Monday, August 5, 2013
STEAM
TURBINE INSTALLATIONS HAVE DOMINATED LNG CARRIER PROPULSION AND ELECTRIC POWER
GENERATION FOR THE PAST FORTY YEARS. THE EASE WITH WHICH THESE INSTALLATIONS
CAN UTILIZE BOIL-OFF GAS AND THEIR APPARENT RELIABILITY HAVE KEPT THEM IN A
POSITION THAT HAS LONG BEEN LOST TO
DIESEL ENGINES IN ALL OTHER SEGMENTS OF THE SHIPPING INDUSTRY. STEAM TURBINE
INSTALLATIONS ARE HOWEVER NOT VERY EFFICIENT.
THIS HAS A NEGATIVE IMPACT ON BOTH THE SHIP’S OPERATING ECONOMY AND ITS EXHAUST
GAS EMISSIONS. EXACTLY THESE ISSUES
PLAY AN INCREASINGLY IMPORTANT ROLE IN LNG SHIPPING TODAY.
ENCOURAGED
BY THE LATEST DEVELOPMENTS IN ITS GAS ENGINE TECHNOLOGY, WÄRTSILÄ STARTED
LOOKING FOR A MORE ECONOMIC AND ENVIRONMENTALLY FRIENDLY WAY TO POWER LNG CARRIERS. MACHINERY
ALTERNATIVES WITH TWO- AND FOUR-STROKE DIESEL, HIGH PRESSURE GAS-DIESEL AND LOW-PRESSURE DUAL-FUEL ENGINES, IN MECHANICAL
AND ELECTRIC PROPULSION ARRANGEMENTS, WITH AND WITHOUT BOIL-OFF RE-LIQUEFACTION WERE STUDIED. DUAL-FUEL-ELECTRIC
INSTALLATIONS WERE FOUND TO BE THE MOST ATTRACTIVE ALTERNATIVE TO STEAM TURBINE INSTALLATIONS.
INTRODUCTION
AFTER
HAVING BEEN PREDOMINANTLY FLARED OFF OR RE-INJECTED FOR DECADES,
NATURAL GAS IS
PLAYING AN INCREASINGLY IMPORTANT ROLE
IN GLOBAL ENERGY
CONSUMPTION TODAY. CLEAN
COMBUSTION PROPERTIES AND ABUNDANT RESERVES ARE THE MAIN BENEFACTORS FOR THIS
EVOLUTION FROM UNSOLICITED BY-PRODUCT OF OIL PRODUCTION TO PREFERRED ENERGY
SOURCE.
WITH
NATURAL GAS RESERVES OFTEN LOCATED FAR AWAY FROM ENERGY CONSUMERS AND PIPELINES
EXPENSIVE OR IMPRACTICAL TO BUILD, SEABORNE TRANSPORTATION OF NATURAL GAS IS ON
THE RISE AS WELL. THE MOST ECONOMIC AND COMMON WAY TO TRANSPORT NATURAL
GAS BY SEA
IS IN LIQUEFIED
FORM. LIQUEFIED NATURAL GAS (LNG) IS TODAY TRANSPORTED BY A FLEET OF SOME
TWO-HUNDRED DEDICATED LNG CARRIERS.
WITH
SEABORNE TRANSPORTATION OF LNG EXPECTED TO DOUBLE WITHIN THIS DECADE, A VAST
EXPANSION OF THE LNG CARRIER FLEET IS IMMINENT. AT THE SAME TIME, INCREASING
CARGO VOLUMES OFFER THE POSSIBILITY TO APPLY ECONOMIES OF SCALE, AND SHIPS ARE
ABOUT TO SIGNIFICANTLY GROW IN SIZE. THESE CIRCUMSTANCES CREATE THE NEED
TO VERIFY THE
TECHNICAL SOLUTIONS THAT HAVE BEEN APPLIED IN LNG CARRIERS SO FAR.
