The current world usage of LNG isabout 110 million tons per year andcurrent analysis indicates this grow-

ing in the next 10 years by 70 to 100 milliontons. This may lead to demand for morethan 80 new LNG carriers by 2010, in addi-tion to those already under construction.Traditionally LNG carriers have been pro-pelled by steam turbines, as this has provedto be a simple and reliable solution forthese ships. However the relatively lowefficiency of the propulsion system (<30%)and possible increased size of future LNGcarriers has initiated investigations into uti-lizing alternative propulsion systems.

Electric propulsion has as a result ofthese investigations been shown to be anattractive alternative with high overallefficiency (~42%). The first three LNG car-riers with electric propulsion are nowunder construction at Chantiers de l'At-lantique in France for the French ownerGaz de France, where ABB is deliveringthe propulsion system on the second andthird vessel, which are the world's firstvessels with full size cargo capacity(154,000 m3) utilizing an electric propul-sion system.

In the last years the power plant for thecargo handling system has changed from a440 V system to a medium voltage system(3.3 kV or 6.6 kV) due to increased installedpower for the cargo pumps as the ship sizeshave increased. Electric propulsion wouldbe the natural extension utilizing thealready installed medium voltage powersystem also for powering the propeller.

With the development of the Dual Fuel(DF) engines, which can operate both ongas and diesel oil, electric propulsion hasbecome an attractive solution, due toincreased efficiency, increased cargo capac-ity and reduced operational costs. The boil-off gas (BOG) is used directly in the medi-um speed engines, and the propulsionpower is transferred to the propellers bythe electric power and propulsion plant.

Other essential criteria for selecting apropulsion system are the availability andreliability of the propulsion equipment.LNG carriers have long time charters withfixed schedules, and a high penalty of notmaking the schedule in time. The avail-ability of the conventional steam turbinehas proven to be very high, and any alter-native solution to be considered mustcomply with at least the same level ofavailability. With the inbuilt redundancyin the electric propulsion system andusing reliable components these require-ments are met.

Why Electric Propulsion?The concept of electric propulsion is notnew, the idea originated more than 100years ago. However, with the possibilityto control electrical motors with variablespeed in a large power range with com-pact, reliable and cost-competitive solu-tions, the use of electrical propulsion hasemerged in new application areas during

the 80's and 90's. Electric propulsion with gas turbine or

diesel engine driven power generation isused in hundreds of ships of various typesand in a large variety of configurations.With the new-buildings for Gaz de France,electric propulsion is now entering alsothe LNG ship. The main arguments forelectric propulsion system are described inthe following subsections.

Propulsion efficiency: At rated loadthe reported efficiency of the DF enginesare typically about 47%. Including thetransmission losses, which are in the rangeof 8-10%, the overall efficiency is approxi-mately 43% calculating from fuel con-sumption to propeller shaft power. Thereported efficiency for the steam propul-sion unit (including boilers, steam turbineand gear) is typically less than 30%. Forpart load operations as in manoeuvringmode or operation in areas with speedrestrictions the efficiency differencebetween steam and electric propulsion iseven higher. This is due to the power plantprinciple where the power generation partconsist of several engines operating in par-allel and an optimum number of primemovers is always selected to match theload demand from the propellers and shipservice load. Figure 1 gives an illustrationof the efficiency.

Total installed power: On the conven-tional LNG carriers today there are 3 - 4auxiliary generators for the cargo han-dling plant. The total power is about 10 -12 MW with medium voltage switchboardinstallations. The majority of this power isonly utilized while the ship is at the termi-nal for cargo unloading.

For vessels with electric propulsion onecommon power plant is utilized for bothpropulsion and cargo unloading. Thismeans that the total amount of installedpower can be reduced with electricpropulsion systems because the cargounloading plant and the propulsion plantare not used simultaneously. For examplean LNG carrier with a requirement of 26MW of propulsion power and 10 MW forcargo unloading would require about 39

MW of installed power capacity (includ-ing 10% turbine margin) for the conven-tional steam turbine ships. Using electricpropulsion the dimensioning factor for thepower plant would be the propulsionpower plus the ship service load. Assum-ing a ship service load of 2500 kW theinstalled power could typically be 35 MW(including 8% electrical losses and 10%engine margin). This means a reduction ofabout 11% of installed power capacity.

