T he current world usage of LNG is about 110 million tons per year and current analysis indicates this grow- ing in the next 10 years by 70 to 100 million tons. This may lead to demand for more than 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 proved to be a simple and reliable solution for these ships. However the relatively low efficiency of the propulsion system (<30%) and possible increased size of future LNG carriers has initiated investigations into uti- lizing alternative propulsion systems. Electric propulsion has as a result of these investigations been shown to be an attractive alternative with high overall efficiency (~42%). The first three LNG car- riers with electric propulsion are now under construction at Chantiers de l'At- lantique in France for the French owner Gaz de France, where ABB is delivering the propulsion system on the second and third vessel, which are the world's first vessels with full size cargo capacity (154,000 m 3 ) utilizing an electric propul- sion system. In the last years the power plant for the cargo handling system has changed from a 440 V system to a medium voltage system (3.3 kV or 6.6 kV) due to increased installed power for the cargo pumps as the ship sizes have increased. Electric propulsion would be the natural extension utilizing the already installed medium voltage power system also for powering the propeller. With the development of the Dual Fuel (DF) engines, which can operate both on gas and diesel oil, electric propulsion has become an attractive solution, due to increased 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 propulsion power is transferred to the propellers by the electric power and propulsion plant. Other essential criteria for selecting a propulsion system are the availability and reliability of the propulsion equipment. LNG carriers have long time charters with fixed schedules, and a high penalty of not making the schedule in time. The avail- ability of the conventional steam turbine has proven to be very high, and any alter- native solution to be considered must comply with at least the same level of availability. With the inbuilt redundancy in the electric propulsion system and using reliable components these require- ments are met. Why Electric Propulsion? The concept of electric propulsion is not new, the idea originated more than 100 years ago. However, with the possibility to control electrical motors with variable speed in a large power range with com- pact, reliable and cost-competitive solu- tions, the use of electrical propulsion has emerged in new application areas during the 80's and 90's. Electric propulsion with gas turbine or diesel engine driven power generation is used in hundreds of ships of various types and in a large variety of configurations. With the new-buildings for Gaz de France, electric propulsion is now entering also the LNG ship. The main arguments for electric propulsion system are described in the following subsections. Propulsion efficiency: At rated load the reported efficiency of the DF engines are typically about 47%. Including the transmission losses, which are in the range of 8-10%, the overall efficiency is approxi- mately 43% calculating from fuel con- sumption to propeller shaft power. The reported efficiency for the steam propul- sion unit (including boilers, steam turbine and gear) is typically less than 30%. For part load operations as in manoeuvring mode or operation in areas with speed restrictions the efficiency difference between steam and electric propulsion is even higher. This is due to the power plant principle where the power generation part consist of several engines operating in par- allel and an optimum number of prime movers is always selected to match the load demand from the propellers and ship service load. Figure 1 gives an illustration of the efficiency. Total installed power: On the conven- tional LNG carriers today there are 3 - 4 auxiliary generators for the cargo han- dling plant. The total power is about 10 - 12 MW with medium voltage switchboard installations. The majority of this power is only utilized while the ship is at the termi- nal for cargo unloading. For vessels with electric propulsion one common power plant is utilized for both propulsion and cargo unloading. This means that the total amount of installed power can be reduced with electric propulsion systems because the cargo unloading plant and the propulsion plant are not used simultaneously. For example an LNG carrier with a requirement of 26 MW of propulsion power and 10 MW for cargo unloading would require about 39 MW of installed power capacity (includ- ing 10% turbine margin) for the conven- tional steam turbine ships. Using electric propulsion the dimensioning factor for the power plant would be the propulsion power plus the ship service load. Assum- ing a ship service load of 2500 kW the installed power could typically be 35 MW (including 8% electrical losses and 10% engine margin). This means a reduction of about 11% of installed power capacity. Electric variable speed drive: Basically a speed controlled electric motor (by fre- quency converter) has full torque avail- able from 0 to max. RPM (revolutions per minute). The RPM range is from max. neg- ative to max. positive RPM. There are practically no limitations in RPM and torque performance, other than those imposed by the mechanical parts in the propulsion and prime mover system. The converter is set to protect the mechanical systems by controlling the torque speed and power increase/decrease accordingly. With these possibilities of the propul- sion drive system, there will be a positive impact on the manoeuvrability and crash stop situations. For the manoeuvrability the impact will rise from the fact that the propeller can be run at any RPM, and RPM changes can be fast because of the possibility to give full torque (pos./neg.) at any RPM. The crash stop situations will improve by the fact that the electric motor and converter is able to absorb reverse power from the propeller by electric brak- ing and hence be able to change RPM direction faster than for the traditional steam turbine propulsion. Redundancy: Electrical power genera- tion and distribution systems must be con- figured to meet the redundancy require- ments of the installation. Classification societies have strict rules on how the pow- er plant shall be configured for their dif- ferent notations; however, the societies do not have common rules. With electric propulsion 50% redundan- cy is easily obtained for the electrical pow- er part, i.e. at least 2 propulsion motors with separate feeding (converter + trans- former), and at least a 2-split switchboard system. With this redundancy concept, the available propulsion torque or power will be a minimum of 50% after any single fail- ure in the electrical system. The conse- quence of loss of ship speed is however much lower and typically the ship should be able to operate with a ship speed not less than 70% of the maximum in case of any single failure in the electrical system. Diesel-mechanical system: Diesel- mechanical systems with two stroke engines and Fixed Pitch Propellers (FPP) are also considered as alternative propul- sion for future LNG carriers. This propul- sion concept is commonly used in other commercial tanker fleets, so this also needs to be considered as an option for LNG carriers. The main obstacles so far have been the handling of BOG and the reliability compared to today's steam propulsion. Using two-stroke engines for propul- sion would require the installation of a reliquefaction plant for handling the BOG. This would mean additional installed power capacity, and also additional con- tinuous power consumption of about 5 MW at seagoing conditions. The total installed power would then increase com- pared to steam propulsion and even more compared to electric propulsion. As the reliability requirements for LNG carriers are extremely high, there is reluc- tance in the market to use two stroke engines for propulsion of LNG carriers con- sidering the single screw case. Two stroke engines are therefore mainly considered for twin 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 the power plant is essential. The power plant consists of several medium speed gas/diesel engines, which drive the electri- cal generators. The generators are connect- ed in a common grid, to the main electrical switchboard. All loads, including propul- sion, thrusters, auxiliaries, and ship sys- tems are normally fed from this grid, and the total load is shared between the run- ning generators. The configuration of the power plant is based upon total installed power, operating modes, flexibility and redundancy requirements, and equipment cost. The number and rating of power gen- erating sets must be optimized to achieve the desired flexibility in view of fuel econo- my for prime movers, operational profile and service factor. The purpose is to achieve optimal loading of the running generator sets during the different opera- tion modes. The voltage level of the main power plant is selected so that load current and short circuit levels are kept within the technical limits of the equipment, and also to optimize the total cost of the installation. For installed power lev- els above 8-10 MW, medium voltage switchboards are the best technical and cost optimal solutions providing arc-proof solutions with high safety with regard to damages and injuries. With increasing total power, the power generating voltage will 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 voltage levels 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 Carriers Full-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 to propeller shaft of a Dual Fuel Electric Propulsion plant compared to a Steam Propulsion plant. Power and propulsion for LNG carriers p11-12revised.qxd 15/10/2004 11:05 Page 1