FOR THE
PAST FORTY YEARS, STEAM TURBINE INSTALLATIONS HAVE DOMINATED ONBOARD LNG
CARRIERS. THE EASE WITH WHICH STEAM TURBINE INSTALLATIONS CAN BURN BOIL-OFF GAS
AND THEIR APPARENT RELIABILITY HAVE KEPT THEM IN THE CONTROLLING POSITION THAT
HAS BEEN TAKEN
OVER BY DIESEL
ENGINE INSTALLATIONS A LONG TIME AGO IN ALL OTHER SEGMENTS OF THE
SHIPPING INDUSTRY.
A TYPICAL
STEAM TURBINE INSTALLATION CONSISTS
OF TWO BOILERS, MOST COMMONLY
FIRED WITH BOIL-OFF GAS AND HEAVY FUEL OIL (HFO). A STEAM TURBINE IS DRIVING A
FIXED-PITCH PROPELLER THROUGH A HIGH-SPEED GEARBOX. TWO ADDITIONAL STEAM
TURBINES AND ONE OR TWO DIESEL ENGINES ARE DRIVING ALTERNATORS TO GENERATE
ELECTRIC POWER.
STEAM
TURBINE INSTALLATIONS ARE HOWEVER NOT VERY EFFICIENT. THIS HAS A NEGATIVE
IMPACT ON THE OPERATING ECONOMY AND EXHAUST GAS EMISSIONS OF THE SHIP. EXACTLY
THESE ISSUES PLAY AN INCREASINGLY IMPORTANT ROLE IN LNG SHIPPING THESE DAYS.
MARKET REQUIREMENTS
AN
ATTRACTIVE ALTERNATIVE SHOULD BE OUTPERFORMING THE STEAM TURBINE INSTALLATION
WITH RESPECT TO ITS APPARENT DISADVANTAGES, WHILE AT THE SAME TIME AT LEAST
MATCHING IT WITH RESPECT TO ITS ADVANTAGES. IT IS THEREFORE IMPORTANT TO STUDY
THESE ADVANTAGES AND DISADVANTAGES.
THE MAIN
REASON TO REMAIN FAITHFUL TO THE STEAM TURBINE INSTALLATION IN LNG CARRIERS IS
THE EASE WITH WHICH THEY CAN BURN BOIL-OFF GAS.
BOIL-OFF
GAS IS AN UNAVOIDABLE BY-PRODUCT OF THE SEABORNE LNG TRANSPORTATION CONCEPT. A
SMALL AMOUNT OF CARGO, APPROXIMATELY 0.13% PER DAY IN LADEN CONDITION, IS LEFT
TO EVAPORATE IN ORDER TO CONTROL TEMPERATURE AND PRESSURE IN THE SHIP’S CARGO
TANKS. BOTH QUANTITY AND QUALITY OF THE BOIL-OFF GAS ARE SUBJECT TO VARIATION.
- OPERATING ECONOMY
ALTHOUGH STEAM TURBINE INSTALLATIONS CAN UTILIZE BOIL-OFF
GAS VERY EASILY, THEY DO NOT USE IT VERY EFFICIENTLY. LOSSES IN THE BOILERS,
STEAM TURBINE, HIGH-SPEED REDUCTION GEAR AND
SHAFTING BRING THE
EFFICIENCY OF THE
PROPULSION MACHINERY TO A
LEVEL BELOW 29% AT
FULL LOAD. THE EFFICIENCY OF THE ELECTRIC POWER
GENERATION MACHINERY IS BELOW 25% AT FULL LOAD. PART-LOAD EFFICIENCIES OF BOTH
THE PROPULSION AND ELECTRIC POWER GENERATION MACHINERY ARE EVEN LOWER.
SUCH LOW MACHINERY EFFICIENCIES LEAD TO A SUBSTANTIAL AMOUNT
OF HFO BEING REQUIRED FOR COMPLEMENTING THE AVAILABLE BOIL-OFF GAS. IN LADEN
CONDITIONS, SOME 50% OF THE SHIP’S ENERGY REQUIREMENT IS COVERED WITH HFO. IN
BALLAST CONDITION, THIS SHARE GROWS TO 80%.
ALSO FOR LNG CARRIERS, LIKE FOR ANY OTHER KIND OF SHIP, FUEL
COSTS ARE ONE OF THE MOST IMPORTANT COMPONENTS OF THE SHIP’S OPERATING COSTS.