Electric variable speed drive: Basicallya speed controlled electric motor (by fre-quency converter) has full torque avail-able from 0 to max. RPM (revolutions perminute). The RPM range is from max. neg-ative to max. positive RPM. There arepractically no limitations in RPM andtorque performance, other than thoseimposed by the mechanical parts in thepropulsion and prime mover system. Theconverter is set to protect the mechanicalsystems by controlling the torque speedand power increase/decrease accordingly.

With these possibilities of the propul-sion drive system, there will be a positiveimpact on the manoeuvrability and crashstop situations. For the manoeuvrabilitythe impact will rise from the fact that thepropeller can be run at any RPM, andRPM changes can be fast because of thepossibility to give full torque (pos./neg.)at any RPM. The crash stop situations willimprove by the fact that the electric motorand converter is able to absorb reversepower from the propeller by electric brak-ing and hence be able to change RPMdirection faster than for the traditionalsteam turbine propulsion.

Redundancy: Electrical power genera-tion and distribution systems must be con-figured to meet the redundancy require-ments of the installation. Classificationsocieties have strict rules on how the pow-er plant shall be configured for their dif-ferent notations; however, the societies donot have common rules.

With electric propulsion 50% redundan-cy is easily obtained for the electrical pow-er part, i.e. at least 2 propulsion motorswith separate feeding (converter + trans-former), and at least a 2-split switchboardsystem. With this redundancy concept, theavailable propulsion torque or power willbe a minimum of 50% after any single fail-ure in the electrical system. The conse-quence of loss of ship speed is howevermuch lower and typically the ship shouldbe able to operate with a ship speed not lessthan 70% of the maximum in case of anysingle failure in the electrical system.

Diesel-mechanical system: Diesel-mechanical systems with two strokeengines and Fixed Pitch Propellers (FPP)are also considered as alternative propul-sion for future LNG carriers. This propul-sion concept is commonly used in othercommercial tanker fleets, so this alsoneeds to be considered as an option forLNG carriers. The main obstacles so farhave been the handling of BOG and the

reliability compared to today's steampropulsion.

Using two-stroke engines for propul-sion would require the installation of areliquefaction plant for handling the BOG.This would mean additional installedpower capacity, and also additional con-tinuous power consumption of about 5MW at seagoing conditions. The totalinstalled power would then increase com-pared to steam propulsion and even morecompared to electric propulsion.

As the reliability requirements for LNGcarriers are extremely high, there is reluc-tance in the market to use two strokeengines for propulsion of LNG carriers con-sidering the single screw case. Two strokeengines are therefore mainly considered fortwin skeg alternatives, however with a rela-tive large machinery weight increase com-pared to the electric propulsion machinery.The vibration levels for the two stroke solu-tion are also higher than for the convention-al system and also higher than for the elec-trical system where the prime movers are 4-stroke dual fuel engines.

For all vessels with electric propulsion thepower plant is essential. The power plantconsists of several medium speedgas/diesel engines, which drive the electri-cal generators. The generators are connect-ed in a common grid, to the main electricalswitchboard. All loads, including propul-sion, thrusters, auxiliaries, and ship sys-tems are normally fed from this grid, andthe total load is shared between the run-ning generators. The configuration of thepower plant is based upon total installedpower, operating modes, flexibility andredundancy requirements, and equipmentcost. The number and rating of power gen-erating sets must be optimized to achievethe desired flexibility in view of fuel econo-my for prime movers, operational profileand service factor. The purpose is toachieve optimal loading of the runninggenerator sets during the different opera-tion modes.

The voltage level of the main powerplant is selected so that load current andshort circuit levels are kept within thetechnical limits of the equipment, and alsoto optimize the total cost of the installation. For installed power lev-els above 8-10 MW, medium voltageswitchboards are the best technical andcost optimal solutions providing arc-proofsolutions with high safety with regard todamages and injuries. With increasingtotal power, the power generating voltagewill increase also, e.g. to 6.6 kV or 11 kV.For full size LNG carriers and future larg-er size LNG carriers the optimum voltagelevels are 6.6 kV and 11 kV, respectively.

