DUAL-FUEL-ELECTRIC LNG CARRIER PROPULSIONBarend Thijssen, Sales Director, Wärtsilä Ship Power Solutions, Finland SUMMARY 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, highpressure gas-diesel and low-pressure dual-fuel engines, in mechanical and electric propulsion arrangements, with and without boil-off reliquefaction, were studied. Dual-fuel-electric installations were found to be the most attractive alternative to steam turbine installations. The first dual-fuel-electric LNG carrier, Gaz de France Energy, will take to the sea later this year and two more dualfuel-electric LNG carriers are on order. 1. 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 (Fig. 1). Fig. 1 - Steam turbine propulsion 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. 2. 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. 8 OTHERS Ships with steam turbine installations have rather poor manoeuvring characteristics.2 ENVIRONMENTAL-FRIENDLINESS As all other segments of the shipping industry have made the switch to diesel engine power during the last three decades.4 RELIABILITY Except for some well-documented problems with highspeed reduction gears. 2.5 REDUNDANCY Since the nineteen-seventies. good manoeuvring characteristics become increasingly important. The timing can easily be made to coincide with the wet.and two-stroke diesel engines. 2. are taking diesel engine technology yet one step further. steam turbine. gasdiesel engine. As the buffer capacity of such a supply chain is limited and punctual cargo operations are important. this share grows to 80%. 3. 2. In laden conditions. reliable and safe diesel engines. diesel engine installations have become dominant in all shipping segments. Onboard LNG carriers. 2. is left to evaporate in order to control temperature and pressure in the ship’s cargo tanks. steam turbine installations have a very decent safety record. and can be expected to get even more attention in the near future. like the application of common rail fuel injection on both four.3 SAFETY 3. Steam turbine installations require a modest amount of well-schedulable maintenance. In ballast condition. In LNG shipping. The safety of crew. This poses a crewing challenge which can even reflect in manning costs.1 OPERATING ECONOMY Although steam turbine installations have proven reliable. These reliquefaction plants require a substantial amount of electric power to operate and are costly. approximately 0. Part-load efficiencies of both the propulsion and electric power generation machinery are even lower.13% per day in laden condition. Losses in the boilers. except LNG shipping. 2. The efficiency of the electric power generation machinery is below 25% at full load. like for any other kind of ship. 2. they do not use it very efficiently. the boil-off gas on LNG carriers has to be reliquefied in an onboard reliquefaction plant and fed back into the ship’s cargo tanks. dual-fuel engines and gas turbines. steam turbine installations have proven reliable in operation. As diesel engines can only burn liquid fuels like marine diesel oil (MDO) and HFO. it is common practice to tie up small fleets of ships on long-term charters on fixed routes with fixed sailing schedules. No major calamities have been reported. Also for LNG carriers. these kinds of trades require ships with amply redundant machinery. A small amount of cargo.6 ‘MAINTAINABILITY’ Due to the nature of the LNG trade.and dry-docking intervals of the ship.7 ‘CREWABILITY’ Although steam turbine installations can utilize boil-off gas very easily.Boil-off gas is an unavoidable by-product of the seaborne LNG transportation concept. 2. heavy and have only been applied in the marine environment on a very limited scale. fuel costs are one of the most important components of the ship’s operating costs. some 50% of the ship’s energy requirement is covered with HFO. MACHINERY ALTERNATIVES Alternative machinery installations for LNG carriers could potentially be built around diesel engines. the pool of experienced and skilled steam engineers is rapidly shrinking. Both quantity and quality of the boil-off gas are subject to variation. CO2 emissions are already the focus of attention these days. Such low machinery efficiencies lead to a substantial amount of HFO being required for complementing the available boil-off gas. The latest developments. high-speed reduction gear and shafting bring the efficiency of the propulsion machinery to a level below 29% at full load. 2. it is also important that maintenance of the machinery installation does not interfere with the sailing schedule of the ship or influence its performance. . they do not have too much redundancy incorporated. ship and environment is of utmost importance. When considering transits in light ice conditions