2. ENVIRONMENTAL-FRIENDLINESS
THE LOW EFFICIENCY AND THE NEED TO USE LARGE AMOUNTS OF HFO HAVE A NEGATIVE IMPACT ON THE SHIP’S CO2 AND
SOX EMISSIONS. CO2 EMISSIONS
ARE ALREADY THE FOCUS OF ATTENTION
THESE DAYS, AND CAN BE EXPECTED TO GET EVEN MORE
ATTENTION IN THE NEAR FUTURE.
GAS
TANKERS ATTRACT LOTS OF ATTENTION FROM SAFETY REGULATORS WORLDWIDE. THE SAFETY
OF CREW, SHIP AND ENVIRONMENT IS OF
UTMOST IMPORTANCE. ONBOARD LNG
CARRIERS, STEAM TURBINE INSTALLATIONS HAVE A VERY DECENT SAFETY RECORD. NO
MAJOR CALAMITIES HAVE BEEN REPORTED.
4. MAINTAINABILITY
DUE TO THE
NATURE OF THE LNG TRADE, IT IS ALSO IMPORTANT THAT MAINTENANCE OF THE MACHINERY
INSTALLATION DOES NOT INTERFERE WITH THE SAILING SCHEDULE OF THE SHIP OR
INFLUENCE ITS PERFORMANCE.
STEAM
TURBINE INSTALLATIONS REQUIRE A MODEST AMOUNT OF WELL-SCHEDULABLE MAINTENANCE.
THE TIMING CAN EASILY BE MADE TO COINCIDE WITH THE WET- AND DRY-DOCKING
INTERVALS OF THE SHIP.
MACHINERY ALTERNATIVES
ALTERNATIVE MACHINERY
INSTALLATIONS FOR LNG
CARRIERS COULD POTENTIALLY BE BUILT AROUND DIESEL ENGINES, GAS-DIESEL
ENGINE, DUAL-FUEL ENGINES AND GAS TURBINES.
1. DIESEL
ENGINE ALTERNATIVES
SINCE THE
NINETEEN-SEVENTIES, DIESEL ENGINE INSTALLATIONS HAVE BECOME DOMINANT IN ALL
SHIPPING SEGMENTS, EXCEPT LNG
SHIPPING. EXPERIENCE GAINED
FROM THOUSANDS OF DIESEL ENGINE INSTALLATIONS IN SERVICE HAS
RESULTED IN THE DEVELOPMENT OF HIGHLY-EFFICIENT, RELIABLE AND SAFE DIESEL
ENGINES. THE LATEST DEVELOPMENTS, LIKE THE APPLICATION OF COMMON RAIL FUEL
INJECTION ON BOTH FOUR- AND TWO-STROKE DIESEL ENGINES, ARE TAKING DIESEL ENGINE
TECHNOLOGY YET ONE STEP FURTHER.
AS DIESEL
ENGINES CAN ONLY BURN LIQUID FUELS LIKE MARINE DIESEL OIL (MDO) AND HFO, THE
BOIL-OFF GAS ON LNG CARRIERS HAS TO BE RE LIQUEFIED IN AN ONBOARD RELIQUEFACTION
PLANT AND FED BACK INTO THE SHIP’S CARGO TANKS. THESE RELIQUEFACTION PLANTS REQUIRE A SUBSTANTIAL AMOUNT OF ELECTRIC POWER TO OPERATE AND ARE COSTLY, HEAVY AND HAVE
ONLY BEEN APPLIED IN THE MARINE ENVIRONMENT ON A VERY LIMITED SCALE.
PLANT AND FED BACK INTO THE SHIP’S CARGO TANKS. THESE RELIQUEFACTION PLANTS REQUIRE A SUBSTANTIAL AMOUNT OF ELECTRIC POWER TO OPERATE AND ARE COSTLY, HEAVY AND HAVE
ONLY BEEN APPLIED IN THE MARINE ENVIRONMENT ON A VERY LIMITED SCALE.