Figure 2 shows the most typical config-urations studied for the future LNG carri-ers with electric propulsion, single pro-

LNG journal September/October 2004 page 11

LNGjournal

Electric propulsion for LNG CarriersFull-size LNG carriers with dual fuel diesel engines and electric propulsion are now under construction in France.The authors present the benefits and design features of electric propulsion systems in LNG shipping applications.

Jan Fredrik Hansen and Rune Lysebo, ABB AS Marine group, Norway

Figure 1 Efficiency from fuel supply topropeller shaft of a Dual Fuel ElectricPropulsion plant compared to a Steam

Propulsion plant. Power andpropulsion forLNG carriers

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peller with medium speed motors andgearbox and twin propellers with directslow speed motors. Other configurationssuch as using medium speed motors withgear also for the twin propeller case andtwo tandem slow speed motors on the sin-gle propeller case are also being consid-ered. As shown in the configuration draw-ings the 50% redundancy is kept all theway down to the propulsion motors, andbesides from the different configurationsof the propulsion motors the electric pow-er generation, distribution and propulsionconverter configurations are the same forboth propulsion alternatives.

Being the major consumer, the propul-sion drive is also the main source of har-monic currents. By correct dimensioningof power plant generators and propulsiondrives it is possible to get the harmonicdistortion level within the classificationlimits without the use of additional filter-ing. The most efficient and reliable meas-ure is to increase the pulse number of thedrive system. For the LNG carriers withpropulsion power of more than 20 MWthis can be applied with minor costimpact. Using the ACS 6000 convertersfrom ABB, standard diode bridge rectifiersare used giving a constant high power fac-tor to the network and by combination ofthe rectifier bridges in 12- and 24-pulsesolution the harmonic distortion level isminimized. With 24-pulse solution theTHD (Total Harmonic Distortion) level ofthe main voltage is normally below 5%.

Fuel consumption andemissions

One of the main drivers to shift fromsteam to electric propulsion system is theimproved efficiency as illustrated in Fig-ure 1. Operational costs of LNG vesselsconsist of several parameters such as fuel

costs, maintenance costs, crewcosts (including training), shipcapital cost, etc. As the fuel costsor fuel consumption are a majorpart of this, the improvement ofefficiency by going electric willlead to significant savings onfuel consumption and hence theoperational costs.

To visualize these savings anexample calculation is givenbelow for a typical operationprofile of 7500 hours per yearwith full speed (19.5 knots). Halfof this is assumed as laden voy-age and half is assumed ballastvoyage. The time for manoeu-vering and harbor operations are

not included in the calculations. Calcula-tions are performed for three propulsionalternatives: Steam-turbine, two-strokemechanical with reliquefaction and duel-fuel electric propulsion. For the steam tur-bine propulsion vessel supplementaryHeavy Fuel Oil (HFO) is assumed used inthe boilers in addition to the BOG, and 5MW power consumption for the relique-faction plant. For the electric propulsionvessel forced boil off is assumed to supple-ment the natural BOG. A typical vessel of140,000 m3 is assumed with a propulsionpower of 26 MW in laden condition and 25MW in ballast condition.

The results are given in Figure 3 and4 and show a clear advantage of the dual-fuel electric propulsion system regardingfuel consumption and emissions. For thefuel consumption both the two-stroke andelectric propulsion alternative are superiorto the steam solution. This is basically dueto the increased efficiency of the propul-sion plant. Even including the electricaltransmission losses the electric solutionhas also lower fuel consumption than thetwo-stroke solution because the relique-faction plant additionally requires about 5MW continues power during normaloperation of the carrier.

Over the last years a growing focus onthe environmental conditions has lead to astronger focus on utilizing cleaner fuelsystems. Using LNG as fuel gives clearbenefits for the emissions of NOx and SOxas indicated in Figure 4. Utilizing forcedboil off gas also for the steam propulsionsystem would lead to lower emissions ofSOx and NOx. The CO2 emissions arestrongly related to the fuel consumptionand together with the lower carbon con-tent in LNG compared to HFO and MarineDiesel Oil (MDO) this gives a clear advan-tage for the electric propulsion system

with respect to CO2 emissions.