and cargo operations offshore. Experience gained from thousands of diesel engine installations in service has resulted in the development of highly-efficient. The low efficiency and the need to use large amounts of HFO have a negative impact on the ship’s CO2 and SOx emissions.1 DIESEL ENGINE ALTERNATIVES Gas tankers attract lots of attention from safety regulators worldwide. dual-fuel engines can accept all gas qualities seen in LNG shipping. Exh Exh Exh When running on gas. As the LNG trade sets high standards with respect to ‘maintainability’ and redundancy. More propulsion redundancy and operational flexibility can be offered by applying multiple four-stroke diesel engines driving controllable-pitch propellers through reduction gears.2 GAS-DIESEL ENGINE ALTERNATIVES Wärtsilä brought its first four-stroke gas-diesel engine.Gas-diesel engine working principle .3 DUAL-FUEL ENGINE ALTERNATIVES Gas-diesel engines act according to the Diesel principle and can virtually burn any possible mixture of gas and liquid fuel. the dual-fuel engine acts as a normal diesel engine. A further enhancement can be realized by applying electric propulsion. dual-fuel engines act according to the Otto principle. Wärtsilä also tested a two-stroke gas-diesel engine in the mid nineteen-eighties (Fig.Sulzer two-stroke gas-diesel engine Except for the reliquefaction plant. Having one engine out of operation for maintenance will however still have a substantial impact on the ship’s service speed. Without additional equipment like SCR units or direct water injection. The project was however discontinued when commercially feasible applications could not be identified. or by using the tuning possibilities of electronically-controlled. common rail two-stroke engines. 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. large gas compressors are required to boost the gas pressure to the appropriate level. NOx emissions are substantial. with an output of 410 kilowatt per cylinder. As the gas is mixed with air before compression starts (Fig. the required injection pressure is high. 4). 2). In addition to running on gas. a gas pressure of about 5 bar is sufficient. dual-fuel engines can run on MDO. especially on LNG carriers. was introduced in 1991. the Wärtsilä 32GD. This impact can be reduced by selecting controllable-pitch propellers. but certainly not excellent. Close to topdead-centre a very small amount of MDO is injected in order to trigger ignition. SOx emissions are high too. In close cooperation with its licensee Diesel United. 3. while for two-stroke gas-diesel engines some 250 bar is deemed sufficient. Provided that an adequate gas supply system is installed. The exhaust emissions of two-stroke engine installations are reasonable. As the mixture of gas and liquid fuel is injected into the combustion chamber during air compression (Fig. LNG machinery installations based on gas-diesel engines look fairly similar to concepts based on conventional diesel engines. The larger Wärtsilä 46GD. to the market in 1987. 3 . The application of electric propulsion will at the same time result in a higher part-load efficiency. When running on MDO. respectively. As boil-off gas is generated at atmospheric pressure. As an inevitable consequence of using HFO as a fuel. the presence of high-pressure gas in the engine room is a major safety concern. These compressors require a substantial amount of electric power to operate and are costly and heavy. Fig. 2 . 3). Devices for locking or disconnecting the propeller shafts will be necessary to enable maintenance activities on one engine while sailing. the most simple and straightforward diesel engine installation onboard an LNG carrier will likely feature twin two-stroke engines. with only a few restrictions to the quality of the gas. with an output of 975 kilowatt per cylinder. Fig. a gas pressure of around 350 bar is required.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. In order to keep the complexity low and the operational flexibility high. 3. each in directdrive to a fixed-pitch propeller. Additionally. This gas pressure is in the same range as the gas pressures in steam turbine installations. For four-stroke gasdiesel engines. electric power will likely be generated by a group of four-stroke diesel generating sets. Fig. Two-stroke diesel engines have a high efficiency. 4 . The need for gas compression turned out to be a too high burden for the operating economy of the ship. sixteen. is available in six. 