Maritime Prime Movers, Contemporary and Futuristic
Maritime Prime Movers, Contemporary and Futuristic by Louis Lemos
As a radical alternative to sail or steam power, evolution of Diesel-Electric propulsion plants for marine applications can be traced back to the world's first Diesel-powered ship, the M.V. VANDAL, a 244 foot oil tanker, which, according to the Russian Maritime Register of Shipping, was designed under the direction of K. W. Hagelin for Naphtaproduktions Gesellschaft Gebruder Nobel. The M.V. VANDAL was built by the Sormovs Shipyard at Nizhniy Novgorod in Russia in 1903, with a Length of 74.50 meters, Beam of 9.60 meters, Loaded Draft of 1.83 meters and a Loaded Displacement of 700 long tons. Her main propulsion plant consisted of three 3-cylinder single-acting Diesel engines of the four-stroke cycle non-reversible type built by AB Diesels Motorer of Sickla, Sweden, (the predecessor of Atlas-Copco), under license from Rudolph Diesel These engines were designed with a bore of 299 mm. (11.77"), stroke of 430 mm., (16.93“) and rated at 120 bhp at 250 rpm. Each of the three main Diesel engines was directly connected to an 85 kW D.C. generator which in turn powered a reversible electric propulsion motor rated at 75 kW, directly connected to its respective propeller shaft to provide a speed of 7 knots fully loaded. This arrangement was attributable to the fact that direct-reversible Diesel engines had not yet been built. Power for the electric steering gear, anchor windlass and a fire pump, plus navigation and accommodation lighting was furnished by an auxiliary generator. Two electrically driven cargo pumps were powered from one of the main propulsion generators. Her relatively shallow draft was required to ensure safe passage through the rivers and canals of her trading route between the Caspian Sea and the port of St. Petersburg.
Direct Current (D.C.) has been used for marine Diesel-Electric propulsion plants since 1903 almost exclusively until it was eventually displaced by the trend toward Alternating Current (A.C.) more than fifty years later. As of the early 1960's there was a gradual but persistent shift towards the adoption of A.C. for power generation, propulsion control and drive purposes. This was mainly due to the trend toward higher propulsion power in excess of 10 MW per shaft, for which the corresponding weight and space requirements of D.C. machinery became unacceptable. However, since the early eighties, subsequent developments in electrical technology have made A.C. systems far more efficient, much lighter and more compact, besides requiring less maintenance than their D.C. counterparts, thereby contributing to greater economy of operation. For main propulsion power requirements in excess of 20 MW per shaft, two or more propulsion motors can be installed either in tandem on a common shaft, or geared to the shaft in parallel, reflecting the inherent flexibility of Diesel-Electric propulsion. Alternatively, there has been considerable growth in the adoption of "Azimuthing Propulsion Pods", consisting of an electric propulsion motor encased in a hydro-dynamically designed housing, directly connected to a propeller, and capable of rotating on a vertical axis through 360 degrees. The above mentioned advantages of A.C. systems however are not limited to Diesel Electric main propulsion plants but are equally applicable to other machinery systems served by the concept of a sea-going "Power Plant", such as dynamic positioning thrusters, and petroleum cargo pumps for shuttle tankers; bow and stern thrusters, air conditioning plants, refrigeration systems and passenger elevators aboard large cruise liners.
It is generally agreed that Diesel-Electric drive offers many advantages over steam turbine plants, medium speed Diesel geared drive, or slow speed Diesel direct drive. Although initial cost is somewhat higher, actual operating expenses are lower due to reduced engine room manning, reduced maintenance work load and greater fuel economy. This in turn ensures maximum efficiency and reduced air pollution by nitrogen oxides in the exhaust emissions, as a result of constant speed, optimized combustion and more stable engine loading. Furthermore, changing from the ahead to the astern direction is accomplished simply by reversing the rotation of the propulsion motors. For special purpose vessels such as missile, satellite and antisubmarine tracking ships; hydrographic, oceanographic, seismographic and fisheries research ships, the feasibility of cruising at slow speed for prolonged periods of time without imposing a corresponding penalty on the prime mover(s), is yet another area in which the flexibility of Diesel-Electric drive has proven itself to be a worthy alternative to competitive conventional drive arrangements. In addition to the improved fuel economy afforded by Diesel-Electric propulsion, in the case of vessels fitted with electrically-powered Azimuthing propulsion pods, there are several other benefits worthy of note such as the following:
1) The installation of azimuth propulsion pods (A.P.P.), completely dispenses with the need for conventional long drive shafts, shaft tunnels, steering gear and rudders.