THE MOST
SIMPLE AND STRAIGHTFORWARD DIESEL
ENGINE INSTALLATION FOR A SHIP THE SIZE OF A CONVENTIONAL LNG CARRIER OR
LARGER WOULD BE A SINGLE TWO-STROKE ENGINE IN DIRECT-DRIVE TO A SINGLE
FIXED-PITCH PROPELLER. AS THE LNG TRADE SETS HIGH STANDARDS WITH RESPECT TO
‘MAINTAINABILITY’ AND REDUNDANCY, THE
MOST SIMPLE AND
STRAIGHTFORWARD DIESEL ENGINE INSTALLATION ONBOARD AN LNG CARRIER WILL LIKELY
FEATURE TWIN TWO-STROKE ENGINES, EACH IN DIRECT-
DRIVE TO A FIXED-PITCH PROPELLER. IN ORDER TO KEEP THE COMPLEXITY LOW AND THE OPERATIONAL FLEXIBILITY HIGH, ELECTRIC POWER WILL LIKELY BE GENERATED BY A GROUP OF FOUR-STROKE DIESEL GENERATING SETS.
DRIVE TO A FIXED-PITCH PROPELLER. IN ORDER TO KEEP THE COMPLEXITY LOW AND THE OPERATIONAL FLEXIBILITY HIGH, ELECTRIC POWER WILL LIKELY BE GENERATED BY A GROUP OF FOUR-STROKE DIESEL GENERATING SETS.
DEVICES FOR
LOCKING OR DISCONNECTING THE PROPELLER SHAFTS WILL BE NECESSARY TO ENABLE
MAINTENANCE ACTIVITIES ON ONE ENGINE WHILE SAILING.HAVING ONE ENGINE OUT OF
OPERATION FOR MAINTENANCE WILL HOWEVER STILL HAVE A SUBSTANTIAL IMPACT ON THE
SHIP’S SERVICE SPEED. THIS IMPACT CAN BE REDUCED BY SELECTING
CONTROLLABLE-PITCH PROPELLERS, OR BY USING THE TUNING POSSIBILITIES OF
ELECTRONICALLY-CONTROLLED, COMMON RAIL TWO-STROKE ENGINES.
THE EXHAUST
EMISSIONS OF TWO-STROKE ENGINE INSTALLATIONS ARE REASONABLE, BUT CERTAINLY NOT
EXCELLENT.WITHOUT ADDITIONAL EQUIPMENT LIKE SCR UNITS OR
DIRECT WATER INJECTION, NOX
EMISSIONS ARE SUBSTANTIAL. AS AN INEVITABLE CONSEQUENCE OF USING HFO AS A FUEL,
SOX EMISSIONS ARE HIGH TOO.
MORE
PROPULSION REDUNDANCY AND OPERATIONAL FLEXIBILITY CAN BE OFFERED BY APPLYING
MULTIPLE FOUR-STROKE DIESEL ENGINES
DRIVING CONTROLLABLE-PITCH PROPELLERS
THROUGH REDUCTION GEARS. A FURTHER ENHANCEMENT CAN BE REALIZED BY APPLYING
ELECTRIC PROPULSION. THE
APPLICATION OF ELECTRIC
PROPULSION WILL AT THE SAME TIME RESULT IN A HIGHER PART-LOAD EFFICIENCY.
2. GAS-DIESEL ENGINE ALTERNATIVES
GAS-DIESEL ENGINES ACT ACCORDING TO THE DIESEL PRINCIPLE AND
CAN VIRTUALLY BURN ANY POSSIBLE MIXTURE OF GAS AND LIQUID FUEL, WITH ONLY A FEW
RESTRICTIONS TO THE QUALITY OF THE GAS.
AS THE MIXTURE OF GAS AND LIQUID FUEL IS INJECTED INTO THE
COMBUSTION CHAMBER DURING AIR COMPRESSION, THE REQUIRED INJECTION PRESSURE IS
HIGH. FOR FOUR-STROKE GAS-DIESEL ENGINES, A GAS PRESSURE OF AROUND 350 BAR IS REQUIRED,
WHILE FOR TWO-STROKE GAS-DIESEL ENGINES SOME 250 BAR IS DEEMED SUFFICIENT.