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. whereas their non-quantifiable characteristics were discussed and compared together with major LNG carrier owners.3. When comparing the operational economy of the various alternatives.and nine-cylinder inline and twelve-. The most attractive alternative to the traditional steam turbine installation turned out to be dual-fuel-electric machinery. The quantifiable characteristics of the other alternatives were compared using a specially developed comparison tool. managers and shipyards over the past few years. with a power of up to 350 kilowatt per cylinder. but the need to reliquefy the boil-off gas gives installations featuring this type of engines a higher total energy consumption.Dual-fuel engine working principle (gas mode) In case the supply of gas is interrupted. without loss of engine power or speed. 6) are multiple dual-fuel generating sets. which have proven their reliability in various demanding marine installations. In addition to this inherited reliability. Other consumers Cargo pumps Converter & Transformer G Gas FPP M E-motors M G G Converter & Transformer Cargo pumps Other consumers G Dual-fuel generator sets Liquid fuel Fig. DUAL-FUEL-ELECTRIC LNG CARRIERS Fig. operators. 5. The transfer from diesel to gas mode is carried out fully-automatic on demand.4 Exh Exh Exh GAS TURBINE ALTERNATIVES Gas turbine installations could also potentially be applied in LNG carriers. emissions of dual-fuel installations are lower than those of steam turbine.and eighteen-cylinder Vee-form configurations.and eighteen-cylinder Veeform configurations. 6 . was brought to the market in 1996. it is important to take the whole machinery installation into account. an installation featuring twin two-stroke engines. Their rather low efficiency at part-load. Central in the dual-fuel-electric propulsion and electric power generation solution (Fig. With an output of 950 kilowatt per cylinder. As a result of higher efficiency and cleaner fuel. burning clean fuel has a positive impact on reliability. In combination with an electric propulsion system. eight.Dual-fuel-electric machinery . 5 .and nine-cylinder inline and twelve. 5) was launched in 1998. The first Wärtsilä dual-fuel engine. This engine. The number and size of these sets of course largely depends on the ship size and speed. 4. The larger Wärtsilä 50DF (Fig.Wärtsilä 6L50DF dual-fuel engine The Wärtsilä 32DF and 50DF dual-fuel engines have been derived from the Wärtsilä Vasa 32 and Wärtsilä 46 diesel engines respectively. Low-pressure dual-fuel technology is only available on four-stroke engines. difficulty in coping with high ambient temperatures. the dual-fuel engine automatically transfers to diesel mode. diesel and gas-diesel installations. a reliquefaction plant. but also on the envisaged operating philosophy. need for high gas pressure and the required special skills and procedures for maintenance. each in direct-drive to a fixed-pitch propeller. the Wärtsilä 32DF. and a group of fourstroke diesel generating sets emerged. As a runner up but at clear distance to dualfuel-electric machinery. it delivers between 6 to 17 megawatt at full load. This engine is available in six-. dualfuel installations achieve optimum performance and high efficiency at virtually any load. Exhaust gas emissions 5. A two-stroke diesel engine installation will have a propulsion machinery efficiency of about 48% and the efficiency of the electric power generation machinery will be about 41%.Dual-fuelstroke electric Steam Two. For the larger ships.An LNG carrier with a cargo capacity of some 150’000 m3 will typically require one six. . Carrying more cargo volume is enabled by the fact that the dual-fuel-electric solution saves engine room space (Fig. but will consume a substantially higher amount of electric power due to the presence of the liquefaction plant. Even when using a small part of the cargo as fuel.and four nine-cylinder engines. 7 . 150% SOx 125% CO2 NOx 100% 75% 50% 25% 0% Steam Two. a dual-fuel-electric LNG carrier will deliver more cargo to the unloading port in this way.Dual-fuelstroke electric Steam Two.and three twelvecylinder Wärtsilä 50DF engines. All other machinery alternatives suffer from the use of HFO.1 OPERATING ECONOMY a steam turbine installation. 8). 5. the choice of fuel is up to operator. Firstly. Twin ‘low-speed’ electric motors mounted on the same shaft can be selected to drive the propeller without assistance of a gearbox alternatively.2 ENVIRONMENTAL-FRIENDLINESS When exclusively using natural and forced boil-off gas as fuel. Two ‘high-speed’ electric propulsion motors drive a fixed-pitch propeller through a reduction gear. forced boil-off gas is cheaper than alternative fuels. Fuel ‘bunkers’ weight is thus reduced.and four twelve-cylinder Wärtsilä 50DF dual-fuel engines. 9 .Operating costs 5. twinscrew arrangements can be selected without significantly increasing the complexity of the machinery installation. 7). the dual-fuel electric solution shows unrivalled emission values (Fig. 