LNG tankers in particular have a special problem, in that heat leaks to the cargo tanks cause the formation of gas boil-off, i.e., a certain amount of gas is released which must be taken care of, either by recondensing or burning off the gas, or alternately by utilizing the gas as fuel in propulsion machinery and other auxiliary machinery.

To make use of the boil-off as fuel on board an LNG carrier, the most common solution has been to combust the boil-off gas in connection with steam turbine machinery, using the gas as auxiliary f el for the ship's boilers, which burn crude oil. .Operating experience with such machinery is good, but combustion efficiency and bunker oil consumption are not favorable.

The desire to improve the fuel economy of propulsion machinery on board LNG tankers has focused interest on low- rp , "large bore" diesel engines. These engines exhibit very good thermal efficiency, and are also able to run on poor quality (inexpensive) bunker oil. As a means of utilizing the gas boil-off from the cryogenic fluids carried in the ship's cargo tanks, the diesel engine alternative immediately presents itself as a very attractive option. At the outset, it would obviously be an advantage if a dual- fuel diesel engine existed which could run on gas/crude oil in any ratio of admixture with the same high efficiency over its entire spectrum of operation. This would be an especially attractive solution for existing ships where the amount of gas boil-off is high, thus making the use of a condensation plant unreasonably expensive.

Strictly gas-fueled engines are currently available for low and medium-range outputs, i.e., up to a few thousand kW. There is also a marine installation with a large-bore dual- fuel diesel engine of about 15,000 kW output.

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In the case of all of the above types of engines, the gas is allowed to enter the combustion chambers or cylinders at moderate pressure immediately after the compression stroke begins. The gas/air mixture is ignited by spark plugs or "pilot" fuel, i.e., a small amount of diesel injected into the chambers at the moment combustion is desired, thereby obtaining almost instantaneous combustion with an associated high rise in pressure (the Otto process).. As a consequence, the amount of gas must be limited, which in turn means a reduction of efficiency and limited capacity to combust gas. In the case of the above-mentioned large-bore diesel engine, the gas portion must be kept below about 55% of the total fuel requirement at full power. For larger ships with large quantities of gas boil-off, there are no suitable dual-fuel engines which can combust all of the boil-off gas with sufficient margin and the desirable efficiency.

A dual-fuel diesel engine has already been proposed which runs partly on pure crude oil, partly on a high percentage of gas, without unpermissible increases in pressure during combustion. Gas boil-off is utilized in this dual-fuel diesel engine. The boiled-off gas is introduced in a con¬ trolled manner to the combustion chamber during the combus¬ tion process itself so that the increase in pressure is limited, i.e., the result is almost the same as an actual diesel process.

Norwegian Patent Application No. 81 2328 describes a method of utilizing gas boil-off from cryogenic fluids as fuel in a dual-fuel diesel engine on board a ship, wherein the boil-off gas is compressed to a high pressure and introduced to the engine's combustion chamber during combustion by high pressure injection that is controlled in accordance with the combustion pressure. The high pressure injection is preferably electronically or electronically/hydraulically controlled.

The necessary technology for utilizing this method exists. Experiments have been and are being performed with electron¬ ically controlled fuel valves for oil. Also available on the market are diesel engines for special purposes that have electronically controlled supply of liquid fuel. Although cam-driven fuel pumps can be constructed for timed injection of liquid fuel into the cylinders approximately in conformity with the course of combustion, it is not possible to transfer this technology directly for a compressible substance like gas. For both types of fuel, electronic control offers the best possibility for gradual and appropriately timed fuel injection, adjusted in accordance with the course of combustion.

Norwegian Patent Application 81 2328 also describes a system for introducing gas boil-off from a cryogenic fluid as fuel for a dual-fuel diesel engine on board a ship. The system comprises a gas compressor whose suction side is connected to the cargo tank containing the cryogenic fluid, a buffer tank that receives compressed boil-off gas from a compressor, and a conduit leading from the buffer tank to a gas injection valve in the engine, as well as a control means for opening and closing the gas injection valve in accordance with the instant pressure in the associated engine combustion chamber. In practice, a microprocessor can be used to administrate the most important control pulses, which are: the instant pressure in the cylinder, the number of revolutions per minute, the desired output power and the crank position.