WÄRTSILÄ BROUGHT ITS FIRST FOUR-STROKE GAS-DIESEL ENGINE, THE
WÄRTSILÄ 32GD, WITH AN OUTPUT OF 410 KILOWATT PER CYLINDER, TO THE MARKET IN
1987. THE LARGER WÄRTSILÄ 46GD, WITH AN OUTPUT OF 975 KILOWATT PER CYLINDER,
WAS INTRODUCED IN 1991.
EXCEPT FOR THE
RELIQUEFACTION PLANT, LNG
MACHINERY INSTALLATIONS
BASED ON GAS-DIESEL
ENGINES LOOK FAIRLY SIMILAR TO CONCEPTS BASED ON CONVENTIONAL
DIESEL ENGINES.
AS BOIL-OFF GAS
IS GENERATED AT
ATMOSPHERIC PRESSURE, LARGE GAS
COMPRESSORS ARE REQUIRED
TO BOOST THE
GAS PRESSURE TO THE
APPROPRIATE LEVEL. THESE
COMPRESSORS REQUIRE A SUBSTANTIAL AMOUNT OF ELECTRIC POWER TO OPERATE
AND ARE COSTLY AND HEAVY. ADDITIONALLY, THE PRESENCE OF HIGH-PRESSURE GAS IN
THE ENGINE ROOM IS A MAJOR SAFETY CONCERN, ESPECIALLY ON LNG CARRIERS.
EMISSIONS OF GAS-DIESEL ENGINE INSTALLATIONS ARE GENERALLY
LOWER THAN THOSE
OF STEAM TURBINE
AND DIESEL ENGINE INSTALLATIONS AS A RESULT OF HIGHER
EFFICIENCY AND CLEANER FUEL, RESPECTIVELY.
3. DUAL-FUEL ENGINE ALTERNATIVES
WHEN RUNNING ON GAS, DUAL-FUEL ENGINES ACT
ACCORDING TO THE OTTO PRINCIPLE. AS THE GAS IS MIXED WITH
AIR BEFORE COMPRESSION STARTS (FIG. 4), A GAS PRESSURE
OF ABOUT 5 BAR IS SUFFICIENT. THIS GAS PRESSURE IS IN THE
SAME RANGE AS THE GAS PRESSURES IN STEAM TURBINE
INSTALLATIONS. CLOSE TO TOP-
DEAD-CENTRE A VERY SMALL AMOUNT OF MDO IS INJECTED IN ORDER TO TRIGGER IGNITION.
DEAD-CENTRE A VERY SMALL AMOUNT OF MDO IS INJECTED IN ORDER TO TRIGGER IGNITION.
PROVIDED THAT AN ADEQUATE GAS SUPPLY SYSTEM
IS INSTALLED, DUAL-FUEL ENGINES CAN ACCEPT ALL GAS QUALITIES
SEEN IN LNG SHIPPING. IN ADDITION TO RUNNING ON GAS,
DUAL-FUEL ENGINES CAN RUN ON MDO. WHEN RUNNING ON MDO, THE DUAL-FUEL ENGINE ACTS AS A NORMAL DIESEL ENGINE.
IN CASE THE SUPPLY OF GAS IS INTERRUPTED, THE DUAL-FUEL
ENGINE AUTOMATICALLY TRANSFERS TO DIESEL MODE, WITHOUT LOSS OF ENGINE POWER OR
SPEED. THE TRANSFER FROM DIESEL TO GAS MODE IS CARRIED OUT FULLY-AUTOMATIC ON
DEMAND.
LOW-PRESSURE DUAL-FUEL TECHNOLOGY IS ONLY AVAILABLE ON FOUR-STROKE
ENGINES. THE FIRST WÄRTSILÄ DUAL-FUEL ENGINE, THE WÄRTSILÄ 32DF, WAS BROUGHT TO
THE MARKET IN 1996. THIS ENGINE, WITH A POWER OF UP TO 350 KILOWATT PER CYLINDER, IS AVAILABLE IN
SIX- AND NINE-CYLINDER INLINE AND TWELVE-
AND EIGHTEEN-CYLINDER VEE-FORM CONFIGURATIONS.
