Liquefied Natural Gas is an attractive fuel for use in High Speed Sealift Vessels (HSS) as compared to jet fuel. It can readily be used as a fuel for reciprocating engines or gas turbines. Natural gas liquefies at a temperature of -259° F and is, therefore, a cryogenic fuel that must be contained in highly insulated tanks. It is one of the safest fuels available for any transportation application including marine vessels.
LNG has more energy per unit mass than jet fuel, 20,800 Btu/lbm for LNG versus 18,200 Btu/lbm for jet fuel. Jet fuel has more energy per unit volume than LNG with the ratio being 1.7/1. LNG, therefore, requires larger tanks than does jet fuel. The overall mass of the onboard fuel is less for LNG fuel compared to the same energy content of diesel or jet fuel. This would potentially provide a vessel with additional cargo weight capacity.
A basic characteristic of LNG important to marine applications is that LNG is not combustible until it vaporizes. After LNG vaporizes and is warm enough to be combustible it becomes lighter than air and disburses extremely rapidly. The vapor (natural gas) is not flammable below five- percent concentration in air or above fifteen percent. In open air it is extremely difficult to maintain combustibility in disbursing LNG vapor.
LNG has been cheaper than diesel fuel or jet fuel for many years. Although the LNG market is driven by different factors than diesel and jet fuel markets, there has been a price differential favoring LNG for decades. The differential is less when the price of diesel/jet fuel is low, and markedly greater when the price of diesel/jet fuel is high. The price of diesel/jet fuel can be expected to increase at a greater rate than natural gas as resources dwindle.
The United States has enough economically recoverable natural gas to last 35 years at the present rate of consumption. The total recoverable supply, which includes "non-conventional" and other resources that require higher prices to make them economical, would last an estimated 200 years. The world supply of natural gas is even greater and often wasted. In 1980 alone, all companies worldwide flared approximately 7,000 billion cubic feet of natural gas, representing the energy equivalent of over 60 billion gallons of gasoline.
For the near term, one of the major economic advantages of using LNG as a fuel is that it is imported into the United States from foreign sources where natural gas is viewed as a waste product, and is being imported largely at the cost of being liquefied and transported. As a result, the fuel does not fluctuate in price, as does natural gas transmitted in the U.S. pipeline system. A reasonably long-term contract arrangement could be made for imported LNG that would ensure a predictable supply for a HSS, at a favorable price.
Over the last decade, LNG has had a continuous and significant cost advantage over diesel fuel (DF-2). It is anticipated that this cost saving will have a significant positive impact on the operational cost of the HSS vessel.
LNG is transported by large ships (LNG Carriers) bringing the liquid to the U.S. for insertion into the natural gas pipelines after gasification. The technology for handling and storing LNG is now well known and is being adopted in the transportation industry. New technology has been developed and defined particularly in the areas of storage and refueling for trucks and buses. The Houston Transit System has had several hundred buses operating at one time on LNG from a central terminal. Roadway, an interstate trucking company, has adopted LNG as a fuel for their vehicles. Many other cities in the West have adopted LNG for their transit systems and for route vehicles in the urban areas. The state of California has supported many experiments with LNG, proving it to be an economical, and emissions acceptable fuel. Truck fleets in California are being converted to LNG on a large scale and there are even 4 LNG switch engines being used in the rail yards in Los Angeles operated by Union Pacific.
The University of Alabama (UA) has significant experience in evaluating the use of natural gas as an alternative fuel for marine vessels. In 1987, the UA placed a LNG powered 70-ft. shrimp boat into service in the Gulf of Mexico and operated it until 1990. This vessel was built and supported by a consortium of agencies having an interest in developing LNG as a marine fuel. The consortium members contributed cash or components to the construction of the vessel. The consortium included The University of Alabama, Mississippi-Alabama Sea Grant Consortium, the Alabama Gas Co. (Alagasco), Caterpillar, Inc., and others. The LNG was used to freeze the shrimp as well as power the vessel, saving $10,000 per year in ice purchases. That the vessel was a small shrimp boat (300 hp) carrying 5,000 gallons of LNG, is not significant in itself; however, it provided a platform to demonstrate that LNG is a viable fuel for marine transportation.
UA also completed a preliminary design for a natural gas (CNG or LNG) powered 130-ft. crewboat for Exxon in Santa Barbara, California in 1992, and completed a feasibility study for Amoco in New Orleans, LA for a natural gas powered (CNG/LNG) 130-ft. crewboat in 1994. U.S. Coast Guard approved the Exxon preliminary design.