The dual-fuel diesel engine utilized will have two fuel supply systems for each cylinder, and each of these is designed for 100% output. When the engine is running on gas, the gas must be ignited by means of pilot fuel or spark plugs. The oil/gas can be combusted in any ratio, and it will nor¬ mally be advantageous to provide reciprocal control so that all available gas will be combusted at all times and liquid fuel is added only to the degree necessary. If the output

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requirement is so low that there will be excess gas, the excess can be stored by allowing the pressure to rise in the cargo tank or tanks. Spherical tanks are especially well suited in this connection, because they can withstand a certain amount of pressure build-up. This capability is best utilized if a small amount of gas is continually tapped from the tank, to avoid a rapid increase in pressure of non- condensable gas (nitrogenl. It will thus be advantageous to provide a high pressure storage tank in connection with the buffer tank, for example in the form of a bank of gas cylinders. The buffer tank can then be one of the cylinders. Within an economical and practical framework, the capacity of the cylinders can be chosen such that the entire quantity of boil-off gas can be accumulated for a period of several days. When the gas is to be utilized, it is guided to a suitable stage of the compressor, which in that case is a multi-stage compressor. This in itself is a favorable solution, because the compressor stage can be chosen depend¬ ing on the prevailing pressure at any time.

With a dual-fuel diesel engine as described above, utilizing diesel oil and gas boil-off from a cryogenic fluid as fuel, the two fuels being introduced into the engine's combustion chamber via respective valves, it is proposed in accordance with the present invention to provide a valve-controlled conduit connecting a source of inert gas to the engine's gas supply valve, and a valve-controlled conduit connecting the engine's gas valve to atmosphere. Nitrogen is preferably the inert gas utilized.

The invention provides several advantages. In addition to permitting gas to enter the cylinder during normal operation, which is its main task, the gas valve can now be utilized for many direct auxiliary functions.

When the engine is running on pure diesel oil, a controlled amount of inert gas to cool the gas valve can be introduced

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into the combustion chamber through the gas valve. The inert gas is preferably introduced when the piston is near its lower dead center position, and thus will not disturb the operation of the engine.

When a need arises for rapid evacuation of the gas in the conduit system leading from the gas storage tank to the gas valve, the gas valve is opened at the beginning of the engine's compression stroke, with gradual and controlled introduction of inert gas to the conduit system as the boil- off gas is depleted and the pressure decreases. The entire conduit system that normally supplies boil-off gas to the engine will thus be rapidly neutralized (rendered inert) by flushing it with inert gas, and it is not necessary to provide exhaust lines from the valves.

The system can also be used for turning over the main engine by supplying a controlled amount of inert gas sequentially, adapting the pressure to the correct cylinders, depending on the position of the piston and the desired direction of rotation. By increasing the pressure, the valves and the control system can also be used as an engine starter. For this purpose, it may be preferable to use available high pressure compressors, and possibly replace the inert gas with air. The latter procedure requires special routines and fail-safe mechanisms to prevent any admixture of air and hydrocarbon gases.

Another possible engine starter procedure for use when the engine is running on gas, i.e., when the supply system and valves are pressurized by the boil-off gas fuel, is as follows: The cylinders will be filled with air at various pressures, depending on the position of the piston and how long the engine has been at a standstill. By introducing fuel gas which could for -instance be ignited by spark plugs, instant combustion can be obtained in one or a few suitable cylinders to start the engine.

The following auxiliary functions have been discussed in the preceding paragraphs:

- Effective cooling of the gas valves by injection of inert gas - Rapid evacuation of gas from the conduit system and render¬ ing the system inert, by means of the gas valves that are open to the engine cylinders

- Turning over the engine

- Starting the engine with high pressure inert gas/air - Starting the engine with boil-off gas and an ignition source

It is very easy to change the control of the gas valves as necessary for the above purposes, simply by expanding the microprocessor programs.

The invention will be discussed in greater detail with reference to the accompanying drawings, wherein

Figure 1 is a schematic drawing of the system according to the invention, and

Figure 2 shows a modified embodiment of a system in accordance with the invention.

On both drawings, the engine room, i.e., the arrangement therein, is on the left-hand side while the cargo region is on the right. The difference between the two systems is mainly on the engine room side. For components that are the same on both figures, the same reference numbers are used.