AS A RESULT
OF HIGHER EFFICIENCY
AND CLEANER FUEL,EMISSIONS OF DUAL-FUEL INSTALLATIONS ARE
LOWER THAN THOSE OF STEAM TURBINE, DIESEL AND GAS-DIESEL INSTALLATIONS.
IN COMBINATION WITH AN ELECTRIC PROPULSION SYSTEM, DUAL- FUEL
INSTALLATIONS ACHIEVE OPTIMUM PERFORMANCE AND HIGH EFFICIENCY AT VIRTUALLY ANY
LOAD.
4. GAS TURBINE ALTERNATIVES
GAS TURBINE INSTALLATIONS COULD ALSO POTENTIALLY BE APPLIED IN
LNG CARRIERS. THEIR RATHER LOW EFFICIENCY AT PART-LOAD, DIFFICULTY IN COPING
WITH HIGH AMBIENT TEMPERATURES, NEED FOR HIGH GAS PRESSURE AND THE REQUIRED
SPECIAL SKILLS AND PROCEDURES FOR MAINTENANCE, MAKE THEM LESS ATTRACTIVE.
SELECTION OF ALTERNATIVES
THE FEASIBILITY OF GAS-DIESEL ENGINES FOR PROPULSION AND
ELECTRIC POWER GENERATION
ONBOARD LNG CARRIERS
WAS STUDIED BY WÄRTSILÄ AND OTHERS SOME TEN YEARS AGO. THE NEED FOR GAS
COMPRESSION TURNED OUT TO BE A TOO HIGH BURDEN FOR THE OPERATING ECONOMY OF THE
SHIP.
THE QUANTIFIABLE CHARACTERISTICS OF THE OTHER ALTERNATIVES
WERE COMPARED USING A SPECIALLY DEVELOPED COMPARISON TOOL, WHEREAS THEIR
NON-QUANTIFIABLE CHARACTERISTICS WERE DISCUSSED AND COMPARED TOGETHER WITH
MAJOR LNG CARRIER OWNERS, OPERATORS, MANAGERS AND SHIPYARDS OVER THE PAST FEW
YEARS.
WHEN COMPARING THE OPERATIONAL ECONOMY OF THE VARIOUS
ALTERNATIVES, IT IS IMPORTANT TO TAKE THE WHOLE MACHINERY INSTALLATION INTO
ACCOUNT. TWO-STROKE DIESEL ENGINES HAVE A HIGH EFFICIENCY, BUT THE NEED TO
RELIQUEFY THE BOIL-OFF GAS GIVES INSTALLATIONS FEATURING THIS TYPE OF ENGINES A
HIGHER TOTAL ENERGY CONSUMPTION.
THE MOST ATTRACTIVE ALTERNATIVE TO THE TRADITIONAL STEAM TURBINE
INSTALLATION TURNED OUT TO BE DUAL-FUEL-ELECTRIC MACHINERY. AS A RUNNER UP BUT
AT CLEAR DISTANCE TO DUAL-FUEL-ELECTRIC
MACHINERY, AN INSTALLATION
FEATURING TWIN TWO-STROKE
ENGINES, EACH IN DIRECT-DRIVE TO A FIXED-PITCH PROPELLER, A RELIQUEFACTION
PLANT, AND A GROUP OF FOUR-STROKE DIESEL GENERATING SETS EMERGED.
DUAL-FUEL-ELECTRIC LNG CARRIERS
CENTRAL IN THE DUAL-FUEL-ELECTRIC PROPULSION AND ELECTRIC
POWER GENERATION SOLUTION ARE MULTIPLE DUAL-FUEL GENERATING SETS. THE NUMBER
AND SIZE OF THESE SETS OF COURSE LARGELY DEPENDS ON THE SHIP SIZE AND SPEED,
BUT ALSO ON THE ENVISAGED OPERATING PHILOSOPHY.