NATURAL GAS POWERED MARINE VESSELS
Two Canadian passenger/auto ferries, M.V."KLATAWA" and "KULLEET" have been operating in the Vancouver, Canada area since 1985. These two ferries have successfully operated sixteen hours daily, nonstop, 165 days a year. The ferries are double ended, 155 ft. in length and are propelled by two 300-hp 3406B Caterpillar diesel engines converted to pilot ignition natural gas.
The MV Accolade II, operating in Adelaide, Australia, is a self-loading limestone carrier of 108.63 meters in length. Propulsion is provided by two, 6-cylinder, Fuji, LG32X engines. These are pilot-ignited engines, developing a 1,650 BHP each. The Accolade is the oldest natural gas powered marine vessel and is still in operation.
The University of Alabama designed, built, and operated a LNG powered shrimp boat, MV MERV I. The 70-ft. vessel had a 5,100-gallon capacity of LNG with an additional 4,400 capacity of diesel fuel. The engine was a pilot ignited 3406B Caterpillar. The MERV I was operated in the Gulf of Mexico from June of 1987 until the winter of 1989.
The LNG Carrier, MV Venator operates on a reciprocating natural gas engine. This engine is a specifically adapted version of the Sulzer 7 RNE90 2-stroke design. The Venator operates in the Western Pacific area.
The ferryboat, MV Virginia was built by Tidewater Regional Transient Ferry (TRT). The ferry is currently being evaluated and is powered by a CNG system using a Caterpillar 300-hp. spark ignited engine. The vessel was built for improved air quality impacting both land and water.
Marintek of Norway has been studying the use of LNG for a new ferry in their ferry system. Construction will begin in 1997 with operation projected for 1998. It is being built for environmental reasons and because Norway has more natural gas available than diesel fuel.
The New York Department of Transportation is undergoing a two-year study to evaluate the use of CNG or LNG as a fuel for converting four Kennedy Class Ferries to dual fuel.
In San Antonio, Texas the small boats that carry tourists around on the river have been converted to natural gas reducing both noise and air pollution. No major problems have been experienced with the conversion.
General Electric states that the efficiency of a gas turbine is the same for LNG or jet fuel. However, fuel economy can be improved with LNG by controlling inlet air-cooling. Since the liquid has to be gasified to an acceptable temperature for combustion, a great deal of cooling is available that will allow controlling inlet air temperature to the turbines. This means the turbines can operate at their optimum design efficiency regardless of ambient air temperature. LNG produces approximately a ton of air conditioning per 100 hp. When LNG is used as a fuel for reciprocating engines (pilot-ignition diesel) a fuel efficiency improvement is possible in excess of ten percent as compared to diesel fuel.
Much has been said and published about the major advantages of maintenance avoidance using natural gas as a fuel in engines, but so far the experience with marine vessels is limited to the two ferryboats in British Columbia. Prior to being converted to natural gas, the diesel engines were overhauled on a 17,000-hour cycle. After the conversion, the overhaul cycle was changed to 50,000 hours and consideration has been given to increasing the cycle to 70,000 hours. GE states that the operating life of a gas turbine doubles using natural gas as compared to jet fuel. Turbine overhaul is typically 15% of operating cost.
LNG can be stored on a vessel in vacuum insulated tanks, foam insulated tanks, or membrane tanks. LNG is normally stored in cylindrical tanks to permit boil-off management by allowing the tank to become pressurized during operation. Normally a cylindrical tank will be near ambient pressure when it is full. It will build pressure if it is allowed to sit unused before it begins to vent natural gas to control tank pressure. This is to provide some standby time without venting if a vehicle has to stop for a period of time in the course of its operation. The ideal is to design the tanks and the operation of the HSS so that the fuel can be used at a rate that would minimize pressure buildup.
To minimize the cost of the LNG tanks, the vessel can be designed to utilize boil-off for auxiliary power requirements such a gensets. With good engineering design, there should be no reason why a HSS should ever vent natural gas to the atmosphere, primarily because of the anticipated high rate of fuel use.
There are several options for handling LNG in a HSS. The most obvious is to mount the tanks permanently within the hull of the vessel and refueling it with conventional hose apparatus. This technology is available from several sources including the bulk LNG transport industry. Fuel transfer time is relatively short and efficient and could be engineered to match turn-around times for cargo transfer. Another option is to use the "cooperage" approach in which spent tanks are removed from the vessel and replaced with full tanks. The space for the tanks would be isolated from the rest of the vessel and a hatch provided to facilitate swapping the spent tank for the full tank. To maintain interior cryogenic temperatures and to minimize heat cycling, LNG tanks are never completely emptied. Although LNG tanks are capable of heat cycling for a long period of time, heat cycling is normally unnecessary because it is very easy to leave some LNG in a spent tank to maintain its internal temperature.