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. 25% 0% Steam Twostroke Dual-fuelelectric Fig 8. .3 SAFETY Fig. Adding the cheaper fuel of the dual-fuel-electric LNG carrier to the equation. and a ship of 250’000 m3 cargo capacity will do with two six. The generated electric power is fed to an electric drive fairly similar to those used on contemporary cruise ships. Secondly. the ship will be able to carry more cargo weight. The recent introduction of double-wall gas piping on the Wärtsilä 50DF will further increase the safety of the solution. safety studies including hazard identification. and at a given displacement. With several potential customers and class. 125% 100% 75% 50% As dual-fuel engines have the ability to run on both gas and MDO.Dual-fuelstroke electric Fig. either used uniquely or in combination with natural boil-off gas. An LNG carrier with a cargo capacity of 200’000 m3 will typically require two six. it is lighter than alternative fuels. compared to 29% and 25% respectively for 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. 9). this solution clearly excels in terms of operating costs (Fig.Dual-fuel-electric machinery The efficiency of the propulsion machinery of a dualfuel-electric LNG carrier is approximately 41% and the efficiency of the electric power generation machinery is around 44%. as more or less all primary functions of the system are distributed over more than one component. 5. with an aggregate power of 12 megawatt. a diesel engine that has proven its reliability in various demanding marine applications. 5. to further validate its performance (Fig. 6. potentially distributed over multiple engine rooms. Maintenance of dual-fuelelectric installations is more costly than of steam turbine installation. and twelve more engines are about to follow.Petrojarl I .5 REDUNDANCY Electric propulsion systems are in their essence highly redundant. two eighteen-cylinder Wärtsilä 32DF engines.8 OTHERS Eleven Wärtsilä dual-fuel engines are in service at sea. due to the availability of full propeller torque at zero speed and excellent manoeuvring characteristics. There is no need for crew members with exceptional skills or experience. such as cruise ships.2 DUAL-FUEL ENGINES AT SEA Dual-fuel-electric installations can be operated and maintained by diesel engine crews. Spain.7 ‘CREWABILITY’ Seventeen Wärtsilä dual-fuel engines. is currently nearing completion and is scheduled to enter commercial operation in November this year.4 RELIABILITY The first dual-fuel-electric ships running on LNG.1 DUAL-FUEL ENGINES ON LAND 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.Wärtsilä 6L50DF endurance test engine 6. The dual-fuel-electric installation provides excellent propulsion characteristics for navigation in ice. 5. 10 . In addition to this. the United Kingdom. The dual-fuel-electric installation features multiple generating sets. are in operation in land-based power plants in Turkey. with a combined output of more than one-hundred megawatt. are running on natural gas from the Glitne oil and gas field onboard Petroleum Geo-Services’ FPSO Petrojarl I (Fig. MARKET INTRODUCTION Wärtsilä dual-fuel engines have already collected a vast number of running hours in installations on land and at sea without any significant problems. Gaz de France Energy. has twin transformers and converters. 11 . 5. but does no harm to the ship’s operating economy. Dual-fuel-electric installations can easily cope with the power requirements of dynamic positioning systems. 6. as an increasing amount of offshore LNG terminals is envisaged. India. 5. Viking Energy and Stril Pioner. the United States of America and Spain. a Wärtsilä 6L50DF dual-fuel engine is undergoing an endurance test at Wärtsilä Ibérica in Bermeo. 10). are in operation since 2002 and the first dual-fuel-electric LNG carrier. The Wärtsilä 50DF has been derived from the Wärtsilä 46.2 (a) Petrojarl 1 Off shore Norway. This might become a valuable feature. Fig. 6. Denmark. Russia.6 ‘MAINTAINABILITY’ Fig. have been conducted to further validate the safety of the solution. 11).FMEA and hazardous operations studies. and features twin electric propulsion motors with double windings. The use of gas in the Wärtsilä 50DF as compared to HFO in the Wärtsilä 46 further enhances this inherited reliability. with an aggregate power of 22. was delivered to Simon Møkster of Norway by the same shipyard during the same year (Fig.Viking Energy 6. 14 . will have a cargo capacity of . 15). These engines. Natural boil-off gas complemented by forced boil-off gas will serve as fuel in normal operating conditions.Stril Pioner 6.Gaz de France Energy Fig.Sendje Ceiba FPSO 6. the French utility Gaz de France placed on order for a 74’130 m3 dual-fuel-electric LNG carrier at the French shipyard Chantiers de l’Atlantique.3 DUAL-FUEL-ELECTRIC LNG CARRIERS Three dual-fuel-electric LNG carriers have been ordered so far.6. 