se-lected according to the LNG carrier sizeand the amount of BOG to be handled.
Design natural BOG rates are typi-cally in the range of 0.135 to 0.15%per day of tanker liquid capacity.During steady-state loaded voyage aBOG rate of 0.10 to 0.12% may be ex-pected. During ballast voyages theamount of BOG naturally evaporatingmay increase by forced evaporation.
Three compressor sizes are suffi-cient to cover the needs for LNG car-riers ranging from 5.1 to 9.5 x 106
MMcf (145,000 to 270,000 m3) capac-ity. As an example, a 7.421 MMcf (210,000 m3) LNG carrier will makeavailable a BOG mass flow of 12,335lb./h (5600 kg/h) that can be han-dled by a compressor with 2146 hp(1600 kW) of shaft power.
In order to handle different amountsof BOG, made available naturally from
the tanks and the amount of gaseousfuel required by the engines, the com-pressors have to be provided with a
very robust and simple control system.This control system must take into
account two main factors: the amountof BOG to be handled during theladen voyage, which is much higherthan that available during the ballast
voyages [also the cryogenic tempera-ture can vary substantially, from -256°to -40°F (-160° to -40°C)] and the de-livery pressure to the engines, whichcan reach peaks of 4348 psi (300
bar). In practice, delivery pressurecan vary from 2674 psi (150 bar)
when the engines are running at low load to 3841 psi (265 bar) and 113°F(45°C) when engines are at full load.Furthermore, if the engines are notrunning, the compressor has to han-dle the gas at low pressure, 58 to 94psi (4 to 6.5 bar) (after the first com-pression stage) to the gas combustionunit (GCU).
Capacity control by valve unloadingis extensively employed in LNG receiv-ing terminals where very large varia-tions of BOG are experienced duringLNG transfer from ships to storagetanks. The capacity of the 6LP250-5SLaby-GI compressor can be reducedsimply and efficiently to 50% in onestep by the use of nitrogen-actuatedsuction valve unloaders unloading onehalf of the double-acting cylinders.
Addi tional stepless regulation re-quired to control compressor capacity corresponding to the rate of BOG andthe demand of the engines, is pro-
vided by returning the gas from thedischarge to the compressor suctionby using several bypass valves.
Combining the two systems enablesthe maximum capacity flexibility (100
to 0%) to be obtained.If the amount of natural BOG is
higher than that required by the en-gines, resulting in a higher than ac-ceptable suction pressure, the controlsystem will send the excess gas to theGCU via the side stream of the firstcompression stage. Safety relief valvesare provided at the discharge of eachcompression stage to protect thecylinders and the entire gas loopagainst overpressure.
Installation of such compressors on-board a ship poses additional prob-
lems as compared to land-based in-stallations because of the flexiblestructure of the ship as opposed to aconcrete foundation block.
Burckhardt, as a consequence, engi-neers the whole compression system — starting from the static and dynamic me-chanical analysis, thermal stress analy-sis, pulsation analysis of the compressoritself and of the auxiliary system con-sisting of the gas piping upstream anddownstream of the compressor, pulsa-tion vessels, gas intercoolers, etc.Included are all components that areconsidered standard parts of the com-pressor supplier’s responsibility.
The compressor is mounted on asteel baseframe having a 36.1 x 23 ft.(11 x 7 m) footprint and designed tobe supported by the ship structure.The main motor drive is designed forhazardous conditions and is inte-grated into the machinery room with-out requiring a dividing bulkhead.
The improved design of the fully bal-anced Laby-GI crank gear develops
very low unbalanced forces and mo-ments compared to a conventional reci-procating compressor. Maintenance in-
s Detail of cylinder head for aMAN ME-GI gas-injected, dual-fuel propulsion engine showinglocation of components neces-sary to convert the engine fromstraight diesel to gas-injected,dual-fuel operation.
terventions are recommended every 20,000 to 24,000 hours. BOG is very clean and an excellent gas to be com-pressed and the lifetime expectancy for
valve plates can be estimated at 24,000hours. Maintenance intervention for
valve replacement will take approxi-mately seven to nine hours includingcompressor shutdown and isolation.
A GI retrofit on an LNG carrier pro- vided with a MAN ME-C two-stroke
engine propulsion system is feasible.These vessels go into dock for generalmaintenance every five years. MANDiesel requires the order for the new components (cylinder heads, controlsystem, etc.) two years in advance inorder to perform the retrofit. At thesame time the compressor house of the reliquefaction plant on the topdeck is enlarged to make room for thenew BOG compressors, thus limitingthe off-hiring period. s
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s Plant layout for a Burckhardt Compression, electric motor drive four-cylinder Laby 4LP250-5S_1 compressor.
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