AN LNG CARRIER WITH A CARGO CAPACITY OF SOME 150’000M3
WILL TYPICALLY REQUIRE
ONE SIX- AND
THREE TWELVE-CYLINDER WÄRTSILÄ
50DF ENGINES. AN LNG CARRIER WITH A CARGO CAPACITY OF 200’000 M3
WILL TYPICALLY REQUIRE TWO SIX- AND FOUR
NINE-CYLINDER ENGINES, AND
A SHIP OF 250’000 M3 CARGO CAPACITY WILL
DO WITH TWO SIX- AND FOUR TWELVE-CYLINDER WÄRTSILÄ 50DF DUAL-FUEL ENGINES.
THE GENERATED ELECTRIC POWER IS FED TO AN ELECTRIC DRIVE FAIRLY
SIMILAR TO THOSE USED ON CONTEMPORARY CRUISE SHIPS. TWO ‘HIGH-SPEED’
ELECTRIC PROPULSION MOTORS
DRIVE A FIXED-PITCH PROPELLER
THROUGH A REDUCTION
GEAR. TWIN ‘LOW-SPEED’ ELECTRIC
MOTORS MOUNTED ON THE SAME SHAFT CAN BE SELECTED TO DRIVE THE PROPELLER WITHOUT
ASSISTANCE OF A GEARBOX ALTERNATIVELY. FOR THE LARGER SHIPS, TWIN-SCREW ARRANGEMENTS CAN BE SELECTED WITHOUT
SIGNIFICANTLY INCREASING THE COMPLEXITY OF THE MACHINERY INSTALLATION.
1.
1. OPERATING ECONOMY
AS DUAL-FUEL ENGINES HAVE THE ABILITY TO RUN ON BOTH GAS AND
MDO, THE CHOICE OF FUEL IS UP TO OPERATOR. SEVERAL INDEPENDENT STUDIES HAVE
HOWEVER CONFIRMED THAT FORCING ADDITIONAL BOIL-OFF GAS TO COMPLEMENT THE
NATURAL BOIL-OFF GAS IS THE WAY TO PROFIT MOST FROM THE POTENTIAL OF THE DUAL-FUEL-ELECTRIC
SOLUTION. FIRSTLY, FORCED BOIL-OFF GAS IS CHEAPER THAN ALTERNATIVE FUELS.
SECONDLY, IT IS LIGHTER THAN ALTERNATIVE FUELS. FUEL ‘BUNKERS’ WEIGHT IS THUS
REDUCED, AND AT A GIVEN DISPLACEMENT, THE SHIP WILL BE ABLE TO CARRY
MORE CARGO WEIGHT. CARRYING MORE CARGO VOLUME IS ENABLED BY THE FACT THAT THE DUAL-FUEL-ELECTRIC SOLUTION SAVES ENGINE ROOM SPACE . EVEN WHEN USING A SMALL PART OF THE CARGO AS FUEL, A DUAL-FUEL-ELECTRIC LNG CARRIER WILL DELIVER MORE CARGO TO THE UNLOADING PORT IN THIS WAY.
MORE CARGO WEIGHT. CARRYING MORE CARGO VOLUME IS ENABLED BY THE FACT THAT THE DUAL-FUEL-ELECTRIC SOLUTION SAVES ENGINE ROOM SPACE . EVEN WHEN USING A SMALL PART OF THE CARGO AS FUEL, A DUAL-FUEL-ELECTRIC LNG CARRIER WILL DELIVER MORE CARGO TO THE UNLOADING PORT IN THIS WAY.
THE EFFICIENCY OF THE PROPULSION MACHINERY OF A
DUAL-FUEL-ELECTRIC LNG CARRIER IS APPROXIMATELY 41% AND THE EFFICIENCY OF THE
ELECTRIC POWER GENERATION MACHINERY IS AROUND 44%, COMPARED TO 29% AND 25%
RESPECTIVELY FOR
2. ENVIRONMENTAL-FRIENDLINESS
WHEN EXCLUSIVELY USING NATURAL AND FORCED BOIL-OFF GAS AS
FUEL, THE DUAL-FUEL ELECTRIC SOLUTION SHOWS UNRIVALLED EMISSION VALUES . ALL
OTHER MACHINERY ALTERNATIVES SUFFER FROM THE USE OF HFO, EITHER USED UNIQUELY
OR IN COMBINATION WITH NATURAL BOIL-OFF GAS.