2) The space thus saved can be used for revenue earning purposes.
3) If required, the A.P.P. system can also be used to maneuver a vessel during berthing or un-docklng.
4) The A.P.P. system produces much less vibration and is much quieter than line-shaft propulsion.
Except for maneuvering during docking and un-docking, sea-going ships spend most of their time operating at a speed of "full ahead" in the open sea. Harbor tug boats, on the other hand, invariably operate in close quarters; with due regard to the relative proximity of other ships moored at adjacent piers, and the traffic of passing vessels,and are required to exert pushing or pulling power in short bursts of high energy, In order to safely maneuver their ship into its designated mooring space. These short intermittent bursts of high energy, of brief duration but repetitive occurrence, tend to detract from the ideal cycle of constant speed, steady load and stable combustion conditions thereby resulting in rapidly fluctuating cylinder temperatures, incomplete combustion and consequent abnormal exhaust emissions. Exhaust Gas Emissions Tests conducted in Europe have shown that compared to medium speed geared-drive Diesel engines operating in a variable speed mode, a significant reduction in nitrogen oxide emissions can be achieved by a Diesel-Electric powered vessel operatlng for extended periods at partial loads, while the main propulsion Diesel-generators are operating at constant speed. With a variable speed engine operating at approximately 30% of maximum continuous rating (mcr) , such as may occur when a cargo ship is entering or leaving port; or the slow speed operational routine of special purpose vessels mentioned above, the exhaust emission of nitrogen oxides peaks around 2,230 ppm, which iS 280% higher than the NOx emissions from an engine operating at 100% of mcr. While conventional wisdom appears to fault marine Diesel engines for their exhaust emissions far more than their highway or railroad counterparts, what is apparently ignored, is the fact that far less power is required to move a given tonnage of cargo for a given distance by river barge than either a truck or train requires to haul the same total load over the same distance. Since river barges are usually "Fleeted" in groups of four, twelve or more per tow, pushed by a single river push boat, it requires considerably less engine horsepower to move a ton of cargo by river barge for a given distance. Therefore, in terms of fuel consumption per ton/mile, river boats burn much less than that required by either a conventional forty foot truck trailer or railroad box car to cover the same distance. Consequently, it follows that river boat Diesel engine exhaust emissions are likewise, commensurably less. Recent studies have shown that, as opposed to exhaust emissions from ship-docking tugboats, the nitrogen-oxide emissions from riverine Tugboat Diesel engine exhaust, estimated at 0.470 grams per ton/mile, are about 27.70% lower than that of railroad trains and 35.60% lower than that of highway trucks. Other exhaust pollutants such as Hydrocarbons, Particulate Matter and Carbon Monoxide are also found to be proportionally lower, in terms of grams per ton/ mile, in favor of water-borne transportation.
Meanwhile, several major ports within California are requiring visiting ships to shut down their Diesel-generators while at pier-side, and use "Shore Power" instead, from transformers connected to the municipal electrical grid. The current trend is to develop alternative forms of energy including wind, water and solar power, innovative forms of prime movers such as electric battery power, hybrid versions consisting of internal combustion engines combined with battery power, fuel cell engines and non-fossil fuels such as hydrogen. However, just as automobiles and trucks in somewhat modified configuration will inevitably constitute the primary form of highway transportation for the foreseeable future, internal combustion engines, also in modified format, are likely to be in use for several years to come, both for land-based vehicles and sea-going ships. Initially, one may expect the major changes to be from spark-ignited gasoline engines and compression-ignition Diesel engines, to spark-ignited gas engines, burning either Liquefied Natural Gas (LNG),Compressed Natural Gas (CNG), Propane or derivatives thereof, for which the current Diesel engine basic configuration is most adaptable, until it is replaced by either a fuel cell or alternative form of prime mover of comparable power range. Currently there is a fleet of car/passenger ferries operating off the coast of Norway powered by Rolls-Royce Bergen Diesel engines burning LNG exclusively. It is reported that the exhaust from these engines is practically free of toxic emissions. Hence, the major differences in vehicular and marine prime movers may probably be in the form of gaseous liquid fuels of a non-fossil nature, requiring relatively minor structural or design changes.