Figure 1 shows a cargo tank (spherical tankl 1 containing LNG on board a ship. Gas boil-off is drawn off at the top of the tank and guided through a conduit 2 to a four-stage compressor 3 where the gas is compressed to a suitable pressure, say 200 bar. The compressor is driven by an electric motor (not illustrated).. Cooling is necessary after the third and fourth stages, and this occurs in the heat exchangers 4.

In practice, two compressors will be utilized, each having the capacity to take the entire amount of boil-off. gas. The power requirement for compressing the normal boil-off will be about 600 kW in a practical embodiment. At normal operation, all the boil-off gas will be led to the engine, here repre¬ sented by engine cylinders 5 and 6, through a manifold 7. The engine thus uses the boil-off gas as fuel. The amount of boil-off delivered to the engine is determined by the tank pressure and will be essentially constant, with small, slow changes depending on weather conditions.

Changes in the power needs for the ship's propulsion will be controlled by means of the diesel oil (the necessary diesel oil supply arrangement is not shown in the drawings) , down to a power consumption corresponding to the minimum amount of pilot diesel oil needed for ignition (about 5-9%) , plus the boil-off gas. When the power requirement sinks below this level, the quantity of boil-off gas supplied to the engine is reduced. Excess boil-off gas then goes to the buffer tanks 8. In a practical embodiment, say a 130,000 m LNG tanker with 0.11% boil-off per 24 hours, 12 cylinders that are 6 meters long and have an internal diameter of 1.25 meters will have sufficient capacity to hold the entire amount of boil-off for 6 hours. To supplement the buffer tanks 8, some of the excess boil-off can be stored in the spherical tanks 1, since a slight pressure build-up is permissible in these tanks. Therefore, it is possible to store the boil-off gas for a period of time well in excess of 6 hours and not have to rely on utilizing the gas as fuel. The actual period of time will depend on the ambient condi¬ tions and the pressure in the tanks 1. -Assuming a tank pressure of 1.03 bar, an ambient temperature of 25°C, calm seas and safety valves set at 1.25 bar, this combination of buffer tanks and pressure build-up in the cargo tanks would give sufficient storage capacity to store all boil-off for a period of about 30 hours.

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The spherical tanks 1 will constitute a major part of the storage capacity for the boil-off, but the provision of the buffer tanks 8 will ensure a continuous flow of boil-off gas from the tanks 1. This is necessary to prevent a sudden increase in the tank pressure, which would occur if the boil-off were completely shut off.

The 30-hour period of time mentioned above can be increased if the boil-off is also used as fuel for auxiliary machinery, such as generator motors and inert gas systems. For safety reasons, however, no more than one gas-utilizing piece of machinery should be found in the engine room. The spherical cargo tanks can also be constructed to withstand a higher pressure than 1.25 bar.

When the power requirement has been low for a period of time and then increases, the pressure in the buffer tanks and/or the cargo tanks will be reduced by increasing the gas flow to the maximum possible rate. Both of the above-mentioned compressors can then be run in parallel.

The normal amount of boil-off will constitute about 60% of the total fuel for the main machinery when the ship is laden and about 40% in the ballast state. These figures are based on a 130,000 m LNG tanker with a cruising speed of 18 knots. The time necessary to reduce the pressure in the buffer and cargo tanks from the maximum to atmospheric pressure will be about 46 hours when the ship is carrying cargo and about 32 hours under ballast. These times can be reduced if the boil- off is used as fuel for auxiliary machinery in addition to its use in the main engine.

The gas in the buffer tanks 8 is supplied to a compressor stage at a suitable pressure to minimize the compressor work when the stored boil-off is used as fuel.

Figure 1 shows, a vent hood 9 arranged over the machinery. In both Figures 1 and 2, the engine room and the cargo area are

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separated by a bulkhead 10. Between the bulkhead 10 and the hood 9, the gas conduit 7 is a double-walled pipe, and a stream of air flows in the annular space between the inner and outer pipes. Gas leaks will be discovered by a gas detector 11 which is connected to a ventilator 12. The ventilator- 12 draws air through the conduit 13, which is a continuation of the conduit in the engine room that surrounds the gas conduit 7. An air conduit 14 also extends from the hood 9 to a ventilator 15 which also is connected to a gas detector 16.

The gas conduit 7 will have a relatively small cross section owing to the high gas pressure. The total amount of gas in the system on the downstream side of the valve block 17, in the practical embodiment illustrated, will constitute only

3 15 kg, which corresponds to about 20 m atmospheric pressure.