In an HSS the weight of jet fuel tanks can be minimized by utilizing the external skin of the vessel and internal bulkheads as a part of the tanks. The total weight of LNG cylinders is likely to be larger than that of jet fuel tanks. However, if one or two LNG tanks are used compared to multiple tanks for jet fuel the amount of material required for the multiple jet fuel tanks may equal or exceed the LNG tank weight even though the jet fuel tanks are using portions of the existing hull structure. More importantly, the weight of LNG for the same energy content is about 15 percent less than for DF-2. This will more than offset any tankage weight increase and allow for additional cargo capacity.
LNG is arguably the safest of all fuels available for marine power. One of the basic characteristics of LNG that supports this argument is that it does not burn in liquid form and when gasified it is lighter than air and disburses very rapidly to the atmosphere. It is not combustible in air in a concentration below five percent or above fifteen percent. Therefore, ventilation can be used very effectively to prevent any leaks that might occur from reaching the lower flammability limit (LFL). If the vessel were struck with high explosive ordinance, no collateral explosions would occur because of the presence of LNG fuel tanks. Collateral explosions would be likely with the presence of jet fuel tanks.
In other situations where damage releases significant amounts of jet fuel to the water, the fuel tends to spread rapidly and fire is possible creating a great hazard to crew in the water. By contrast LNG in water may not spread significantly but vaporizes rapidly to the atmosphere. There is little likelihood of damage to sensitive coastal environments. The main hazard with LNG would be to crew that might come into contact with it in the water or might be close enough to be asphyxiated as the LNG rapidly vaporizes. Because of the high activity of LNG in seawater, the option to avoid it is great and its risk area is much smaller than that of jet fuel.
LNG is one of the safest and most cost-effective fuels available for powering an HSS. In the near term, well-established technology is available to design, build and operate an HSS. This technology is available from a variety of sources, but has never been packaged in the form of a HSS. Other marine vessels currently operate on natural gas and codes are presently available from the National Fire Protection Association that may be adopted by US Coast Guard and other standards groups. There are no significant barriers that would prevent the development of a LNG powered HSS. Most of the work necessary in the near term is to develop a comprehensive feasibility evaluation identifying currently technology and providing systems integration for a complete LNG/HSS. This would include economic evaluation and identification of sources for technology that could be used with the HSS. In the short term, the evaluation of high-tech foam insulated pressurized LNG tanks needs to be completed along with some preliminary design evaluations for membrane tanks with accompanying economic analysis.
Additional work needs to be done on designing inlet air cooling systems for gas turbines utilizing chill from LNG fuel. Some preliminary testing in the laboratory would be useful to work out the control systems and to demonstrate feasibility of the technology.
Methods of designing catamaran hulls that will allow the use of the cooperage method for refueling the HSS would be useful. This work could look at how to design the hatches and tank rooms so that the fuel system would not impinge upon the mission of the vessel and still maintain hull integrity at a reasonable cost.
Other areas of research could include methods for managing boil-off using gensets, air conditioning systems, freezers for food, inlet air cooling for gas turbines, or wasting small amounts of energy with opposing bow thrusters. Other boil-off utilizations may come to mind with time.
Long-term research could include evaluating the effects of variations in LNG fuel composition and their impact on the operation of gas turbines. Natural gas is largely methane but contains varying amounts of other gases having higher heating values such as propane, butane, and ethane. These higher heating value gases are usually less than ten percent of total composition. The tolerance of gas turbines to the variation in fuel mixture needs to be evaluated to provide for the establishment of fuel standards.
Development should progress in designing mounting systems for LNG tanks to meet the rigors of marine operating conditions. Basic research needs to be completed on the high tech foam materials for insulating LNG tanks to continually improve the characteristics of the foam and the technologies used to apply the foam to LNG tanks. Since the tanks are insulated using robotics, a great deal can be done to improve the efficiency of tank construction reducing overall costs. The major cost of any kind of LNG vehicle is the cost of the LNG tanks. Anything that can be done to reduce this cost will make the use of LNG more cost effective.
For questions concerning this report contact: C. Everett Brett Ph.D.
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