A good example of large scale propulsion plant conversion from twin screw steam turbine (circa 1969) rated at 55,000 shp each, to upgraded Diesel-Electric (circa 1986)consisting of nine Diesel generators rated at 14,445 bhp each, is that of the "QUEEN ELIZABETH II," accomplished in April 1987 at an estimated cost of $53,000,000.00, resulting in the world's most powerful Diesel-Electric propulsion plant of any merchant vessel afloat. Her two 60 cycle synchronous propulsion motors were rated at 44 MW each at 144 rpm, capable of producing a speed of 33 knots. An interesting but otherwise unpublished bonus to this machinery conversion lay in the fact that waste heat recycled from the Diesel engine exhaust, plus heat recovered from the Diesel engine jacket cooling water, was used to evaporate 1,000 tons of fresh water per day from sea water, resulting in a reduction of fuel consumption of over 200 tons per day. In order to minimize atmospheric contamination by marine engine exhaust emissions, new rules recently established and/or proposed, require ships (of all flags) to switch from conventional heavy fuel, (380 centistokes), and from No.2 Diesel fuel to a distillate fuel with a very low sulfur content, known as Ultra-Low Sulfur Fuel (U.L.S.F.), while navigating within Sulfur Emission Control Areas (SECAs), adjacent to the continental coastline, in designated areas. These rules have been developed by the International Maritime Organization (IMO) of the United Nations Organization. The California Air Resources Board (CARB) has also attempted to develop and enforce similar rules applicable within 24 miles from the coast, however, the U.S. Environmental Protection Agency (EPA), has blocked California from enforcing such legislation. The ultimate objective of such legislation is to impose ever-more strict limits of sulfur oxide emissions over a period of time within the SECA areas, with a current target sulfur cap of 1.0% by Januaryl, 2010, followed by a progressive reduction to 0.10% by January 1, 2015.
It is worthy of note that conventional naval submarines originally employed Diesel-Electric propulsion, using the main engines for propulsion while simultaneously using the Diesel-driven electric generators to recharge their batteries. For submerged operation the Diesel engines were shut down and de-clutched from the generators, which then became propulsion motors powered from the batteries. Contemporary nuclear submarines however, while equipped with an auxiliary diesel generator, rely upon the heat generated by the nuclear reactor to generate steam that is then fed to a steam turbine for propulsion purposes. Their ability to remain submerged for up to ninety days at a time is attributable to their capability of drawing in seawater, extracting oxygen from the seawater to replenish their own atmosphere, and disposing of the hydrogen by pumping it back overboard. It is foreseeable that this capability may eventually be extended to surface vessels powered by Hydrogen Fuel Cell engines. In this case they may be capable of drawing in seawater, and extracting the hydrogen to feed the fuel cell, using a form of hydrolysis. However, the current process for doing so is considered to be energy-intensive and it may require more in-depth research to find a less expensive method. The basic requirements for a Hydrogen Fuel Cell are 1) fuel, 2) oxidant, and 3) an electrolyte plus a negative anode and a positive anode.
Currently there are several different kinds of electrolyte available in which the corresponding operating temperatures and electrochemical reactions may vary according to the chemical constituency of the electrolyte involved. In a typical Polymer Electrolyte Membrane Fuel Cell (PEMPC), such as that specifically developed for Space Missions of the 1960's, the system operates at 80 degrees Centigrade as follows:
* Hydrogen is fed into the anode, which is the electrically negative post of the fuel cell.
* In the center of the fuel cell the electrolyte absorbs an electron from the hydrogen atom, using it to make
electricity.
* The cathode, as the electrically positive post of the fuel cell, is where the electrons recombine with the hydrogen
and oxygen to make water, which is the exhaust effluent.
This transition of protons and electrons is known as the ionic conduction mechanism that characterizes a Fuel Cell. The protons travel through the electrolyte toward the cathode, while the electrons move through an external circuit before recombining with protons to form water.
If conversion to fuel cell power were to become mandatory, given the eventual phase-out of fossil fuels, conventional power plants of existing vessels could be replaced by fuel-cell power plants with electric drive motors. This phase of the project would probably require an installation mounting cradle compatible with the foot print of the new Fuel Cell plant and electric drive motor assembly. Feasibility of such transplant would of course be contingent upon successful reduction of current fuel-cell systems to a size compatible with that of existing I.C. engines, of comparable power range. The original Diesel fuel tanks would have to be replaced with specifically designed high pressure hydrogen fuel tanks, similar to the new LNG tanks under consideration. Meanwhile, the entire Diesel fuel refinery, distribution, storage and retail infrastructure will have to be redesigned to cope with hydrogen and/or LNG instead. Studies along these lines are currently in progress coordinated by the California Fuel Cell Partnership consisting of British Petroleum, Chevron-Texaco, Exxon-Mobil, Shell, California Air Resources Board and the United States Department of Energy (D.O.E.). Fuel Cell propulsion is being viewed within D.O.E. as the transportation technology of the future, based on the findings of studies being conducted by the Argonne National Laboratory.