This corresponds to about 10 seconds' consumption of boil- off gas.

As mentioned above, the arrangement shown in Figure 2 is in principle constructed in the same way as that in Figure 1. The difference is that in the engine room the hood 9 is replaced by a double-walled conduit guide 18 extending all the way down to the top of the machinery. A ventilator 19 is connected to this double-walled pipe system 18 via a conduit 20 and draws air through this conduit. Any gas will be detected by the gas detector 21.

The arrangement in Figure 2 has the advantage that, unlike the arrangement in Figure 1, gas cannot slip out beneath the hood and enter the engine room, but it has the drawback that maintenance will be more complicated, because the injection valves and various shut-off valves and instruments will be located within the outer pipe or cover.

In Figures land 2, the gas conduit 7 can be supplied with inert gas from an inert gas source N ₂ , usually nitrogen. The gas conduits can thus be flushed with nitrogen all the

way through to the valve seat of the injection valves. The injection valves, i.e., the gas valves, are in both figures designated by numerals 22,23 (for the engine cylinder 5) and 24,25 (for the engine cylinder 6)..

Anywhere that the respective conduits are open to the atmos¬ phere, the conduits are depicted with curved ends to symbolize release of gas to the atmosphere.

The mode of operation and the various possible utilizations for the two arrangements shown in Figures 1 and 2 will be obvious to a skilled person, since standard international symbols are used for the components in the figures. Never¬ theless, a brief description of some of the possible uses of the system follows .

It will be especially necessary to cool the gas valves when the valves are not in use, i.e., when the engine is running on oil alone. The supply of boil-off gas to the gas conduit 7 is then shut off and nitrogen can be introduced through the conduit 7 into the combustion chambers . Only a small amount of inert gas is needed for cooling, and it is intro¬ duced through the valves every time the piston is near the lower dead center position. The expansion of the inert gas flowing through the valve will cause effective cooling of the valve.

The gas conduit system can be neutralized by flushing it with nitrogen. By shutting off the supply of gas from the buffer tanks 8 ₇ the gas in the system can be evacuated very quickly, simply by allowing the gas to enter the engine cylinders. This is preferably done by changing the injection time to the beginning of the compression stroke, when the counterpressure is low. Thereafter, the engine runs in a transient phase like a normal dual-fuel engine _. As the gas gradually is depleted in the conduit system and the pressure decreases, nitrogen gas is supplied. The entire system is

thereby rapidly made inert, and it is not necessary to utilize an exhaust line from the valves.

By controlling the inert gas to effect a sequential supply of inert gas at suitable pressure to the correct engine cylinders, depending on the position of the piston and the desired direction of rotation, the system of the invention can also be used to turn over the engine. By increasing the pressure, the valves and associated conduit system can also be used as an engine starter. It may in this case be advisable to use a high pressure compressor, and perhaps replace the inert gas with air. Such a procedure (not shown in the drawings) will require special routines and fail-safe mechanisms to prevent admixture of air and hydrocarbon gas.

Another possible starting procedure for use under gas pres¬ sure, i.e., when the gas supply system and the valves are pressurized by gas, is a procedure wherein the cylinders are filled with air at varying pressures, depending on the position of the piston and how long the engine has been at a standstill. By introducing gas which could be ignited by spark plugs, one can obtain almost instantaneous combustion in one or a few suitable cylinders, and start up the engine.

The use of two valves 22,23 and 24,25 in the respective engine cylinders 5, 6 is advantageous because it allows one to proceed by introducing gas. first through one valve and thereafter also through the other valve, thereby obtaining a desirable slow introduction of gas followed by an increased supply. This procedure is beneficial for combustion.

The system of the invention also permits the combustion gas to be analyzed in a better way, by removing gas at the desired position of the piston and just for a moment, thereby obtaining an analysis at the instant of combustion and not, as is otherwise the case, in the exhaust phase.

Figures 1 and 2 show only one cargo tank (spherical tank) 1, but it should be obvious that this represents all of the cargo tanks, as there are usually four or five large tanks on an LNG carrier. Increased safety is an important advan¬ tage obtained with the invention, because the valves can be cooled, and above all, because the conduit system can be flushed with inert gas while the engine is running.