Reliability and AvailabilityFor LNG Carriers the main task is to deliv-er the LNG cargo at the terminal at thescheduled time. Due to limited storagecapacity at the terminal the whole LNGsupply system is based on regular arrivalof the LNG carriers. The vessels are alsonormally chartered for 20-30+ years toensure a stable and reliable supply ofLNG. In that respect the propulsion sys-tem is not allowed to fail in such a waythat the vessel will be late arriving at theterminal. When introducing the electricalpropulsion system as an alternative to theconventional steam propulsion system itmust be proven to have the same or high-er level of availability.

ABB has long experience with reliableelectric propulsion systems for manytypes of vessels, and has delivered tomore than 350 vessels ranging fromcruise vessels, ferries, ice breakers, shut-tle tankers, drilling vessels/rigs, fieldsupport vessels, etc. The propulsion pow-er has been varying up to more than 20MW for a single propulsion motor. Usingthe experience from these projects theelectric propulsion system on LNG carri-ers is designed to meet the highest stan-dards of safety and reliability.

For Gaz de France's first two 154,000 m3LNG carriers with electric propulsion,ABB is delivering the propulsion drivesystem which consist of propulsionmotors, frequency converters, propulsiontransformers and the MV (Medium Volt-age) switchboards.

The vessels are being built at Chantiersde l'Atlantique, one of the pioneeringyards with respect to innovative propul-sion systems and technical solutions.Together with Gaz de France, the first

shipowner to order LNG carriers withelectric propulsion, a breakthrough forDual Fuel electric propulsion system hasbeen achieved. More information can alsobe found in [1].

The 2 x 14 MW medium speed propul-sion motors are connected to a commongear-box for driving a single fixed pitchpropeller. Each motor is controlled by anACS 6000SD frequency converter, which isthe latest generation of MV (medium volt-age) drives from ABB utilizing IGCTs(Integrated Gate Commutated Thyristors)as switching devices and the ABB patent-ed DTC (Direct Torque Control) principlefor synchronous motor drive. The DTCcontrol is developed for optimizing thedynamic torque response and minimizingthe torque ripple on the motor shafts,hence leading to minimized machinery-induced vibration and noise levels.

One small LNG carrier with electricpropulsion is already in operation. The1100 m3 LNG carrier "Pioneer Knutsen"was delivered from the Dutch yard Bijlismain March this year, and has been in opera-tion along the Norwegian west-coast sincethen. The main propulsion consists of 2 x900 kW azimuth thrusters driven by lowvoltage frequency controlled motors deliv-ered by ABB. Power production is generat-ed by 2 x 900 kW Mitsubishi gas enginesand 2 x 640 kW Mitsubishi diesel engines.The gas engines are fueled by BOG whilethe diesel-engines are supporting withadditional power if required.

SHIPPING

LNG journal September/October 2004 page 12

Figure 3 Comparison of fuel consumptionFigure 5 Illustration of typical electrical

propulsion arrangementFigure 4 Comparison of emissions

Jan Fredrik Hansen graduat-ed from the Norwegian Insti-tute of Technology with aM.Sc. in Electrical PowerEngineering in 1995. He alsoholds a PhD degree in Engi-neering Cybernetics on the

topic of Modelling and Control of MarinePower Systems (year 2000). He joined ABBMarine in Oslo in 1999 working with electricpropulsion systems. He is now TechnicalAdvisor with the main focus on LNG carriers.

Rune Lysebo graduatedfrom the Norwegian Insti-tute of Technology with aM.Sc. in Power Electron-ics in 1996. He has beenemployed by ABB Marinein Oslo since 1996. Thelast three years he has

worked in the sales department with salesresponsibility for LNG Carriers.

References[1] R Courtay, L Claes and J Sainson,"LNG Carriers Using Gas Fuel Only,Diesel Gas Electric Propulsion", Proc. ofthe 14th International Conference & Exhi-bition on Liquefied Natural Gas, Doha,Qatar, March 2004.

Figure 2 Electric propulsion configurations

Small LNG shipwith electricpropulsion

World's First Full Size LNG carriers with

Electric Propulsion

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