The near-future requirement will be to build enough dedicated hydrogen and/or LNG plants to meet the anticipated demand and it will be interesting to see just how long it may take to actually implement conversion from conventional fuel to the "hydrogen/LNG" fuel logistics infrastructure within the United States, and elsewhere. Currently, the European Union is sponsoring a Methanol based Auxiliary Power System (METHAPU), with the cooperation of Lloyd's Register. However, in the absence of a formal safety program or specific operating rules for Marine Fuel Cell Power Plants, it remains to be seen if the I.M.O. or the International Association of Classification Societies will take the initiative in establishing such essential guidelines. Meanwhile, this entire fuel cell evolution, research, development and adoption may be expected to provide new business prospects and employment opportunities for those willing to accept the challenge.
A prime example is the Norwegian-flagged Offshore-Support ship MV. VIKING LADY, built in 2009 and acclaimed as the "Most Environmentally-Friendly Ship" afloat. She was recently fitted with an MTU Onsite Energy Fuel Cell as part of the Fellow-SHIP Project. The partnership includes Det Norske Veritas; the ship-owner Eidesvik; the System Integrator, Wartsila and MTU Onsite Energy who supplied the 320 kW Fuel Cell powered by LNG. Funding for the FellowSHIP project is provided by the Norwegian Research Council and Innovation Norway. Its aim is to respond to rising concerns about the environmental impact of harmful emissions to air, including NOx, SOx, and C02. In this reqard it is worth noting that the U.S. National Oceanic and Atmospheric Administration (NOAA) and the University of Colorado published a study in 2009, said to be the first to provide an accurate estimate of shipping's total contribution to air particle pollution based on direct measurement. The study concludes that on a global basis, ships emit 0.9 teragrams, or about 2.2 million pounds of particle pollution each year. Further, since more than 70% of shipping traffic takes place within 250 miles of the coastline, emissions represent a significant health concern for coastal communities. The above mentioned vessel, MV. VIKING LADY is a harbinger of maritime environmental preservation given that her advanced on-board technology, compared with a traditional ship, allows for a reduction in harmful emissions equal to the emissions from 22,000 automobiles annually .
The adoption of rules requiring ships to switch from conventional heavy fuel (380 centistokes), and from No.2 Diesel fuel to a distillate fuel with very low sulfur content, known as Ultra-Low Sulfur Fuel (ULSF), when transiting within Sulfur Emission Control Areas (SECAs), adjacent to the continental coastline in designated areas, is not without its problems. Although the intent is to minimize toxic exhaust gas emissions, there have been incidents involving ULSF that adversely affects cylinder liner lubrication, particularly of the large bore slow-speed main propulsion Diesel engines. In addition thereto, the diminished lubricity of ULSF has been cited as the apparent cause of fuel injection pump binding of generator engines, resulting in the loss of power. It has also been determined that bio-diesel fuel blends are detrimental to elastomer sealing materials used in certain fuel transfer pump oil seals, believed to be attributable to acidity of the bio-diesel due to oxidation. While various countermeasures are being considered with varying degrees of success, there are evidently limits to which such corrective solutions are feasible and effective, without intro- ducing adverse effects of their own. For example, while the efficacy of SCR systems in reducing the level of NOx in exhaust gas emissions has been well proven, there is also the problem of slow speed operation. This is due to the fact that when an SCR-equipped main propulsion engine is operating at slow speed and low load, such as occurs when maneuvering in and out of port, especially in cold ambient temperatures, the reaction of ammonia with sulfur oxides produces ammonia bisulphate (ABS) that forms a viscous compound and clogs the catalyst. To cope with this problem during long periods of slow speed or idling, some owners have installed a separate after-burner within the SCR system to periodically raise the exhaust temperature high enough to avoid the formation of ammonia bisulphate. It is important to note that in the case of marine Diesel-Electric propulsion plants incorporating SCR systems on the engines, this type of problem does not exist since the engines are operating at constant normal speed regardless of the speed or load of the propulsion motors.
