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Petromin marine and offshore Jan-Feb Issue

prime mover is a diesel

prime mover is a diesel engine, then it is called Diesel-Electrical Propulsion. The most commonly used diesel electrical propulsion systems are not a new concept. In the past these systems were usually diesel engine driven D.C generators that supplied power to D.C motors. Their applications were generally limited to vessels that required a degree of low speed maneuvering. Layout of diesel electric propulsion in ship Vessels such as ferries, harbor tugs, and various other applications used diesel electrical systems for features that were not available in mechanical systems at that time like speed control and maneuverability. To date, electrical propulsion systems have been used mainly for specialized vessels rather than for cargo ships in general. These include dredgers, tugs, trawlers, lighthouse tenders, cable ships, ice breakers, research ships, floating cranes, and vessels for the offshore industries. Electrical-drive systems have made substantial progress in recent years. Types of Diesel Electric Propulsion The two systems dominating the market today are Frequency controlled A.C Motors and SCR controlled D.C Motors. Frequency controlled A.C Motor drive system were generally more cost effective below 500 H.P and SCR controlled D.C motors systems at the higher end. The offshore drilling industries favor SCR controlled DC drives. Modern SCR and frequency controlled systems have efficiencies approaching 97% in power conversion. The selection of one over the other is an application issue. The deep draft cruise ship industry, due to the high hotel-like power requirements, is adopting highpower diesel electrical propulsion systems in most of its new builds. Both technologies have a proven record of efficiencies and reliability. For a direct current propulsion motor, the electrical power may be from one or more DC generators or may be form an alternator and then delivered through a rectifier as a DC supply. The power for direct current motors is limited to about 8 MW, and so AC machines are used for high outputs unless an effort is made to install DC motors in tandem. The rectification scheme can incorporate speed control and a means of reversing. Electric power generation and propulsion is a solution optimized for an environmentally aware era, and which answers shipyard and ship-owner’s needs for fuel economy, higher flexibility (both for installation and operation), better comfort on board, increased availability and an inherent capacity to be configured for fault tolerance and graceful degradation. Power for AC propulsion motor is supplied obviously by an alternator; the prime movers may be a diesel engine, a gas turbine, or a boiler and steam turbine installation. The choice of diesel electrical system as the power source for a propulsion system of a vessel has nothing to do with hydrodynamic efficiency. The propulsion system of a vessel provides thrust to move the vessel and is still chosen by the de- 44 January to February 2018

signer based on merits for the vessel’s application. Conventional propellers, controllable pitch propellers, azimuthing Z drives, transverse tunnel thrusters, and low speed water jet systems can a small or relatively small percentage of total power requirements, research vessels with special maneuvering requirements, and gaming vessels where speed is inconsequential (such as a gaming vessel operating in a river). be driven with equal effectiveness by a diesel-electrical system. When space and propulsion machinery limitations either Diesel-electrical propulsion becomes viable when the installed KW for propulsion approaches or is exceeded by the exclude the use of direct diesel or adversely affect the construction costs resulting from using direct diesels: KW installed for other purposes. The convenience of electric power distribution makes it possible to optimally locate the primary power source, i.e. diesel generators, exclusive of 1. Vessels with hull and struts too small to accommodate diesel engines, access, ventilation, etc. consideration as to whether it is for propulsion, thrusters, or cargo handling purposes. A large variation in propulsion power requirements, such as long periods of low speed 2. Vessels with potential trim problems, such as stern wheelers, where machinery need to be located forward to avoid trim problems. operation or the necessity to shift power from main propulsion to thrusters for dynamic positioning purposes, can also justify diesel electric systems. Modern turbo-charged diesel engines are efficient over a relatively narrow operating load and RPM range. They are not suitable for long period of low speed, low load, low RPM, high torque requirements for reversing large propellers. Modern generator systems with load sharing, auto-start, and load shedding features make it possible to efficiently utilize the installed horsepower of a diesel electrical system. Advantages of Diesel Electric Propulsion Diesel electrical propulsion can overcome the following design problems: When propulsive or station-keeping power requirements are January to February 2018 45

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