3. SAFETY
A ‘SAFETY CONCEPT’
FOR DUAL-FUEL-ELECTRIC MACHINERY ONBOARD LNG CARRIERS HAS BEEN
DEVELOPED BY WÄRTSILÄ TO MAKE SURE THAT THE SAFETY OF THE INSTALLATION COMPLIES
WITH CLASS AND AT LEAST MATCHES THE SAFETY OF STEAM TURBINE INSTALLATIONS. THE
RECENT INTRODUCTION OF DOUBLE-WALL GAS PIPING ON THE WÄRTSILÄ 50DF WILL FURTHER INCREASE THE SAFETY OF THE
SOLUTION. WITH SEVERAL POTENTIAL CUSTOMERS AND CLASS, SAFETY STUDIES INCLUDING
HAZARD IDENTIFICATION, FMEA AND HAZARDOUS OPERATIONS STUDIES, HAVE BEEN CONDUCTED
TO FURTHER VALIDATE THE SAFETY OF THE SOLUTION.
4. RELIABILITY
THE WÄRTSILÄ 50DF HAS BEEN DERIVED FROM THE WÄRTSILÄ 46, A DIESEL
ENGINE THAT HAS
PROVEN ITS RELIABILITY IN VARIOUS DEMANDING MARINE
APPLICATIONS, SUCH AS CRUISE SHIPS. THE USE OF GAS IN THE WÄRTSILÄ 50DF AS
COMPARED TO HFO IN THE WÄRTSILÄ 46 FURTHER ENHANCES THIS INHERITED RELIABILITY.
5. REDUNDANCY
ELECTRIC PROPULSION SYSTEMS ARE IN THEIR ESSENCE HIGHLY REDUNDANT,
AS MORE OR LESS ALL PRIMARY FUNCTIONS OF THE SYSTEM ARE DISTRIBUTED OVER MORE
THAN ONE COMPONENT. THE
DUAL-FUEL-ELECTRIC
INSTALLATION FEATURES MULTIPLE GENERATING SETS,
POTENTIALLY DISTRIBUTED OVER
MULTIPLE ENGINE ROOMS, HAS TWIN TRANSFORMERS AND CONVERTERS, AND
FEATURES TWIN ELECTRIC
PROPULSION MOTORS WITH
DOUBLE
WINDINGS.
WINDINGS.
6. ‘MAINTAINABILITY’
CASE STUDIES FOR VARIOUS CUSTOMERS HAVE SHOWN THAT THE REQUIRED
MAINTENANCE ON DUAL-FUEL-ELECTRIC INSTALLATIONS CAN EASILY BE CARRIED OUT
WITHOUT AFFECTING THE SHIP’S OPERATIONAL PERFORMANCE. MAINTENANCE
OF DUAL-FUEL-ELECTRIC
INSTALLATIONS IS MORE COSTLY THAN OF STEAM TURBINE INSTALLATION, BUT DOES NO
HARM TO THE SHIP’S OPERATING ECONOMY.
7. ‘CREWABILITY’
DUAL-FUEL-ELECTRIC
INSTALLATIONS CAN BE
OPERATED AND MAINTAINED BY DIESEL
ENGINE CREWS. THERE IS NO NEED FOR CREW MEMBERS WITH EXCEPTIONAL SKILLS OR
EXPERIENCE.
8. OTHERS
THE
DUAL-FUEL-ELECTRIC
INSTALLATION PROVIDES EXCELLENT PROPULSION CHARACTERISTICS FOR
NAVIGATION IN ICE, DUE TO THE AVAILABILITY OF FULL PROPELLER TORQUE AT ZERO
SPEED AND EXCELLENT MANOEUVRING CHARACTERISTICS.
DUAL-FUEL-ELECTRIC INSTALLATIONS CAN EASILY COPE WITH THE
POWER REQUIREMENTS OF
DYNAMIC POSITIONING SYSTEMS. THIS MIGHT BECOME A VALUABLE
FEATURE, AS AN INCREASING AMOUNT OF OFFSHORE LNG TERMINALS IS ENVISAGED.