16). is currently nearing completion and is scheduled to enter service in November 2004. it is also the first ship running on LNG. Gaz de France Energy will primarily trade between Algeria and France. is equipped with four six-cylinder Wärtsilä 32DF dual-fuel engines. with a combined output of 8 megawatt. Both ships are on charter to Statoil and are stationed in the port of Bergen in Norway. will power the ship to a service speed of 17.3 (a) Gaz de France Energy 6. Using hot water vaporizers. In February 2002. In case no gas is available. Fig. Four six-cylinder Wärtsilä 50DF dual-fuel engines.2 (b) Sendje Ceiba One eighteen-cylinder Wärtsilä 32DF dual-fuel engine. 13). 14). This ship. Added to the fact that Viking Energy is the first ship to apply dual-fuel-electric machinery. 12). Fig. Provalys (Fig. This ship. Stril Pioner. is running on natural gas from the Ceiba oil and gas field off shore Equatorial Guinea onboard Bergesen’s FPSO Sendje Ceiba (Fig.8 megawatt. 13 .3 (b) Provalys Viking Energy’s sister ship.2 (c) Viking Energy and Stril Pioner The platform supply vessel Viking Energy (Fig.5 knots. 12 . Gaz de France ordered a second DFelectric LNG carrier at Chantiers de l’Atlantique. In September 2003. delivered to Eidesvik of Norway by Kleven Verft of Norway in 2002. 15 . Gaz de France Energy (Fig. are driving two azimuthing thrusters through an electric drive. natural gas is forced to boil off from a 220 m3 insulated LNG fuel tank underneath the ship’s deck. the engines will run on MDO. rated at 6 megawatt. Fig. Article reproduced with the kind permission of The Royal Institution of Naval Architects from their Design and Operation of Gas Carriers Conference September 2004. With the market introduction of low-pressure. Fig. a joint-venture of NYK of Japan (60%) and Gaz de France (40%) ordered a sister ship to Provalys at Chantiers de l’Atlantique. In addition. and a third ship is in the order book. . three twelve. and Norway and France. Port-to-port sailing distances are ranging from several days to several weeks. as well as floating 9. ACKNOWLEDGEMENTS The author wishes to thank Chantiers de l’Atlantique for the computer generated images of Gaz de France Energy and Provalys. 7. dual-fuel engines are under consideration for application in various kinds of offshore installations. regasification units.9 megawatt. The ship will primarily trade between Egypt and France. selling and delivering ship power installations to the marine industry. 16 . Wärtsilä Ship Power Solutions takes a holistic approach in engineering. from Delft University of Technology in the Netherlands and joined Wärtsilä in 1997. he now leads the sales activities of Wärtsilä Ship Power Solutions from Raisio in Finland. Also in Provalys. These engines are currently in production at Wärtsilä Italia in Trieste. In this ship. Dual-fuel engines in combination with an electric drive have turned out to be the most attractive alternative to the traditional steam turbine installation. a second vessel is on the building blocks. marketing. with a combined output of 39. Installations for LNG carriers are one of the cornerstones of Wärtsilä Ship Power Solutions. More orders for dual-fuel-electric LNG carriers are imminent. which is scheduled for delivery at the end of 2006. Dual-fuel-electric machinery is presently being evaluated by ship owners and shipyards around the world for a vast number of LNG carrier newbuilding projects.3 (c) Chantiers de l’Atlantique #P32 In July 2004. especially in terms of operating economy and environmental friendliness. while MDO will serve as fuel in conditions were no gas is available. The first dual-fuel-electric LNG carrier is about to enter commercial operation. After assignments in Switzerland and Greece. AUTHORS’ BIOGRAPHY Barend Thijssen graduated as a Naval Architect M. The cargo capacities of the envisaged ships are ranging from the conventional 150’000 m3 to 200’000 and even 250’000 m3. will generate the required electric power to give the ship a service speed of 19 knots.153’500 m3 and upon delivery at the end of 2005 be the largest LNG carrier afloat.Provalys 6. including floating liquefaction units.and one six-cylinder Wärtsilä 50DF dual-fuel engines. CONCLUSIONS Steam turbine installations have dominated LNG carrier propulsion and electric power generation for decades because no suitable alternatives were available. forced boil-off gas will complement the natural boil-off gas in normal operation conditions. four-stroke dualfuel engines came the chance to challenge the steam turbine dominance.Sc. 8.
Report "Dual Fuel Electric Lng Carrier Propulsion"