Current statistics have shown that there is an increasing trend towards Dual-Fuel Diesel engines, notably in new vessels being built in Europe. Both MAN-B&W and Wartsila are leading the field in Dual-Fuel (DF) technology, as it applies to both main propulsion and auxiliary engines. In many cases, "Dual-Fuel-Electric" is now being specified by ship-owners to ensure even higher efficiency. Apart from the inherent flexibility of DF-E propulsion, there are several advantages to be considered not the least of which, especially in the case of multiple-engine installations, is the considerable fuel economy attainable from selective use of power to meet immediate needs, in addition to minimizing wear and tear of those engines retained in standby mode. In response to the prospect of even stricter air emission standards within the foreseeable future, the choice of Liquefied Natural Gas (LNG) as a primary fuel is gaining popularity. This trend is particularly evident among LNG Carriers equipped with Dual-Fuel main propulsion plants, wherein "boil-off gas" and reqasified LNG from the cargo tanks contribute to higher energy efficiency while minimizing undesirable exhaust emissions. Evidence supporting this contention includes reduction of about 75% in NOx emissions due to the lean burn combustion process; a reduction of nearly 20% in C02 emissions given the lower carbon content of natural gas compared to that of liquid fuels, and in the absence of sulfur in natural gas, there are no SOx emissions either. The above mentioned advantages were highlighted by the Classification Society Germanischer Lloyd during a forum on "Gas as a ship fuel", held in Hamburg in March, 2010. A spokesman for GL announced that "There appears to be general consensus on developing gas engines that can be used on vessels operating regular or shorter routes". He further stated that "LNG fuel is a viable alternative to heavy fuel oil". In support of this trend the International Maritime Organization subcommittee on bulk liquids and gases is expected to publish interim guidelines later this year on the use of natural gas as a marine fuel. Thus, as the demand for LNG increases, we can expect to see additional LNG bunkering facilities around the world.
FOOT NOTE: 1 - FUEL CELLS - Since writing the above, guidelines for the safe operation of Fuel Cells for marine propulsion purposes have been proposed by the world's largest Classification Society, Bureau Veritas, according to BV Product Manager Gijsbert de Jong. The intent being to establish a regulatory framework within which, building and testing of prototype Fuel Cell systems can be safely conducted while ensuring that the technology is developed and applied in accordance with safe performance-criteria, In his comments Mr. de Jong stated that "BV's guidelines for the safe application of fuel cells on ships take into account all relevant existing IMO conventions and guidelines, together with a wide range of international non-marine standards. They reflect BV's inhouse knowledge and expertise, and could have important commercial - as well environmental - implications for ship-owners and operators." He further explained "The object of the BV guidelines is to provide criteria for the arrangement and installation of machinery for propulsion and auxiliary purposes, using fuel cell installations, which have an equivalent level of integrity in terms of safety, reliability and dependability as that which can be achieved with new and comparable conventional oil fuelled main and auxiliary machinery. The guidelines currently have preliminary status and are subject to internal and external review. After taking into account all relevant feed-back, they will be published as a Bureau Veritas Guidance Note entitled "Guidelines for Fuel Cell systems on board commercial ships."
FOOT NOTE: 2 - FUEL CELL FUELS - Of the various fuels available for use in Fuel Cell systems, hydrogen appears to be the most logical, given that (a), it is non-toxic, (b),yields a higher ratio of chemical energy per unit mass than that available from natural gas, and (c), it is abundant as an unlimited resource in atomic form. In addition to which, hydrogen is non-polluting. Liquefied Natural Gas (LNG) which is now widely used in many Dual-Fuel marine Diesel engines, is also a strong contender for Fuel Cell use. Currently, there is an on-going initiative in Holland known as the Green Tug project, piloted by the Offshore Ship Designers' Group including participation by Bureau Veritas, featuring a hydrogen-powered Fuel Cell tugboat designed for near-zero exhaust emissions level, and estimated to increase propulsion efficiency by almost seventy percent compared to that of a conventional Diesel-direct-drive plant.
FOOT NOTE: 3 - TURBO-CHARGERS - Given the world-wide adverse effects of the on-going economic turmoil and the escalating price of fossil fuels, many Ship-owners are now operating their vessels at somewhat slower speeds, in some cases down to 12 knots, referred to as "Slow Steaming", as an effective way of reducing overall fuel consumption. In support of this new strategy the major Diesel engine manufacturers have developed modification kits to enable their main propulsion engine exhaust turbochargers to operate safely at reduced speed for prolonged periods. Basically this involves installing remotely-actuated shut-off valves within the exhaust manifold immediately upstream of the turbo-charger, and within the scavenge air trunk immediately down-stream of the compressor, along with a by-pass valve to allow the turbo-charger rotating element to continue running at a somewhat reduced but fairly constant r.p.m. Resultant fuel consumption reduction is claimed to be from a low of 6 to a high of 12 grams per kiloWatt hour (6-12g/kWh), depending upon the engine model and horse power range. In view of the uncertainty of the current duration of adverse economic effects and the on-going need for such countermeasures, it remains to be seen if orders for future new-builds will include specifications for lower overall horse power main propulsion engines.
Louis Lemos can be reached at [email protected]